3 years ago

“Conducted Tour” Migration of Li+ during the cis/trans Stereoinversion of α-Arylvinyllithiums

“Conducted Tour” Migration of Li+ during the cis/trans Stereoinversion of α-Arylvinyllithiums
Petra Böhrer, Jakob Ruhdorfer, Claudia Behringer, Ernst Lattke, Rudolf Knorr, Ulrich von Roman
A “conducted tour” migration keeps a mobile client on a profitable route even though an occasional side-step may seem attractive. A stereochemical manifestation of such a migration had been suggested by Donald J. Cram (1964), and we present now a different example that concerns the cis/trans stereoinversion of monomeric H2C=C(Li)-aryl compounds: Upon tetrahydrofuran (THF)-assisted heterolysis of the Li−C bond with formation of a solvent-separated ion pair (SSIP), the unchained “mobile client” Li+(THF)4 is proposed to surmount the rim of the electronically fixed aryl group and to disdain the less encumbered pathways across the H2C=C region. This interpretation is based on knowledge from a previously published series of monomeric α-arylalkenyllithiums in combination with two new members: 4-(α-lithiovinyl)-2,2-dimethylbenz[f]indane (1) revealed both a barrier against α-aryl rotation and a route-distinguishing retardation as compared with the corresponding migration-dependent cis/trans stereoinversion rate constant of 1-(α-lithiovinyl)naphthalene (2). Monomeric and dimeric ground states of 1 and 2 and their microsolvation numbers were determined by using the recently developed primary and secondary NMR criteria. Separation and migration—no digressions: Quasibenzyl-anion resonance is stronger in the linear transition state (see scheme) than in the two tri-solvated, angular ground states (CIP and CIP′), because immobilization of the fourth THF ligand at Li increases the charge delocalization from C(α) to C(4). The excess negative charge (∂−) at C(4) appears to favor the migration of Li+(THF)4 across the C(3)/C(4) region of the conformationally fixed α-aryl group.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201702353

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