Cooperative Heterobimetallic Substrate Activation Enhances Catalytic Activity and Amplifies Regioselectivity in 1,4-Hydroboration of Pyridines

Synthesis of Quinoline-Based Pt-Sb Complexes with L- or Z-Type Interaction: Ligand-Controlled Redox via Anion Transfer

A series of Pt-Sb complexes with two or three L-type quinoline side arms were prepared and studied. Two ligands, tri(8-quinolinyl)stibane (SbQ3, Q = 8-quinolinyl, 1) and 8,8'-(phenylstibanediyl)diquinoline (SbQ2Ph, 2), were used to synthesize the PtII-SbIII complexes (SbQ3)PtCl2 (3) and (SbQ2Ph)PtCl2 (4). Chloride abstraction with AgOAc provided the bis-acetate complexes (SbQ3)Pt(OAc)2 (5) and (SbQ2Ph)Pt(OAc)2 (6). To better understand the electronic effects of the Sb moiety, analogous bis-chloride complexes were oxidized to an overall formal oxidation state of +7 (i.e., Pt + Sb formal oxidation states = 7) using dichloro(phenyl)-λ3-iodane (PhICl2) and 3,4,5,6-tetrachloro-1,2-dibenzoquinone (o-chloranil) as two-electron oxidants. Depending on the oxidant, different conformational changes occur within the coordination sphere of Pt as confirmed by single-crystal X-ray diffraction and NMR spectroscopy. In addition, the nature of Pt-Sb interactions was evaluated via molecular and localized orbital calculations.

Synthesis and utility of N-boryl and N-silyl enamines derived from the hydroboration and hydrosilylation of N-heteroarenes and N-conjugated compounds

Catalytic hydroboration and hydrosilylation have emerged as promising strategies for the reduction of unsaturated hydrocarbons and carbonyl compounds, as well as for the dearomatization of N-heteroarenes. Various catalysts have been employed in these processes to achieve the formation of reduced products via distinct reaction pathways and intermediates. Among these intermediates, N-silyl enamines and N-boryl enamines, which are derived from hydrosilylation and hydroboration, are commonly underestimated in this reduction process. Because these versatile intermediates have recently been utilized in situ as nucleophilic reagents or dipolarophiles for the synthesis of diverse molecules, an expeditious review of the synthesis and utilization of N-silyl and N-boryl enamines is crucial. In this review, we comprehensively discuss a wide range of hydrosilylation and hydroboration catalysts used for the synthesis of N-silyl and N-boryl enamines. These catalysts include main-group metals (e.g., Mg and Zn), transition metals (e.g., Rh, Ru, and Ir), earth-abundant metals (e.g., Fe, Co, and Ni), and non-metal catalysts (including P, B, and organocatalysts). Furthermore, we highlight recent research efforts that have leveraged these versatile intermediates for the synthesis of intriguing molecules, offering insights into future directions for these invaluable building blocks.

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Lanthanide/B(C6F5)3-Promoted Hydroboration Reduction of Indoles and Quinolines with Pinacolborane

We have developed a lanthanide/B(C6F5)3-promoted hydroboration reduction of indoles and quinolines with pinacolborane (HBpin). This reaction provides streamlined access to a range of nitrogen-containing compounds in moderate to excellent yields. Large-scale synthesis and further transformations to bioactive compounds indicate that the method has potential practical applications. Preliminary mechanistic studies suggest that amine additives promote the formation of indole-borane intermediates, and the lanthanide/B(C6F5)3-promoted hydroboration reduction proceeds via hydroboration of indole-borane intermediates with HBpin and in situ-formed BH3 species, followed by the protodeborylation process.

Gold-Manganese Bimetallic Redox Coupling with Light

The classical Au(I)/Au(III) redox couple chemistry has been limited to constructing C-C and C-X bonds, and thus, the exploration of the elementary reaction of gold redox coupling is very significant to enrich its organometallic features. Herein, we report the first visible-light-mediated, external oxidant-free Au(I)/Au(III) redox couple using commercially available Mn2(CO)10 to generate Mn-Au(III)-Mn intermediates for bimetallic redox coupling. A wide range of structurally diverse heterodinuclear and polynuclear L-Au(I)-Mn-L' complexes (19 examples, up to >99% yields) are readily constructed, providing a robust strategy for the concise construction of Au-Mn complexes under mild reaction conditions. The mechanistic studies together with DFT calculations support the radical oxidative addition of •Mn(CO)5 to gold and bimetallic reductive elimination mechanisms from highly active Mn-Au(III)-Mn species, representing an important step toward an elementary reaction in gold chemistry research. Furthermore, the resulting Au-Mn complexes exhibit unique catalytic activity, with which divergent reductive coupling of nitroarenes can readily afford azoxybenzenes, azobenzenes, and hydrazobenzenes in moderate to good yields.

