Theoretical study on thermal decomposition kinetics of allyl formates in the gas phase

The study of the kinetics and mechanisms of the gas-phase thermal decarboxylation of allyl formate and 2-methylallyl formate was carried out by means of electronic structure calculations using Density Functional Theory (DFT) functionals: B3LYP, B3PW91, MPW1PW91 and PBEPBE. Two possible molecular mechanisms were investigated: a concerted [1,5] hydrogen shift through a six-centered cyclic transition state, and a [1,3] hydrogen shift mechanism through a four-centered cyclic transition state. Calculation results suggest these reactions to be unimolecular and proceed in a concerted mechanism through a six-membered cyclic transition state (TS) geometry. Despite the similar activation energies, differences were found when comparing the two elimination processes. In the case of allyl formate, the reaction shows small predominance of the change in the terminal allyl moiety bond order and the CO_ester bond breaking. Conversely, for 2-methylallyl formate, the process is highly controlled by the new CO double bond development leading to carbon dioxide formation. Moreover, in the 2-methylallyl formate decomposition, the α-carbonyl hydrogen transfer to the terminal allyl carbon to produce 2-methyl propene is at a very early stage in the TS compared to allyl formate.