Oxymetallation. Part 15. Oxidation of hex-1-ene by thallium(III) trifluoroacetate in methanol

Abstract

Hex-1-ene reacts with thallium(III) acetate in methanol to afford 2-methoxyhexylthallium diacetate which under goes no appreciable decomposition when kept in methanol at room temperature for one week, but whic, upon treatment with solid potassium bromide and a catalytic amount of 2,6-dimethyl-18-crown-6, cleanly affords a high yield of 1-bromo-2-methoxyhexane (1). By contrast, the methoxythalliation adduct obtained with thallium(III) trifluoroacetate undergoes oxidative dethalliation fairly rapidly (ca. 85% dethalliation after 2 h) to give 1,2-dimethoxyhexane (2) and 2-methoxyhexanol (3) as major products. It is suggested that compounds (2) and (3) arise by facile ligand exchange followed by transfer of methoxy-or hydroxy-groups from thallium to C-1 in an S_Ni process. If the methoxythalliation is carried out with thallium(III) trifluoroacetate in dichloromethane with methanol (2 equiv.) the oxidative dethalliation is complete after only 1 h, but 1-methoxyhexan-2-ol (4) and 2-methoxy-1-trifluoroacetoxyhexane (5) are obtained as additional products. It is suggested that ligand exchange is not effective under these conditions and that dethalliation via an oxonium ion, which leads to compounds (2)–(4), competes with the S_Ni process which affords compound (5). Only small amounts (<10%) of hexan-2-one are obtained in the oxidations.