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Molybdenum(VI) Nitrido Complexes with Tripodal Silanolate Ligands. Structure and Electronic Character of an Unsymmetrical Dimolybdenum μ-Nitrido Complex Formed by Incomplete Nitrogen Atom Transfer. Inorg Chem 2024; 63:8376-8389. [PMID: 38663089 PMCID: PMC11080062 DOI: 10.1021/acs.inorgchem.4c00762] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2024] [Revised: 04/05/2024] [Accepted: 04/11/2024] [Indexed: 05/07/2024]
Abstract
In contrast to a tungsten nitrido complex endowed with a tripodal silanolate ligand framework, which was reported in the literature to be a dimeric species with a metallacyclic core, the corresponding molybdenum nitrides 3 are monomeric entities comprising a regular terminal nitride unit, as proven by single-crystal X-ray diffraction (SC-XRD). Their electronic character is largely determined by the constraints imposed on the metal center by the podand ligand architecture. 95Mo nuclear magnetic resonance (NMR) and, to a lesser extent, 14N NMR spectroscopy allow these effects to be studied, which become particularly apparent upon comparison with the spectral data of related molybdenum nitrides comprising unrestrained silanolate, alkoxide, or amide ligands. Attempted nitrogen atom transfer from these novel terminal nitrides to [(tBuArN)3Mo] (Ar = 3,5-dimethylphenyl) as the potential acceptor stopped at the stage of unsymmetric dimolybdenum μ-nitrido complex 13a as the first intermediate along the reaction pathway. SC-XRD, NMR, electron paramagnetic resonance, and ultraviolet-visible spectroscopy as well as magnetometry in combination with density functional theory allowed a clear picture of the geometric and electronic structure of this mixed-valent species to be drawn. 13a is formally best described as an adduct of the type [(Mo[O])+III-(μN)-III-(Mo[N])+VI], S = 1/2 complex with (Mo[O])+III in the low-spin configuration, whereas related complexes such as [(AdS)3Mo-(μN)-Mo(NtBuAr)3] (19; Ad = 1-adamantyl) have previously been regarded in the literature as mixed-valent Mo+IV/Mo+V species. The spin population at the two Mo centers is uneven and notably larger at the more reduced Mo[O] atom, whereas the only spin present at the (μN) bridge is derived from spin polarization.
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2
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Elucidating the Electronic Nature of Rh-based Paddlewheel Catalysts from 103 Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations. Chemistry 2024; 30:e202301846. [PMID: 37721802 DOI: 10.1002/chem.202301846] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2023] [Revised: 09/18/2023] [Accepted: 09/18/2023] [Indexed: 09/20/2023]
Abstract
The tremendous importance of dirhodium paddlewheel complexes for asymmetric catalysis is largely the result of an empirical optimization of the chiral ligand sphere about the bimetallic core. It was only recently that a H(C)Rh triple resonance 103 Rh NMR experiment provided the long-awaited opportunity to examine - with previously inconceivable accuracy - how variation of the ligands impacts on the electronic structure of such catalysts. The recorded effects are dramatic: formal replacement of only one out of eight O-atoms surrounding the metal centers in a dirhodium tetracarboxylate by an N-atom results in a shielding of the corresponding Rh-site of no less than 1000 ppm. The current paper provides the theoretical framework that allows this and related experimental observations made with a set of 19 representative rhodium complexes to be interpreted. In line with symmetry considerations, it is shown that the shielding tensor responds only to the donor ability of the equatorial ligands along the perpendicular principal axis. Axial ligands, in contrast, have no direct effect on shielding but may come into play via the electronicc i s ${cis}$ -effect that they exert onto the neighboring equatorial sites. On top of these fundamental interactions, charge redistribution within the core as well as the electronict r a n s ${trans}$ -effect of ligands of different donor strengths is reflected in the recorded 103 Rh NMR shifts.
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3
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NMR spectroscopy of a 18O-labeled rhodium paddlewheel complex: Isotope shifts, 103Rh-103Rh spin-spin coupling, and 103Rh singlet NMR. J Chem Phys 2024; 160:014305. [PMID: 38174793 DOI: 10.1063/5.0182233] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2023] [Accepted: 12/10/2023] [Indexed: 01/05/2024] Open
Abstract
Despite the importance of rhodium complexes in catalysis, and the favorable 100% natural abundance of the spin-1/2 103Rh nucleus, there are few reports of 103Rh nuclear magnetic resonance (NMR) parameters in the literature. In part, this is the consequence of the very low gyromagnetic ratio of 103Rh and its dismal NMR sensitivity. In a previous paper [Harbor-Collins et al., J. Chem. Phys. 159, 104 307 (2023)], we demonstrated an NMR methodology for 1H-enhanced 103Rh NMR and demonstrated an application to the 103Rh NMR of the dirhodium formate paddlewheel complex. In this paper, we employ selective 18O labeling to break the magnetic equivalence of the 103Rh spin pair of dirhodium formate. This allows the estimation of the 103Rh-103Rh spin-spin coupling and provides access to the 103Rh singlet state. We present the first measurement of a 18O-induced 103Rh secondary isotope shift as well as the first instance of singlet order generated in a 103Rh spin pair. The field-dependence of 103Rh singlet relaxation is measured by field-cycling NMR experiments.
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4
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Catalytic asymmetric cationic shifts of aliphatic hydrocarbons. Nature 2024; 625:287-292. [PMID: 38200298 PMCID: PMC10781632 DOI: 10.1038/s41586-023-06826-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2023] [Accepted: 11/02/2023] [Indexed: 01/12/2024]
Abstract
Asymmetric catalysis is an advanced area of chemical synthesis, but the handling of abundantly available, purely aliphatic hydrocarbons has proven to be challenging. Typically, heteroatoms or aromatic substructures are required in the substrates and reagents to facilitate an efficient interaction with the chiral catalyst. Confined acids have recently been introduced as tools for homogenous asymmetric catalysis, specifically to enable the processing of small unbiased substrates1. However, asymmetric reactions in which both substrate and product are purely aliphatic hydrocarbons have not previously been catalysed by such super strong and confined acids. We describe here an imidodiphosphorimidate-catalysed asymmetric Wagner-Meerwein shift of aliphatic alkenyl cycloalkanes to cycloalkenes with excellent regio- and enantioselectivity. Despite their long history and high relevance for chemical synthesis and biosynthesis, Wagner-Meerwein reactions utilizing purely aliphatic hydrocarbons, such as those originally reported by Wagner and Meerwein, had previously eluded asymmetric catalysis.
