Gas-Phase Reactions of Bis(η5-cyclopentadienyl)methylzirconium Cations with Ketones and Aldehydes
journal contributionposted on 29.01.2007, 00:00 by Alexander A. Aksenov, Cesar S. Contreras, David E. Richardson, John R. Eyler
The reactions of the bis(η5-cyclopentadienyl)methylzirconium cation (1) with ketones and aldehydes in the gas phase have been studied by Fourier transform ion cyclotron resonance mass spectrometry. Reactions of 1 with a majority of the ketones studied resulted in consecutive addition of one and two substrate molecules and/or elimination of alkanes. Deuterium-labeled substrates and methylzirconocene were used to investigate the mechanistic details. On the basis of DFT calculations, the key products of the elimination reaction(s) were identified as η3-enolate complexes formed via an insertion/elimination mechanism. Similar product ion structures are postulated for the reaction of 1 with aldehydes in cases where these complexes are either the only or the major reaction product. A multiple-step mechanism is proposed, which involves migratory insertion of an aldehyde molecule into the methylzirconocene cation, followed by β-H elimination, and, via a six-membered cyclic transition state, formation of the resulting enolate complex. When a β-H elimination pathway is not available for ketones, the reaction is instead proposed to proceed via direct nucleophilic attack of the metal-bound alkyl preceded by a fast migratory insertion equilibrium.