Epoxidation of acyclic chiral allylic alcohols with peroxy acids: Spiro orplanar butterfly transition structures? A computational DFT answer

The mechanism of the epoxidation of two chiral allylic alcohols, i.e., 3-me thyl-3-buten-2-o1 and (Z)-3-penten-2-ol, with pepoxyformic acid has been in vestigated by locating 20 transition structures with the B3LYP/6-31G* metho d and by evaluating their electronic energy also at the B3LYP/6-311+G**/B3L YP/6-31G* theory level. Relative stability of TSs, as far as electronic ene rgy is concerned, is basis set dependent; moreover, it also depends on entr opy and solvent effects. Free enthalpies, calculated by using electronic en ergy at the higher theory level and with inclusion of solvent effects, indi cates that syn,exo TSs, where the olefinic OH group hydrogen bonds the pero xy oxygens of the peroxy acid, outweigh syn,endo TSs, where the peroxy acid carbonyl oxygen is involved in hydrogen bonding. In the former TSs the per oxy acid moiety maintains its planar geometry while in the latter ones a st rong out-of-plane distortion of peroxy acid is observed. This distortion ma kes it viable an unprecedented 1,2-H shift, as a possible alternative to th e 1,4-H shift, for the peroxy acid hydrogen. In fact, for one syn,endo TS I RC analysis demonstrated that the 1,2-H shift mechanism is actually operati ve. The geometry of all TSs substantially conforms to a spiro (i.e., with t he peroxy acid plane almost perpendicular to the C=C bond axis) butterfly o rientation of the reactants while no TS resembles, even loosely, the planar butterfly structure. Theoretical threo/erythro epoxide ratios are in fair accord with experimental data. Calculations indicate that three epoxides de rive mostly from TSs in which the olefinic OH assumes an outside conformati on while erythro epoxides originate from TSs with the OH group in an inside position. Computational findings do not support the qualitative TS models recently proposed for these reactions.