Selective Reductions. 28. The Fast Reaction of Lithium Aluminum Hydride with Alkyl Halides in Tetrahydrofuran. A Reappraisal of the Scope of the Reaction

Article abstract of DOI:10.1021/jo00341a018

Clear solutions of lithium aluminum hydride in tetrahydrofuran reduce organic halides of different structural characteristics rapidly and quantitatively, commonly at 25 deg C, to the corresponding hydrocarbons.The rates of reduction of organic halides observed in the present study are far faster than those realized by previous workers employing slurries of lithium aluminum hydride.The reaction exhibits the typical characteristics of a S_N2 substitution reaction, as revealed by the approximate rate studies with a representative series of alkyl halides: (I > Br > Cl; benzylic ca. allylic > primary > secondary > tertiary; n-butyl > isobutyl >> neopentyl; cyclopentyl > 2-octyl > cycloheptyl > cyclooctyl > cyclohexyl).Allyl and benzyl bromides are reduced instantaneously. n-Alkyl iodides and bromides are reduced rapidly and quantitatively in 15-30 min at 25 deg C. n-Alkyl chlorides and secondary alkyl bromides, including the highly resistant cyclohexyl bromide, underwent facile reduction to completion in refluxing THF.Surprisingly, the organic halides that are resistant to normal nucleophilic substitution reactions - bromobenzene, β-bromostyrene, and tert-butyl bromide - all exhibit considerable reactivity toward lithium aluminum hydride.Thus, the present study has led to a better understanding of the scope and applicability of lithium aluminum hydride for hydrodehalogenation of alkyl halides.