Mechanistic studies of the phosphine-catalyzed homodimerization of ketoketenes

The mechanism of PBu3-catalyzed homodimerization of ketoketenes has been explored and compared with that of the previously reported trialkylphosphite-mediated reactions. NMR studies of the PBu3-catalyzed reaction implicated the involvement of tetravalent phosphonium intermediates. Phosphonium intermediates in the catalytic cycle were trapped through reaction with trimethylsilyl chloride and 4-chlorobenzaldehyde, and the resulting products were characterized. A method for the stoichiometric generation of phosphonium enolates was developed as a result of these studies. No evidence was obtained for the involvement of pentacovalent phosphorane intermediates in trialkylphosphine-catalyzed ketoketene homodimerization reactions, in contrast with the mechanism of the trialkylphosphite-mediated homodimerization of dimethylketene. An X-ray crystal structure analysis of methylphenylketene dimer showed that it possesses Z-geometry about the exocyclic olefin.