Chiral synthesis via organoboranes. 27. Remarkably rapid and exceptionally enantioselective (approaching 100% ee) allylboration of representative aldehydes at -100.degree. under new, salt-free conditions

TLDR: In the absence of magnesium salts (from the synthesis of the reagents), this paper showed that B-allylditerpenylborane reagents (Ter 2 *BCH 2 CH=CH 2, 1-3) react with representative aldehydes (RCHO, R=Me, n-Pr, iPr, t-Bu, vinyl, and Ph) practically instantaneously at −100 o C to give homoallylic alcohols (R*CH(OH)CH 2 CH =CH 2 ) with optical purities approapching 100%

Abstract: In the absence of magnesium salts (from the synthesis of the reagents), our chiral B-allylditerpenylborane reagents (Ter 2 *BCH 2 CH=CH 2 , 1-3) react with representative aldehydes (RCHO, R=Me, n-Pr, i-Pr, t-Bu, vinyl, and Ph) practically instantaneously at −100 o C to give homoallylic alcohols (R*CH(OH)CH 2 CH=CH 2 ) with optical purities approapching 100% ee. The exceptional reaction rate achieved at −100 o C indicates that these allylborations are among the fastest reactions presently known to the organic chemist. The short reaction time adopted (≤0.5 h) greatly facilitates maintaining the reaction temperature at −100 o C. In this way, B-allyldiisopinocampheylborane ( d Ipc 2 BAll, d 1) gives homoallylic alcohols of ≥96-99% ee, B-allylbis(4-isocaranyl) borane (4- d Icr 2 BAll, 2) affords alcohols of ≥98% ee and B-allylbis(2-isocaranyl) borane (2- d Icr 2 BAll, 3) provides alcohols of ≥99% ee