A BETA,BETA-SHIELDED VINYLLITHIUM EXAMPLE FOR A QUANTIFICATION OF STRUCTURE, MONOMER-DIMER EQUILIBRIUM, AND SOME REACTIVITY PARAMETERS

Crystalline 2-(lithiomethylene)-1,1,3,3-tetramethylindan (3) forms the etherate 11 as a centrosymmetric dimer with a LiCLiC four-membered ri ng. The carbanionic terminal of each of the two CC double bonds is tet racoordinated, being bonded at almost equal distances to the two lithi um cations above and below the plane containing both the double bond a nd the vinylic hydrogen atom. For each lithium cation, tetracoordinati on is achieved by bonding to two carbanion centers, to the oxygen atom of one ether molecule, and (weakly) to one of the methyl groups. This unstrained dimeric structure is retained in tert-butyl methyl ether b ut not for all molecules in tetrahydrofuran (THF). In THF monomeric 3 predominates at low temperatures and dimerizes endothermically. The tw o components in equilibrium have characteristically different NMR spec tra, in part due to charge polarization by the lithium cation. Their e quilibration is the fastest mechanism of C-Li bond breaking. The monom er (15 % at +30 degrees C) in THF is the active base for an allylic de protonation and shows the expected degree of reactivity. Addition of d imeric 3 to di-tert-butyl ketone is rapid, yet measurable and selectiv e; monomeric 3, however, might be more selective.