Stereochemical studies on the amination of arenes with ammonia and alkylamines via photochemical electron transfer
Photoamination of 9-methoxyphenanthrene (1) with ammonia in the presence of m-dicyanobenzene (DCNB) gives cis- and trans-9-amino-10-methoxy-9,10-dihydrophenanthrene (6a, b) in a ratio of 75 : 25. The stereochemistry of 6a has been confirmed by X-ray crystallographic analysis of its acetamide. The photoamination of 1 with isopropylamine and 9-ethoxyphenanthrene (2) with tert-butylamine gives exclusively N-substituted cis-9-amino-10-alkoxy-9,10-dihydrophenanthrene. Also, the photoamination of phenanthrene (4) and anthracene (5) proceeds with trans addition of ammonia with high selectivities. The photoamination proceeds by nucleophilic addition of amine to the cation radical of the arene, generated from photochemical electron transfer to DCNB. The resulting aminated cation radicals undergo reduction by an anion radical of DCNB, followed by protonation to give final products. Selective trans additions of ammonia and alkylamines to arenes are suggested to arise from the protonation of the aminated anion.