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.