Photochemical reactions of aromatic compounds. Part 44. Mechanisms for direct photoamination of arenes with ammonia and amines in the presence of m-dicyanobenzene

Abstract

Mechanistic details of the efficient photoamination of arenes (ArH) with ammonia or aliphatic primary amines (RNH₂) in the presence of m-dicyanobenzene (DCNB) in 9:1 acetonitrile–water have been analysed by kinetics. The initation process of the photoamination is electron transfer from excited singlet ArH to DCNB to generate the cation radical of ArH (ArH⁺˙), to which RNH₂ undergoes nucleophilic addition. The rate constant (k_N) for the nucleophilic addition to the phenanthrene cation radical depends on R, varying from 3 × 10⁷ dm³ mol^–1s^–1 for NH₃to 8.9 × 10⁸ dm³ mol^–1s^–1 for Bu^tNH₂. A plot of log k_Nversus the Taft σ* parameter is linear with a slope of –2.1, demonstrating a substantial positive charge on RNH₂ in the transition state in line with the proposed mechanism. Photoamination with Me₂NH is very inefficient for naphthalene, 2-methoxynaphthalene, and phenanthrene, and is attributed to electron exchange between ArH⁺˙ and Me₂NH being competitive with nucleophilic addition. Anthracene is efficiently photoaminated with Me₂NH but inefficiently with Et₂NH. Efficient photoamination requires that the observed oxidation potential of the amines is more positive by 0.3–0.4 eV than that of ArH.