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Knorr, Rudolf; Lattke, Ernst; Ruhdorfer, Jakob; Ferchland, Kathrin; Roman, Ulrich von (2018): Why is cis/trans stereoinversion with Li+(THF)(4) migration across the phenyl ring of alpha-lithiostyrene accelerated by two ortho-methyl groups? In: Tetrahedron, Vol. 74, No. 14: pp. 1621-1631
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Common wisdom might anticipate that two methyl groups placed on a molecular migration route should act as an impediment. However, the "conducted tour" migration of Li+(THF)(4) across the aryl ring ("pi-route") during the cis/trans stereoinversion of alpha-arylvinyllithiums had been found to occur with practically equal velocities in the presence of either one or two ortho-alkyl substituents. We now report that the omission of both ortho-methyl groups retards the stereoinversion process. In order to arrive at an answer to the title question, we investigate the aggregation equilibria and microsolvation states of ortho, ortho'-unsubstituted alpha-lithiostyrenes by means of approved secondary NMR criteria. Beyond such necessary knowledge about the ground-state properties, we provide kinetic evidence showing that the retarded cis/trans stereoinversion of alpha-lithiostyrene proceeds by the pseudomonomolecular, ionic mechanism with Li+(THF)(4) migration.