Multinuclear NMR Study of the Solution Structure and Reactivity of Tris(trimethylsilyl)methyllithium and its Iodine Ate Complex

The extreme steric bulk of tris(trimethylsilyl)methyl derivatives (1-X) provides interesting structural and dynamic behavior for study. Dynamic NMR studies on 1-SePh and 1-I showed restricted rotation around the C-Si bonds of each trimethylsilyl groups. An extensive multinuclear NMR study of natural abundance and (6)Li and (13)C enriched 1-Li revealed three species in THF-containing solvents, a dimer 1T, and two monomers, the contact ion pair 1C, and solvent separated ion pair 1S. Observed barriers for interconversion of 1-Li aggregates were unusually high (Delta G(double dagger) ca. 9 kcal/mol for exchange of 1S and 1C, Delta G(41)(double dagger) = 16.4 kcal/mol for exchange of 1T with 1C and 1S), allowing For study of reactivity of each aggregate individually. We call show that 1S is at least 50 times as, reactive as 1C and at least 5 x 10(10) times as reactive as 1T toward MeI. The large difference in reactivity allowed further study on the mechanism of the lithium-iodine exchange of 1-I with 1-Li and characterization of the intermediate iodine ate complex 4. Additional calibrations are presented for the sensitive yet. chemically inert (13)C NMR chemical shift thermometer 1-H.