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5
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From the Glovebox to the Benchtop: Air-Stable High Performance Molybdenum Alkylidyne Catalysts for Alkyne Metathesis. J Am Chem Soc 2023; 145:26993-27009. [PMID: 38032858 PMCID: PMC10722517 DOI: 10.1021/jacs.3c10430] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2023] [Revised: 11/03/2023] [Accepted: 11/06/2023] [Indexed: 12/02/2023]
Abstract
Molybdenum alkylidynes endowed with tripodal silanolate ligands belong to the most active and selective catalysts for alkyne metathesis known to date. This paper describes a new generation that is distinguished by an unprecedented level of stability and practicality without sacrificing the chemical virtues of their predecessors. Specifically, pyridine adducts of type 16 are easy to make on gram scale, can be routinely weighed and handled in air, and stay intact for many months outside the glovebox. When dissolved in toluene, however, spontaneous dissociation of the stabilizing pyridine ligand releases an active species of excellent performance and functional group tolerance. Specifically, a host of polar and apolar groups, various protic sites, and numerous basic functionalities proved compatible. The catalysts are characterized by crystallographic and spectroscopic means, including 95Mo NMR; their activity and stability are benchmarked in detail, and the enabling properties are illustrated by advanced applications to natural product synthesis. For the favorable overall application profile and ease of handling, complexes of this new series are expected to replace earlier catalyst generations and help encourage a more regular use of alkyne metathesis in general.
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Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes. Angew Chem Int Ed Engl 2023; 62:e202313578. [PMID: 37769154 DOI: 10.1002/anie.202313578] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2023] [Revised: 09/28/2023] [Accepted: 09/28/2023] [Indexed: 09/30/2023]
Abstract
Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.
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7
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Bi-Catalyzed Trifluoromethylation of C(sp 2)-H Bonds under Light. J Am Chem Soc 2023; 145:25538-25544. [PMID: 37963280 PMCID: PMC10690797 DOI: 10.1021/jacs.3c10333] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2023] [Revised: 11/01/2023] [Accepted: 11/03/2023] [Indexed: 11/16/2023]
Abstract
We disclose a Bi-catalyzed C-H trifluoromethylation of (hetero)arenes using CF3SO2Cl under light irradiation. The catalytic method permits the direct functionalization of various heterocycles bearing distinct functional groups. The structural and computational studies suggest that the process occurs through an open-shell redox manifold at bismuth, comprising three unusual elementary steps for a main group element. The catalytic cycle starts with rapid oxidative addition of CF3SO2Cl to a low-valent Bi(I) catalyst, followed by a light-induced homolysis of Bi(III)-O bond to generate a trifluoromethyl radical upon extrusion of SO2, and is closed with a hydrogen-atom transfer to a Bi(II) radical intermediate.
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8
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Taming secondary benzylic cations in catalytic asymmetric S N1 reactions. Science 2023; 382:325-329. [PMID: 37856595 DOI: 10.1126/science.adj7007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2023] [Accepted: 09/05/2023] [Indexed: 10/21/2023]
Abstract
Benzylic stereogenic centers are ubiquitous in natural products and pharmaceuticals. A potentially general, though challenging, approach toward their selective creation would be asymmetric unimolecular nucleophilic substitution (SN1) reactions that proceed through highly reactive benzylic cations. We now report a broadly applicable solution to this problem by identifying chiral counteranions that pair with secondary benzylic cations to engage in catalytic asymmetric C-C, C-O, and C-N bond-forming reactions with excellent enantioselectivity. The critical cationic intermediate can be accessed from different precursors via Lewis- or Brønsted acid catalysis. Key to our strategy is the use of only weakly basic, confined counteranions that are posited to prolong the lifetime of the carbocation, thereby avoiding nonproductive deprotonation pathways to the corresponding styrene.
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9
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The 103Rh NMR spectroscopy and relaxometry of the rhodium formate paddlewheel complex. J Chem Phys 2023; 159:104307. [PMID: 37698193 DOI: 10.1063/5.0165830] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2023] [Accepted: 08/15/2023] [Indexed: 09/13/2023] Open
Abstract
The nuclear magnetic resonance (NMR) spectroscopy of spin-1/2 nuclei with low gyromagnetic ratio is challenging due to the low NMR signal strength. Methodology for the rapid acquisition of 103Rh NMR parameters is demonstrated for the case of the rhodium formate "paddlewheel" complex Rh2(HCO2)4. A scheme is described for enhancing the 103Rh signal strength by polarization transfer from 1H nuclei, which also greatly reduces the interference from ringing artifacts, a common hurdle for the direct observation of low-γ nuclei. The 103Rh relaxation time constants T1 and T2 are measured within 20 min by using 1H-detected experiments. The field dependence of the 103Rh T1 is measured. The high-field relaxation is dominated by the chemical shift anisotropy mechanism. The 103Rh shielding anisotropy is found to be very large: |Δσ| = 9900 ± 540 ppm. This estimate is compared with density functional theory calculations.
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Oxidative Addition of Aryl Electrophiles into a Red-Light-Active Bismuthinidene. J Am Chem Soc 2023; 145:18742-18747. [PMID: 37603853 PMCID: PMC10472430 DOI: 10.1021/jacs.3c06651] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2023] [Indexed: 08/23/2023]
Abstract
The oxidative addition of aryl electrophiles is a fundamental organometallic reaction widely applied in the field of transition metal chemistry and catalysis. However, the analogous version based on main group elements still remains largely underexplored. Here, we report the ability of a well-defined organobismuth(I) complex to undergo formal oxidative addition with a wide range of aryl electrophiles. The process is facilitated by the reactivity of both the ground and excited states of N,C,N-bismuthinidenes upon absorption of low-energy red light.
