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 (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 (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.