Selective Reductions. 31. Lithium Triethylborohydride as an Exceptionally Powerful Nucleophile. A New and Remarkably Rapid Methodology for the Hydrogenolysis of Alkyl Halides under Mild Conditions

Article abstract of DOI:10.1021/jo00166a031

Lithium triethylborohydride exhibits enormous nucleophilic power in S_N2 displacement reactions with alkyl halides, far more powerful than the other common nucleophiles, such as n-butyl mercaptide (14 times), thiophenoxide (20 times), borohydride (1E4 times), and nitrate (1E7 times).The reaction follows second-order kinetics and exhibits typical characteristics of a nucleophilic substitution of the S_N2 type.In addition to being the best nucleophile, it is the most powerful nucleophilic reducing agent available for the reduction of alkyl halides, far more powerful and cleaner than lithium aluminum hydride and lithium borohydride.Even hindered alkyl halides, such as cyclohexyl bromide, neopentyl bromide, and exo-norbornyl bromide, undergo facile reduction to the corresponding alkanes in > 96 percent yield with this reagent.Consequently, the new reagent provides a highly useful and simple means as a probe for studying S_N2 displacement reactions and also for the facile dehalogenation of hindered alkyl halides where this is required in synthetic transformations.The corresponding deuterated derivative, lithium triethylborodeuteride, conveniently synthesized from lithium deuteride and triethylborane, is useful for the stereospecific introduction of deuterium in the molecule.