Journal of the American Chemical Society p. 943 - 947 (1986)
Update date:2022-08-17
Topics:
Aoyama, Yasuhiro
Fujisawa, Takeshi
Watanabe, Takamichi
Toi, Hiroo
Ogoshi, Hisanobu
(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 RhIII porphyrin (RhIII + BH4- -> RhIII-BH4) followed by a rapid borane transfer from the adduct to ketone to give dialkoxyborane and hydridorhodium species (RhIII-BH4 + 2R1R2C=O -> Rh-H + HB(O-CHR1R2)2).In the subsequent step, the Rh-H species undergoes oxidation with O2 back to RhIII with concomitant "hydrolisis" of dialkoxyborane to alcohol (Rh-H + O2 + HB(O-CHR1R2)2 -> RhIII + 2R1R2CHOH + BO2-).Essentially, autorecycling RhIII and Rh-H act as a "borane" generator and proton source, respectively, in a catalytic manner.Furthermore, the RhIII-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.
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