Synthesis and Tautomerism of 1,5-Bis(alkylamino)-4H-benzo[a]phenothiazin-4-ones

The equimolecular condensation of 2-aminobenzenethiol (1) with 4,8-bis(alkylamino)-1,5-naphthoquinones (2) in ethanol in the presence of HCl gave near-infrared-absorbing 1,5-bis(alkylamino)-4H-benzo[a]phenothiazin-4-ones (3), which existed as predominant tautomers in various solvents. The reaction mechanism was proposed to involve pathways through the O-protonation of 2, followed by the attack of 1 on the resulting ion, the subsequent oxidation, and the acid-catalyzed intramolecular cyclization of the quinonoid intermediate. The tautomeric equilibria of 2, 3, and 5,8-dihydroxy-1,4-naphthoquinone favor the formation of an interconverting hydrogen atom of a higher electron density. The solvatochromic effect of 3 was analyzed by means of the linear solvation energy relationships.