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

The transition structures for the [3,3] sigmatropic rearrangements of a variety of substituted allyl vinyl sulfides (X=CH) and vinylthioethanimines (X=N) H2C=CR-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 (vinylthioethanimine). 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.