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 CO 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.
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