A computational study-of the hydroxy-group directivity in the peroxyformic acid epoxidation of the chiral allylic alcohol (Z)-3-methyl-3-penten-2-ol:: Control of threo diastereoselectivity through allylic strain and hydrogen bonding

Eight transition structures for the epoxidation of the chiral allylic alcohol (Z)-3-methyl-3-penten-2-ol (1) with peroxyformic acid have been computed by the B3LYP density functional method with 6-31G(d) and 6-31G(d,p) basis sets. The four lowest-energy transition structures and their respective pre-reaction Clusters were fully re-optimized by employing 6-311+G(d,p) and correlation:consistent; polarized valence triple-zeta cc-pZTV basis sets. The relative energies of the transition structures were found to be highly sensitive-to the basis set applied. The transition state for three product formation, anti-(2S,3R,4S)-TS-3f, with the lowest total energy (at B3LYP/611+G(d,p) and BSLYP/AUG-cc-pZTV) of all the TSs examined, has a planar peracid moiety and is a precursor for the 1,4 migration of the peracid hydrogen atom H-a to the peroxy oxygen atom O-4. The use of different basis sets does not affect markedly the geometry of the anti-(2S,3R,4S)-TS-3f transition structure: The transition state for erythro epoxidation, syn-(2R,3R,4S)-TS-3a, is 0.9 kcal/mol higher in energy and possesses a nonplanar peracid approaching the C=C bond in a manner intermediate between spiro and planar. The relative energy and nonplanarity of this syn transition structure is highly sensitive to the basis set applied. With the smaller basis set, 6-31G(d,p), it is actually the lowest-energy TS and the peracid moiety is significantly skewed. The contribution of the four lowest energy transition,structures 3a, 3b, 3e, and 3f to the threo/erythro product ratio has been assessed through an extended Curtin-Hammet principle analysis of this multi-transition state reaction. It has been found that this approach agrees well with the experimental threo/erythro product ratio, in particular when the corrections for a solvent effect are made within the self-consistent isodensity polarized continuum model (SCI-PCM).