A Density-Functional Study of the Mechanism for the Diastereoselective Epoxidation of Chiral Allylic Alcohols by the Titanium Peroxy Complexes

The epoxidation of three stereolabeled methyl-substituted chiral allylic alcohols with 1,2A and/or 1,3A allylic strain, namely 3-methylbut-3-en-2-ol (1a), pent-3-en-2-ol (1b), and 3-methylpent-3-en-2-ol (1c), have been studied by the density-functional theory method, B3LYP/6-31+G(d,p). For each substrate we calculated the two prereaction complexes with Ti(OH)4/MeOOH (the oxidant model for Ti(O-i-Pr)4/t-BuOOH), their threo and erythro transition states for oxygen transfer, and the corresponding product complexes. For substrate 1a, the erythro transition state is 0.91 kcal/mol of lower energy than the threo one; for substrates 1b and 1c, the threo compared to the erythro transition states are by 1.05 and 0.21 kcal/mol more favorable, respectively. The threo/erythro product ratios have been estimated from the computed free energies for the competing threo and erythro transition states 3a−c in CH2Cl2 solution to be 12:88 (1a), 92:8 (1b), and 77:23 (1c), which are in good accordance with the experimental value...