Thiourea dioxide, formation

A one-pot Hantzsch reaction in aqueous medium without any solvent or catalyst is known for the synthesis of 1,4-dihydropyridines. Tamaddon and coworkers have reported the synthesis of 1,4-dihydropyridines 107 by the reaction of aldehydes 51 and methyl/ethyl acetoacetates 106 in aqueous ammonium carbonate at 55-60 °C (Scheme 35) [88]. Recently, a similar study has also been carried out by Yang and coworkers to obtain dihydro-pyridines [89]. Another example of dihydro pyridine ring formation in water employs methyl/ethyl acetoacetates and aromatic aldehydes with 6-amino-l,3-dimethyluracil in the presence of thiourea dioxide as the catalyst [90]. The utilization of water as a solvent and indium(III) chloride as a promoter for the formation of dihydropyridine ring is reported by Khurana and coworkers in the reaction of with 6-amino-l,3-dimethyluracil, aldehydes, and 1,3-diketones [91]. 

The Hurd-Mori reaction,where a tosylhydrazone is converted by thionyl chloride to the corresponding thiadiazole, involves the formation of a 1,2,3-thiadiazole-3,3-dioxide. In one example, this type of compound was isolated and subsequently deoxygenated with thiourea <1991PS175>. There have been no further reports of S-linked sulfoxide or sulfone derivatives of 1,2,3-thiadiazoles since the publication of CHEC-II(1996). 

Due to the absorption of UV radiation by the reactant nucleophiles and the solvent in the zone of interest, application of classical spectrophotometric methods to follow the mechanism of formation of imidazo[4,5-f]-[l,2,5]thiadiazoles 51 from reaction of the parent thiadiazoles 1,1-dioxides 89 and ureas/thioureas was restricted. Consequently, details of the mechanism and equilibrium constants were established by CV <2004JP01091, 2003JP0220>. It was found that the reaction proceeded by a stepwise mechanism with an intramolecular second step as outlined in Scheme 4. In solution, this reaction was reversible, and the equilibrium could be shifted to the side of the reactants by a temperature increase. For example, a 3.54mM solution of 51b in DMF, with a 0.1 M NaC104 as supporting electrolyte, presented a featureless CV between 0.9 and —2.8 V which corroborated with the... 

The kinetics of the oxidation of l-phenyl-2-thiourea by chlorite, in aqueous acidic media, are strongly influenced by the pH and show a complex acid dependence. The proposed mechanism involves HOC1 as a major intermediate whose autocatalytic production determines the observed kinetics of the reaction. The oxidation involved the formation of two stable intermediates, the sulfinic acid and the sulfonic acid, on the pathway towards total desulfurization to form phenylurea. A comprehensive 29-reaction scheme has been proposed to describe the observed complex kinetics.96 The oxidation of trimethylthiourea (TMTU) by chlorite in slightly acidic media is very fast. The oxidation of TMTU proceeds through the formation of sulfinic acid then to the sulfoxylate anion. The direct reaction of chlorine dioxide and TMTU is autocatalytic and is also inhibited by acid. A series of 28 reactions have been proposed to describe the mechanism.97... 

The alkyl-, alky laryl- and diarylcarbodiimides are the diimides derived from carbon dioxide, however, no direct formation of carbodiimides from amines and carbon dioxide is known. Interestingly, carbodiimides can be obtained from amines and carbon dioxide via a switteri-onic titanium complex (see Section 2.2.8). The major starting materials for the synthesis of carbodiimides are isocyanates, 1,3-disubstituted ureas or 1,3-disubstituted thioureas. The synthesis of isocyanates requires the use of the toxic carbonyl chloride or its oligomers. A book on the synthesis and reactions of isocyanates appeared in 1996. ... 

Considering that the chemical reactivity of carboxylic acids is similar to that of carbonic acids, as is observed in amide and ester formation, we have attempted the substitution of carbon dioxide for carboxylic acids in the coupling reaction with amines by using phosphites in pyridine or imidazole, and found that ureas are in fact produced in good yields (Eq. (4))6. Similarly, carbon disulfide reacts with amines to yield the corresponding thioureas (Eq. (5)). 

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