Microsolvation and sp2-stereoinversion of monomeric α-(2,6-di-tert-butylphenyl)vinyllithium as measured by NMR

• Published in: Beilstein journal of organic chemistry Vol. 10; no. 1; pp. 2521 - 2530
• Main Authors: Knorr, Rudolf, Knittl, Monika, Rossmann, Eva C
• Format: Journal Article
• Language: English
• Published: Trakehner Str. 7-9, 60487 Frankfurt am Main, Germany Beilstein-Institut 29.10.2014
• Subjects: 13C,6Li NMR coupling; Chemistry; Full Research Paper; ion pair intermediate; monomeric alkenyllithiums; pseudoactivation parameters; sp2-stereoinversion mechanism; THF catalysis
• ISSN: 1860-5397; 1860-5397
• DOI: 10.3762/bjoc.10.263

The β-unsubstituted title compound dissolves in THF as a uniformly trisolvated monomer, whereas it forms exclusively disolvated monomers in tert -butyl methyl ether, Et 2 O, 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 13 C, 6 Li NMR coupling constants. An aggregated form was observed only with Et 2 O (2 equiv) in toluene as the solvent. The olefinic geminal interproton coupling constants of the H 2 C= 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 1 H 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 α-aryl group within the solvent-separated ion pair.