Structural and Rate Studies of the Formation of Substituted Benzynes

The key elimination step for the formation of 3-substituted and 3,6-disubstituted benzynes from 2-haloaryllithiums displays a pronounced solvent-dependent regioselectivity. All 2-haloaryllithiums with electron withdrawing groups in the 6 position are shown by 6Li and 13C NMR spectroscopic studies to be monomers in THF. DFT computational studies implicate trisolvates. Rate studies reveal that LiF eliminates via monomer-based pathways requiring THF dissociation whereas LiCl eliminates via nondissociative pathways. Elimination to form 3-chloro- and 3-fluorobenzyne from 2-chloro-6-fluorophenyllithium displays a pronounced solvent-dependent regioselectivity that is traced to competing solvent-dissociative and nondissociative dissociative pathways for the elimination of LiCl and LiF, respectively.