TOWARD PLANAR TETRACOORDINATE CARBON IN THE PUCKERED LADDER STRUCTURES OF CHELATED CYCLOPROPENYLLITHIUM AGGREGATES

Computational predictions of planar tetracoordinate carbon R(1)R(2)C(a lpha)Li(2) arrangements in the cyclopropenyllithium dimer have now rea lized experimentally in derivatives with additional chelating ligands. The dimer structures of two substituted cyclopropenyllithiums bearing lithium alkoxide and lithium amide side arms, C-alpha and O dilithiat ed -tert-butylhydroxymethyl)-3,3-dimethylcyclopropene (8) as well as C -alpha and N dilithiated -butylaminodimethylsilyl)-3,3-dimethylcyclopr opene (9), were characterized by single-crystal X-ray diffraction. The complexes of 8 with tetramethylethylenediamine (TMEDA), (8 . TMEDA)(2 ), and with THF, (8 . 2THF)(2), show ladder-type dimeric aggregation. The tetracoordinate R(1)R(2)C(alpha)Li(2) carbon environments in both crystal structures have a high degree of planarization (with 35-39 deg rees twist angles), partly due to chelation of one of the lithiums by the O- moiety. Compound 9 crystallizes as a dimeric THF solvate, (9 . 2THF)(2) with a puckered ladder structure. Chelation of one of the lit hiums by the N- moiety in the individual (9 . 2THF)2 units results in R(1)R(2)C(alpha)Li(2) environments which approach planarity even more closely (17 and 30 degrees twist angles). Bridging of the vinylic C-al pha-C-beta bonds by the chelated lithiums in (9 . 2THF)(2) results in an unexpected feature, the nearly planar tetracoordinate environment o f C-beta (the cyclopropene carbon next to ther lithiated site)! Becke3 LYP/6-31G calculations on mixed-anion (MeLi . LiOH)(2) model aggregat es are consistent with the experimental findings that dimerization of both (8 . TMEDA)(2) and of (8 . 2THF)(2) involves the Li-O rather than the Li-C bond. In contrast (but also consistent with the model comput ations and with the steric environments), the (9 . 2THF)(2) dimer is f ormed via Li-C rather than Li-N bond aggregation.