Hydroboration. 91. Improved Procedure for the Synthesis of Optically Pure Bis-adducts, N,N,N',N''-Tetramethylethylenediamine-2-organylapoisopinocampheylboranes, from the Corresponding 2-Organylapopinenes of Lower Optical Purity. Conversion of These Adducts into 2-Organylapoisopinocamp...

Article abstract of DOI:10.1021/jo00088a016

The higher analogues of α-pinene, such as 2-ethyl-, 2-n-propyl-, 2-isobutyl-, 2-phenyl-, and 2-isopropylapopinenes, were treated with borane-methyl sulfide (BMS) in the stoichiometry of 1.2:1, respectively, in anhydrous tetrahydrofuran (THF) at 25 deg C, obtaining an equilibrium mixture of 2-organylapoisopinocampheylboranes (2-R-apoisopinylborane, RapBH2) as major and bis(2-organylapoisopinocampheyl)borane as minor constituents.Treatment of the equilibrium mixture of boranes with tetramethylethylenediamine (TMEDA) at 34 deg C readily and selectively provided the optically pure bis(2-organylapoisopinocampheylborane)*TMEDA adducts, (RapBH2)2*TMEDA, in satisfactory yield.This reaction is quite general and appears to be broadly applicable to the conversion of the sterically bulkier 2-organylapopinenes (2-R-apopinenes) with widely varied steric requirements and lower optical purity (87-92percent ee) into the corresponding optically pure (RapBH2)2*TMEDA derivatives, approaching >/= 99percent ee.Treatment of the crystalline bis-adduct in ethyl ether with BF3*EE precipitates (BF3)2*TMEDA and provides RapBH2 of essentially >/= 99percent ee, enantiomerically purer than the 2-R-apopinenes of 87-92percent ee utilized for these preparations.