Dioxazine synthesis involves two distinct stages: dianil formation and dianil cycli — zation.


Dianil Formation

The primary intermediate (generically referred to as the dianil), a 2,5-diarylamino — 1,4-benzoquinone (5), where X = H or Cl or NHCOCH3, is obtained by the reac­tion of an appropriately substituted aromatic primary amine with a suitably func­tionalized quinone. An acid scavenger (e. g. sodium acetate) is included in any pro­cess utilizing a chlorinated quinone to neutralize the cogenerated hydrogen chlo­ride, which otherwise would proportionately deplete the amine component.

For example, the dianil of Pigment Violet 37 (5, B = D = OCH2CH3, A = NHCOPh, E = H, X = NHCOCH3) is formed by the reaction of 2,5-diethoxy-4-ben- zoylaminoaniline (6), (B = D = OCH2CH3, A = NHCOPh) with 2,5-diacetylamino- 3,6-dichloro-1,4-benzoquinone (7), (X = NHCOCH3). Similarly the reaction be­tween 3-amino-9-ethylcarbazole (9) and 2,3,5,6-tetrachloro-1,4-benzoquinone (10) affords the dianil precursor of Pigment Violet 23 (5), where A and E correspond to the fused indole moiety and B = D = H, X = Cl.

The solvent employed in the first stage may be different from that used in the second (cyclization) stage. In a process of that type the dianil is isolated prior to dioxazine formation being conducted in the second solvent. Obviously, overall effi­ciency is improved by conducting each stage sequentially as a one-pot process in a common solvent (e. g. o-dichlorobenzene).


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