Combined Experimental and Computational Studies of Heterobimetallic Bi−Rh Paddlewheel Carboxylates as Catalysts for Metal Carbenoid Transformations

In Situ Observation of Elusive Dirhodium Carbenes and Studies on the Innate Role of Carboxamidate Ligands in Dirhodium Paddlewheel Complexes: A Combined Experimental and Computational Approach

Carboxamidates as equatorial ligands in dirhodium paddlewheel catalysts are widely believed to increase selectivity at the expense of reactivity. The results of the combined experimental and computational approach described in this paper show that one has to beware of such generalizations. First, 103Rh NMR revealed how strongly primary carboxamidates impact the electronic nature of the rhodium center they are bound to; at the same time, such ligands stabilize donor/acceptor carbenes by engaging their ester carbonyl group into peripheral interligand hydrogen bonding. This array benefits selectivity as well as reactivity if maintained along the entire reaction coordinate of a catalytic cyclopropanation. In settings where the hydrogen bond needs to be distorted for the reaction to proceed, however, it constitutes a significant enthalpic handicap. Representative examples for each scenario were analyzed by DFT; in both cases, the cyclopropanation step rather than carbene formation was found to be turnover-limiting. While this conclusion somehow contradicts the literature, it implied that the direct observation of highly reactive dirhodium carbenes in truly catalytic settings might be possible, even though the intermediates carry olefinic sites amenable to intramolecular cyclopropanation. Such in situ monitoring by NMR is without precedent, yet it was successful with the homoleptic catalyst [Rh2(OPiv)4] as well as with its heteroleptic sibling [Rh2(OPiv)3(acam)] comprising an acetamidate (acam); in the latter case, the carbene bound to the rhodium atom at the [O3N]-face was observed, which concurs with the computational data that this species is stabilized by the forecited interligand hydrogen bonding.

Elucidating the Electronic Nature of Rh-based Paddlewheel Catalysts from 103 Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations

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.

Borane catalyzed transesterification of tert-butyl esters using α-aryl α-diazoesters

The B(C6F5)3-catalyzed transesterification of a series of 3-alkenyl-oxindoles and other unsaturated tert-butyl esters with aryl-diazo esters is reported. This protocol is facile and generally high yielding proceeding under mild conditions and is remarkably chemoselective leaving the CC bonds intact.

[3+3]-Annulation of Cyclic Nitronates with Vinyl Diazoacetates: Diastereoselective Synthesis of Partially Saturated [1,2]Oxazino[2,3-b][1,2]oxazines and Their Base-Promoted Ring Contraction to Pyrrolo[1,2-b][1,2]oxazine Derivatives

A rhodium(II)-catalyzed reaction of cyclic nitronates (5,6-dihydro-4H-1,2-oxazine N-oxides) with vinyl diazoacetates proceeds as a [3+3]-annulation producing bicyclic unsaturated nitroso acetals (4a,5,6,7-tetrahydro-2H-[1,2]oxazino[2,3-b][1,2]oxazines). Optimization of reaction conditions revealed the use of Rh(II) octanoate as the preferred catalyst in THF at room temperature, which allows the preparation of target products in good yields and excellent diastereoselectivity. Under basic conditions, namely, the combined action of DBU and alcohol, these nitroso acetals undergo ring contraction of an unsaturated oxazine ring into the corresponding pyrrole. Both transformations can be performed in a one-pot fashion, thus constituting a quick approach to oxazine-annulated pyrroles from available starting materials, such as nitroalkenes, olefins, and diazo compounds.

B(C6F5)3-catalyzed cyclopropanation of 3-alkenyl-oxindoles with diazomethanes

Spirocyclopropane-oxindoles are key motifs in biologically active compounds and are versatile synthetic intermediates. Herein, we report a metal-free, B(C₆F₅)₃ catalyzed cyclopropanation of 3-alkenyl-oxindoles with diazomethanes. This provides 25 variants of spirocyclopropane-oxindole derivatives. These spirocyclopropane-oxindole products were obtained in good to excellent yields (up to 99%) and high diastereoselectivities (up to 20 : 1 d.r.) under mild reaction conditions and could be performed on a gram scale.

