OXIDATIVE HAIR DYEING 121 • kl N-' "-.d" -F OH k..•r • H•.N' • "0 •"•'•/'•0 FI•N'•K•v •u t-I0 •'•-•0 (V:D .,,'/ ( V ) I NFIz •' "nc•gentc• •(slow) $ ( ]: X) Ox, brown polymer H•N' • lqO' • '-0 '• 'N,FI• (X) green Figure 7, •echa•ism ooe •eactio• ooe p-be•zoqui•o•e aiimi•e with •esorci•o] Kinetic studies (17), and in particular the dependence of reaction rate up- on pH, have shown that the reaction with hydrogen-bearing m-diamines fol- lows the mechanism shown in Fig. 9. This involves rate-controlling electro- philic attack by the diiminium ion on the neutral m-diamine to give the triaminodiphenylamine (XXIII) which undergoes rapid oxidation to the 2- aminoindamine (XXIV). Starting from p-phenylenediamine, 4 equivalents of oxidant are consumed per molecule of dye formed. We have now studied the reaction of diimines with 2,4-diaminoanisole, and with 2,4-diamino-5-methylanisole (XXV) and 2,4-diamino-l,5-dimethoxyben- zene (XXVI). The first of these gives 2-amino-5-methoxyindamine (XXVII) H•N• H•N" • '-NH H/N' • -NHz (XXYlI) (XXV) R:Me (XXV I) R:MeO

1'2'2 JOUBNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.5 42 2O TIME IN MIN. i I i I 50O 600 Wavelenglh, nm Figure 8. Spectrophotometric course of reaction of p-benzoquinone diimine (2x 10-*M) and resorcinol (1 x 10-•M) at pH 9.57 and 30øC in the presence of oxygen. Solution is colorless at ti•ne zero HN'""',,•' H:•N' • 'NH•, I-L,N' • I-I•N' • 'NH•. / (xxlH) ( fo•t)/,• di-irnine -I- or' H• N •'•'•,"' FInN •'•-'"•NH H•Oz (XXlV) Figure 9. Mechanism o[ reaction o[ p-benzoquinonc diimine with 4-equivalent m-diamine couplers and behaves as a normal 4-equivalent coupler. The other two compounds react with diimine to give the corresponding 2-aminoindamine in which one of the methoxy groups has been eliminated in the course of the reaction. Stoichiometry studies show that, starting from p-diamine, only two equivalents of oxidant are consumed per molecule of dye formed. Since this amount of oxidant is required to oxidize one molecule of p-diamine to the diimine which must react with the m-diamine to give