CHIRAL SYNTHESIS VIA ORGANOBORANES .27. REMARKABLY RAPID AND EXCEPTIONALLY ENANTIOSELECTIVE (APPROACHING 100-PERCENT EE) ALLYLBORATION OF REPRESENTATIVE ALDEHYDES AT 100-DEGREES-C UNDER NEW SALT-FREE CONDITIONS

In the absence of magnesium salts (from the synthesis of the reagents), our chiral B-allylditerpenylborane reagents (Ter2*BCH2CH=CH2, 1-3) react with representative aldehydes (RCHO, R = Me, n-Pr, i-Pr, t-Bu, vinyl, and Ph) practically instantaneously at -100-degrees-C to give homoallylic alcohols (R*CH(OH)CH2CH=CH2) with optical purities approaching 100% ee. The exceptional reaction rate achieved at -100-degrees-C indicates that these allylborations are among the fastest reactions presently known to the organic chemist. The short reaction time adopted (less-than-or-equal-to 0.5 h) greatly facilitates maintaining the reaction temperature at -100-degrees-C. In this way, B-allyldiisopinocampheylborane (d(Ipc)2BAll, d1) gives homoallylic alcohols of greater-than-or-equal-to 96-99% ee, B-allylbis(4-isocaranyl)borane (4-d(Icr)2BAll, 2) affords alcohols of greater-than-or-equal-to 98% ee and B-allylbis(2-isocaranyl)borane (2-d(Icr)2BAll, 3) provides alcohols of greater-than-or-equal-to 99% ee. The enantioselectivities (greater-than-or-equal-to 99% ee) achieved in allylboration by the reagent 3 are essentially perfect, making this one of the most stereoselective reactions currently known in organic chemistry.