Chiral Bismuth-Rhodium Paddlewheel Complexes Empowered by London Dispersion: The C-H Functionalization Nexus

Heterobimetallic [BiRh] tetracarboxylate catalysts endowed with 1,3-disilylated phenylglycine paddlewheels benefit from interligand London dispersion. They were originally designed for asymmetric cyclopropanation but are now shown to perform very well in asymmetric C-H functionalization reactions too. Because of the confined ligand sphere about the derived donor/acceptor carbenes, insertions into unhindered methyl groups are kinetically favored, although methylene units also react with excellent levels of asymmetric induction; even gaseous ethane is a suitable substrate. Moreover, many functional groups in both partners are tolerated. The resulting products are synthetically equivalent to the outcome of traditional asymmetric ester alkylation, allylation, benzylation, propargylation and aldol reactions and therefore constitute a valuable nexus to more conventional chemical logic.

In-situ Kinetic Studies of Rh(II)-Catalyzed C-H Functionalization to Achieve High Catalyst Turnover Numbers

Detailed kinetic studies on the functionalization of unactivated hydrocarbon sp3 C-H bonds by dirhodium-catalyzed reaction of aryldiazoacetates revealed that the C-H functionalization step is rate-determining. The efficiency of this step was increased by using the hydrocarbon as solvent and using donor/acceptor carbenes with an electron-withdrawing substituent on the aryl donor group. The optimum catalyst for these reactions is the tetraphenylphthalimido derivative Rh2(R-TPPTTL)4 and a further beneficial refinement was obtained by using N,N'-dicyclohexylcarbodiimide as an additive. Under the optimum conditions with a catalyst loading of 0.001 mol %, effective enantioselective C-H functionalization (66-97% yield, 83-97% ee) was achieved of cycloalkanes with a range of aryldiazoacetates as long as the aryldiazoacetate was not to sterically demanding. The reaction with cyclohexane using a catalyst loading of 0.0005 mol % could be recharged twice with additional aryldiazoacetate, resulting in an overall dirhodium catalyst turnover number of 580,000.

Effect of Tethered, Axially Coordinated Ligands (TACLs) on Dirhodium(II,II) Catalyzed Cyclopropanation: A Linear Free Energy Relationship Study

Hammett correlation experiments were used to determine the influence of dirhodium(II,II) paddlewheel complexes with tethered, axially coordinated ligands (TACLs) on the selectivity of rhodium carbenoids in competitive cyclopropanation reactions. The results suggest that dirhodium(II,II) paddlewheel complexes with TACLs are less sensitive to changes in electronics and reduce selectivity in cyclopropanation reactions with acceptor-substituted rhodium carbenoids. Also, Hammett plots with aryl diazoacetates resulted in a nonlinear downward curvature, suggesting a change in the rate-limiting step of the carbene transfer reaction.

Triple Resonance Experiments for the Rapid Detection of 103Rh NMR Shifts: A Combined Experimental and Theoretical Study into Dirhodium and Bismuth-Rhodium Paddlewheel Complexes

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.

Bismuth species in the coordination sphere of transition metals: synthesis, bonding, coordination chemistry, and reactivity of molecular complexes

This contribution is focused on bismuth species in the coordination sphere of transition metals. In molecular transition metal complexes, three types of Bi-M bonding are considered, namely dative Bi→M interactions (with Bi acting as a donor), dative Bi←M interactions (with Bi acting as an acceptor) and covalent Bi-M interactions (M = transition metal). Synthetic routes to all three classes of compounds are outlined, the Bi-M bonding situation is discussed, trends in the geometric parameters and in the coordination chemistry of the compounds are addressed, and common spectroscopic properties are summarized. As an important part of this contribution, the reactivity of bismuth species in the coordination sphere of transition metal complexes in stoichiometric and catalytic reactions is highlighted.

A New Ligand Design Based on London Dispersion Empowers Chiral Bismuth-Rhodium Paddlewheel Catalysts

Heterobimetallic bismuth-rhodium paddlewheel complexes with phenylglycine ligands carrying TIPS-groups at the meta-positions of the aromatic ring exhibit outstanding levels of selectivity in reactions of donor/acceptor and donor/donor carbenes; at the same time, the reaction rates are much faster and the substrate scope is considerably wider than those of previous generations of chiral [BiRh] catalysts. As shown by a combined experimental, crystallographic, and computational study, the new catalysts draw their excellent application profile largely from the stabilization of the chiral ligand sphere by London dispersion (LD) interactions of the peripheral silyl substituents.

