Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part XV. Reactions of certain aryl 2-nitrophenyl sulphides and their 2-azido-analogues leading to evidence for the intermediacy of 2,3-dihydro-1,3,2-benzothiazaphosph(V)oles (thiazaphosphoranes) and to the formation of dihydrodimethylphenothiazinones and pyrimido- and pyrido-[1,2-b]indazoles

Abstract

Deoxygenation by triethyl phosphite and, in two cases, by trimethyl phosphite, of aryl 2-nitrophenyl sulphides [aryl = 2,6-Me₂C₆H₃, 2,4,6-Me₃C₆H₂, 2,6-(MeO)₂C₆H₃, 2,4,6-(MeO)₃C₆H₂, 2,5-(MeO)₂C₆H₃, 2-MeO·C₆H₄, or 4-MeC₆H₄] gave, in addition to various heterocycles described previously, appreciable yields (6–87%) of dialkyl N-2-alkylthiophenyl-N-arylphosphoramidates, e.g.(9), where the alkyl group is derived from the phosphite and the aryl groups are as above. Some of these phosphoramidates were previously wrongly assigned the isomeric dialkyl N-alkyl-N-(2-arylthiophenyl)phosphoramidate structures, e.g.(6), and as such were previously considered to be unimportant and expected by-products [ArNO₂+(RO)₃P →(RO)₂P(O)NRAr]. The present reassignment of structure points to the intermediacy of 2,2-dihydro-1,3,2-benzothiazaphosph(V)oles, e.g.(14), analogous to the oxazaphospholes (2) obtained from the deoxygenation of the corresponding aryl 2-nitrophenyl ethers (1), thus indicating mechanistic unity for the two series of reactions.

Deoxygenation of 4-hydroxy-2,6-dimethylphenyl 2-nitrophenyl sulphide (25) and its 2-hydroxy-4,6-dimethylphenyl analogue (26) gave 4-(2-ethylthioanilino)-3,5-dimethylphenol (29)(25%) and 2-(2-ethylthioanilino)-3,5-dimethylphenol (33)(21%), respectively, together with 4-ethoxy-2′-ethylthio-2,6-dimethyldiphenylamine (30)(24%) in the former case. These are hydrolysis products of the corresponding N-2-alkylthiophenylphosphoramidates. Spiro-dienones of the type (34) were not detected, but the corresponding 2-azidophenyl analogues gave the novel 4,4a-dihydro-1,4a-dimethylphenothiazin-3-one (36)(28%) and a mixture of 2,4a- and 4,4a-dihydro-3,4a-dimethylphenothiazin-1-one (4)(6.5%) and (41)(2.3%).

Deoxygenation of 2-nitrophenyl 2-pyridyl and 2-pyrimidyl sulphides by triethyl phosphite gave, respectively, pyrido[1,2-b]indazole (44)(2%) and the novel pyrimido[1,2-b]indazole (46)(5%), via cyclisation accompanied by desulphurisation not previously observed during phosphite deoxygenations.