Synthesis and Structure of Ethynylsilyllithiums

Ethynylsilyllithiums [R1R2(R3CC)SiLi:  R1 = R2 = t-BuMe2Si, R3 = Me3Si (5a); R1 = R2 = t-BuMe2Si, R3 = Ph (5b); R1 = R2 = R3 = Me3Si (5c); R1 = t-BuMe2Si, R2 = Ph, R3 = Me3Si (5d); R1 =
t-BuMe2Si, R2 = Me3SiCC, R3 = Me3Si (5e)] were prepared by the sila-metalation reactions of the
corresponding hydrosilanes using tert-butyllithium in THF and trapped by MeI, Me3SiCl, and i-Pr3SiCl
in high yields. X-ray structural analysis of a single crystal of 5a has revealed that 5a adopts a dimer form
(7a) containing one THF molecule per dimer. Dimer 7a is a syn dimer with a planar four-membered
ring made by two lithium atoms and two anionic silicon atoms; one lithium atom is coordinated by two
ethynyl π groups and the other by a THF molecule, indicative of the importance of the coordination of
ethynyl π groups for the stabilization of the silyllithium. 7Li NMR analysis indicates that the dimeric
structure of 5a (7a) is maintained in toluene, but 5a is monomeric in THF. Rates and activation
parameters of facile intramolecular exchange of the two lithium atoms in 7a observed in toluene-d8 were
determined by the dynamic 7Li NMR. The DFT calculations of a model ethynylsilyllithium [(H3Si)2(H3SiCC)SiLi, 12] have shown that dimer 13, with a similar geometry to 7a, is more stable than the monomeric
form. The optimized monomeric form of 12 (12‘) has an ethynylsilyllithium-type structure with a largely
bent Si−CC skeleton, suggesting significant intramolecular π+ coordination to Li+.