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Keywords:

  • alcohols;
  • carbonic acids;
  • homogeneous catalysis;
  • oxidation;
  • rhodium;
  • transfer hydrogenation

Abstract

The amino diolefin complex [Rh(trop2NH)(TMIY)]+(OTf) (OTf=CF3SO3; trop=5-H-dibenzo[a,d]cyclohepten-5yl), incorporating N-heterocyclic carbene ligand 1,3,4,5-tetramethylimidazole-2-ylidene (TMIY), has been prepared. The structure is determined by single-crystal X-ray diffraction, which shows a butterfly structure with the NH and TMIY groups in the apical and the two olefinic binding sites in the equatorial position of a trigonal bipyramid with one vacant site in the equatorial plane. This complex can be deprotonated to give the amide [Rh(trop2N)(TMIY)], which cleaves H2 heterolytically across the Rh[BOND]N bond to give the amino hydride [RhH(trop2NH)(TMIY)]. In dimethylsulfoxide (DMSO), both the amide and amino hydride complexes are active as catalysts in the dehydrogenative coupling (DHC) of various primary alcohols with water to give the corresponding acids under aerobic conditions. O2 serves as hydrogen acceptor. One O atom is converted into water while the other is transferred to DMSO as oxygen acceptor, to yield Me2SO2. Hence, the net reaction RCH2OH+2 O2+2 DMSO[RIGHTWARDS ARROW]RCOOH+2 Me2SO2+H2O is catalyzed by the butterfly-shaped rhodium amide complexes.