"Conducted Tour" Migration of Li+ during the cis/trans Stereoinversion of [alpha]-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 [alpha]-arylalkenyllithiums in combination with two new members: 4-([alpha]-lithiovinyl)-2,2-dimethylbenz[f]indane (1) revealed both a barrier against [alpha]-aryl rotation and a route-distinguishing retardation as compared with the corresponding migration-dependent cis/trans stereoinversion rate constant of 1-([alpha]-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.