Well-Defined, Versatile and Recyclable Half-Sandwich Nickelacarborane Catalyst for Selective Carbene-Transfer Reactions

Catalytic carbene-transfer reactions constitute a class of highly useful transformations in organic synthesis. Although catalysts based on a range of transition-metals have been reported, the readily accessible nickel(II)-based complexes have been rarely used. Herein, an air-stable nickel(II)-carborane complex is reported as a well-defined, versatile and recyclable catalyst for selective carbene transfer reactions with low catalyst loading under mild conditions. This catalyst is effective for several types of reactions including diastereoselective cyclopropanation, epoxidation, selective X-H insertions (X = C, N, O, S, Si), particularly for the unprotected substrates. This represents a rare example of carborane ligands in base metal catalysis.

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M-E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

1,2,4-Triazolato paddlewheel dibismuth complexes with very short Bi(ii)–Bi(ii) bonds: bismuth(iii) oxidation of 1,2,4-triazolato anions into neutral N-1,2,4-triazolyl radicals

Bismuth(iii) oxidation of 1,2,4-triazolato anions allowed paddlewheel 1,2,4-triazolato dibismuth complexes to be isolated and the reaction involved the neutral N-1,2,4-triazolyl radicals that were evidenced by EPR analysis.

Intramolecular azavinyl carbene-triggered rearrangement of furans

An intramolecular rhodium-catalyzed reaction of 1-tosyl-1,2,3-triazoles with furans has been explored. The tosylimino functionality was found to play a significant chemical role participating in the subsequent domino-transformations of a key reaction intermediate. One of the reaction pathways leads to valuable 2-formyl- and 2-acetylpyridine building blocks, which could be obtained in one-pot starting from easily accessible (furan-2-ylmethyl)propargyl amines. An original method for the synthesis of highly functionalized indolizines from the obtained pyridines has been proposed.

A bromo-capped diruthenium(i,i) N-heterocyclic carbene compound for in situ bromine generation with NBS: catalytic olefin aziridination reactions

A bromo-capped diruthenium(i,i) complex activates NBS to produce bromine in situ, and thus catalyses bromine-mediated olefin aziridination reactions.

How Dirhodium Catalyst Controls the Enantioselectivity of [3 + 2]-Cycloaddition between Nitrone and Vinyldiazoacetate: A Density Functional Theory Study

The origin of enantioselectivity in the dirhodium-catalyzed [3 + 2]-cycloaddition of nitrone and vinyldiazoacetate has been investigated using dispersion-corrected density functional theory. Taking a more realistic account of bulky ligands in models of the dirhodium catalyst when investigating its catalytic behavior is crucial for describing the effects resulting from a high level of asymmetric induction. More than one active site can be located and the extra reactivity is provided by an electron-donation interaction between the substrate and an additional Rh2L4 catalyst.

Mechanistic Insight into the Intramolecular Benzylic C-H Nitrene Insertion Catalyzed by Bimetallic Paddlewheel Complexes: Influence of the Metal Centers

The intramolecular benzylic C-H amination catalyzed by bimetallic paddlewheel complexes was investigated by using density functional theory calculations. The metal-metal bonding characters were investigated and the structures featuring either a small HOMO-LUMO gap or a compact SOMO energy scope were estimated to facilitate an easier one-electron oxidation of the bimetallic center. The hydrogen-abstraction step was found to occur through three manners, that is, hydride transfer, hydrogen migration, and proton transfer. The imido N species are more preferred in the Ru-Ru and Pd-Mn cases whereas coexisting N species, namely, singlet/triplet nitrene and imido, were observed in the Rh-Rh and Pd-Co cases. On the other hand, the triplet nitrene N species were found to be predominant in the Pd-Ni and Pd-Zn systems. A concerted asynchronous mechanism was found to be modestly favorable in the Rh-Rh-catalyzed reactions whereas the Pd-Co-catalyzed reactions demonstrated a slight preference for a stepwise pathway. Favored stepwise pathways were seen in each Ru-Ru- and Pd-Mn-catalyzed reactions and in the triplet nitrene involved Pd-Ni and Pd-Zn reactions. The calculations suggest the feasibility of the Pd-Mn, Pd-Co, and Pd-Ni paddlewheel complexes as being economical alternatives for the expensive dirhodium/diruthenium complexes in C-H amination catalysis.

