A computational study of the mechanism of the unimolecular elimination of alpha,beta-unsaturated aldehydes in the gas phase
The mechanism for the decarbonylation of (E)-2-butenal and (E)-2-methyl-3-pheny-2-propenal was studied with different levels of ab initio and DFT methods. Reactants, products and transition structures were optimized for two kinds of reaction channel: a one-step reaction which involves a three-membered cyclic transition state, and a two-step reaction which involves an initial four-membered cyclic transition state. According to our calculations, these two possible mechanisms entail similar energetic costs, and there are only small differences depending on the reactant. The elimination of (E)-2methyl-3-pheny-2-propenal yields different products depending on the channel followed. Only one of the three possible one-step mechanisms leads directly to (E)-beta-methylstyrene (the main product according to experiment). This fact is reasonably well reproduced by our results, since the corresponding transition state gave rise to the lowest activation Gibbs free energy.
elimination decarbonylation ab initio dft reaction mechanism