Kinetic and mechanistic studies on the formation and reactions of early-transition-metal-ketene complexes

Article abstract of DOI:10.1021/om00095a009

A series of complexes of vanadocene or molybdenocene with unsymmetrical ketenes were prepared, either by reaction of the various ketenes with vanadocene itself or by reaction with the molybdenocene phosphine complex (C₅H₅)₂Mo(PEt₃). All of the complexes exhibited the expected ketene C=O bonding mode, and all reactions were very specific in their formation of the facial isomer with metallocene fragment located on the side of the smaller ketene substituent. Kinetic studies were used to assess the sensitivity of the incoming vanadocene to steric and electronic effects, with the latter found to dominate. Kinetic studies and activation parameters for reaction of Cp₂Mo(PEt₃) with EtPhC=C=O indicated a second-order associative mechanism, proposed to involve a nucleophilic attack of the metal center on the ketene central carbon in the rate-limiting step. Lastly, reactions of the bound ketenes with nucleophiles (alkyllithiums or Grignard reagents) proceeded readily to either ketone or aldehyde enolates (the latter via transfer of a β-hydride from the alkyl); the clean production of only Z enolates from the unsymmetrical ketenes is indicative of a metal-mediated internal attack of nucleophile on the bound ketene.