| Abstract
| - 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 with6-31G(d) and 6-31G(d,p) basis sets. The four lowest-energy transition structures and their respectivepre-reaction clusters were fully re-optimized by employing 6-311+G(d,p) and correlation-consistentpolarized valence triple-ζ cc-pZTV basis sets. The relative energies of the transition structureswere found to be highly sensitive to the basis set applied. The transition state for threo productformation, anti-(2S,3R,4S)-TS-3f, with the lowest total energy (at B3LYP/611+G(d,p) and B3LYP/AUG-cc-pZTV) of all the TSs examined, has a planar peracid moiety and is a precursor for the 1,4migration of the peracid hydrogen atom Ha to the peroxy oxygen atom O4. The use of differentbasis 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 energyand possesses a nonplanar peracid approaching the CC bond in a manner intermediate betweenspiro and planar. The relative energy and nonplanarity of this syn transition structure is highlysensitive 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 energytransition stuctures 3a, 3b, 3e, and 3f to the threo/erythro product ratio has been assessed throughan extended Curtin−Hammet principle analysis of this multi-transition state reaction. It has beenfound that this approach agrees well with the experimental threo/erythro product ratio, in particularwhen the corrections for a solvent effect are made within the self-consistent isodensity polarizedcontinuum model (SCI-PCM).
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