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

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.