Alkenes are prepared by the hydroboration of enamines followed by an elimination reaction to form the alkene. This process has wide applicability and is useful for the stereospecific synthesis of [Z] isomers. It is preferred to use 9-borabicyclo[3.3.1 nonane as the hydroborating agent and to use methanol to catalyze the elimination reaction.
The present invention relates to novel, powerful reducing agents, lithium aminoborohydrides which are prepared by addition of BH.sub.3 .cndot.THF to the corresponding dialkylamine at 25.degree. C. to give the intermediate amineborane complex. Subsequent deprotonation by strong base, e.g. n-BuLi, yields the aminoborohydride quantitatively. Lithium aminoborohydrides are powerful reducing agents, comparable in strength to lithium aluminum hydride. The activity is determined by the dialkylamine. Lithium pyrrolidinoborohydride has unique activity and selectivity in its reducing properties. Esters, lactones and anhydrides are reduced cleanly at 25.degree. C. to give the corresponding alcohols, while carboxylic acids are not reduced. Test reductions show that lithium pyrrolidinoborohydride is also capable of reducing a wide range of functional groups including amides, epoxides, oximes, nitriles and halides.
The present invention relates to a process for the synthesis of chiral enantiomerically pure .beta.-amino alcohols which are extraordinarily important as therapeutic agents for the treatment of a variety of human disorders and as chiral auxiliaries in organic synthesis. The hydroboration of enamines is a versatile and convenient method for the synthesis of both racemic and enantiomerically pure .beta.-amino alcohols in high yields. This methodology enables the synthesis of virtually any .beta.-amino alcohol. Hydroboration of these enamines with chiral organic borohydrides, e.g. either mono- or diisopinocampheylborane, followed by oxidation with aqueous or solid NaOH/30% H.sub.2 O.sub.2 or Me.sub.3 NO, gives the corresponding chiral .beta.-amino alcohol. Enantiomeric excesses ranged from 60% for reactions run at 25.degree. C. to greater than 99% for reactions run at -25.degree. C.
