ORGANOMETALLIC COMPOUNDS OF GROUP-III .50. HIGH REGIOSELECTIVITY IN THE ALTERNATIVE, HALOGENATIVE CLEAVAGES OF TERMINAL EPOXIDES WITH LEWIS ACID METAL-HALIDES

The regioselectivity in cleaving the epoxide of a typical terminal olefin to isomeric halohydrins by various metal halides (MX(n)) was investigated. 1,2-Epoxydecane (2a), was subjected to cleavage by LiCl, LiI, Mg(TMP)Br, AlCl3, AlBr3, AlI3, i-BuAlCl2, i-Bu2AlCl, TiCl4, TiBr4, and Ti(NEt2)Br2 in donor and nondonor media with various mixing and temperature conditions and the yield and the ratio of the 1-halo-2-decanol (3a) and 2-halo-1-decanol (4a), obtained upon hydrolysis, were determined. The results can be understood in terms of two limiting models for the transition state: (a) for forming 4a, the complex of 2a and MX(n) undergoes opening to form the more stable, secondary carbenium ion-like transition state or (b) for forming 3a, the complex of 2a and MX(n) undergoes nucleophilic attack by X- at the more accessible primary carbon. The regularities in the observed regioselectivities can thus be summarized: Strongly Lewis acidic conditions, such as TiX4 in alkanes, favor the formation of 4a, while Lewis bases and Lewis basic conditions, such as TiX2(NEt2)2, favor the formation of 3a. In this study, reagents and conditions have been discovered with which the individual halohydrin isomers can be synthesized in high yield and with about 95% regioselectivity.