THEORETICAL-STUDY OF THE THIO-CLAISEN REARRANGEMENT - CAN VINYLTHIOETHANIMINE UNDERGO A [3,3]-SIGMATROPIC SHIFT

An ab initio study of the [3,3]-sigmatropic rearrangements of allyl vi nyl sulfide 1 and vinylthioethanimine 2 has been carried out, To this end, geometries of stationary points were optimized at the HF, DFT (B3 LYP) and MP2 (fc) levels using the 6-31G basis set, while relative en ergies were estimated using post-HF calculations up to similar to QCIS D(T)/6-311 + G(2df,2p)//MP2/6-31G level and corrected for zero-point energies. Progress of the charge distributions and delocalization inte ractions (NBO analysis) along the reaction have been investigated by e mploying the intrinsic coordinate method. Solvent effects on these pro cesses 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 trans-2 rearrangement being respectively 4.5 and 10.0 kcal mol(-1) hig her than for the rearrangement of compound 1. The thio-Claisen rearran gement is calculated to be slightly exothermic (Delta(r)H = - 1.5 kcal mol(-1) at our best level of calculation) while the rearrangement fro m vinylthioethanimine is calculated to be endothermic [Delta(r)H(cis-2 ) = 7.0 kcal mol(-1); Delta(r)H(trans-2) 9.1 kcal mol(-1)]. Unlike the thio-Claisen rearrangement, the results obtained considering the pola rizable continuum approach indicate that no significant rate enhanceme nt of rearrangement from compound 2 can be expected on going from the gas phase into solution. NBO analysis is a tool for the mechanistic st udy of these rearrangements and is a guideline in the research of elec tronic effects in order to improve their feasibility.