Catalytic Reactions of Metalloporphyrins. 1. Catalytic Modifikation of Borane Reduction of Ketone with Rhodium(III) Porphyrin as Catalyst

Article abstract of DOI:10.1021/ja00265a017

(Octaethyl- or tetraphenylporphyrinato)rhodium(III) chloride shows an efficient catalysis in the aerobic reduction of ketone with NaBH4 in THF, BH4^- + 2R₁R₂C=O + O2 ->2R₁R₂CHOH + BO2^-.The initial step in the catalytic cycle is the rate-determining complexation of BH4^- with Rh^III porphyrin (Rh^III + BH4^- -> Rh^III-BH4) followed by a rapid borane transfer from the adduct to ketone to give dialkoxyborane and hydridorhodium species (Rh^III-BH4 + 2R₁R₂C=O -> Rh-H + HB(O-CHR₁R₂)2).In the subsequent step, the Rh-H species undergoes oxidation with O2 back to Rh^III with concomitant "hydrolisis" of dialkoxyborane to alcohol (Rh-H + O2 + HB(O-CHR₁R₂)2 -> Rh^III + 2R₁R₂CHOH + BO2^-).Essentially, autorecycling Rh^III and Rh-H act as a "borane" generator and proton source, respectively, in a catalytic manner.Furthermore, the Rh^III-BH4 complex capable of transferring borane to ketone lacks what is characteristic of free borane, i.e., facile oxidation with O2 and ready hydrolysis with H2O.Thus, the present system provedes a highly efficient, catalytic modification of synthetic reactions of borane in the presence of oxygen.