American Chemical Society
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Dimeric [3,3-Dimethyl-2-(trimethylsilyl)cyclopropenyl]- lithium−Tetramethylethylenediamine:  Distortion of the Cyclopropenyl Geometry Due to Strong Rehybridization at the Lithiated Carbon

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posted on 1996-02-07, 00:00 authored by Klas Sorger, Paul von Ragué Schleyer, Dietmar Stalke
The reaction of 3,3-dimethyl-1-(trimethylsilyl)cyclopropene with n-BuLi in the presence of 1 equiv of tetramethylethylenediamine (TMEDA) affords [3,3-dimethyl-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 P21/n, a = 1879.9(2) pm, b = 1035.0(2) pm, c = 2045.5(2) pm, β = 113.198(9)°, V = 3.6580(8) nm3, Z = 4, [C28H62Li2N4Si2]). Although dimeric unsolvated cyclopropenyllithium was computed (Becke3LYP/6-31G*) to have two planar tetracoordinate carbon (R1R2CLi2) fragments, (2·TMEDA)2 adopts a perpendicular (“tetrahedral”) structure due to lithium solvation and the steric crowding of the lithium ligands. Lithiation at C(α) of the cyclopropenyl ring in 2 lengthens the vicinal and shortens the distal C−C bonds due to the rehybridization at the lithiated carbon. This is confirmed both by the natural localized molecular orbital carbon hybridizations and by the large coupling constant, 1J13C-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 halogen and C(Li)O carbenoids, the lithiated cyclopropenes do not have carbenoid nature.