Syntheses via c-b from Diphenylamines

3-Substituted diphenylamines react in the same way, and the formation of small amounts of 4-substituted phenothiazines is observed, not unusual for thionation reactions (57MI393, 59BCJ294). 

Nitrodiphenylamines and, surprisingly, Al-phenyltoluidines, did not react in this way to yield phenothiazines (57MI393). Also surprising is the failure of thionation in the case of 2-chlorodiphenylamine, compared to the normal behavior of 4-chloro-diphenylamine (59BCJ294). 

Other C-substituted diphenylamines bearing electron-donor substituents (OH, OCH3, NH2, etc.) underwent cyclization to chlorinated phenothiazines by SOQ2, the chlorine atom generally entering ortho to the preexistent substituents (33GEP562268). 

N-substituted diphenylamines also yielded phenothiazines on treatment with SOCI2 (33GEP562268). In this way, A-acetyldiphenylamine was quantitatively converted into tetrachlorophenothiazine, while A-phenyldiphenylamine (triphenyla-mine) gave 3,7-dichloro-10-(4 -chlorophenyl)-phenothiazine in 91% yield. In the first case, the chlorine atoms in positions 1 and 9 promote the removal of the acyl group from position 10 in the second case, the bulky N-substituent prevents the replacements of hydrogen from positions 1 and 9 by chlorine. The behavior of A-methyl-diphenylamine is intermediate in this respect 3,7-dichloro-lO-methylphenothiazine was isolated in 7% yield, along with tetrachlorophenothiazine (68AHC325). 

The reaction of diphenylamines with SOBr2 and SO2CI2 leads only to halogenated diphenylamines (57MI393). 

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