STERIC INHIBITION OF SYNERGISTIC RADICAL STABILIZING EFFECTS

Equilibrium acidities in Me2SO for 14 a-N-morpholinyl-, 6 a-N-piperidinyl-, and 7 a-cyclohexylarylacetonitriles and the oxidation potentials of their conjugate bases have been determined. The increased pKHA 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 p 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, EOX(A-), versus pKHA for the a-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 a-cyclohexylphenylacetonitrile by a-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. © 1990, American Chemical Society. All rights reserved.