Portal de Periódicos da CAPES

Toward Planar Tetracoordinate Carbon in the Puckered Ladder Structures of Chelated Cyclopropenyllithium Aggregates

1996; American Chemical Society; Volume: 118; Issue: 29 Linguagem: Inglês

10.1021/ja9603362

1943-2984

Klas Sorger, Paul von Ragué Schleyer, Roland Fleischer, Dietmar Stalke,

Synthesis and characterization of novel inorganic/organometallic compounds

Computational predictions of planar tetracoordinate carbon R1R2CαLi2 arrangements in the cyclopropenyllithium dimer have now been realized experimentally in derivatives with additional chelating ligands. The dimer structures of two substituted cyclopropenyllithiums bearing lithium alkoxide and lithium amide side arms, Cα and O dilithiated 1-(di-tert-butylhydroxymethyl)-3,3-dimethylcyclopropene (8) as well as Cα and N dilithiated 1-(tert-butylaminodimethylsilyl)-3,3-dimethylcyclopropene (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 R1R2CαLi2 carbon environments in both crystal structures have a high degree of planarization (with 35−39° 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 lithiums by the N- moiety in the individual (9·2THF)2 units results in R1R2CαLi2 environments which aproach planarity even more closely (17 and 30° twist angles). Bridging of the vinylic Cα−Cβ bonds by the chelated lithiums in (9·2THF)2 results in an unexpected feature, the nearly planar tetracoordinate environment of Cβ (the cyclopropene carbon next to the lithiated site)! Becke3LYP/6-31G* calculations on mixed-anion (MeLi·LiOH)2 model aggregates 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 computations and with the steric environments), the (9·2THF)2 dimer is formed via Li−C rather than Li−N bond aggregation.