1,2- vs 1,4-Regioselectivity of Lithiated Phenylacetonitrile toward α,β-Unsaturated Carbonyl Compounds. 1. Monomer−Dimer Equilibrium of Lithiated Phenylacetonitrile Ion Pairs in Solution and Structure Determination of the Species by Spectroscopic Methods and ab Initio Calculations. Evidence of a Lithium Bridged Monomeric Ion Pair

• Published in: Journal of organic chemistry 1998-05, Vol.63 (10), p.3287-3294
• Main Authors: Strzalko, Tekla, Seyden-Penne, Jacqueline, Wartski, Lya, Corset, Jacques, Castella-Ventura, Martine, Froment, Françoise
• Format: Article
• Language: English
• Subjects: Chemical Sciences
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo9614497

New contributions from near-infrared FT-Raman and IR spectroscopies to the structure of [PhCHCNLi] n (1) in solution evidence that only two species, a monomeric ion pair 2 and a dimeric one 4, are in equilibrium in media of low dielectric constants such as THF, THF−toluene, or THF−hexane solvent mixtures. The equilibrium position is directly related to the dielectric constant of the medium, and the integrated molar absorption coefficient of each species is determined. The THF-solvated dimer 4 has a structure very close to that of the TMEDA-solvated dimer 3 in the solid state, the two phenyl groups being in an anti arrangement and the nitrogen atom of the C1‘N bond interacting with the two lithium cations. The comparison of ab initio calculations with IR and 13C NMR data of the lithiated monomer 2 shows that its structure is a bridged one in which the lithium cation interacts mainly with the C2‘ carbon in the α position to the nitrile group, with the ipso C3‘ carbon of the phenyl group, and to a lesser extent with the C4‘ carbon of the phenyl group. Low-temperature 13C NMR indicates a lower rotational barrier of the phenyl group for the monomeric lithiated ion pair 2 than for the free anion.