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11
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Direct Regioselective Dehydrogenation of α-Substituted Cyclic Ketones. Angew Chem Int Ed Engl 2023; 62:e202307081. [PMID: 37337974 DOI: 10.1002/anie.202307081] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2023] [Revised: 06/19/2023] [Accepted: 06/20/2023] [Indexed: 06/21/2023]
Abstract
We disclose a highly regioselective, catalytic one-step dehydrogenation of α-substituted cyclic ketones in the presence of 2,3-dichlorobenzo-5,6-dicyano-1,4-benzoquinone (DDQ). The high regioselectivity originates from a phosphoric acid-catalyzed enolization, selectively affording the thermodynamically preferred enol, followed by the subsequent oxidation event. Our method provides reliable access to several α-aryl and α-alkyl substituted α,β-unsaturated ketones.
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12
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Toward a Formyl-to-Phenyl Conversion: An Unexpected Photochemical Fulvene Rearrangement. Angew Chem Int Ed Engl 2023; 62:e202303119. [PMID: 37329283 DOI: 10.1002/anie.202303119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2023] [Revised: 06/16/2023] [Accepted: 06/16/2023] [Indexed: 06/19/2023]
Abstract
Toward a conversion of aldehydes into arenes, we designed a sequence involving the initial reaction of an aldehyde to give a fulvene, followed by photochemical and platinum-catalyzed rearrangements into a Dewar benzene derivative, which finally isomerizes into the targeted arene. While computational studies support the plausibility of this route, we found that fulvene irradiation resulted in an unexpected isomerization into a spiro[2.4]heptadiene. This unusual photorearrangement has been investigated mechanistically and provides access to a variety of spiro[2.4]heptadienes with different substituents.
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Abstract
Since its discovery more than a century ago, the Friedel-Crafts reaction has manifested itself as a powerful method for the introduction of carbon substituents to arenes. Despite its potential generality, the scope of the reaction is intrinsically limited by the arene's nucleophilicity, which has previously restrained the applicability of asymmetric variants to activated substrates. To overcome this fundamental limitation, we report herein an asymmetric Friedel-Crafts reaction of unactivated, purely hydrocarbon arenes, alkoxybenzenes, and heteroarenes with N,O-acetals to give enantioenriched arylglycine esters. Highly regio- and stereoselective C-C bond formation was achieved using strong and confined Brønsted acid organocatalysts, enabling the first asymmetric catalytic Friedel-Crafts reaction of simple alkylbenzenes.
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14
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Bismuth-Catalyzed Amide Reduction. Angew Chem Int Ed Engl 2023:e202306447. [PMID: 37283567 DOI: 10.1002/anie.202306447] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2023] [Revised: 06/05/2023] [Accepted: 06/05/2023] [Indexed: 06/08/2023]
Abstract
In this article we report that a cationic version of Akiba's Bi(III) complex catalyzes the reduction of amides to amines using silane as hydride donor. The catalytic system features low catalyst loadings and mild conditions, en route to secondary and tertiary aryl- and alkylamines. The system permits that functional groups such as alkene, alkyne, ester, nitrile, furan, and thiophene remain intact. Kinetic studies on the reaction mechanism result in the identification of a reaction network with an important product inhibition that is in agreement with the experimental reaction profiles.
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Dihydrogen and Ethylene Activation by a Sterically Distorted Distibene. Angew Chem Int Ed Engl 2023:e202302071. [PMID: 37265121 DOI: 10.1002/anie.202302071] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2023] [Revised: 04/21/2023] [Accepted: 05/30/2023] [Indexed: 06/03/2023]
Abstract
Herein, we report the synthesis of a sterically distorted distibene ([4]2) and its transition-metal-like reactivity towards two fundamental feedstock chemicals: H2 and ethylene. Although [4]2 exhibits an unusually long Sb=Sb distance and noticeable backbone distortion in the solid state, NMR data suggest that [4]2 remains predominantly as a dimer in solution, even at high temperatures. However, it was proposed that the elusive reactivity of [4]2 toward H2 and ethylene results from reversible dissociation of [4]2 into the transient stibinidene ([4]), which could be observed by NMR spectroscopic techniques.
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Bismuth radical catalysis in the activation and coupling of redox-active electrophiles. Nat Chem 2023:10.1038/s41557-023-01229-7. [PMID: 37264103 PMCID: PMC10396954 DOI: 10.1038/s41557-023-01229-7] [Citation(s) in RCA: 8] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2022] [Accepted: 05/03/2023] [Indexed: 06/03/2023]
Abstract
Radical cross-coupling reactions represent a revolutionary tool to make C(sp3)-C and C(sp3)-heteroatom bonds by means of transition metals and photoredox or electrochemical approaches. However, the use of main-group elements to harness this type of reactivity has been little explored. Here we show how a low-valency bismuth complex is able to undergo one-electron oxidative addition with redox-active alkyl-radical precursors, mimicking the behaviour of first-row transition metals. This reactivity paradigm for bismuth gives rise to well-defined oxidative addition complexes, which could be fully characterized in solution and in the solid state. The resulting Bi(III)-C(sp3) intermediates display divergent reactivity patterns depending on the α-substituents of the alkyl fragment. Mechanistic investigations of this reactivity led to the development of a bismuth-catalysed C(sp3)-N cross-coupling reaction that operates under mild conditions and accommodates synthetically relevant NH-heterocycles as coupling partners.
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Synthesis and isolation of a triplet bismuthinidene with a quenched magnetic response. Science 2023:eadg2833. [PMID: 37200451 DOI: 10.1126/science.adg2833] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2022] [Accepted: 05/04/2023] [Indexed: 05/20/2023]
Abstract
Large Spin-Orbit Coupling (SOC) is an intrinsic property of the heavy-elements that directly affects the electronic structures of the compounds. Herein we report the synthesis and characterization of a mono-coordinate bismuthinidene featuring a rigid and bulky ligand. All magnetic measurements (SQUID, NMR) point to a diamagnetic compound. However, multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated (76%) by a spin-triplet. The apparent diamagnetism is explained by an extremely large SOC induced positive zero-field-splitting of more than 4500 cm-1 that leaves the MS = 0 magnetic sublevel thermally isolated in the electronic ground state.
