Structural and kinetic aspects of the retro-cheletropic ene reactions of 2,2-dimethyl but-3-enal, 1-methyl-6-methylenecyclohexa-2,4- diene-1-carbaldehyde and their deuterated derivatives were investigated using a variety of computational methods. Theoretical calculations were carried out with ab initio and DFT methods at the RHF, MP2 and B3LYP levels of the theory, using the 6-31G* basis set. Vibrational frequency analysis confirmed the stationary states including the transition state (TSs) structures. Intrinsic reaction coordinate calculations show the localized TSs connect with the corresponding minima associated with the reactants and the products. The mechanistic studies on the retro-cheletropic ene processes rejected the stepwise mechanism and confirmed a five-membered cyclic TS for each reaction. The conversion of the reactants to the products proceeds through an asynchronous concerted mechanism. Theoretical calculations indicate that the reactions display a primary kinetic isotope effect of 3.8 and 8.75 for the studied reactions at 563.15 and 357.15 K, respectively. Calculated activation parameters especially MP2 results are in good agreement with the experimental values. 2,2-Dimethyl but-3-enal; 1-Methyl-6-methylenecyclohexa-2,4-diene-1-carbaldehyde; Retro-cheletropic ene reaction; Ab initio and DFT methods; Asynchronous concerted mechanism

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