Photochemical Reactions of Arenecarbonitriles with Aliphatic Amines. 2. Effect of Amine Structure on Aminyl vs. α-Aminoalkyl Radical Formation

The photochemical reactions of 9-phenanthrenecarbonitrile with several secondary and primary amines and triethylamine have been investigated. Rate constants for fluorescence quenching by secondary and primary amines decrease with amine α-alkylation in spite of increased electron-donor ability of the more substituted amines. This trend is attributed to a steric effect on the equilibrium constant for reversible exciplex formation. The reactions of secondary and primary amines in nonpolar solvent result in selective N-H vs. α-C-H transfer from amine to arene, via a singlet exciplex. The ratio of α-C-H/N-H transfer increases with α-alkylation and increased solvent polarity and is larger for secondary vs. primary amines. The behavior of the intermediate free radical pairs is also dependent upon amine structure and solvent polarity.