Hydrometalation. 6. Evaluation of Lithium Hydride as a Reducing Agent and Hydrometalation Agent

Article abstract of DOI:10.1021/jo01294a024

The reaction of activated lithium hydride with carbonyl compounds (aldehydes, ketones, esters, and enones), alkenes, and alkynes in the presence of transition-metal halides were investigated.Significant reaction involving the above substrates was accomplished only when an equimolar amount of VCl3 was used in conjunction with the lithium hydride.Aldehydes were reduced to their corresponding alcohols in high yield (95-97 percent), and esters were also reduced to their corresponding alcohols (93-95 percent) with a small amount (5-7 percent) of the corresponding aldehyde (of the carboxylic acid portion) formed.Reductions of cyclohexanones were highly stereoselective.In this connection, 4-tert-butylcyclohexanone was reduced to the axial alcohol in 96 percent yield and with 82 percent stereoselectivity.The only enone to be reduced was cinnamaldehyde, which gave the 1,2-reduction product in 90 percent yield.Terminal olefins were reduced to alkanes whereas internal olefins were completely unreactive.In this connection, 1-octene was reduced to octane in 95 percent yield.In this example, 30 percent deuterium incorporation of the product was observed when hydrolysis was effected with D2O.Such a result indicates formation of the intermediate octyllithium in 30 percent yield.Since alkynes and internal olefins are not reduced at all with LiH and VCl3 and terminal olefins are, reduction of enynes and dienes with LiH and VCl3 could serve as a selective reduction method for the reduction of a terminal double bond in the presence of a triple bond and also for the selective reduction of a terminal double bond in the presence of an internal double bond.