Acyl Transfer Reactions in the Gas Phase. Ion-Molecule Chemistry of Vinyl Acetate

Article abstract of DOI:10.1021/ja00381a020

An ion cyclotron resonance study of the ion-molecule reactions of vinyl acetate with methanol in the gas phase has revealed the formation of structurally different ions having the composition of protonated vinyl acetate.Deuterium-labeled reactants (CD3CO2CH=CH2 with CH3OH or CD3OD) gave product ions showing incorporation of up to two deuteriums in the vinyl group, indicating coexistence and interconversion of O-protonated and C-protonated vinyl acetate.Evidence was also obtained for a third MH⁺ ion for which the proposed structure is protonated 3-oxobutanal.This ion is believed to be formed by attack of CH3CO⁺ at the terminal vinylic carbon with loss of the ester acyl group as ketene.The ion reacts with methanol to give m/z 101 by loss of water.In contrast, protonated vinyl acetate reacts with methanol by an acyl transfer process to give (AcOCH3)H⁺, m/z 75.The related ion chemistry of vinyl propanoate, vinyl 2,2-dimethylpropanoate, and isopropenyl acetate is also described.Each of the acyl transfer reactions observed is consistent with formation of intermediate acylium ion complexes rather than tetracovalent addition complexes.Attempts were made to generate tetracovalent intermediates by an independent route from ortho esters of vinyl acetate.Dissociation of the ortho ester CH3C(OCH=CH2)2(OCH3) to dioxacarbocations was the dominant reaction, but the product ions were unreactive with H2O, CH3OH, or t-C4H9OH.The mechanistic implications of these results are discussed.