Chiral synthesis via organoboranes. Part 32. Synthesis of B-(cycloalk-2-enyl)diisopinocampheylboranes of high enantiomeric purity via the asymmetric hydroboration of cycloalka-1,3-dienes. successful asymmetric allylborations of aldehydes with B-(cycloalk-2-enyl)diisopinocampheylboranes

Abstract

The hydroboration of cycloalka-1,3-dienes (C_nH_2n–4, n= 6, 7 and 8) with diisopinocampheylborane, (^dIpc₂BH or ^IIpc₂BH) at –25°C provides highly enantiomerically pure B-(cycloalk-2-enyl)diisopinocampheylboranes, ^{I or d}Ipc₂BC_nH_2n–3( 93% ee). Surprisingly, these allylic borane derivatives retain their stereochemical integrity at –25 °C although such compounds are capable of undergoing racemization through rapid allylic rearrangements. Furthermore, the B-(cycloalk-2-enyl)diisopino-campheylboranes achieve allylborations of aldehydes at –78 °C and afford 1 -(cycloalk-2-enyl)alkan-1- ols in 90–95% ee and 100%syn-diastereoselectivity.