Mechanistic evaluation of metal-catalyzed hydrogen-transfer processes: The Shvo catalyst as an example of computational unravelling

Hydrogen-transfer processes allow hydrogenation under mild conditions reaching high selectivity. They use readily available solvents such as 2-propanol instead of molecular H(2). The mechanism of these processes can be classified into: (a) Direct hydrogen-transfer or Meerwein-Ponndorf-Verley (MPV) reduction and (b) Hydridic route, with the formation of a metal-hydride species. The latter can be divided into both dihydridic and hydridic routes depending on the catalytic species involved, a dihydride or a monohydride, respectively. Within the monohydridic route the reaction can go through an inner-sphere mechanism (with substrate coordination) or through an outer-sphere mechanism (without coordination to the metal substrate). The proposed mechanisms for the hydrogen-transfer processes catalyzed by several metal complexes within the literature are reviewed, paying special attention to its computational analysis concerning the factors affecting the mechanism such as ligand lability and alkoxide stability. Afterwards, we focus on one of the most paradigmatic hydrogen-transfer processes catalyzed by the Shvo complex, It presents great versatility applied in a broad range of hydrogen-transfer processes to polar (ketones, imines) and non-polar (alkene, alkynes) bonds, and even successfully heterogenized by a sol-gel process. (C) 2009 Elsevier B.V. All rights reserved.