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

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.