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Abstract
Despite recent advancements in the development of catalytic asymmetric electrophile induced lactonization reactions of olefinic carboxylic acids, the archetypical hydrolactonization has long remained an unsolved and well-recognized challenge. Here, we report the realization of a catalytic asymmetric hydrolactonization using a confined imidodiphosphorimidate (IDPi) Brønsted acid catalyst. The method is operationally simple, scalable, and compatible with a wide variety of substrates. Its potential is showcased with concise syntheses of the sesquiterpenes (-)-boivinianin A and (+)-gossonorol. Through in-depth physicochemical and DFT analyses, we derive a nuanced picture of the mechanism and enantioselectivity of this reaction.
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Synthesis, Isolation, and Characterization of Two Cationic Organobismuth(II) Pincer Complexes Relevant in Radical Redox Chemistry. J Am Chem Soc 2023; 145:5618-5623. [PMID: 36854169 PMCID: PMC10021010 DOI: 10.1021/jacs.2c12564] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/02/2023]
Abstract
Herein, we report the synthesis, isolation, and characterization of two cationic organobismuth(II) compounds bearing N,C,N pincer frameworks, which model crucial intermediates in bismuth radical processes. X-ray crystallography uncovered a monomeric Bi(II) structure, while SQUID magnetometry in combination with NMR and EPR spectroscopy provides evidence for a paramagnetic S = 1/2 state. High-resolution multifrequency EPR at the X-, Q-, and W-band enable the precise assignment of the full g- and 209Bi A-tensors. Experimental data and DFT calculations reveal both complexes are metal-centered radicals with little delocalization onto the ligands.
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Organocatalytic DYKAT of Si-Stereogenic Silanes. J Am Chem Soc 2023; 145:4994-5000. [PMID: 36826435 PMCID: PMC9999423 DOI: 10.1021/jacs.3c00858] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/25/2023]
Abstract
Chiral organosilanes do not exist in nature and are therefore absent from the "chiral pool". As a consequence, synthetic approaches toward enantiopure silanes, stereogenic at silicon, are rather limited. While catalytic asymmetric desymmetrization reactions of symmetric organosilicon compounds have been developed, the utilization of racemic silanes in a dynamic kinetic asymmetric transformation (DYKAT) or dynamic kinetic resolution (DKR) would significantly expand the breadth of accessible Si-stereogenic compounds. We now report a DYKAT of racemic allyl silanes enabled by strong and confined imidodiphosphorimidate (IDPi) catalysts, providing access to Si-stereogenic silyl ethers. The products of this reaction are easily converted into useful enantiopure monohydrosilanes. We propose a spectroscopically and experimentally supported mechanism involving the epimerization of a catalyst-bound intermediate.
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Catalytic asymmetric synthesis of cannabinoids and menthol from neral. Nature 2023; 615:634-639. [PMID: 36859552 PMCID: PMC10033408 DOI: 10.1038/s41586-023-05747-9] [Citation(s) in RCA: 7] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2022] [Accepted: 01/20/2023] [Indexed: 03/03/2023]
Abstract
The selective conversion of natural or synthetic neral to (1R,6S)-trans-isopiperitenol would enable and expedite sustainable routes to menthol1,2 and cannabinoids3-5. However, this reaction has been considered impossible because its product is more reactive to the required acid catalysts than its starting material, resulting in several side products6-9. We now show that an unsymmetric, strong and confined chiral acid, a highly fluorinated imino-imidodiphosphate, catalyses this process with excellent efficiency and selectivity. Expanding the method to other α,β-unsaturated aldehydes could enable access to new cannabinoids and menthol derivatives not readily accessible previously. Mechanistic studies suggest that the confined catalyst accomplishes this reaction by binding the product in an unreactive conformation, thereby preventing its decomposition. We also show how (1R,6S)-trans-isopiperitenol can be readily converted to pharmaceutically useful cannabinoids and menthol, each in the shortest and most atom-economic routes so far.
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Bio-Inspired Deaminative Hydroxylation of Aminoheterocycles and Electron-Deficient Anilines. Angew Chem Int Ed Engl 2023; 62:e202212219. [PMID: 36479796 PMCID: PMC10107619 DOI: 10.1002/anie.202212219] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2022] [Indexed: 11/24/2022]
Abstract
Among the tools available to chemists for drug design of bioactive compounds, the bioisosteric replacement of atoms or groups of atoms is the cornerstone of modern strategies. Despite the undeniable interest in amino-to-hydroxyl interchange, enzymatic deaminative hydroxylation remains unmatched. Herein, we report a user friendly and safe procedure to selectively convert aminoheterocycles to their hydroxylated analogues by means of a simple pyrylium tetrafluoroborate salt. The hydroxylation step relies on a Lossen-type rearrangement under mild conditions thus avoiding the use of strong hydroxide bases. In addition to biorelevant heterocycles, the deaminative hydroxylation of electron-deficient anilines was also demonstrated. Finally, mechanistic experiments allowed the identification of the key intermediates, thus unveiling a rather unusual mechanism for this formal aromatic substitution.
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Selective oxidation of silanes into silanols with water using [MnBr(CO) 5] as a precatalyst. Chem Sci 2022; 14:54-60. [PMID: 36605749 PMCID: PMC9769106 DOI: 10.1039/d2sc05959b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2022] [Accepted: 11/18/2022] [Indexed: 12/03/2022] Open
Abstract
The development of earth-abundant catalysts for the selective conversion of silanes to silanols with water as an oxidant generating valuable hydrogen as the only by-product continues to be a challenge. Here, we demonstrate that [MnBr(CO)5] is a highly active precatalyst for this reaction, operating under neutral conditions and avoiding the undesired formation of siloxanes. As a result, a broad substrate scope, including primary and secondary silanes, could be converted to the desired products. The turnover performances of the catalyst were also examined, yielding a maximum TOF of 4088 h-1. New light was shed on the debated mechanism of the interaction between [MnBr(CO)5] and Si-H bonds based on the reaction kinetics (including KIEs of PhMe2SiD and D2O) and spectroscopic techniques (FT-IR, GC-TCD, 1H-, 29Si-, and 13C-NMR). The initial activation of [MnBr(CO)5] was found to result from the formation of a manganese(i) hydride species and R3SiBr, and the experimental data are most consistent with a catalytic cycle comprising a cationic tricarbonyl Mn(i) unit as the active framework.
