R. Arnaud et al., THEORETICAL-STUDY OF THE THIO-CLAISEN REARRANGEMENT - CAN VINYLTHIOETHANIMINE UNDERGO A [3,3]-SIGMATROPIC SHIFT, Perkin transactions. 2, (10), 1996, pp. 2065-2071
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.