Reaction Mechanism Studies: 2. Reductive Elimination of 1,2-Dihalides With Lithium Aluminum Hydride

1,2-Dibromides and bromohydrin p-toluenesulphonates of favorable geometry are readily reduced by lithium aluminum hydride in good yield to the corresponding olefin. Small quantities of water are found to have a temporaryacceleratingeffect on the reduction. The normal course of the reaction is a trans elimination, but when steric factors inhibit this, then substitution or cis elimination reactions may occur. It is shown that the cis elimination in the case of cis-1,2-dibromocyclohexane cannot involve reduction to the monobromide followed by dehydrobromination to the olefin. The reaction of some other 1,2-dihalides and related compounds with lithium aluminum hydride, and the reaction of a 1,2-dibromide with a number of other complex hydrides are reported.A general scheme for the mechanism of the olefin-forming elimination reaction is proposed. The limits that this study places on the possible positions of the reductive elimination of 1,2-dibromides with lithium aluminum hydride within this general scheme are discussed.