Pentasulfides

The reaction of phosphorus pentasulfide with a-acylamino carbonyl compounds of type Ilia also yields thiazoles. Even more commonly, a mercaptoketone is condensed with a nitrile of type IVa or a-mercaptoacids or their esters with Schiff bases. This ring closure is limited to the thiazolidines. In the Va ring-closure type, /3-mercaptoalkylamines serve as the principal starting materials, and ethylformate is the reactant that supplies the carbon at the 2-position of the ring. These syntheses constitute the most important route for the preparation of many thiazolidines and 2-thiazohnes. In the Vb t3fpe of synthesis, one of the reactant supplies only the carbon at the 5-position of the resultant thiazole. Then in these latter years new modern synthetic methods of thiazole ring have been developed (see Section 7 also Refs. 515, 758, 807, 812, 822). 

This reaction is explosive and proceeds in low yield (—21%) because of the instability of the thioformamide that is destroyed as soon as it is cyclized with 1 (113,491). The thioformamide is better prepared directly in the reaction mixture by condensing phosphorus pentasulfide and for-mamide at room temperature, in dioxane solution, according to reaction 1 (491,492),... 

When chloroacetaldehyde is condensed with higher thioamides prepared from amides and phosphorus pentasulfide according to Schwarz s method (222), 2-substituted thiazoles are obtained (4, 10,"22, 175). 

Aromatic thioamides can be prepared as described in the literature by different ways, either by S -> O exchange between the corresponding benzamides and phosphorus pentasulfide in pyridine solution in the presence of triethylamine (65, 646) as strong base, or by action of H2S on the appropriate nitrile with pyridine and triethylamine solvents using the method of Fairfull et al. (34, 374, 503). In this reaction, thioacetamide in acidic medium can also be used as a H2S generator with dimethylform-amide as the solvent (485). 

The reaction can be carried out in two steps (641). First, equimolar amounts of amide and phosphorus pentasulfide are mixed under stirring in dioxane, the temperature being kept below 45°C. After 20 minutes, the a-halocarbonyl compounds (in dioxane solution) are added in small portions. At the end of the addition the temperature reaches 80 to 100°C, and the reaction mixture is kept at this temperature for another hour. 

Thiazolium salts with alkyl (103, 722), arylalkyl (116), aryl (305), or heteroaryl (96) substituents on the nitrogen have been also prepared by this procedure. As in the thiazole series, N-substituted thioamides can be formed directly in the reaction mixture from phosphorus pentasulfide and N-substituted amides (127). These methods are important in the synthesis of thiamine 102 (vitamin Bj) (Scheme 45). 

This reaction was first described by Gabriel in 1910 (40), when he warmed an acylaminoketone (197a) with an equimolecular amount of phosphorus pentasulfide. The reaction (Scheme 103) is similar to the preparation of other five-membered oxygen- and sulfur-containing rings from 1,4-dicarbonyl compounds. 

Arylthiazoles were prepared either from w-formaminoaceto-phenones and phosphorus pentasulfide in 70% yield (47, 641) or by treating thioformaminoketones with concentrated sulfuric acid in water (344). Thioformaminoacetophenone itself was obtained by the action of potassium dithioformate on aminoacetophenone (251). 

The Gabriel s synthesis is also applicable when a polysubstituted thiazole is required (381, 550). Thus 2,4,5-trisubstituted thiazoles are obtained by treating the corresponding a-acylaminoketones with phosphorus pentasulfide for a few minutes at 100°C (550) or at higher temperature for heavier substituents (381). (Table 11-31). 

TABLE 11-31. THIAZOLES FROM -ACYLAMINOKETONES AND PHOSPHORUS PENTASULFIDE (Gabriel s synthesis)... 

Tliiazoles were usually obtained by treating the corresponding a-acylaminnketones with phosphorus pentasulfide for a few minutes at 10O°C... 

Alkoxythiazole derivatives (202) are formed in a similar manner by the action of phosphorus pentasulfide on a-acylamino esters (201) (64, 334, 711). 

Oxazoles (283) heated with phosphorus pentasulfide can be converted to thiazoles. In this manner, Keyer and Brooker obtained 2-methyl-5-phenylthiazoles (10), Ri = Me, R2 = H, R3 = Ph in 85% yield (Scheme 152) (389). 

