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

Article abstract of DOI:10.1021/jo00087a016

Two higher analogues of α-pinene, 2-isobutyl- and 2-isopropylapopinenes, promising chiral auxiliaries for asymmetric hydroboration, were readily synthesized from α-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*SMe2 (BMS) and BH3*THF, in order to develop a convenient procedure for the synthesis of mono- and bis(2-organylapoisopinocampheyl)boranes <2-R-apoisopinylborane (RapBH2) and bis(2-R-apoisopinyl)borane (Rap2BH), 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 RapBH2 in >/=95percent yield, while a yield of >/=90percent was realized in the case of 2-isobutylapopinene.However, α-pinene, and 2-ethyl- and 2-n-propylapopinenes smoothly reacted with boranes to form a mixture of RapBH2 and Rap2BH.Under the reaction conditions employed, 2-ethyl-, 2-n-propyl-, 2-isobutyl-, 2-phenyl-, and 2-isopropylapopinenes failed to produce clean Rap2BH.The rate of hydroboration decreased significantly with increase in bulk of the organyl group at the 2-position of the apopinene.