Hydroboration. 94. Rates of Hydroboration of 2-Organylapopinenes with 9-Borabicyclo[3.3.1]nonane, Providing B-(2-Organylapoisopinocampheyl)-9-borabicyclo[3.3.1]nonanes, Potentially Valuable for the Asymmetric Reduction of Prochiral Ketones

Five representative enantiomerically pure, hindered terpenes, derived from α-pinene, namely 2-organylapopinenes (2-R-apopinenes, R = Et, Pr, i-Bu, Ph, and i-Pr) have been treated with 9-borabicyclo[3.3.1]nonane (9-BBN) in a 1:1 molar ratio in THF at 24 °C and the rate of hydroboration followed. Increasing the bulk of the 2-R group from the 2-methyl of α-pinene (Ipc, 2-methylapopinene) to 2-ethyl- (Eap), to 2-propyl- (Prap), to 2-isobutyl- (i-Bap), to 2-phenyl- (Pap), and to 2-isopropyl- (i-Prap) significantly lowers the rate of hydroboration with 9-BBN. Thus, the rate of hydroboration of α-pinene with 9-BBN is faster than the rates for the 2-R-apopinenes studied. The sterically bulkier 2-isobutyl-, 2-phenyl-, and 2-isopropylapopinenes reveal a significantly slower rate of hydroboration with 9-BBN. At an elevated temperature, 65 °C, the reaction of 9-BBN (1.0 equiv) with a slight excess of optically pure 2-isobutyl- and 2-phenylapopinenes (1.10−1.20 equiv), under neat conditions, is facilitated to provide the desired B-(2-organylapoisopinocampheyl)-9-borabicyclo[3.3.1]nonanes (2-organyl = isobutyl- and phenyl) in quantitative yield. Unfortunately, this synthesis failed for 2-isopropylapopinene. Fortunately, an indirect synthesis proved satisfactory. Treatment of enantiomerically pure (2-isopropylapoisopinocampheyl)borane, i-PrapBH2, conveniently synthesized from 2-isopropylapopinene, and 1,5-cyclooctadiene (1,5-COD), provided, after thermal isomerization, the desired 1:1 adduct [B-(2-Rap)-9-BBN; 2-Rap = 2-isopropylapoisopinyl skeleton] in quantitative yield. Consequently, five of the 2-R-apopinenes, R = Et, Pr, i-Bu, Ph, and i-Pr, have been successfully converted into the corresponding B-(2-Rap)-9-BBN derivatives.