Reactions of organotin hydrides with lithium diisopropylamide and organolithiums. Reactivities of the intermediates and of the lithium hydride produced

Article abstract of DOI:10.1021/om00146a021

Equimolar lithium diisopropylamide (LDA) and trimethyltin hydride (TMTH) react in tetrahydrofuran (THF) to form diisopropylamine and (trimethylstannyl)lithium, but in diethyl ether or hexanes 2 mol of TMTH is required for complete reaction and the products are diisopropylamine, hexamethylditin, and lithium hydride. When organic halides are present in this reacting system, reduction to alkane or substitution to form the trimethylalkyltin may occur depending on the nature of the halide. These and other observations suggest that (trimethylstannyl)lithium is formed as an intermediate yielding the tetraalkyltin. Studies on the products and stoichiometries of the reductions of alkyl bromides in ether and hexanes suggest that three reducing agents may be involved: TMTH, [Me₃Sn(H)N-i-Pr₂]^-, and [Me₃SnSn(H)Me₃]^-. The latter predominates in ether, and either or both of the others predominate in hexanes. Formation of methylcyclopentane from 1-bromo-5-hexene suggests involvement of a free radical mechanism. When methyllithium is used instead of LDA in the reaction with TMTH, the products are tetramethyltin and lithium hydride. This reaction can also be diverted to reduction by the presence of primary bromides. Aryl bromides react in both systems, but the yields of either substitution or reduction products are low. The lithium hydride formed in these reactions is extremely reactive as a base as shown by a brief study of its reaction with weak carbon acids and amines and as a nucleophile by its reaction with hexamethylditin to form (trimethylstannyl)lithium in THF.