Microsolvation, Dimerization, and sp2-Stereoinversion of Monomeric α-(2,6-Diisopropylphenyl)vinyllithium

Abstract

The title compound is entirely monomeric in THF as the solvent, but its two ortho-diisopropyl substituents (in contrast to two tert-butyl groups) do not suffice to prevent its dimerization in Et2O: At −107 °C, the disolvated dimer (43% of the total material in monomeric formula units) was accompanied by the disolvated (15%) and the trisolvated monomers (42%). Increasing temperatures reduced trisolvation by Et2O to disolvation and diminished also the combined monomer portions (10% at +25 °C); hence, dimerization in Et2O is endothermic (+3.1 kcal mol–1) but favored by a strongly positive entropy. Microsolvation by only one tert-butyl methyl ether (t-BuOMe) ligand per Li cation of the dimer in t-BuOMe solution was discovered through NMR integration; this disolvated dimer was also observed in cyclopentane as the solvent from −85 to at least +25 °C (hence, no monomer, no higher aggregate). Aryl rotations about the (partial) single bond to the carbanionic center C-α were assessed from the temperatures of NMR signal coalescences of the diastereotopic methyls in an isopropyl group: they are slowest in THF as the solvent because the monomer is then always trisolvated, and delocalization of the negative charge from C-α into the aromatic π-system is therefore most efficient (hence, more rotation-resistant). Microsolvation controls also the cis/trans sp2-stereoinversion, which is faster in THF than in the other three solvents because only one additional THF ligand must temporarily be immobilized (performing THF catalysis) on the way to the tetrasolvated, ionic transition state that involves rehybridization (close to sp) of C-α (pseudoactivation entropy ΔSψ⧧ = −23.6 cal mol–1 K–1). Most of the pertaining microsolvation numbers became available through their empirical relationship with the measured one-bond 13C–6Li NMR coupling constants.