Nitrene-induced aromatic rearrangements

Abstract

It has been shown in this investigation that thermal decomposition of mono-substitutedaryl 2-azidophenyl sulphides and deoxygenation of mono-substitutedaryl 2-nitrophenyl sulphides both proceed via a re-arrangement involving five-membered ring attack of the first-formed nitrene followed by a sigmatropic sulphur shift to give the corresponding phenothiazines. By using deuterium labelling, it has been further shown that the rearrangement occurs even in the unsubstituted 2-nitrophenyl phenyl sulphide. Decomposition of 2,6-disubatitutedaryl 2-azidophenyl sulphides and deoxygenation of 2,6-disubstitutedaryl 2-nitrophenyl sulphides led to some very interesting chemistry. For example, 2,6-dimethyl-phenyl 2-azidophenyl sulphide and 2,6-dimethylphenyl 2-nitrophenyl sulphide gave 5,11-dihydro-4-methyldibenzo[b,e][1,4] thiazepine rather than the isomeric 10,11-dihydro-4-methyldibenzo[b,f][1,i4] thiazepine, while 2,6-dimethoxyphenyl 2-azidophenyl sulphide and 2,6-dimethoxy-phenyl 2-nitrophenyl sulphide both gave 1-methoxyphenothiazine and 1,2-dimethoxyphenothiazine via a rearrangement involving a novel l,4-methoxyl shift. That these and the other examples described herein may be explained by a mechanism analogous to that postulated for the phenothiazine rearrangement was shown by the isolation of 1,4a-dicarbethoxy-4H-phenothiazine, a product analogous to the intermediate postulated, from the deoxygenation of 2,6-dicarbethoxy-phenyl 2-nitrophenyl sulphide.