1,3,4-Thiadiazolium

The ring nitrogens react with electrophiles to afford either 1,3,4-thiadiazolium salts or l,3,4-thiadiazol-2(37/)-ones depending on the tautomerisability of the substituents at the C-2 or C-5 positions. While N-alkylation is the most common electrophilic reaction of 1,3,4-thiadiazoles, reactions with acyl and cyanogen halides as well as Mannich salts have also been reported. 

Amino-5-methyl-l,3,4-thiadiazole 48 reacts with chloroacetone to give the N-alkylated thiadiazolimine 49 (Equation 4) <2000AF550> and iV-alkylation of the 2,5-diphenyl- and 2,5-dimethyl-l,3,4-thiadiazole 50a and 50b with trimethylsilylmethyl trifluoromethanesulfonate gave the corresponding 1,3,4-thiadiazolium salts 51 (Equation 5) <2002J(P 1)2851 >. A comprehensive study of the quarternization of the 2,5-disubstituted thiadiazoles has been covered in CHEC(1984) <1984CHEC(6)545>. 

Deprotonation of 1,3,4-thiadiazolium salts affords carbenes that can be trapped with aromatic isocyanates to yield spirocyclic compounds. These reactions have been reviewed in CHEC(1984) <1984CHEC(6)545> and CHEC-11(1996) <1996CHEC-II(4)379>. 

The sulfide groups in mesoionic 1,3,4-thiadiazolium salts are activated toward nucleophilic substitution. The mercapto substituent of the thiadiazolium salt 117 can be displaced by cyclohexylamine to afford the 2//-thiadiazol-imine 118 (Equation 37) <2004BML4607>. 

Conclusions about mesomeric forms arose from X-ray analysis of mesoionic A -[2-(5-methyl-l,3,4-thiadiazolo[3,2- ]pyri-dinio)]acetamidate 28 (Scheme 6). Among the six possible delocalized structures, a large contribution of the 1,3,4-thiadiazolium structures 28a and 28b was observed, rather than pyridinium structures. These two mesomeric forms are in agreement with the high double-bonded character of the S-C6 bond <1996BCJ1769>. 

Thiadiazol-2-enes (A nhydro-2-alkyl-1,3,4-thiadiazolium Hydroxides) (261)... 

Mit Ethanol erfolgt bei l,3,4-Oxadiazolium-2-thiolaten substituentenabhangig entweder Ringumwandlung zu 1,3,4-Thiadiazolium-2-olaten oder Ringspaltung ohne nachfolgende Cyclisierung223. 

Zur Azogruppen-Ubertragung auf 1,3,4-Thiadiazolium-2-thiolat kann auch Dimethoxy-carbonyl-diazen verwendet werden4 z.B. ... 

The l,3,4-thiadiazolo[3,2-a]quinazolinium perchlorates (365) were prepared by cyclocondensation of 2-chlorocyclohex-l-enecarboxaldehyde (363) with 2-amino-1,3,4-thiadiazolium perchlorates (364) (73KGS1285). 

Like the oxadiazolium salts (288), 1,3,4-thiadiazolium salts (296) can also react with hydrazines to afford 1,4-disubstituted l,4-dihydro-l,2,4,5-tetrazines (290). By this method, 1,4-dihydro-1,2,4,5-tetrazines with four different substituents can be prepared this seems to be the only method for preparation of such compounds, and proceeds in yields of 70-99%. The 4-amino-l,2,4-triazoles were isolated as side-products (74ZOR377, 71DOK(200)134, 71MIP22100). 

Methylthio-1,3,4-thiadiazolium salts (297) were treated with hydrazines in the ratio 1 2 to afford l,4-dihydro-l,2,4,5-tetrazine-6(5//)-thiones (298). In most cases the acyclic intermediate could not be isolated (73CZ565,73CZ566,01CB2311). 

The deprotonation rate of 1,3,4-thiadiazolium salts is about 104 times faster than for thiazolium salts. The rate equation is first order in both substrate and OD and the mechanism involves rate determining proton abstraction by OD followed by fast reaction with D20 to give the exchanged product (74AHC(16)14). 

Similarly, pyrido[2,1 -b]-1,3,4-thiadiazolium-2-amenates are obtained by cyclization of carbodiimides, generated in situ, from thioureas. ... 

These compounds can in principle, with suitable modifications, be prepared by all the methods (a)-(d) in the previous section. In this case (c) would have to be applied to 1,3,4-thiadiazolium ions, e.g. 82. An interesting modification of method (b) has been described by Meyer and Cummings. l,l-Dimethyl-2-p-chlorothiobenzoyl-hydrazine (92) reacted with carbonyl chloride to give a product. 

Thiadiazolium 4-Methyl-5-methylthio- -(4-chlor-phenyla-menat) E8d. 213 [H3CS CS-N(CH3) — NH2 + Ar3PBr2 Ar-NCO]... 

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