Sulfur and its compounds are among the oldest and most widely used pesticides. Elemental sulfur is especially effective as a dust for the control of mites attacking citms, cotton, and field crops and as a protectant against chiggers, Trombicula spp., attacking humans. Sulfur also is a valuable fungicidal diluent for other dust insecticides and is used in wettable form as a spray mixture. Time sulfur has been a standard dormant spray for the control of the San Jose Quadraspidiotuspemiciosus and for other scales and various plant diseases. Time sulfur is a water-soluble mixture of calcium pentasulfide,... 

Thioketenes can be prepared in several ways, from carboxyHc acid chlorides by thionation with phosphoms pentasulfide [1314-80-3] 2 5 ketene dithioacetals by -elimination, from l,2,3-thiadia2oles with flash pyrolysis, and from alkynyl sulfides (thioacetylenes). The dimeri2ation of thioketenes to 2,4-bis(alkyHdene)-l,3-dithietane compounds occurs quickly. They can be cleaved back pyrolyticaHy (63). For a review see Reference 18. 

Properties and Reactions. Phosphorus combines with sulfur to form the binary tetraphosphoms trisulfide [1314-85-8] (phosphoms sesquisulfide), P4S2, (1) tetraphosphoms pentasulfide [12137-70-1] ( )j tetraphosphoms heptasulfide [12037-82-0] P) and phosphoms(V)... 

Phosphoms compounds are manufactured for a variety of uses, either direcdy or as intermediates in the production of other compounds. Manufacturing of the largest-volume products is summari2ed in Table 15, and prices are given in Table 16. Phosphoms trichloride and phosphoms pentasulfide are the compounds in highest demand. Phosphoms trichloride production increased steadily from 1985 through the mid-1990s. Up to 36% of PCl is used for pesticide products. 

Zinc dithiophosphates, which serve as antioxidants (qv) and antiwear agents in lubricants, are prepared by reaction of amyl alcohol and phosphoms pentasulfide followed by treatment with 2inc sulfate (43). 

Dithiophosphates. These compounds (13) are made by reaction of an alcohol with phosphoms pentasulfide, then neutralization of the dithiophosphoric acid with a metal oxide. Like xanthates, dithiophosphates contain no nitrogen and do not generate nitrosamines during vulcanization. Dithiophosphates find use as high temperature accelerators for the sulfur vulcanization of ethylene—propylene—diene (EPDM) terpolymers. 

Manufacture. The manufacture of sulfur dichloride is similar to that of sulfur monochloride, except that the last stage of chlorination proceeds slowly and must be conducted at temperatures below 40°C. The preparation of a high assay sulfur dichloride requites special techniques, eg, continuous chlorination duriag distillation or distillation with traces of phosphoms trichloride or phosphoms pentasulfide [1314-80-3] (162—164). Cmde product containing 80 wt % sulfur dichloride to which is added 0.1 wt % phosphoms trichloride can be distilled to yield a 98—99 wt % pure sulfur dichloride, which can be stored for weeks at room temperature without appreciable change. 

Based on a mixture of animal and vegetable oils plus synthetic esters. Di-/-nonyl pentasulfide (DTNPS) is based on mercaptan. 

Antimony trioxide and sodium antimonate are added to specialty glasses as decolorizing and fining agents, and are used as opacifiers in porcelain enamels. Antimony oxides are used as white pigments in paints, whereas antimony trisulfide and pentasulfide yield black, vermilHon, yeUow, and orange... 

Arsenic pentasulfide (arsenic(V) sulfide), As S q, is stable in air up to 95°C, but at higher temperatures begins to dissociate into arsenous sulfide and sulfur. It is prepared by the fusion of arsenic with sulfur foUowed by extraction with ammonia and reprecipitation at low temperatures by addition of hydrochloric acid. Arsenic pentasulfide is precipitated at low temperatures from strongly acidic arsenate solutions by a rapid stream of hydrogen sulfide. It is hydrolyzed by boiling with water, yielding arsenous acid and sulfur. Salts derived from a number of thioarsenic acids are formed from arsenic pentasulfide and alkaH metal sulfides. 

Phosphorus pentasulfide in pyridine can be used also for simultaneous substitution of oxygen and chlorine in polysubstituted pyridazinones. For example, 4,5-dichloro- and... 

In some cases oxo compounds have been converted directly to the corresponding thiones with phosphorus pentasulfide. 

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