Cobalt-Catalyzed Regioselective 1,2-Hydroboration of N-Heteroarenes

Regioselective hydroboration of pyridines to 1,2-dihydropyridines remains a significant challenge for the synthesis of valuable nitrogenous bioactive molecules. Herein, we report a base free ligand-controlled cobalt-catalyzed 1,2-hydroboration of pyridines and quinolines with very low catalyst loading under neat reaction conditions. The choice of sterically demanding N-heterocyclic ligands led to the 1,2-regioselectivity and the scope was demonstrated by the N-heterocycles having a variety of functional groups. The preliminary mechanistic studies corroborate that the two ligands followed a distinct catalytic cycle with Co(I) as an active species.

Intermediates, Isolation and Mechanistic Insights into Zinc Hydride-Catalyzed 1,2-Regioselective Hydrofunctionalization of N-Heteroarenes

The conjugated bis-guanidinate-supported zinc hydride [{LZnH}₂; L = {(ArHN) (ArN)-C═N-C═(NAr) (NHAr); Ar = 2,6-Et₂-C₆H₃}] (I)-catalyzed highly demanding exclusive 1,2-regioselective hydroboration and hydrosilylation of N-heteroarenes is demonstrated with excellent yields. This protocol is compatible with many pyridines and N-heteroarene derivatives, including electron-donating and -withdrawing substituents. Catalytic intermediates, such as [(LZnH) (4-methylpyridine)] IIA, [(L'ZnH) (4-methylpyridine) IIA', where L' = CH{(CMe) (2,6-Et₂C₆H₃N)}₂)], LZn(1,2-DhiQ) (isoquinoline) III, [L'Zn(1,2-DhiQ) (isoquinoline)] III', and LZn(1,2-(3-MeDHQ)) (3-methylquinoline) V, were isolated and thoroughly characterized by NMR, HRMS, and IR analyses. Furthermore, X-ray single-crystal diffraction studies confirmed the molecular structures of compounds IIA', III, and III'. The NMR data proved that the intermediate III or III' reacted with HBpin and gave a selective 1,2-addition hydroborated product. Stoichiometric experiments suggest that V and III independently reacted with silane, yielding selective 1,2-addition of mono- and bis-hydrosilylated products, respectively. Based on the isolation of intermediates and a series of stoichiometric experiments, plausible catalytic cycles were established. Furthermore, the intermolecular chemoselective hydroboration reaction over other reducible functionalities was studied.

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

Heterobi- and multimetallic complexes providing close proximity between several metal centers serve as active species in artificial and enzymatic catalysis, and in model systems, showing unique modes of metal-metal cooperative bond activation. Through the rational design of well-defined, unsymmetrical ligand scaffolds, we create a convenient approach to support the assembly of heterometallic species in a well-defined and site-specific manner, preventing them from scrambling and dissociation. In this perspective, we will outline general strategies for the design of unsymmetrical ligands to support heterobi- and multimetallic complexes that show reactivity in various types of heterometallic cooperative bond activation.

Small molecule activation with bimetallic systems: a landscape of cooperative reactivity

There is growing interest in the design of bimetallic cooperative complexes, which have emerged due to their potential for bond activation and catalysis, a feature widely exploited by nature in metalloenzymes, and also in the field of heterogeneous catalysis. Herein, we discuss the widespread opportunities derived from combining two metals in close proximity, ranging from systems containing multiple M-M bonds to others in which bimetallic cooperation occurs even in the absence of M⋯M interactions. The choice of metal pairs is crucial for the reactivity of the resulting complexes. In this context, we describe the prospects of combining not only transition metals but also those of the main group series, which offer additional avenues for cooperative pathways and reaction discovery. Emphasis is given to mechanisms by which bond activation occurs across bimetallic structures, which is ascribed to the precise synergy between the two metal atoms. The results discussed herein indicate a future landscape full of possibilities within our reach, where we anticipate that bimetallic synergism will have an important impact in the design of more efficient catalytic processes and the discovery of new catalytic transformations.

