Copper-catalyzed regio- and stereo-selective hydrosilylation of terminal allenes to access (E)-allylsilanes

How to Achieve Hydrogenation/Hydrofunctionalization via Metal Hydride Complexes

ConspectusMetal hydride (M-H) complexes have garnered widespread attention in the synthesis of fine chemicals, materials, agrochemicals, and pharmaceuticals owing to the remarkable reactivity of the M-H bonds. Specifically, M-H complexes are active intermediates that catalyze hydrogen-transfer reactions, leading to efficient hydrogenation and hydrofunctionalization of C═C/C═X (X = O or N) bonds in unsaturated organic substrates for the formation of new carbon-hydrogen, carbon-carbon, and carbon-heteroatom bonds.Our research group has long studied M-H transformation mechanisms, with significant advancements over the past decade. For this Account, we have drawn on our extensive expertise to investigate the mechanisms governing numerous M-H transformation-driven reactions, including the hydrogenation of inert C═X bonds in unsaturated compounds, the hydrofunctionalization of C═C/C═X bonds, dehydrogenative coupling, and C-H functionalizations. On the basis of these mechanistic investigations, we developed a series of representative M-H transformation models, which offer robust theoretical guidance for modulating the reactivity and selectivity of M-H complexes in hydrogenation and hydrofunctionalization.Our Account begins with the structures and properties of M-H complexes, which lead to homolytic and heterolytic cleavage in reactions with different conditions, showcasing the remarkable versatility of metal hydride reactivity. Based on these principles, three transformation modes are discussed. First, hydride transfer of low-oxidation-state M-H complexes is chiefly engaged because the hydrogen atom attached to the metal has a high electron density and is strongly nucleophilic. In this case, a hydrogen atom serves as a hydride to transfer from the metal center to the electropositive center of the substrate through the following pathways: (a) insertion of an unsaturated bond into the M-H bond; (b) direct hydride transfer from the metal center to the electrophilic site of an unsaturated bond; (c) σ-bond metathesis; and (d) oxidative hydrogen migration. Reductive elimination might also occur when the oxidation state of the metal center increases and the metal center becomes electron-deficient. This usually regenerates the low-oxidation-state catalytic species while producing C/X'-H bonds. Notably, metal hydride hydrogen atom transfer (MHAT) is an advanced approach to radical-type hydrofunctionalizations. MHAT is usually induced by (a) a one-electron redox process enabled by a paramagnetic metal or (b) low M-H bond dissociation energy (BDE) values. Two possible types of MHAT (i.e., spontaneous and passive), which lead to different regioselectivities, are proposed. This article provides a detailed account of the strategies and mechanisms related to the reactivity and selectivity of M-H bond transformations, thus offering valuable guidance for the rational design of novel M-H complexes and reaction systems.

Nickel-catalyzed stereo-controlled 2,3-hydrosilylation of 1,1-disubstituted allenes

Directing regioselectivity and stereoselectivity in allene reactions has long been a significant challenge due to the multiple reactive pathways available. In this study, we report the development of a Ni-catalyzed regio- and stereoselective 2,3-hydrosilylation of 1,1-disubstituted allenes. Stereoselectivity was precisely controlled through the strategic modulation of ligand-induced steric effects and non-covalent interactions. Phenyl dibenzophosphole as the ligand enabled the selective formation of (Z)-allylsilanes, while tricyclohexylphosphine favored the production of (E)-allylsilanes. This work highlights the critical role of ligand-induced steric and non-covalent interactions in dictating regio- and stereoselectivity, offering new insights into Ni(ii) catalysis for stereoselective hydrosilylation.

Photo-induced ring-maintaining hydrosilylation of unactivated alkenes with hydrosilacyclobutanes

Increasing attention has been paid to silacyclobutanes because of their wide application in ring opening and ring extension reactions. However, the synthesis of functionalized silacyclobutanes remains an unmet challenge because of the limited functional group tolerance of the reactions with organometallic reagents and chlorosilacyclobutanes. Herein, we report a conceptually different solution to this end through a visible-light-induced metal-free hydrosilylation of unactivated alkenes with hydrosilacyclobutanes. A wide range of unactivated alkenes with diverse functional groups including the base-sensitive acid, alcohol and ketones participated in this reaction smoothly. In particular, the first hydrosilylation reaction of alkenes with dihydrosilacyclobutane provides a facile access to various functionalized alkyl monohydrosilacyclobutanes. Unsymmetrical dialkyl silacyclobutanes have also been synthesized through consecutive hydrosilylation with dihydrosilacyclobutane in one pot. The mechanism study reveals that the Lewis basic solvent could promote the generation of strained silyl radicals by direct light irradiation without a redox-active photocatalyst and the thiol catalyst plays an important role in accelerating the reaction.

