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Knorr, Rudolf; Knittl, Monika; Rossmann, Eva C. (2014): Microsolvation and sp²-stereoinversion of monomeric α-(2, 6-di-tert-butylphenyl)vinyllithium as measured by NMR. In: Beilstein Journal of Organic Chemistry, Vol. 10: pp. 2521-2530
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Abstract

The beta-unsubstituted title compound dissolves in THF as a uniformly trisolvated monomer, whereas it forms exclusively disolvated monomers in tert-butyl methyl ether, Et2O, TMEDA, or toluene with TMEDA (1.4 equiv). This was established at low temperatures through the observation of separated NMR signals for free and lithium-coordinated ligands and/or through the patterns and magnitudes of C-13, Li-6 NMR coupling constants. An aggregated form was observed only with Et2O (2 equiv) in toluene as the solvent. The olefinic geminal interproton coupling constants of the H2C= part can be used as a secondary criterion to differentiate between these differently solvated ground-states (3, 2, or <2 coordinated ligands per Li). Due to a kinetic trisolvation privilege of THF, the cis/trans sp(2)-stereoinversion rates could be measured through analyses of H-1 NMR line broadening and coalescence only in THF as the solvent: The pseudomonomolecular (because THF-catalyzed),ionic mechanism is initialized by a C-Li bond heterolysis with the transient immobilization of one additional THF ligand, followed by stereoinversion of the quasi-sp(2)-hybridized carbanionic center in cooperation with a "conducted tour" migration of Li+(THF)(4) along the alpha-aryl group within the solvent-separated ion pair.