Organic Synthesis via Transition Metal Complexes. 112. Reactivity Enhancement of Group VI Fischer Carbene Complexes by Copper and Rhodium Catalysts:  Experimental Proof of a Carbene Rhodium Complex as an Intermediate

Rhodium(I) compounds were found to efficiently catalyze reactions of Fischer carbene complexes. It was shown that the thermally quite unreactive tungstaoctatetraenes 14a−d were readily transformed into spiro-fused vinylcyclopentadienes 15a−d in the presence of catalytic amounts of RhCl3·3H2O in MeOH or [(COD)RhCl]2. A reactivity enhancement was observed also for the transformation of the tungsten complex 8a into vinylcyclopentadiene 10a. Experimental proof for the intermediacy of carbene rhodium complexes in these reactions was provided by preparation of the rhodaoctatetraene 16c from the reaction of the tungsten compound 14c with a stoichiometric amount of [(COD)RhCl]2. The carbene rhodium compound 16c was transformed into the vinylcyclopentadiene 15c thermally in a smooth reaction and showed a catalytic activity similar to [(COD)RhCl]2. Not only rhodium but also copper compounds were found to be good catalysts for the π-cyclization of tungstaoctatetraenes.