Cyclization of the Dianil

Dioxazines are obtained by the cyclization of the corresponding dianils (5), a pro­cess that may be conducted by means of:

(a) Cyclocondensation (dealcoholation) of 5 when D = OR (R usually methyl or ethyl) (Scheme 12.1).

The reaction probably proceeds through the enol form of the dianil 5, sub­sequently generating the cyclic ether by displacement of ethanol (when D = OCH2CH3). Since cyclization is regiospecific, directed by the nucleofugic alkoxy group, this synthetic method is selected for the preparation of specifically substi­tuted dioxazines [3]. Cyclizations of this type are normally performed by heating at 170-175 oC for 4-5 h in solvents like o-dichlorobenzene in the presence of acidic agents such as benzoyl chloride. For example, the crude form of Pigment Violet 37 (8), is produced by this method [5] from its corresponding dianil 5, (B = D = OCH2CH3, A = NHCOPh, E = H, X = NHCOCH3).

Cyclization can be effected in the absence [3] of acidic agents, but substantially higher temperatures (> 200 °C) typically are required.

(b) Oxidation (cyclodehydrogenation) of 5, when D = H.

This process, also conducted in solvents like o-dichlorobenzene, at similar tem­peratures (170-175 °C), in the presence of acidic agents, usually arylsulfonyl chlo­rides, is the typical method of synthesis for Pigment Violet 23 crude. However, the nature of this cyclization reaction is more complex than the cyclocondensation process, but can formally be represented as shown in Scheme 12.2.




O Cl

Cl O

X 7

Scheme 12.2.

The process is probably initiated by cycloisomerization of the dianil to the diphenox — azine 11, which oxidizes (dehydrogenates) to the azaquinone 12. Cycloisomerization of the latter to the dihydrotriphenodioxazine 13, followed by its oxidation, (dehydro­genation) then affords the corresponding triphenodioxazine in the final step.

Arylsulfonyl chlorides are typically employed as the cyclization agents in the oxi­dative route to dioxazines. In the manufacture of Pigment Violet 23 crude the most commonly employed agents are benzene — or p-toluene-sulfonyl chloride.

Some literature articles describing the synthesis of dioxazines refer to the sulfonyl chlorides as catalysts, however, since they are consumed during the reaction, a more accurate description is cyclization agent.

Generally, sulfonyl chlorides are not considered as oxidizing agents, but some examples involving p-toluenesulfonyl chloride are known [6]. Therefore, during the cyclooxidation of the dianil 5 the sulfonyl chloride must be reduced. Since hydrogen chloride is a by-product of the reaction it is reasonable to assume the formation of an arylsulfonate ester (14) during the initial cyclization. Subsequent oxidation/cyclization, during which the sulfonate is reduced and expelled as an arylsulfinate (the oxidation state of S changing from +5 to +3), affords the dihydro­triphenodioxazine 15 (Scheme 12.3).

Although the composition of the oxidizing species involved in the dehydrogena­tion of 15 is not known with certainty, the participation of the arylsulfinic acid is clearly implicated by the formation of several specific by-products. The most prob­able structures, based on analysis by mass spectrometry, are the following thioether derivatives of the parent dioxazine (where X = H or X = Cl) [7]:

When the cyclizing agent is benzenesulfonyl chloride, R = H; when p-toluene — sulfonyl chloride is used, R = CH3. Consequently, the composition of the thioether group indicates which sulfonyl chloride has been employed in the synthesis.

Formation of the thioethers may be rationalized by assuming that some oxida­tion is effected by the arylsulfinic acid, which subsequently is reduced to the corre­sponding sulfenic acid (the oxidation state of S changing from +3 to +1). The inherent instability of the sulfenic acid is manifested [8] by its self-condensation to a thiosulfinate which, under the reaction conditions, disproportionates to afford a thiosulfonate and corresponding disulfide (as illustrated below Scheme 12.4). Since a disulfide will promote oxidation (dehydrogenation) during the facile reduction to its thiol components (the oxidation state of S changing from 0 to -1), the aryldisulfide probably competes with the arylsulfinic acid during the oxidation of the dihydrotriphenodioxazine 15. The relatively nucleophilic arylthiol then can lead to the formation of a thioether by reaction with an appropriate species of the dianil-dioxazine reaction sequence (presumably involving one of the more soluble species), most likely by substitution of chlorine. The probability that a process like this occurs during dioxazine synthesis is supported by the odor of methyl mercap­tan that becomes very evident when methanesulfonyl chloride is employed as the cyclizing agent [7].

(c) Oxidation/cyclization by dehydrochlorination.

A dioxazine nucleus may also be formed, in principle, from its dianil precursors 5 or 11, where X = Cl, by an alternative route involving dehydrochlorination, gen­erating a product devoid of chlorine. This is illustrated in the following generic outline. The azaquinone, formed by elimination of HCl from the tautomer of the cycloisomerized dianil, is in equilibrium with the isomeric dihydrotriphenodiox- azine, which can eliminate a second molecule of HCl through the intermediacy of its equilibrating tautomer, to afford the dechlorinated product. In addition, it is interesting to observe that a monochlorinated dioxazine may also form during the sequence shown in Scheme 12.5 if dehydrogenation occurs at the dihydrotriphe — nodioxazine stage.

A process, presumably related to this, dominates during the synthesis of carba — zole violet when the arylsulfonyl chloride cyclizing agents are replaced by the cor­responding sulfonic acids [9] (or by various carboxylic acids). The resulting prod­uct is not completely dechlorinated, indicating that some oxidation by the dehy­drogenation route is occurring also. A product with a chlorine content of approxi-

mately 4% (theoretical chlorine content of Pigment Violet 23 « 12%), consisting mainly of 3, X1 =X2 = H, with appropriate contributions from 3, X1 = Cl, X2 = H and X1 = X2 = Cl, respectively, to afford that composition, is produced.

The value of this mode of cyclization is demonstrated by the shade of the fin­ished products; as the chlorine content decreases the subsequent pigments exhibit increasingly redder shades of violet. Thus the product containing chlorine at the 4% level (available in crude form as Sandorin Violet 2R-CSA) affords the reddest shade version of Pigment Violet 23 available commercially.

It is important to note that regular Pigment Violet 23 typically contains a small proportion of both the monochloro and the dechlorinated compounds, indicating that even under normal synthesis conditions some of the alternative cyclization is competing to a minor extent.


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