Expanding the family of heterobimetallic Bi-Rh paddlewheel carboxylate complexes via equatorial carboxylate exchange

Five novel homoleptic heterobimetallic bismuth(II)-rhodium(II) carboxylate complexes--BiRh(TPA)4 (1), BiRh(but)4 (2), BiRh(piv)4 (3), BiRh(esp)2 (4), and BiRh(OAc)4 (5)--were synthesized in good yields by equatorial ligand substitution starting from BiRh(TFA)4 (TPA = triphenylacetate, but = butyrate, piv = pivalate, esp = α,α,α',α'-tetramethyl-1,3-benzenedipropionate, OAc = acetate, and TFA = trifluoroacetate). We report here (1)H and (13)C{(1)H} NMR spectra and cyclic voltammograms for complexes , and IR spectra for all complexes. Irreversible redox waves appear between -1.4 to -1.5 V for [BiRh](3+/4+) couples and 1.3 to 1.5 V vs. Fc/Fc(+) for [BiRh](4+/5+) couples for complexes indicating a wide range of stability for the compounds. The X-ray crystal structure of reveals a Bi-Rh distance of 2.53 Å.

Asymmetric sequential Au(i)/chiral tertiary amine catalysis: an enone-formation/cyanosilylation sequence to synthesize optically active 3-alkenyloxindoles from diazooxindoles

A gold-catalyzed enone-formation and asymmetric cyanosilylation sequence is developed, providing enantioenriched 3-alkenyloxindoles from diazooxindoles, furans and trimethylsilyl cyanide (TMSCN).

Bioinspired Synthesis of a Sedaxane Metabolite Using Catalytic Vanadyl Acetylacetonate and Molecular Oxygen

A bioinspired synthesis of the sedaxane metabolite 2 from intermediate 3 using catalytic VO(acac)2 and O2 is described. Intermediate 3 was synthesized starting from 2-bromostyrene in four steps. The inner cyclopropyl ring of 3 was assembled with trans geometry using a highly diastereoselective Nishiyama cyclopropanation, and the outer hydroxycyclopropyl ring was installed using the Kulinkovich cyclopropanation. Additionally, conversion of 3 into 2 was demonstrated in in vitro microbial culture experiments consisting of bacteria and fungi.

Paddlewheel 1,2,4-diazaphospholide dibismuthanes with very short bismuth-bismuth single bonds

One-electron oxidation of the 1,2,4-diazaphospholide anion [3,5-R2dp](-) by BiCl3 generated several remarkable paddlewheel dibismuthanes [L2(Bi-Bi)L2] (L = η(1),η(1)-3,5-R2dp, R = tBu, iPr, or Ph) with very short Bi-Bi single bond lengths (2.7964(4)-2.8873(3) Å).

Mechanism and stereoselectivity of the Rh(ii)-catalyzed cyclopropanation of diazooxindole: a density functional theory study

A density functional theory study was performed to understand the detailed mechanisms and stereoselectivity of the rhodium(ii)-catalyzed cyclopropanation reactions with diazooxindole and styrene.

Versatile reactivity of Pd-catalysts: mechanistic features of the mono-N-protected amino acid ligand and cesium-halide base in Pd-catalyzed C–H bond functionalization

The C–H functionalization strategies, complexity in Pd-catalyzed chemical transformations, unprecedented Pd-clustering, base (Cs-halide) and weakly coordinated amino acid ligand effects.

D2-symmetric dirhodium catalyst derived from a 1,2,2-triarylcyclopropanecarboxylate ligand: design, synthesis and application

Dirhodium tetrakis-(R)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh(2)(R-BTPCP)(4)) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C-H functionalization/Cope rearrangement were achieved using Rh(2)(R-BTPCP)(4) as catalyst. The advantages of Rh(2)(R-BTPCP)(4) include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a D(2)-symmetric arrangement, but when the carbenoid binds to the catalyst, two of the p-bromophenyl groups on the ligands rotate outward to make room for the carbenoid and the approach of the substrate to the carbenoid.