Hydroboration. 44. Diisopinocampheylborane of High Optical Purity. Asymmetric Synthesis via Hydroboration with Essentially Complete Asymmetric Induction

The reaction of borane with α‐pinene in tetrahydrofuran attains equilibrium in 3 days at 0°. With 15% excess (+)‐α‐pinene ([α] D 26.5 + 47.99°), the formation of diisopinocampheylborane is essentially quantitative (‐99.5%). The longer reaction time is accompanied by a selective incorporation of the major isomer into the reagent. Thus, (+)‐α‐pinene, dehydroborated from diisopinocampheylborane with triethylamine, exhibits an optical rotation of [α] D 23.5 + 51.0°, indicating an optical purity of 99.8%, and the isopinocampheol, obtained following the oxidation of the reagent, showed [α] D 23 −35.1° ( c 10, benzene), a higher rotation than any realized previously. The hydroboration of cis ‐2‐butene with this diisopinocampheylborane in tetrahydrofuran at −25° provided 2‐butanol of high optical purity, [α] D 22.5 −13.29°, an optical purity of 98.4%. Thus this reaction proceeds with nearly complete asymmetric induction.