Substituent effects on the gas phase reactivity of alkyl allyl sulfides, a theoretical study

The potential energy surface of the reaction C3H5SCH2X→C3H6+XCH=S was studied for the various X substitutions (X=CH3, H, Cl, F) at the high level of the theory (G3MP2 method) using the 6-31G(d) basis set. It was demonstrated that CH3, H, Cl and F substitution, respectively, accelerates the reaction (This order for the substitutions will be called X=CH3-to-F throughout this paper). Barrier height changes from ΔE≠=167.76 to 148.63 kJ mol−1, because of the electronic effects introduced upon the substitution. It was shown that the X=CH3-to-F substitution in the C6-position posses two types of contribution to the calculated energy barriers and bond lengths at the transition state (TS). First, it facilitates the TS complex formation, consequently reduces the barrier height. Second, it reduces the H1-C2 bond length, the major component of the reaction coordinate at the TS, and increases the rate of the reaction. Charge analysis shows that the acceptor substituent (F) induces more positive character on C6 atom at the Transition state (TS), while donor substituent (CH3) induces the less positive one, therefore, the reactivity decreases. HOMO-LUMO energy differences confirmed the activation energy and reactivity order. The G3MP2 results show the capability of this level of calculation to predict the reactivity of the intramolecular retro-ene reaction correctly.