1.2.4- Thiadiazoles, formation rearrangements

The scope of this mechanism for the reaction of a series of nucleophiles with hydroperoxides has been reviewed and a linear free-energy plot of ogkz against logArj derived for acid-catalysed oxidations. The reaction of l-aIkyl-3-aryl-thioureas with HgOa yields 1,2,4-thiadiazoles which rearrange in acidic solutions to thiazolyl-guanidines. The formation of amidinothioureas as precursors of the thiadiazoles has, however, been established. 

The synthesis of the benzoimidazo[l,2- ][l,2,3]thiadiazole 61 can be explained using the same mechanistic model to that used for the Hurd-Mori reaction. The amino benzimidazole 58 when treated with thionyl chloride at reflux affords the benzoimidazo[l,2-r ][l,2,3]thiadiazole 61. If, however, the reactant 58 is treated with thionyl chloride at room temperature, the chloromethyl derivative 59 is formed. This derivative was then transformed into product 61 on reflux with thionyl chloride. The proposed mechanism for the formation of product 61 is for the initial formation of the sulfoxide 60, which then undergoes a Pummerer-like rearrangement, followed by loss of SO2 and HC1 to give the c-fused 1,2,3-thiadiazole 61 (Scheme 7) <2003TL6635>. 

Thiocarbonyl ylides are both nucleophilic and basic compounds (40,41,86). For example, adamantanethione (5)-methylide (52) is able to deprotonate its precursor, the corresponding 2,5-dihydro-1,3,4-thiadiazole, and a 1 1 adduct is formed in a multistep reaction (40,86). Thioxonium ion (56) (Scheme 5.22) was proposed as a reactive intermediate. On the other hand, thiofenchone (S)-methylide (48) is not able to deprotonate its precursor but instead undergoes electrocyclization to give a mixture of diastereoisomeric thiiranes (41,87,88). The addition of a trace of acetic acid changes the reaction course remarkably, and instead of an electrocyclization product, the new isomer 51 was isolated (41,87) (Scheme 5.18). The formation of 51 is the result of a Wagner-Meerwein rearrangement of thioxonium ion 49. 

The photolysis of 1,2,4-oxadiazoles in the presence of sulfur nucleophiles has been shown to afford 1,2,4-thiadiazoles. N—S bond formation between the ring species and the sulfur nucleophile is thought to account for the observed products.96 A review has appeared which includes an account of the rearrangement of 1,2,3-thiadiazoles to other heterocycles such as 1,2,3-triazoles and 1,2,3,4-thiatriazoles.97... 

The thermal reaction of 6-amino-4-oxopyrano[3,4-d][l,2,3]thiadiazoles (32) leads to 6-hydroxy-4-oxo-[l,2,3]thiadiazolo[4,5-c]pyridines (33) and 2-cyano-2-(l,2,3-thiadia-zol-5-yl)acetamide (34).58 Formation of (33), a Dimroth-type rearrangement, proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate and its recyclization on to the amido group to form the pyridin-2-one cycle. 

Thiadiazoles extrude nitrogen and thiirenes can be formed, but their lifetime is fleeting as they rearrange to thioketenes Scheme 3 shows the eventual formation of propadienethione 33 from 32 <1988JA789, 1990GPL (168)1>. 

Amidines, N-(l,2,4-thiadiazol-5-yl)-, rearrangement, 56, 103 Amidoximes, 1,2,4-oxadiazol-3-yl-, rearrangements, 56, 55 Amidyl radicals, see Radicals, nitrogen Amination, asymmetric, of carboxylic acids by chiral nitroso compds, 57, 41 Amines, catalysis of 3-acyl-1,2,4-oxadiazole arylhydrazone rearrangement by, 56, 87 Amines, thionitroso-, formation, 55, 20 Aminium cation radicals, see Radicals, nitrogen... 

Elimination of nitrogen by irradiation of argon matrix-isolated 4,5-bis(methoxycarbonyl)-l,2,3-thiadiazole (38) resulted in the almost quantitative formation of bis(methoxycarbonyl)thiirene (39), confirming earlier predictions that electron-withdrawing substituents should stabilize the thiirene ring. Benzoselenirene (40) has similarly been obtained and identified spectroscopically on irradiation of matrix-isolated 1,2,3-benzoselenadiazole (41) rearrangement to the fulveneselone (42) is observed on further irradiation. 

A literature report57 indicated that 3-amino-5-phenyl-l,2,4-oxadiazole 76 (R = Ph) reacted with ethoxycarbonylisothiocyanate in refluxing ethyl acetate to give the stable thiourea 79. A reexamination30 of this reaction showed that the product was the rearranged 1,2,4-thiadiazole 80 probably obtained through the intermediate formation of 79. 

The formation of this product and some kinetic data were taken as evidence for valence isomerization of the dithiins. The minor product (27) from the methyl-ation of 3-nitro-2,5-diphenyl-l,4-dithiin appears not to be stable, and it rearranges via (28) and (29) to (30a), which is methylated to give the isolated product (30b).Upon photolysis of cycloalkene-thiadiazoles (31), thiophens (32) are formed, in yields of 7—31%, together with 1,4-dithiins, 1,3-dithioles, and 1,2,4,5-tetrathians. ... 

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