STRUCTURAL STUDY AND REACTIVITY OF CARBANIONS IN SOLUTION - PHENYLACETONITRILES AND 3-PYRIDYLACETONITRILES

From the IR and C-13 NMR spectra of Li- or K-associated anionic species 1A,M formed from phenylacetonitrile in THF, it appears that monomeric tight ion pairs and THF-solvated aggregates are in equilibrium at 0.25 M concentration; the 0.025 M THF solution of IA,Li only contains the monomer, the Li cation being likely located near the N atom of the CN moiety. In toluene-THF or hexane-THF solvent mixtures, solvated aggregates are predominant In toluene-hexane, only desolvated aggregates are seen. Addition of increasing amounts of HMPA to THF solutions shows the formation of various Li solvates among which, according to low-temperature P-31 and Li-7 NMR determinations, the Li-di HMPA externally solvated ion pairs predominate. Solvent- or cryptand-separated ion pairs can also be characterized in DMSO or THF m the presence of cryptand. Li- or K-associated 3-pyridylacetonitrile anionic species 2A,M behave as IA,M in toluene, in DMSO, and in THF in the presence of HMPA or of cryptand. In THF, as for IA,Li, 2A,Li monomeric tight ion pairs are in equilibrium with THF-solvated aggregates at c = 0.25 M concentration, the Li cation interacting both with the CN moiety and the pyridyl ring; however, at c = 0.025 M aggregates are still present. IA,M and 2A,M (M = Li, K) aggregates or IA,Li and 2A,Li externally HMPA solvated ion pairs lead to predominant or exclusive 1,4-addition with benzylideneacetone. In THF 1,2-addition takes place with IA,Li while 2A,Li leads to exclusive 1,4 addition; this discrepancy could be due to the fact that the equilibrium between monomeric tight ion pairs and aggregates is different in the two cases and to the different structure of the tight ion pairs, especially the strength of the anion-cation interaction.