Gas-Phase Reactions of Bis(η⁵-cyclopentadienyl)methylzirconium Cations with Ketones and Aldehydes

Abstract

The reactions of the bis(η 5 -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 η 3 -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.

Dehydrogenation and demethanation of 2-methylpropane and propane in the gas-phase by the 16-electron complex [Ru(bipy)2(CO)]2+* chemically activated by the association of [Ru(bipy)2]2+ and CO

“…C-H bond activation by larger transition metal ion complexes has also been studied recently. [49][50][51][52][53] The Uggerud group investigated the mechanism for C-H bond activation of ethylene by 18e − Rh(III) and 16e − Rh(III) complexes inside the ICR cell in which H 2 elimination was observed in the reaction of ethylene and Cp-Rh(C 2 H 4 -μH) + complexes. 54 DFT calculations showed that vinylic C-H bond activation is an energetically unfavorable process.…”

Gas-phase reactions of the bis(η5-cyclopentadienyl)methylzirconium cation with imines

“…Such comparison of gas-phase and condensed phase results and bridging the gap between them has many advantages. Most importantly, gas-phase studies can furnish information which can lead to much better understanding of the role of solvent, ancillary ligand [22,[25][26][27][28][29][30][31][32]. Also, the detection of elusive reaction intermediates may be possible in such gas-phase experiments.…”

“…A recent gas-phase study of the reactivity of the bis(g 5 -cyclopentadienyl)methylzirconium cation involved simple ketones and aldehydes [30]. It was found that reactions with a majority of the ketones resulted in consecutive addition of one or two substrate molecules and/or elimination of alkanes.…”

“…In the case of aldehydes and ketones, group Y is the carbonyl oxygen. It was shown that this mechanism encompasses reactions with both aldehydes, and, with the appropriate modifications, ketones [30].…”