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Bio‐Inspired Deaminative Hydroxylation of Aminoheterocycles and Electron‐Deficient Anilines. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202212219] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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25
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Abstract
The development of unconventional strategies for the activation of ammonia (NH3) and water (H2O) is of capital importance for the advancement of sustainable chemical strategies. Herein we provide the synthesis and characterization of a radical equilibrium complex based on bismuth featuring an extremely weak Bi-O bond, which permits the in situ generation of reactive Bi(II) species. The ensuing organobismuth(II) engages with various amines and alcohols and exerts an unprecedented effect onto the X-H bond, leading to low BDFEX-H. As a result, radical activation of various N-H and O-H bonds─including ammonia and water─occurs in seconds at room temperature, delivering well-defined Bi(III)-amido and -alkoxy complexes. Moreover, we demonstrate that the resulting Bi(III)-N complexes engage in a unique reactivity pattern with the triad of H+, H-, and H• sources, thus providing alternative pathways for main group chemistry.
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Abstract
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In this article, we describe a combined experimental
and theoretical
mechanistic investigation of the C(sp2)–F bond formation
from neutral and cationic high-valent organobismuth(V) fluorides,
featuring a dianionic bis-aryl sulfoximine ligand. An exhaustive assessment
of the substitution pattern in the ligand, the sulfoximine, and the
reactive aryl on neutral triarylbismuth(V) difluorides revealed that
formation of dimeric structures in solution promotes facile Ar–F
bond formation. Noteworthy, theoretical modeling of reductive elimination
from neutral bismuth(V) difluorides agrees with the experimentally
determined kinetic and thermodynamic parameters. Moreover, the addition
of external fluoride sources leads to inactive octahedral anionic
Bi(V) trifluoride salts, which decelerate reductive elimination. On
the other hand, a parallel analysis for cationic bismuthonium fluorides
revealed the crucial role of tetrafluoroborate anion as fluoride source.
Both experimental and theoretical analyses conclude that C–F
bond formation occurs through a low-energy five-membered transition-state
pathway, where the F anion is delivered to a C(sp2) center,
from a BF4 anion, reminiscent of the Balz–Schiemann
reaction. The knowledge gathered throughout the investigation permitted
a rational assessment of the key parameters of several ligands, identifying
the simple sulfone-based ligand family as an improved system for the
stoichiometric and catalytic fluorination of arylboronic acid derivatives.
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27
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An Anionic Dinuclear Ruthenium Dihydrogen Complex of Relevance for Alkyne
gem
‐Hydrogenation. Angew Chem Int Ed Engl 2022; 61:e202201311. [PMID: 35363926 PMCID: PMC9322539 DOI: 10.1002/anie.202201311] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2022] [Indexed: 11/12/2022]
Abstract
During an investigation into the fate of ruthenium precatalysts used for light‐driven alkyne gem‐hydrogenation reactions with formation of Grubbs‐type ruthenium catalysts, it was found that the reaction of [(IPr)(η6‐cymene)RuCl2] with H2 under UV‐irradiation affords an anionic dinuclear σ‐dihydrogen complex, which is thermally surprisingly robust. Not only are anionic σ‐complexes in general exceedingly rare, but the newly formed species seems to be the first example lacking any structural attributes able to counterbalance the negative charge and, in so doing, prevent oxidative insertion of the metal centers into the ligated H2 from occurring.
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28
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Design of an Organocatalytic Asymmetric (4 + 3) Cycloaddition of 2-Indolylalcohols with Dienolsilanes. J Am Chem Soc 2022; 144:8460-8466. [PMID: 35523203 PMCID: PMC9121375 DOI: 10.1021/jacs.2c02216] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
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Here we present the
design of a highly enantioselective, catalytic
(4 + 3) cycloaddition of gem-dialkyl 2-indolyl alcohols
and dienolsilanes, enabled by strong and confined IDPi Lewis acids.
The method furnishes novel bicyclo[3.2.2]cyclohepta[b]indoles with up to three stereogenic centers, one of which is quaternary.
A broad substrate scope is accompanied by versatile downstream chemical
modifications. Density functional theory-supported mechanistic studies
shed light on the importance of the in situ generated silylium species
in an overall concerted yet asynchronous cycloaddition.
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29
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An Anionic Dinuclear Ruthenium Dihydrogen Complex of Relevance for Alkyne gem‐Hydrogenation. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202201311] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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30
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Abstract
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gem-Hydrogenation of an internal alkyne with the
aid of [Cp*RuCl]4 as the precatalyst is a highly unorthodox
transformation, in which one C atom of the triple bond is transformed
into a methylene group, whereas the second C atom gets converted into
a ruthenium carbene. In the case of 1,3-enynes bearing a propargylic
steering substituent as the substrates, the reaction occurs regioselectively,
giving rise to vinyl carbene complexes that adopt interconverting
η1/η3-binding modes in solution;
a prototypical example of such a reactive intermediate was characterized
in detail by spectroscopic means. Although both forms are similarly
stable, only the η3-vinyl carbene proved kinetically
competent to insert into primary, secondary, or tertiary C–H
bonds on the steering group itself or another suitably placed ether,
acetal, orthoester, or (sulfon)amide substituent. The ensuing net
hydrogenative C–H insertion reaction is highly enabling in
that it gives ready access to spirocyclic as well as bridged ring
systems of immediate relevance as building blocks for medicinal chemistry.