Ruthenium-Catalyzed α-Regioselective Hydroboration of Allenes

Hydroboration of allenes is powerful and atom-economic approach to the synthesis of organoboranes, such as the highly versatile allylboranes. However, regarding regiocontrol, existing methods uniformly deliver the boron functionality to the less hindered β- or γ-position, but not the α-position. The latter is particularly challenging for allenes with substantial steric difference between the two terminals and lacking electronic bias (e.g., 1,1-disubstituted allenes). Herein we report the first highly efficient ruthenium-catalyzed hydroboration of allenes featuring exclusive α-regioselectivity, providing access to sterically hindered allyl boranes that are limitedly accessible by conventional methods. DFT studies suggested that the unusual α-regioselectivity is attributed to the disfavored reductive elimination at the γ-position due to the high energy cost required to overcome the agostic interaction and rotation of the key π-allyl intermediates. This protocol is also applicable to the previously unprecedented α-hydroalkynylation and underdeveloped α-hydrosilylation of allenes, thus complementing known catalytic systems and providing convenient access to highly congested yet densely-functionalized allyl silanes and skipped enynes bearing a fully-substituted allylic carbon center. It is expected that this ruthenium-catalyzed system can serve as a new platform for the development of other hydrofunctionalization processes with unorthodox selectivity.

Intramolecular Hosomi-Sakurai Reaction for the Synthesis of Benzoxasiloles

A Lewis acid-catalyzed intramolecular Hosomi-Sakurai reaction of o-(allylsilyl)benzaldehyde/ketone has been developed. The reaction proceeds through simultaneous C-Si bond cleavage and C-C bond reconstruction. This protocol provides a rapid approach for the synthesis of allyl-substituted benzoxasiloles under mild conditions.

Cu-Catalyzed Synthesis of 2-Silyl-1,3-butadienes from Allenols and Applications to One-Pot Synthesis of Tetrasubstituted Arylsilanes

A copper-catalyzed chemo- and stereoselective method for the synthesis of (E)-2-silyl-1,3-butadienes from a broad range of allenols using mild Si-B reagents is reported in this study. Our protocol required a short reaction time at ambient temperature to produce the desired dienes in high yields. Synthetic applications are highlighted by the one-pot synthesis of tetrasubstituted arylsilanes from allenols as well as the further functionalization of C-Si bonds.

Decarboxylative Allylation of Silanecarboxylic Acids Enabled by Organophotocatalysis

Herein we present a visible-light-facilitated transition-metal-free allylic silylation reaction under mild conditions. This protocol is enabled by an inexpensive organophotocatalyst and provides efficient and concise synthetic routes to substituted allylsilanes, particularly from readily available allyl sulfones and stable silanecarboxylic acids as silyl radical precursors. Further investigations reveal that this strategy is also generally compatible with vinyl sulfones to access vinylsilanes. The silver catalytic system opens up an alternative entry to realize the decarboxylative allylation of silanecarboxylic acids.

Lithium Triethylborohydride (LiHBEt3)-Promoted Hydrosilylation of Allenes to Prepare (E)-Allylsilanes

A transition-metal-free hydrosilylation of allenes is reported herein by using commercially available lithium triethylborohydride (LiHBEt3) as the catalyst. Both mono- and disubstituted allenes could be hydrosilylated with primary or secondary silanes effectively. This reaction represents an environmental and economic method to prepare (E)-allylsilanes in good yields along with decent selectivities.

Regio- and enantioselective CuH-catalyzed 1,2- and 1,4-hydrosilylation of 1,3-enynes

We report a copper-catalyzed ligand-controlled selective 1,2- and 1,4-hydrosilylation of 1,3-enynes, which furnishes enantiomerically enriched propargyl- and 1,2-allenylsilane products in high yields with excellent enantioselectivities (up to 99% ee). This reaction proceeds under mild conditions, shows broad substrate scope for both 1,3-enynes and trihydrosilanes, and displays excellent regioselectivities. Mechanistic studies based on deuterium-labeling reactions and density functional theory (DFT) calculations suggest that allenylcopper is the dominant reactive intermediate under both 1,2- and 1,4-hydrosilylation conditions, and it undergoes metathesis with silanes via selective four-membered or six-membered transition state, depending on the nature of the ligand. The weak interactions between the ligands and the reacting partners are found to be the key controlling factor for the observed regioselectivity switch. The origin of high enantiocontrol in the 1,4-hydrosilylation is also revealed by high level DLPNO-CCSD(T) calculations.

Enantioselective Nickel-Catalyzed Hydrosilylation of 1,1-Disubstituted Allenes

Here, we report the first example of Ni-catalyzed asymmetric hydrosilylation of 1,1-disubstituted allenes with high level of regioselectivities and enantioselectivities. The key to achieve this stereoselective hydrosilylation reaction was the development of the SPSiOL-derived bisphosphite ligands (SPSiPO). This protocol features broad substrate scope, excellent functional group, and heterocycle tolerance, thus provides a versatile method for the construction of enantioenriched tertiary allylsilanes in a straightforward and atom-economic manner. DFT calculations were performed to reveal the reaction mechanism and the origins of the enantioselectivity.