3- Amino-4-methyl-1,2,4-thiadiazoline

The thermal decomposition of some 3,5-disubstituted-l,2,4-thiadiazoles has been studied and some nonisothermal kinetic parameters have been reported <1986MI239>. Polarographic measurements of a series of methylated 5-amino-l,2,4-thiadiazoles show that thiadiazoles are not reducible in methanolic lithium chloride solution, while thiadiazolines are uniformily reduced at 0.5 = — 1.6 0.02 V. This technique has been used to assign structures to compounds which may exist theoretically as either thiadiazoles or thiadiazolines <1984CHEC(6)463>. The photoelectron spectrum for 1,2,4-thiadiazole has been published <1996CHEC-II(4)307>. 

The condensation of 5-amino-3-methyl-l,2,4-thiadiazole (118) with aliphatic or aromatic nitriles yields 1 1 adducts, which are, according to their H NMR spectra, equilibrium mixtures of (119) and (120) (Scheme 28) <82AHC(32)285>. These adducts are produced by a bond switch at the n-hypervalent sulfur in (121). X-ray analysis of the adduct formed from the reaction of (118) with chloroacetonitrile showed the adduct to exist as (122) in the crystals <81AX(B)185>. Further examples of this type of bond switch at rc-hypervalent sulfur are observed in the reaction of 5-imino-1,2,4-thiadiazolines with various electrophilic reagents (Section 4.08.6.1). 

The scope of the synthesis is greatly widened by the preliminary preparation of the N-halogenoamidines (56), preferably in situ subsequent displacement of halogen by thiocyanate results in the formation of the desired 5-amino-1,2,4-thiadiazoles (58) in good yields (50-80%).6,79, 80 The use of formamidine affords the parent, 5-amino-1,2,4-thiadiazole (58 R = H) while use of AT-methylformamidine (59) leads to 5-imino-4-methyl-Zl4-1,2,4-thiadiazolines (60).81... 

Experiments using a series of methylated 2-amino-1,2,4-thiadia-zoles of unequivocal structure (351-353) show that thiadiazoles (e.g. 351) are not reducible in methanolic lithium chloride solution, while thiadiazolines (e.g. 352-353) are uniformly reduced at E0 = — 1.6 + 0.02V. In aqueous alcoholic tetramethylammonium iodide,... 

Bond Switch at n-Hypervalent Sulphur.—Condensation of S amino-3-methyl-1,2,4-thiadiazole (190) with alkyl or aryl cyanides affords an equilibrium mixture of products (191) and (192). A similar bond switch is observed when the 5-imino-l,2,4-thiadiazoline (193) is treated with the imino-ester MeC(=NH)OEt, as... 

Chemical Properties.—Alkylation. Methylation of 2-amino-5-benzoyl-l,3,4-thiadiazole and its derivatives has been systematically examined. The direction of methylation, which occurs at the N-3 atom of the ring and the exocyclic amino-group, is influenced by the structure of the substrate, the nature of the reagent (Mel, Me2S04, or CH2N2), and the reaction conditions. 2-(Substituted amino)-l,3,4-thiadiazoline-5-thiones (104) are 5-methylated to (105) by one equivalent of methyl iodide, in the presence or absence of alkali. Further methylation occurs at N-3 or N-4, but may lead to mixtures, the composition of which depends on the ratio of the reactants and on the degree of substitution of the 2-amino-group in (105). At 190 C, (105 R = R = Me) isomerizes to (106) ... 

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