J . Am. Chem. SOC.1988, 110, 7802-7806
7802
Planar Tetracoordinate Carbon Atoms Hidden in the Tetrameric Aggregate of (2,6-Dimethoxyphenyl)lithium Sjoerd Harder,+Jaap Boersma,? Lambert Brandsma,**+Albert van Heteren,t Jan A. Kanters,*Walter Bauer,s and Paul von Rag& Schleyer*~~ Contribution from the Department of Metal-mediated Synthesis and Laboratory for Crystal and Structural Chemistry, University of Utrecht, Padualaan 8,3508 TB, Utrecht. The Netherlands, and Institut fur Organische Chemie, Universitat Erlangen- Niirnberg, Henkestrasse 42, 0-8520 Erlangen, FRG. Received February 5, 1988
Abstract: The crystal structure of the (2,6-dimethoxyphenyl)lithium tetramer is comprised of two interacting dimer units. This is shown both by the somewhat greater C-Li distances between rather than within the dimers and by differences in the corresponding I3C4Li coupling constants of the tetramer in toluene solution. While these dimer units fold as a result of their interaction, the local environment of C(ipso) approaches the planar tetracoordination predicted in 1981 for the isolated dimer on the basis of MNDO calculations.
When two lithium atoms are attached to the same carbon atom, there is a remarkable increase in the stability of the planar versus the tetrahedral geometrical arrangement.’ While CH2Li2affords 1,l-dilithiocyclopropane(1) was the first the simplest
( partial structure )
L
2
Q?
-5
6
H,fi H
