Theoretical study of the thio-Claisen rearrangement. Can vinylthioethanimine undergo a [3,3]-sigmatropic shift?

Abstract

An ab initio study of the [3,3]-sigmatropic rearrangements of allyl vinyl sulfide 1 and vinylthioethanimine 2 has been carried out. To this end, geometries of stationary points were optimized at the HF, DFT (B3LYP) and MP2 (fc) levels using the 6-31G* basis set, while relative energies were estimated using post-HF calculations up to ∼ QCISD(T)/6-311 + G(2df,2p)//MP2/6-31G* level and corrected for zero-point energies. Progress of the charge distributions and delocalization interactions (NBO analysis) along the reaction have been investigated by employing the intrinsic coordinate method. Solvent effects on these processes have been simulated by the mean of SCRF computations associated to a continuum model. The results show that 2 is predicted to be less reactive than 1, the calculated enthalpy of activation for cis-2 and tras-2 rearrangement being respectively 4.5 and 10.0 kcal mol^–1 higher than for the rearrangement of compound 1. The thio-Claisen rearrangement is calculated to be slightly exothermic (Δ_rH=–1.5 kcal mol^–1 at our best level of calculation) while the rearrangement from vinylthioethanimine is calculated to be endothermic [Δ_rH(cis-2)= 7.0 kcal mol^–1; Δ_rH(trans-2)= 9.1 kcal mol^–1]. Unlike the thio-Claisen rearrangement, the results obtained considering the polarizable continuum approach indicate that no significant rate enhancement of rearrangement from compound 2 can be expected on going from the gas phase into solution. NBO analysis is a tool for the mechanistic study of these rearrangements and is a guideline in the research of electronic effects in order to improve their feasibility.