N-heterocyclic carbene complexes of Rh(I) and electronic effects on catalysts for 1,2-addition of phenylboronic acid to aldehydes

1,3-Diarylsubstituted imidazolinium salts, (NHC-H)Cl, 3, containing hydrogen or alkyl groups at the 4,5-positions of the imidazolidine ring, served as precursors to rhodium(I) complexes [RhCl(NHC)COD], 4, which were converted into cis-[RhCl(NHC)(CO)(2)] complexes, 5. All compounds prepared were characterized by elemental analyses, H-1 NMR and C-13 NMR. The relative sigma-donor/pi-acceptor strength of the NHC ligands was determined by means of IR spectroscopy of 5. The ability of NHCs in 4 to enchance activity was explored in the 1,2-addition of phenylboronic acid to aldehydes. A good correlation was observed between catalytic activity and the electron-donating power of the NHC ligands. Copyright (C) 2010 John Wiley & Sons, Ltd.