BORANES IN SYNTHESIS .5. THE HYDROBORATION OF ENAMINES WITH MONOALKYLBORANES AND DIALKYLBORANES - ASYMMETRIC-SYNTHESIS OF BETA-AMINO ALCOHOLS OF MODERATE ENANTIOMERIC PURITY FROM ALDEHYDE ENAMINES

The hydroboration of both acyclic and cyclic aldehyde and ketone enamines with such representative mono- and dialkylboranes as thexylborane and dicyclohexylborane, followed by an oxidative workup, yields the corresponding beta-amino alcohols in good to excellent isolated yields. The hydroboration of ketone and aldehyde enamines with the asymmetric hydroboration reagents monoisopinocampheylborane ((d)IpcBH(2)) and diisopinocampheylborane ((d)Ipc(2)BH) was also investigated, (d)Ipc(2)BH is highly effective for the asymmetric hydroboration of acyclic aldehyde enamines, such as 1-(4-morpholino)-3-phenyl-1-propene and 1-(1-pyrrolidino)-1-octene. Oxidation of the intermediate trialkylborane furnishes the corresponding beta-amino alcohols in 50-86% ee. The stereogenic center of the carbinol carbon is consistently enriched in the R-enantiomer when (d)Ipc(2)BH prepared from (+)-alpha-pinene is used as the hydroboration reagent. The enantiomeric excesses of the beta-amino alcohols synthesized in this study were determined by HPLC using a chiral stationary phase. The absolute configurations of some of the beta-amino alcohols synthesized in this study were determined by chiral HPLC comparison with authentic beta-amino alcohols prepared from chiral epoxides of known absolute configuration.