DIMERIC )CYCLOPROPENYL]-LITHIUM-TETRAMETHYLETHYLENEDIAMINE - DISTORTION OF THE CYCLOPROPENYL GEOMETRY DUE TO STRONG REHYBRIDIZATION AT THE LITHIATED CARBON

The reaction of 3,3-dimethyl-1-(trimethylsilyl)cyclopropene with n-BuL i in the presence of 1 equiv of tetramethylethylenediamine (TMEDA) aff ords methyl-2-(trimethylsilyl)cyclopropenyl]lithium(2). While NMR data reveal a monomer in tetrahydrofuran (THF) solution, 2 crystallizes as a dimeric TMEDA solvate, (2 . TMEDA)(2). The structure was determined by single-crystal X-ray diffraction (crystal data: monoclinic, space group P2(1)/n, a = 1879.9(2) pm, b = 1035.0(2) pm, c = 2045.5(2) pm, b eta = 113.198(9)degrees, V = 3.6580(8) nm(3), Z = 4, [C28H62Li2N4-Si-2 ]). Although dimeric unsolvated cyclopropenyllithium was computed (Bec ke3LYP/6-31G) to have two planar tetracoordinate carbon (R(1)R(2)CLi( 2)) fragments, (2 . TMEDA)(2) adopts a perpendicular (''tetrahedral'') structure due to lithium solvation and the steric crowding of the lit hium ligands. Lithiation at C(alpha) of the cyclopropenyl ring in 2 le ngthens the vicinal and shortens the distal C-C bonds due to the rehyb ridization at the lithiated carbon. This is confirmed both by the natu ral localized molecular orbital carbon hybridizations and by the large coupling constant, (1)J(13C-6Li) = 17.6 HZ, observed in THF solution (the usual range for organolithium monomers is 10 and 16 Hz). Despite the strong rehybridization and their relationship to the C(Li)X haloge n and C(Li)O carbenoids, the lithiated cyclopropenes do not have carbe noid nature.