Conventional Lithium Bases as Unconventional Sources of Methyl Anion: Facile Me−Si and Me−C Bond Cleavage in RLi, R₂NLi, and BR₄^- by an Electrophilic Osmium Dihydride

cis,trans-Os(H)2(OTf)(NO)(PiPr3)2 (1-OTf) and several other precursors (1-X) to Os(H)2(NO)(PiPr3)2+ (1+) react with (Me3Si)2NLi, (Me3Si)2CHLi, lithium 2,2,6,6-tetramethylpiperidide (TMPLi), Me3SiCH2Li, and B(CH2SiMe3)4- by a highly unusual, facile β-Me- transfer, the exclusive reaction pathway for the first two in nonpolar solvents. A series of lithium alkyls and alkylamides and organoborate reagents have been examined to reveal widespread occurrence of the direct β-R‘- transfer (R‘ = H, Me) to the Os electrophile, being completely selective for β-H- over β-Me-, with the sole (surprising) exception of NpLi. The β-R‘ elimination was ruled out as the mechanism of the net β-R‘- transfer for two representative RLi cases with R‘ = H, Me, and a single-electron-transfer mechanism was shown to be inoperative for tetraalkylborates. The mechanistic studies also uncovered the important role of Li in RLi and R2NLi, which acts as a potent Lewis acid to abstract the halide/pseudohalide X from 1-X in generating the unsaturated Os species. The proposed intimate mechanism of Me−C and Me−Si bond cleavage is a direct SE2 substitution at carbon with inversion of the Me group, supported by DFT calculations. While the imines formed in the process of C−H and C−Me cleavage are lithiated by, and compete for the Os with, the original base, the unsaturated silicon species formed by Si−Me cleavage react with the remaining base by 1,2-addition of (N,C)-Li, forming intermediates that are also reactive by β-Me- transfer. A complex mixture of Os-free coproducts is obtained in both cases. The structural features of 1+ responsible for its unusual reactivity are discussed.