HYDROBORATION .90. SYNTHESIS OF 2-ISOBUTYL AND 2-ISOPROPYLAPOPINENES - RATES AND STOICHIOMETRY OF THE HYDROBORATION OF 2-ORGANYLAPOPINENES WITH BORANE-METHYL SULFIDE AND BORANE-TETRAHYDROFURAN

Two higher analogues of Lu-pinene, 2-isobutyl- and 2-isopropylapopinenes, promising chiral auxiliaries for asymmetric hydroboration, were readily synthesized from alpha-pinene in good chemical yield. A quantitative study was made of the rates and stoichiometry of the hydroboration of a number of representative 2-organylapopinenes (2-R-apopinenes) with representative boranes, such as BH3.SMe(2) (BMS) and BH3.THF, in order to develop a convenient procedure for the synthesis of mono- and bis(2-organylapoisopinocampheyl)boranes [2-R-apoisopinylborane (RapBH(2)) and bis(2-R-apoisopinyl)borane (Rap(2)BH), respectively],under investigation as possible improved asymmetric hydroborating reagents. It was evident that the sterically bulkier 2-R-apopinenes such as 2-phenyl- and 2-isopropylapopinenes reacted with boranes at room temperature to form essentially RapBH(2) in greater than or equal to 195 % yield, while a yield of greater than or equal to 90 % was realized in the case of 2-isobutylapopinene; However, alpha-pinene, and 2-ethyl- and 2-n-propylapopinenes smoothly reacted with boranes to form a mixture of RapBH(2) and Rap(2)BH. Under the reaction conditions employed, 2-ethyl-, 2-n-propyl-, 2-isobutyl-, 2-phenyl-, and 2-isopropylapopinenes failed to produce clean Rap(2)BH. The rate of hydroboration decreased significantly with increase in bulk of the organyl group at the 2-position of the apopinene.