Asymmetric Radical-Type 1,2-Alkoxy-Sulfenylation of Benzoxazole-2-Thiols to Vinylarenes Catalyzed by Chiral Vanadyl Complexes

Abstract

Chiral vanadyl complex derived from N-salicylidene-tert-butyl-l-glycinate bearing a 3-(2,5-dimethyl)phenyl-5-bromo substituent was first tested for the catalytic feasibility of asymmetric intermolecular 1,2-alkoxy-sulfenylation of styrene with three different types of six- and five-membered ring heteroaromatic thiols in the presence of t-butyl hydroperoxide in methanol at ambient temperature. Among them, 2-mercapto-benzoxazole (BzOxz-SH) was identified as the best candidate. A variety of chiral vanadyl complexes bearing 3-aryl-5-bromo, 3,5-dihalo-, and benzo-fused salicylidene templates were further examined for optimizing yields and enantio-control. The best scenario involved the use of 5 mol % 3,5-dibromo or -dichloro catalyst at 0 °C with BzOxz-SH in MeOH. The asymmetric catalytic cross-coupling reactions proceeded smoothly with enantioselectivities of up to 94% ee of (R)-configuration by using the 3,5-dichloro catalyst for various 1° alcohols by screening through various 4-, 3-, 3,4-, 3,5-, and 2-substituted (including Me/t-Bu, Ph, OR, Cl/Br, OAc, NO₂, C(O)Me, CO₂Me, CN, and benzo-fused) vinylarenes. Further improvement to 96% ee was achieved by the use of 5-methyl-BzOxz-SH. The origin and catalytic mechanism of enantiocontrol through homolytic methoxy delivery to incipient benzylic radical intermediates by vanadyl-bound methoxide were proposed.