The mechanism and kinetic aspects of the retro-ene reaction of allyl n-butyl sulfide and its deuterated derivative were studied using four different types of density functional theory (DFT) methods with eight different levels of the basis sets. Vibrational frequency analysis confirmed that the stationary points include the transition state (TS) structure with only one imaginary frequency. Mechanistic studies on the retro-ene process rejected the step-wise mechanism and confirmed that the reaction proceeds through a six-centered cyclic transition state. Theoretical calculations show that propene elimination from the reactant can occur through an asynchronous concerted mechanism. A primary kinetic isotope effect of 2.21 for the reaction can be determined. Theoretical kinetics and activation energies especially at the B3LYP/6-31G* levels are in good agreement with the experimental values.

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