DIRECT COMPARISON OF THE SOLUTION STRUCTURE OF A C1-SYMMETRICAL PINANE-FUSED CYCLOPENTADIENYLLITHIUM WITH THE STEREOSELECTIVITY OF ITS CAPTURE BY ELECTROPHILES

The verbenone-derived cyclopentadienyl compound (1S,8S)-7,7,9,9-tetramethyltricyclo[6.1.1.0(2,6)]deca-2,5-dienyllithium (VCpLi) yielded a 3:2 mixture of exo and endo deuterio quench products upon reaction with D2O at -78-degrees-C in THF. Stereochemical identification was achieved by NMR analysis of Diels-Alder addition products. Reaction of VCpLi with Me3SiCl under the same conditions gave rise to a 9:1 exo:endo product mixture. Silatropic shifts were observed in these quench products. According to NMR analysis, VCpLi consists nearly exclusively of the exo-Li monomer in THF at +26-degrees-C. However, at -80-degrees-C a ternary equilibrium of an exo-Li monomer, an exo,exo-Li sandwich dimer, and an endo,endo-Li sandwich dimer in a 5.1:2.8:1.0 molar ratio has been detected. Thermodynamic parameters for the monomer-dimer equilibrium are DELTAH-degrees = -3.6 +/- 0.2 kcal/mol and DELTAS-degrees = -15.6 +/- 0.9 eu. Due to ring current effects, unusual upfield Li-6 chemical shifts are observed: delta (ppm) = -7.83 (exo monomer), -12.22 (exo,exo dimer), and -12.25 (endo,endo dimer). Exo/endo assignment of the isomers of VCpLi was achieved by Li-6, H-1 HOESY. MNDO calculations correctly reflect the relative stabilities of the individual isomers of VCpLi as well as the temperature dependence of the monomer-dimer equilibrium.