Steric Inhibition of Synergistic Radical Stabilizing Effects

Article abstract of DOI:10.1021/jo00288a012

Equilibrium acidities in Me2SO for 14 α-N-morpholinyl-, 6 α-N-piperidinyl-, and 7 α-cyclohexylarylacetonitriles and the oxidation potentials of their conjugate bases have been determined.The increased pK_HA values by about 2 units observed in these systems, relative to the corresponding arylacetonitriles, is ascribed to the presence of increased steric constraints in the anions.Hammett plots revealed larger ρ values than for arylacetonitriles (6-7 versus 5.5), pointing to an increased negative charge density on the benzylic carbon atoms in the anions.A plot of anion oxidation potentials, E_ox(A-), versus pK_HA for the α-N-morpholinylarylacetonitriles was linear with a slope near unity, showing that remote substituents for the most part have very little effect on radical stabilities.A 5 and 6 kcal/mol lowering of the BDE of the acidic C-H bond caused by replacing the cyclohexyl group in α-cyclohexylphenylacetonitrile by α-N-morpholinyl or N-piperidinyl groups, respectively, is attributed to the strong donor properties of these amino functions.The effects are only about one-third as large, however, as similar effects in R2NCH2COPh, where the steric effect of Ph is absent and a synergistic effect is believed to be operative.