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 (1.2)A and/or (1.3)A 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 la, the erythro transition state is 0.91 kcal/mol of lower energy than the threo one; for substrates 1b and le, 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 values 22:78 (1a), 91:9 (1b), and 83:17 (1c). The diastereoselectivity of this diastereoselective oxyfunctionalization is rationalized in terms of the competition between (1.3)A and (1.2)A strain and the electronic advantage for the spiro transition state. In addition, solvent effects are also play a role for the diastereoselectivity at the same time.