Moreover, the reaction scales well and lends itself to the formation
of partly or fully deuterated isotopologues. Labeling experiments
in combination with PHIP NMR spectroscopy (PHIP = parahydrogen induced
polarization) confirmed that the reactions are indeed triggered by gem-hydrogenation, whereas kinetic data provided valuable
insights into the very nature of the turnover-limiting transition
state of the actual C–H insertion step.
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31
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Direkte Katalytische
C
‐Glykosylierung von Arenen: beschleunigte Synthese des Remdesivir‐Nucleosids**. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202114619] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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32
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Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne
gem
‐Hydrogenation. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202113827] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
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33
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Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem-Hydrogenation. Angew Chem Int Ed Engl 2021; 61:e202113827. [PMID: 34911159 PMCID: PMC9306539 DOI: 10.1002/anie.202113827] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2021] [Indexed: 11/10/2022]
Abstract
gem‐Hydrogenation of propargyl alcohol derivatives with [CpXRu(MeCN)3]PF6 (CpX=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events.
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34
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Direct and Catalytic C-Glycosylation of Arenes: Expeditious Synthesis of the Remdesivir Nucleoside. Angew Chem Int Ed Engl 2021; 61:e202114619. [PMID: 34856043 PMCID: PMC9305923 DOI: 10.1002/anie.202114619] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2021] [Indexed: 12/01/2022]
Abstract
Since early 2020, scientists have strived to find an effective solution to fight SARS‐CoV‐2, in particular by developing reliable vaccines that inhibit the spread of the disease and repurposing drugs for combatting its effects on the human body. The antiviral prodrug Remdesivir is still the most widely used therapeutic during the early stages of the infection. However, the current synthetic routes rely on the use of protecting groups, air‐sensitive reagents, and cryogenic conditions, thus impeding a cost‐efficient supply to patients. We have, therefore, focused on the development of a straightforward, direct addition of (hetero)arenes to unprotected sugars. Here we report a silylium‐catalyzed and completely stereoselective C‐glycosylation that initially yields the open‐chain polyols, which can be selectively cyclized to provide either the kinetic α‐furanose or the thermodynamically favored β‐anomer. The method significantly expedites the synthesis of Remdesivir precursor GS‐441524 after a subsequent Mn‐catalyzed C−H oxidation and deoxycyanation.
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Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap. Chemistry 2021; 27:14025-14033. [PMID: 34293239 PMCID: PMC8518412 DOI: 10.1002/chem.202102080] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2021] [Indexed: 11/28/2022]
Abstract
Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework ("canopy catalysts") are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo≡CR entities in a stoichiometric triple-bond metathesis event to furnish RC≡CR and the corresponding dinuclear complex, 8, with a Mo≡Mo core. In addition to the regular analytical techniques, 95 Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo-tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris-alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo-oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single-crystal X-ray diffraction.
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36
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Confinement-Controlled, Either syn- or anti-Selective Catalytic Asymmetric Mukaiyama Aldolizations of Propionaldehyde Enolsilanes. J Am Chem Soc 2021; 143:14475-14481. [PMID: 34436899 PMCID: PMC8447262 DOI: 10.1021/jacs.1c07447] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
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Protected aldols
(i.e., true aldols derived from aldehydes) with
either syn- or anti- stereochemistry
are versatile intermediates in many oligopropionate syntheses. Traditional
stereoselective approaches to such aldols typically require several
nonstrategic operations. Here we report two highly enantioselective
and diastereoselective catalytic Mukaiyama aldol reactions of the
TBS- or TES- enolsilanes of propionaldehyde with aromatic aldehydes.
Our reactions directly deliver valuable silyl protected propionaldehyde
aldols in a catalyst controlled manner, either as syn- or anti- isomer. We have identified a privileged
IDPi catalyst motif that is tailored for controlling these aldolizations
with exceptional selectivities. We demonstrate how a single atom modification
in the inner core of the IDPi catalyst, replacing a CF3-group with a CF2H-group, leads to a dramatic switch in
enantiofacial differentiation of the aldehyde. The origin of this
remarkable effect was attributed to tightening of the catalytic cavity
via unconventional C–H hydrogen bonding of the CF2H group.
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37
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Catalytic Hydrodefluorination via Oxidative Addition, Ligand Metathesis, and Reductive Elimination at Bi(I)/Bi(III) Centers. J Am Chem Soc 2021; 143:12487-12493. [PMID: 34358426 PMCID: PMC8377712 DOI: 10.1021/jacs.1c06735] [Citation(s) in RCA: 34] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
Herein, we report a hydrodefluorination reaction of polyfluoroarenes catalyzed by bismuthinidenes, Phebox-Bi(I) and OMe-Phebox-Bi(I). Mechanistic studies on the elementary steps support a Bi(I)/Bi(III) redox cycle that comprises C(sp2)-F oxidative addition, F/H ligand metathesis, and C(sp2)-H reductive elimination. Isolation and characterization of a cationic Phebox-Bi(III)(4-tetrafluoropyridyl) triflate manifests the feasible oxidative addition of Phebox-Bi(I) into the C(sp2)-F bond. Spectroscopic evidence was provided for the formation of a transient Phebox-Bi(III)(4-tetrafluoropyridyl) hydride during catalysis, which decomposes at low temperature to afford the corresponding C(sp2)-H bond while regenerating the propagating Phebox-Bi(I). This protocol represents a distinct catalytic example where a main-group center performs three elementary organometallic steps in a low-valent redox manifold.
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38
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Triple Resonance Experiments for the Rapid Detection of 103Rh NMR Shifts: A Combined Experimental and Theoretical Study into Dirhodium and Bismuth-Rhodium Paddlewheel Complexes. J Am Chem Soc 2021; 143:12473-12479. [PMID: 34351134 PMCID: PMC8377716 DOI: 10.1021/jacs.1c06414] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
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A H(C)Rh triple resonance
NMR experiment makes the rapid detection
of 103Rh chemical shifts possible, which were previously
beyond reach. It served to analyze a series of dirhodium and bismuth–rhodium
paddlewheel complexes of the utmost importance for metal–carbene
chemistry. The excellent match between the experimental and computed 103Rh shifts in combination with a detailed analysis of the
pertinent shielding tensors forms a sound basis for a qualitative
and quantitative interpretation of these otherwise (basically) inaccessible
data. The observed trends clearly reflect the influence exerted by
the equatorial ligands (carboxylate versus carboxamidate), the axial
ligands (solvents), and the internal “metalloligand”
(Rh versus Bi) on the electronic estate of the reactive Rh(II) center.
