Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part IX. The ‘blocked ortho’ effect in reactions of 2,6-di-substituted aryl 2-nitrophenyl and 2,6-disubstituted aryl 2-azidophenyl sulphides: a new series of nitrene-induced aromatic rearrangements

Abstract

In view of the demonstration that deoxygenation of aryl 2-nitrophenyl sulphides with triethyl phosphite and thermolysis of the corresponding 2-azido-derivatives proceed via rearrangement involving a free ortho-position in the aryl group to give phenothiazines, corresponding reactions involving nitro-compounds and azides in which both ortho-positions are blocked have been carried out. These have led to a new series of aromatic rearrangements whereby 2-nitrophenyl 2,6-dimethyl-, 2,6-dimethoxy-, 2,6-dichloro-, 2-chloro-6-methyl-, 2,4,6-trimethyl-, and 2,6-diethoxycarbonylphenyl sulphides and (with the exception of the last-named case) the corresponding 2-azidoderivatives are converted into, respectively, 5,11-dihydro-4-methyldibenzo[b,e][1,4]thiazepine (17), 1- and 1,2-di-methoxyphenothiazines (via novel demethoxylation and 1,4-methoxy-group shift), 1- and 4-chlorophenothiazines, 1- and 4-methylphenothiazines, 5,11-dihydro-2,4-dimethyldibenzo[b,e][1,4]thiazepine (23), and diethyl 4aH-phenothiazine-1,4a-dicarboxylate (50). In the cases of the two chloro-substituted derivatives the patterns of products from the nitro- and azido-compounds were different, suggesting the participation in the former case of a nitrene precursor [ArN–O–P(OEt)₃] as well as, or instead of, a nitrene. In all other cases the patterns of products from both nitro-compound and azide were similar, suggesting a nitrene as the simplest common intermediate. In accord with this, phosphoramidates were also formed from the nitro-compounds. Photolysis of the azides gave only the corresponding amines.

When subject to triethyl phosphite deoxygenation, 2-substituted aryl 2-nitrophenyl sulphides [2-Me, 2-Cl, 2-MeO, 2,4-(MeO)₂ and 2,3-(MeO)₂] also undergo rearrangement to give the corresponding 1-substituted phenothiazines.