Pseudomonomolecular, Ionic sp2-Stereoinversion Mechanism of 1-Aryl-1-alkenyllithiums

Abstract

The trans/cis stereoinversion of the trigonal carbanion centers C-α in a series of monomeric 2-(α-aryl-α-lithiomethylidene)-1,1,3,3-tetramethylindanes (known to be trisolvated at Li) is rapid on the NMR time scales (400 and 100.6 MHz) in THF solution. The far-reaching redistribution of electric charge in the ground-state molecules caused by lithiation (formal replacement of α-H by α-Li) is illustrated through NMR shifts, Δδ. The transition states for stereoinversion are significantly more polar and charge-delocalized than the ground states (Hammett ρ = +5.2), pointing to a mechanism that involves heterolysis of the C–Li bond via a solvent-separated ion pair (SSIP). This requires immobilization of only one additional (the fourth) THF molecule at Li+, which accounts for part of the apparent activation entropies of ca. −23 cal mol–1 K–1 and constitutes a kinetic privilege of THF depending on microsolvation at Li. Thus, the sp2-stereoinversion process is “catalyzed” by the solvent THF; its mechanism is monomo...