A Heterogeneous Iridium Catalyst for the Hydroboration of Pyridines

Sulfated zirconium oxide (SZO) capped with silylium-like ions reacts with (cod)Ir(py)Cl (cod = 1,5-cyclooctadiene; py = pyridine) to form [Ir(cod)py][SZO] (1) and Me₃SiCl. 1 can also be formed in reactions of phosphonium functionalized SZO and [Ir(cod)(OSi(O^tBu)₃]₂, which forms [Ir(cod)P(^tBu)₂Ph][SZO] (2), followed by reaction with pyridine to form 1. FTIR and ¹⁵N{¹H} MAS NMR spectroscopy are consistent with coordination of pyridine in 1 to an electrophilic iridium. 1 is moderately active in the dearomative hydroboration of pyridine. The primary product of this reaction is 1,2-dihydropyridine, which converts to the 1,4-dihydropyridine product at long reaction times. 1 catalyzes the dearomative hydroboration of a variety of substituted pyridines and is also reactive toward pyrazines and N-methylimidazole.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Synthesis of 1,4-Dihydropyridines and Related Heterocycles by Iodine-Mediated Annulation Reactions of N-Cyclopropyl Enamines

The annulation of N-cyclopropyl enamines to produce 1,4-dihydropyridine (1,4-DHP) derivatives is described. In the presence of molecular iodine (I₂), an N-cyclopropyl enamine substrate undergoes iodination, opening of the cyclopropyl ring, and annulation with a second molecule of the substrate to form the 1,4-DHP product. This reaction is amenable to gram-scale operations under mild reaction conditions with no transition metals being required. Further transformations of the 1,4-DHPs leads to related pyridine and bicyclic frameworks.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Transition-Metal-Catalyzed Nucleophilic Dearomatization of Electron-Deficient Heteroarenes

In recent decades, transition-metal-catalyzed nucleophilic dearomatization of electron-deficient heteroarenes, such as pyridines, quinolines, isoquinolines and nitroindoles, has become a powerful method for accessing unsaturated heterocycles. This short review summarizes nucleophilic dearomatizations of electron-deficient hetero­arenes with carbon- and heteroatom-based nucleophiles via transition-metal catalysis. A significant number of functionalized heterocycles are obtained via this transformation. Importantly, many of these reactions are carried out in an enantioselective manner by means of asymmetric catalysis, providing a unique method for the construction of enantio­enriched heterocycles.

1 Introduction

2 Transition-Metal-Catalyzed Nucleophilic Dearomatization of Hetero­arenes via Alkynylation

3 Transition-Metal-Catalyzed Nucleophilic Dearomatization of Heteroarenes­ via Arylation

4 Transition-Metal-Catalyzed Nucleophilic Dearomatization of Heteroarenes­ with Other Nucleophiles

5 Transition-Metal-Catalyzed Nucleophilic Dearomatization with Nucleophiles Formed In Situ

6 Conclusion and Outlook

Manganese-Catalyzed Hydroborations with Broad Scope

Reductive transformations of easily available oxidized matter are at the heart of synthetic manipulation and chemical valorization. The applications of catalytic hydrofunctionalization benefit from the use of liquid reducing agents and operationally facile setups. Metal‐catalyzed hydroborations provide a highly prolific platform for reductive valorizations of stable C=X electrophiles. Here, we report an especially facile, broad‐scope reduction of various functions including carbonyls, carboxylates, pyridines, carbodiimides, and carbonates under very mild conditions with the inexpensive pre‐catalyst Mn(hmds)₂. The reaction could be successfully applied to depolymerizations.

Synthesis and characterization of heteromultinuclear Ni/M clusters (M = Fe, Ru, W) including a paramagnetic (NHC)Ni-WCp*(CO)3 heterobinuclear complex

A diverse range of heteromultinuclear NiI/[MCO] clusters (MCO = CpFe(CO)2, CpRu(CO)2, Cp*W(CO)3) supported by a N-heterocyclic carbene ligand have been synthesized by reacting the NiI precursor, [IPrNi(μ-Cl)]2, with [MCO]- reagents under various conditions. Clusters with Ni2Fe2, NiFe2, Ni2Ru, Ni2Ru2, NiRu2, and Ni2W, and NiW cores were all characterized using NMR and IR spectroscopies and X-ray crystallography. The NiI-containing paramagnetic heterobinuclear species, IPrNi-Wp* (7), was further characterized by EPR spectroscopy and DFT calculations. Notably, unlike previously studied (NHC)CuI-[MCO] derivatives, complex 7 was found to coordinate Lewis bases like 3-chloropyridine to produce (IPr)(3-Clpy)NiWp* (9). Complex 9 further underwent thermolytic C-Cl activation, proposed to involve NHC-free [(3-Clpy)Ni(μ-Wp*)]2 (10), to provide the C-arylated N-heterocyclic carbene product, [IPr(py-3-yl)]+[Cp*WCl2(CO)2]- (11).

