How Microsolvation Numbers at Li Control Aggregation Modes, sp2-Stereoinversion, and NMR Coupling Constants 2JH,H of H2C═C in α-(2,6-Dimethylphenyl)vinyllithium
| Autor: | Claudia Behringer, Rudolf Knorr, Ernst Lattke, Monika Knittl, Ulrich von Roman |
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| Rok vydání: | 2015 |
| Předmět: | |
| Zdroj: | The Journal of Organic Chemistry. 80:6313-6322 |
| ISSN: | 1520-6904 0022-3263 |
| Popis: | The title compound 4 is a trisolvated monomer 4&3THF in THF solution and dimerizes endothermically to form (4&THF)2 with a strongly positive (!) dimerization entropy in toluene as the solvent. In the absence of electron-pair donor ligands, 4 aggregates (>dimer) in hydrocarbon solutions. These results followed from the (13)C-α splitting patterns and the magnitudes of the one-bond (13)C,(6)Li NMR coupling constants in combination with lithiation NMR shifts as secondary NMR criteria. The rate constants of cis/trans sp(2)-stereoinversion could be measured on the (1)H NMR time scale in THF, in which solvent the preinversion lifetime is 0.24 s at 25 °C. This inversion proceeds according to the pseudomonomolecular, ionic mechanism with the typical, strongly negative pseudoactivation entropy. In a different mechanism, the lifetimes are much longer at 25 °C for the dimer (4&t-BuOMe)2 in toluene (ca. 2.5 min) and for donor-free, aggregated 4 in hexane solution (roughly 1 min). The olefinic interproton two-bond coupling constants (2)JH,H of the H2C═CLi part are proposed as an indicator of microsolvation at Li, because they were found to increase linearly with the "explicit" solvation of α-arylvinyllithiums by 0, 1, 2, and 3 electron-pair donor ligands. |
| Databáze: | OpenAIRE |
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