Electrochemical reactions. Part 19. Intermolecular radical substitution during the reduction of 2-halogeno-N-methylbenzanilides

Abstract

The reduction of 2-halogeno-N-methylbenzanilides gives N-methyl-N-phenylcarbamoylphenyl radicals, which (i) react by substitution to yield phenanthridones and biphenyl-2-carboxamides or (ii) are reduced further to the N-methylbenzanilide. Occurrence of the substitution process confirms the presence of phenyl radical intermediates. The maximum yield of intramolecular substitution products is limited by the proportion of syn-amide rotamer present at equilibrium. When the halogen is chlorine, intramolecular trapping of the resulting phenyl radical is an efficient process and only the anti-amide rotamer forms the N-methylbenzanilide. When the halogen is bromine or iodine a significant proportion of the phenyl radical derived from the syn-amide rotamer is also converted by further reduction and protonation into the N-methylbenzanilide. Intramolecular radical substitution leads to both phenanthridones and biphenyl-2-carboxamides when the aniline component of the N-methylbenzanilide does not carry an ortho-substituent. Only the biphenyl-2-carboxamide is formed when an ortho-substituent is present on the aniline component. These results are compared with the results from related radical reactions.