The formation of rearrangement products in the reactions of cyclic alcohols with boron trichloride

Abstract

The reactions of cyclopentylmethanol and cyclohexylmethanol with boron trichloride afford the corresponding dichloroborinates (alkoxydichloroboranes), which decompose thermally to yield mixtures of halide isomers; ring expansion and a 1,2-hydride shift occur. Secondary cyclic alcohols having no ring substituent other than the hydroxy-group, and 1-substituted cycloalkanols, react rapidly at room temperature or below to yield the corresponding halides with no detectable rearrangement. trans-2-Methylcyclohexanol affords the corresponding halide with retention of configuration, together with 15–20% of the tertiary chloride. Subsequent rearrangement of certain products is catalysed by heating with boron trichloride. The 1-methylcycloalkyl chlorides thus afford mixtures with the isomeric secondary methylcycloalkyl chlorides, whilst cycloheptyl chloride undergoes irreversible rearrangement to ring-contracted products. Primary halides are isomerized only slowly by boron trichloride.