ISODICYCLOPENTADIENES AND RELATED MOLECULES .58. STRUCTURAL-ANALYSIS OF A CAMPHOR-DERIVED CYCLOPENTADIENYLLITHIUM COMPOUND BY NMR AND MNDO - A TERNARY EQUILIBRIUM IN TETRAHYDROFURAN

At -107-degrees-C, the camphor-derived cyclopentadienyllithium compound 2 (CCpLi) exists as three different aggregates in tetrahydrofuran solution. These are the exo-lithio monomer 10, the endo-lithio monomer 11, and the endo,endo-lithio dimer 12, in a 57:21:22 molar ratio (0.34 M solution). Approximately the same composition of these three species is present at room temperature. The identification was achieved by 1D and 2D NMR methods. Due to ring-current phenomena, unusual upfield Li-6 NMR chemical shifts are observed (delta = 7.83 ppm for the two monomers 10 and 11 and -12.45 ppm for ''sandwiched'' lithium in dimer 12). MNDO calculations on the individual isomers are in agreement with the experimentally determined relative stabilities. The results rationalize earlier findings concerning the endo stereoselectivity in reactions of 2 with Ti and Zr electrophiles.