Malachite green, synthesis

The most versatile route to lactone color formers is based on the elegant synthesis of Malachite Green lactone described by Haller and Guyot in 1899.24 Scheme 3 illustrates the application of this route for the preparation of CVL. 

The nucleophilic displacement of suitable para substituents in analogues of malachite green is used as a means of synthesis of some important acid dyes. For example, Cl Acid Blue 83 (6.176 R = H) is made by the aldehyde method, using 4-chlorobenzaldehyde and N-ethyl-N-(3-sulphobenzyl)aniline. The resulting leuco base is oxidised to the colour salt,... 

The technical preparation of crystal violet and of its methyl-free parent substance, parafuchsine, almost the oldest of the triphenylmethane dyes, is not so easily explained. As is well known, in this process aniline and p-toluidine are united by oxidation in an acid melt. (In the preparation of fuchsine itself, which contains a methyl group attached to one of the benzene rings, o-toluidine is an additional ingredient.) Although all the phases of this important synthesis have not yet been experimentally established, we may nevertheless explain it on the basis of a dehydrogenation similar to that involved in the formation of malachite green. Moreover, the union of several molecules of base proceeds exactly according to the principle on which indamines are formed (p. 321) (Bucherer). 

Examples of acid-catalyzed carbonyl chemistiy are abundant in synthetic organic chemistry, biochemistry, industrial processes (such as in the synthesis of malachite green eq 5), and in polymer chemistry (such as in the synthesis of bisphenols of derived epoxy and polycarbonate resins eq 6).9... 

Triphenylmethane dyes are usually prepared in two steps 1) condensation of an N,N-dialkylaniline with a benzaldehyde compound and 2) oxidation of the resultant leuco base (27). The synthesis of C.I. Basic Green 4 (Malachite Green) is given as an example in Fig. 13.106. Alternatively, C.I. Acid Green 50 is prepared in three steps 1) condensation of... 

Fig. 13.106. Two-step synthesis of Malachite Green (Basic Green 4). Three-step synthesis of Acid Green 50.

Molybdates are used in a variety of industries. Sodium molybdate is used for the synthesis of pigments such as molybdate chrome orange, which is a homogeneous mixture of lead molybdate, lead chromate and lead sulphate. This use is likely to decline because of concerns about health hazards associated with lead, but phosphomolybdates and phosphotungstomolybdates are used to complex certain dyestuffs to produce pigments. A few of the best-known of these are Malachite Green, Rhodamine Band Methyl Violet, also used as indicators in analytical chemistry. 

In the same year Caro [27] produced methylene blue, basing his process on the sulphuretted hydrogen reaction observed by Lauth in 1876 [28]. Almost simultaneously malachite green was discovered by E. and O. Fischer. [29] and by Doebner [30] and O. N. Witt and Roussin introduced the synthesis of azo-dyes on a manufacturing scale, which proved afterwards to be of such great importance. 

Kashima et al. have explored the synthesis of N-1 substituted, C-2 unsubstituted dihydropyrimidines 39 and 40 through Raney-Ni induced desulfurization of N-1 derivatives 37 and 38 (Scheme 17) (83JCS(P1)1799). However, the 4,6-dimethyl-l-phenylpyrimidin-2-(lH)-thione required an H2 atmosphere at room temperature. The higher oxidation potential of these products than 1-benzyl 1,4-dihydronicotinamide (Ep = 0.700 V), showed their propensity to transfer hydride. Thus, the desulfurized DHPMs 39 as well as 40, reduced malachite green to its leuco derivative (83TL209). 

A second method for the synthesis of triphenylmethane dyes is illustrated with Basic Green 4 (Malachite Green). Benzalde-hyde (one mole) is condensed with two moles of Af,Af-dimethylaniline to form a leuco base, which is oxidized with lead peroxide and treated with hydrochloric acid to give the dye. 

Fig. 6.7. Copper-based catalysts for methanol synthesis. A novel device for controlled precipitation enabled separation of blue from green products. Structural analysis (top left) revealed that the blue products are disordered nanocrystalline materials furnishing poor catalysts. The green products are mixtures of two phases, malachite (violet) and auricalcite (red). By systematically optimizing the reaction conditions it was possible to prepare phase-pure green products and thereby to improve thesynthesisofthe working catalyst based on pure malachite precursors. In the X-ray diffraction pattern (top right), the features are labeled by the Miller Indices, indicating the diffraction lattice plane of the crystal °29 is the diffraction angle.

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