PROTONATION OF PI-ALKENE-RHODIUM(I) COMPLEXES LEADS TO SIGMA-ALKYL-RHODIUM(III) - AN NMR-STUDY

The reaction of Rh(beta-diketonato)(alkene)(2) complexes with CF3SO3H in THF gives Rh(beta-diketonato)(alkyl) species when the alkene is eth ylene, cis-butene or 5-methylene-cycloheptene whereas 1,5-cyclooctadie ne complexes are unreactive. The reaction was followed by means of NMR -spectroscopy at low temperature and the products were characterized i n solution by 2D-NMR techniques. Three possible reaction mechanisms ar e discussed: ligand assisted proton transfer, hydride transfer and dir ect protonation of the alkene. The Rh(III)ethyl species are stable aga inst degradation by beta-hydride elimination. However, Rh(III)(beta-di ketonato)(n-butyl) complexes lose butene rapidly when ethylene is adde d and Rh(beta-diketonato)(ethyl) complexes are formed. A mechanism pre venting the formation of higher oligomers is proposed, where beta-hydr ide elimination is followed by rapid alkene exchange and hydride re-in sertion. (C) 1998 Elsevier Science S.A. All rights reserved.