No abstract

Computational predictions of planar tetracoordinate carbon R 1 R 2 C R Li 2 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, C R and O dilithiated 1-(di-tert-butylhydroxymethyl)-3,3-dimethylcyclopropene (8) as well as C R and N dilithiated 1-(tertbutylaminodimethylsilyl)-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 laddertype dimeric aggregation. The tetracoordinate R 1 R 2 C R Li 2 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 Omoiety. Compound 9 crystallizes as a dimeric THF solvate, (9‚2THF) 2 , with a puckered ladder structure. Chelation of one of the lithiums by the Nmoiety in the individual (9‚2THF) 2 units results in R 1 R 2 C R Li 2 environments which aproach planarity even more closely (17 and 30°twist angles). Bridging of the vinylic C R -C β bonds by the chelated lithiums in (9‚2THF) 2 results in an unexpected feature, the nearly planar tetracoordinate environment of C β (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.

Two different Li environments characterize the solvent‐free, polymeric structure of 1 in the crystal. A tetrameric chain is formed by Li atoms, which are coordinated by two O and three or five C atoms. In contrast, 1 exists as isolated tetramers in THF. The C(Li)O carbenoid character of 1 results in an elongated CO bond and in lithium bridging.

Zwei unterschiedliche Li‐Umgebungen kennzeichnen die solvensfreie, polymere Struktur von 1 im Kristall. Die Tetramerenkette wird durch Li‐Atome aufgebaut, die durch zwei O‐ und drei C‐ bzw. durch fünf C‐Atome koordiniert sind. In THF liegt 1 dagegen tetramer vor. Die C(Li)O‐carbenoide Natur von 1 äußert sich in einer verlängerten C‐O‐Bindung und in überbrückendem Lithium.

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, 1 J 13 C - 6 Li = 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.