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Bifunctional activation of amine-boranes by the W/Pd bimetallic analogs of "frustrated Lewis pairs"

The reaction between basic [(PCP)Pd(H)] (PCP = 2,6-(CH2P(t-C4H9)2)2C6H4) and acidic [LWH(CO)3] (L = Cp (1a), Tp (1b); Cp = η5-cyclopentadienyl, Tp = κ3-hydridotris(pyrazolyl)borate) leads to the formation of bimolecular complexes [LW(CO)2(μ-CO)⋯Pd(PCP)] (4a, 4b), which catalyze amine-borane (Me2NHBH3, t BuNH2BH3) dehydrogenation. The combination of variable-temperature (1H, 31P{1H}, 11B NMR and IR) spectroscopies and computational (ωB97XD/def2-TZVP) studies reveal the formation of an η1-borane complex [(PCP)Pd(Me2NHBH3)]+[LW(CO3)]- (5) in the first step, where a BH bond strongly binds palladium and an amine group is hydrogen-bonded to tungsten. The subsequent intracomplex proton transfer is the rate-determining step, followed by an almost barrierless hydride transfer. Bimetallic species 4 are easily regenerated through hydrogen evolution in the reaction between two hydrides.

Catalytic Reactions by Heterobimetallic Carbonyl Complexes with Polar Metal–Metal Interactions

Heterobinuclear catalysts capable of bimetallic cooperative bond activation provide an alternative pathway to approach the discovery of novel and unique reactivity and selectivity in catalytic transformations, complementing more traditional mononuclear precious metal catalysts. This review summarizes recent advances in homogenous catalysis using heterobimetallic carbonyl catalysts with polar metal–metal interactions.

1 Introduction

2 Hydrogenation and Hydrofunctionalization

3 Carbonylation and Carboxylation

4 Oxidative Transformations

5 Conclusion and Outlook

Unexpected reactivity of a PONNOP 'expanded pincer' ligand

We report the synthesis, characterization and coordination chemistry of a new naphthyridine-derived phosphinite PONNOP expanded pincer ligand. As envisioned, the dinucleating ligand readily binds two copper(i) centers in close proximity, but undergoes an unexpected rearrangement in the presence of nickel(ii) salts to form an interesting PONNP pincer platform.

Synthesis and characterization of heterometallic complexes involving coinage metals and isoelectronic Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- ligands

The chemistry of coinage metal ions with Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- has been explored using Mes₃P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)₅][SbF₆], [(^Et2CAAC)Au-Fe(CO)₅][SbF₆] and [(Mes₃P)Au-Fe(CO)₅][SbF₆] shows that the ligand donor strength towards Au(i) follows the order Mes₃P > ^Et2CAAC > SIPr. These Fe(CO)₅ complexes show significant blue shifts in [small nu, Greek, macron]_CO bands relative to those observed for free Fe(CO)₅ as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)₅ compared to Ag(i) in (SIPr)Ag-Mn(CO)₅. The structural details of Mes₃PAu-Mn(CO)₅ are also presented. [Fe(CO)₄CN]^- afforded CN bridged coinage metal complexes with (IPr*)Au⁺, (SIPr)Ag⁺ and (SIPr)Cu⁺ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)₅]^- and [Fe(CO)₄CN]^- are stronger donors toward Au(i) than Fe(CO)₅. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.

Evidence for Simultaneous Dearomatization of Two Aromatic Rings under Mild Conditions in Cu(I)-Catalyzed Direct Asymmetric Dearomatization of Pyridine

Bis(phosphine) copper hydride complexes are uniquely able to catalyze direct dearomatization of unactivated pyridines with carbon nucleophiles, but the mechanistic basis for this result has been unclear. Here we show that, contrary to our initial hypotheses, the catalytic mechanism is monometallic and proceeds via dearomative rearrangement of the phenethylcopper nucleophile to a Cpara-metalated form prior to reaction at heterocycle C4. Our studies support an unexpected heterocycle-promoted pathway for this net 1,5-Cu-migration beginning with a doubly dearomative imidoyl-Cu-ene reaction. Kinetics, substituent effects, computational modeling, and spectroscopic studies support the involvement of this unusual process. In this pathway, the CuL₂ fragment subsequently mediates a stepwise Cope rearrangement of the doubly dearomatized intermediate to the give the C4-functionalized 1,4-dihydropyridine, lowering a second barrier that would otherwise prohibit efficient asymmetric catalysis.

Computational study of catalyst-controlled regiodivergent pathways in hydroboration of 1,3-dienes: mechanism and origin of regioselectivity

DFT calculations were performed to elucidate the origins of catalyst-controlled regioselectivity in the hydroboration of 2-substituted 1,3-dienes.