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39
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Abstract
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Molybdenum alkylidyne
complexes of the “canopy catalyst”
series define new standards in the field of alkyne metathesis. The
tripodal ligand framework lowers the symmetry of the metallacyclobutadiene
complex formed by [2 + 2] cycloaddition with the substrate and imposes
constraints onto the productive [2 + 2] cycloreversion; pseudorotation
corrects this handicap and makes catalytic turnover possible. A combined
spectroscopic, crystallographic, and computational study provides
insights into this unorthodox mechanism and uncovers the role that
metallatetrahedrane complexes play in certain cases.
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40
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183 W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis. Angew Chem Int Ed Engl 2020; 59:21758-21768. [PMID: 32820864 PMCID: PMC7756321 DOI: 10.1002/anie.202009975] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2020] [Revised: 08/21/2020] [Indexed: 12/12/2022]
Abstract
Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. 1 H,183 W heteronuclear multiple bond correlation spectroscopy, exploiting a favorable 5 J-coupling between the 183 W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over-stabilized and the catalytic turnover brought to a halt. Guided by the 183 W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly π-donating all-alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentate tert-butoxy ligands, in terms of rate and functional-group compatibility.
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41
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Abstract
In this article, we investigated the I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles originally reported by Hillhouse. A detailed mechanistic investigation based on spectroscopic and crystallographic analysis revealed that a putative reductive elimination to forge C(sp3)-OC(sp3) using I2 might not be operative. We isolated a paramagnetic bimetallic NiIII intermediate featuring a unique Ni2(OR)2 (OR = alkoxide) diamond-like core complemented by a μ-iodo bridge between the two Ni centers, which remains stable at low temperatures, thus permitting its characterization by NMR, EPR, X-ray, and HRMS. At higher temperatures (>-10 °C), such bimetallic intermediate thermally decomposes to afford large amounts of elimination products together with iodoalkanols. Observation of the latter suggests that a C(sp3)-I bond reductive elimination occurs preferentially to any other challenging C-O bond reductive elimination. Formation of cyclized THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp3)-I bond. The results of this article indicate that the use of F+ oxidants permits the challenging C(sp3)-OC(sp3) bond formation at a high-valent nickel center to proceed in good yields while minimizing deleterious elimination reactions. Preliminary investigations suggest the involvement of a high-valent bimetallic NiIII intermediate which rapidly extrudes the C-O bond product at remarkably low temperatures. The new set of conditions permitted the elusive synthesis of diethyl ether through reductive elimination, a remarkable feature currently beyond the scope of Ni.
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Abstract
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Herein
we present the catalytic activation of N2O at
a BiI⇄BiIII redox platform. The activation
of such a kinetically inert molecule was achieved by the use of bismuthinidene
catalysts, aided by HBpin as reducing agent. The protocol features
remarkably mild conditions (25 °C, 1 bar N2O), together
with high turnover numbers (TON, up to 6700) and turnover frequencies
(TOF). Analysis of the elementary steps enabled structural characterization
of catalytically relevant intermediates after O-insertion, namely
a rare arylbismuth oxo dimer and a unique monomeric arylbismuth hydroxide.
This protocol represents a distinctive example of a main-group redox
cycling for the catalytic activation of N2O.
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43
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Ruthenium-Catalyzed trans-Hydroalkynylation and trans-Chloroalkynylation of Internal Alkynes. J Am Chem Soc 2020; 142:18746-18752. [PMID: 33095568 PMCID: PMC7660751 DOI: 10.1021/jacs.0c08582] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
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[Cp*RuCl]4 catalyzes the
addition of iPr3SiC≡CX (X = H,
Cl) across internal alkynes with
formation of 1,3-enyne or 1-chloro-1,3-enyne derivatives, respectively;
the reaction follows an unorthodox trans-addition
mode. The well-balanced affinities of the different reaction partners
to the ruthenium catalyst ensure that crossed addition prevails over
homodimerization of the individual components, as can be deduced from
spectroscopic and crystallographic data of various intermediates;
this includes a dinuclear complex in which an internal alkyne bridges
two [Cp*RuCl] fragments.
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44
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Catalytic enantiocontrol over a non-classical carbocation. Nat Chem 2020; 12:1174-1179. [PMID: 32989271 DOI: 10.1038/s41557-020-00558-1] [Citation(s) in RCA: 27] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/27/2019] [Accepted: 08/21/2020] [Indexed: 12/28/2022]
Abstract
Carbocations can be categorized into classical carbenium ions and non-classical carbonium ions. These intermediates are ubiquitous in reactions of both fundamental and practical relevance, finding application in the petroleum industry as well as the discovery of new drugs and materials. Conveying stereochemical information to carbocations is therefore of interest to a range of chemical fields. While previous studies targeted systems proceeding through classical ions, enantiocontrol over their non-classical counterparts has remained unprecedented. Here we show that strong and confined chiral acids catalyse enantioselective reactions via the non-classical 2-norbornyl cation. This reactive intermediate is generated from structurally different precursors by leveraging the reactivity of various functional groups to ultimately deliver the same enantioenriched product. Our work demonstrates that tailored catalysts can act as suitable hosts for simple, non-functionalized carbocations via a network of non-covalent interactions. We anticipate that the methods described herein will provide catalytic accessibility to valuable carbocation systems.
