Competitive cyclisations of singlet and triplet nitrenes. Part 5. Mechanism of cyclisation of 2-nitrenobiphenyls and related systems

Abstract

The conversion of 2-azidobiphenyl into carbazole is considered to involve concerted cyclisation of a singlet nitrene on the basis of the following evidence, (a)N-2-Nitrenophenyl heterocycles are poor sources of carbazole analogues since the concertedness of the cyclisation is impaired by the ‘bridgehead’ nitrogen. (b) 2-Methyl-2′-nitrenobiphenyl gives 4-methylcarbazole (up to 100% yield) under singlet-promoting conditions but increasingly more phenanthridine under conditions producing triplet nitrenes (heavy atom effect; photosensitised formation in acetophenone). By elevation of the photolysis temperature, azo-compound formation is diminished and phenanthridine yields are increased in photosensitised reactions, (c) 2,4,6-Trimethyl-2′-nitrenobiphenyl is shown to give both a carbazole and a phenanthridine at high temperatures (>300 °C) by way of a concerted attack of a singlet nitrene. The above principles are applied to the cyclisation of 2-(2-azidophenyl)-3,5-dimethylthiophen giving 2-methylthieno[3,2-c]quinoline. The role of solvents and additives during nitrene reactions is explored generally; tetrachlorothiophen and 2,3,5-tribromothiophen are solvents of choice for azide decompositions.