Hydrocobaltation reactions of 1,3-dienes. Regioselective hydroxylation of myrcene to geraniol and to (±)-linalool via allylcobaloxime intermediates

Abstract

Hydrocobaltation of myrcene (3) by pyridinatocobaloxime leads to a 2:1 mixture of the E- and Z-allylcobaloxime (4a) and (4b) in good yield. When a solution of the cobaloximes (4a) and (4b) in toluene is heated in the presence of tetramethylpiperidine oxide (TEMPO) the hydroxylamines (7a) and (7b) result, which can be converted into geraniol (8a) and nerol (8b) by reduction using zinc in acetic acid. By contrast, in the presence of molecular oxygen, the allylcobaloxime (4) is converted into the allylperoxycobaloxime (15) which on reduction produces (±)-linalool (16). In addition, when the cobaloxime (15) is heated with TEMPO it undergoes cyclisation to the tetrahydrofuran (17), precursor to (+)-linalool oxide (18). Whereas the 1,3-dienes (20)–(24) all failed to undergo hydrocobaltation reactions, both cobaloximes (27) and (28) were easily obtained from 2-methyl-(25) and 2,3-dimethyl-buta-1,3-diene (26), respectively. Using similar chemistry to that described for compound (4), the allylcobaloxime (27) was smoothly converted into the hydroxylamine (29) and into the epoxy-TEMPO derivative (32).