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45
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183
W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis. Angew Chem Int Ed Engl 2020. [DOI: 10.1002/ange.202009975] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
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46
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The Silicon-Hydrogen Exchange Reaction: A Catalytic σ-Bond Metathesis Approach to the Enantioselective Synthesis of Enol Silanes. J Am Chem Soc 2020; 142:13695-13700. [PMID: 32786813 PMCID: PMC7426905 DOI: 10.1021/jacs.0c06677] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
The use of chiral enol silanes in fundamental transformations such as Mukaiyama aldol, Michael, and Mannich reactions as well as Saegusa-Ito dehydrogenations has enabled the chemical synthesis of enantiopure natural products and valuable pharmaceuticals. However, accessing these intermediates in high enantiopurity has generally required the use of either stoichiometric chiral precursors or stoichiometric chiral reagents. We now describe a catalytic approach in which strongly acidic and confined imidodiphosphorimidates (IDPi) catalyze highly enantioselective interconversions of ketones and enol silanes. These "silicon-hydrogen exchange reactions" enable access to enantiopure enol silanes via tautomerizing σ-bond metatheses, either in a deprotosilylative desymmetrization of ketones with allyl silanes as the silicon source or in a protodesilylative kinetic resolution of racemic enol silanes with a carboxylic acid as the silyl acceptor.
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47
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Geminal parahydrogen-induced polarization: accumulating long-lived singlet order on methylene proton pairs. MAGNETIC RESONANCE (GOTTINGEN, GERMANY) 2020; 1:175-186. [PMID: 37904826 PMCID: PMC10500696 DOI: 10.5194/mr-1-175-2020] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 06/19/2020] [Accepted: 07/27/2020] [Indexed: 11/01/2023]
Abstract
In the majority of hydrogenative parahydrogen-induced polarization (PHIP) experiments, the hydrogen molecule undergoes pairwise cis addition to an unsaturated precursor to occupy vicinal positions on the product molecule. However, some ruthenium-based hydrogenation catalysts induce geminal hydrogenation, leading to a reaction product in which the two hydrogen atoms are transferred to the same carbon centre, forming a methylene (CH 2 ) group. The singlet order of parahydrogen is substantially retained over the geminal hydrogenation reaction, giving rise to a singlet-hyperpolarized CH 2 group. Although the T 1 relaxation times of the methylene protons are often short, the singlet order has a long lifetime, provided that singlet-triplet mixing is suppressed, either by chemical equivalence of the protons or by applying a resonant radiofrequency field. The long lifetime of the singlet order enables the accumulation of hyperpolarization during the slow hydrogenation reaction. We introduce a kinetic model for the behaviour of the observed hyperpolarized signals, including both the chemical kinetics and the spin dynamics of the reacting molecules. Our work demonstrates the feasibility of producing singlet-hyperpolarized methylene moieties by parahydrogen-induced polarization. This potentially extends the range of molecular agents which may be generated in a hyperpolarized state by chemical reactions of parahydrogen.
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48
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Strong and Confined Acids Control Five Stereogenic Centers in Catalytic Asymmetric Diels-Alder Reactions of Cyclohexadienones with Cyclopentadiene. Angew Chem Int Ed Engl 2020; 59:12347-12351. [PMID: 32159921 PMCID: PMC7383742 DOI: 10.1002/anie.202000307] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2020] [Revised: 02/03/2020] [Indexed: 12/14/2022]
Abstract
We describe a highly enantioselective Diels-Alder reaction of cross-conjugated cyclohexadienones with cyclopentadiene, in which five stereocenters are effectively controlled by a strongly acidic and confined imidodiphosphorimidate catalyst. Our approach provides tricyclic products in excellent stereoselectivity. We also report methods to convert the obtained products into useful intermediates and a computational study that aids in gaining deeper insight into the reaction mechanism and origin of stereoselectivity.
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49
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"Canopy Catalysts" for Alkyne Metathesis: Molybdenum Alkylidyne Complexes with a Tripodal Ligand Framework. J Am Chem Soc 2020; 142:11279-11294. [PMID: 32463684 PMCID: PMC7322728 DOI: 10.1021/jacs.0c04742] [Citation(s) in RCA: 42] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
![]()
A new family of structurally well-defined
molybdenum alkylidyne
catalysts for alkyne metathesis, which is distinguished by a tripodal
trisilanolate ligand architecture, is presented. Complexes of type 1 combine the virtues of previous generations of silanolate-based
catalysts with a significantly improved functional group tolerance.
They are easy to prepare on scale; the modularity of the ligand synthesis
allows the steric and electronic properties to be fine-tuned and hence
the application profile of the catalysts to be optimized. This opportunity
is manifested in the development of catalyst 1f, which
is as reactive as the best ancestors but exhibits an unrivaled scope.
The new catalysts work well in the presence of unprotected alcohols
and various other protic groups. The chelate effect entails even a
certain stability toward water, which marks a big leap forward in
metal alkylidyne chemistry in general. At the same time, they tolerate
many donor sites, including basic nitrogen and numerous heterocycles.
This aspect is substantiated by applications to polyfunctional (natural)
products. A combined spectroscopic, crystallographic, and computational
study provides insights into structure and electronic character of
complexes of type 1. Particularly informative are a density
functional theory (DFT)-based chemical shift tensor analysis of the
alkylidyne carbon atom and 95Mo NMR spectroscopy; this
analytical tool had been rarely used in organometallic chemistry before
but turns out to be a sensitive probe that deserves more attention.
The data show that the podand ligands render a Mo-alkylidyne a priori
more electrophilic than analogous monodentate triarylsilanols; proper
ligand tuning, however, allows the Lewis acidity as well as the steric
demand about the central atom to be adjusted to the point that excellent
performance of the catalyst is ensured.
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Abstract
Herein, we report a radical borylation of aromatic amines through a homolytic C(sp2 )-N bond cleavage. This method capitalizes on a simple and mild activation via a pyrylium reagent (Sc Pyry-OTf) thus priming the amino group for reactivity. The combination of terpyridine and a diboron reagent triggers a radical reaction which cleaves the C(sp2 )-N bond and forges a new C(sp2 )-B bond. The unique non-planar structure of the pyridinium intermediate, provides the necessary driving force for the aryl radical formation. The method permits borylation of a wide variety of aromatic amines indistinctively of the electronic environment.
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