Substituent effect on the allyl vinyl sulfide rearrangement (thio-Claisen rearrangement) and the vinylthioethanimine rearrangement. A theoretical study

Abstract

The transition structures for the [3,3] sigmatropic rearrangements of a variety of substituted allyl vinyl sulfides (XCH) and vinylthioethanimines (XN) H₂CCR–S–CH₂–CR′XR″ (R = NH₂; R′ = BH₂, CN; R″ = CF₃) have been located using ab initio and DFT (B3LYP) calculations and the 6-31G* basis set. Relative energies have been estimated using post-HF calculations up to the QCISD(T)/6-31G*//B3LYP/6-31G* level. Solvent effects on these processes have been simulated by means of SCRF computations associated with a continuum model. The results show that combined donor–acceptor disubstitutions (R = NH₂, R′ = BH₂ or CN) improve considerably the reactivity of the allyl vinyl sulfide (vinylthio-ethanimine). Thus, 2-amino-5-cyano disubstitution lowers the enthalpy of activation by 6.6 kcal mol^–1 (9.3 kcal mol^–1) (1 cal = 4.184 J) and increases the exothermicity of the rearrangement by 13.0 kcal mol^–1 (19.1 kcal mol^–1). In addition, as the corresponding saddle points are highly polar in nature, an additional transition state stabilization, even in moderately polar solvents, is predicted by the SCRF calculations.