1.2.4- Thiadiazolium salts, reaction with

The reaction of 1,3,4-thiadiazolium salts 37 with triethylamine affords the heterocyclic carbene 38, which reacts with two equivalents of aryl isocyanates to give spirobicyclic compounds 39 in high yields. ... 

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. 

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>. 

On treatment with ethyl cyanoacetate and base, the fused 1,3,4-oxadiazolium salt 141 underwent ring opening with subsequent closure to the fused pyrazole 143 in low yield, probably via the intermediate 142.146 This reaction proceeds by analogy with the reactions of mono-cyclic oxadiazolium salts,147 but a similar intermediate 145 obtained from the fused thiadiazolium salt 144 failed to cyclize to a bicyclic system under a variety of conditions.146... 

Interaction of the mesoionic thiadiazolium salt (299 Ar = Ph) with hydrazine led to the formation of l,4-dihydro-6-hydrazino-l,2,4,5-tetrazine (300) (76KGS713). The reaction of salts (299) with azodicarboxylate esters was at one time thought to provide the mesomeric... 

A short synthesis of imidazoles 47 was discovered on the basis of the reaction of 3,5-diaryl-l,2,4-dithiazolium triiodides 46 (X = I3) with glycinates or aminoacetonitrile. Intermediate (thiocarbonyl)amidines 48 were oxidized in situ to 1,2,4-thiadiazolium salts 49 by the triiodide anion resulting in imidazoles 47. The same reaction with 3,5-diaryl-l,2,4-dithiazolium perchlorates 46 (X = C104) stops at the formation of amidines 48 due to the lack of an oxidizing counterion (Scheme 3) <1997JOC3480>. 

Acetamide enters a cycloaddition reaction with thiadiazolium salts 128 in the presence of triethylamine with the formation of heteropentalene stmcture 33, for which an isomerization equilibrium with open form 40 was also proposed (see Section 6.03.4) (Scheme 36) <2003HAC95>. 3-A-Methyl-5-(/>-tolylimino)-l,2,4-thiadiazole hydrochlorides 129 form heteropentalene structures 34 in a cycloaddition reaction with CS2 in nearly quantitative yields (Equation 18) <2003HAC95>. Similar stmctures, including selenium containing products, are obtained by the reaction of compounds 129 with iso-, isothio-, and isoselenocyanates <2003HAC95>. 

Similar reactions have been carried out with amidines (67NEP6610627). Thus tri-chloroacetamidine (309) reacts with chlorothioformate esters to produce thioacyl products (310) which are readily oxidized to 5-alkoxy-3-trichloromethyl-l,2,4-thiadiazoles (311). The yields in both steps are about 80% (Scheme 111). The bromine oxidation of N-thiocarbamyl derivatives (312) of cyclic amidines (2-aminopyridine, 2-aminothiazole, 2-aminopyrimidine) yields thiadiazolium salts (313) (71JPR1148). In the 2-aminopyridine series, products of type (230 Scheme 81) are obtained in 20-73% yield when R is an ethoxycar-bonyl group (750PP55). 

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). 

A brief comparison of rearrangements of oxadiazolium and thiadiazolium salts deserves mention. The sequence of reactions represented in Scheme 105 has been well studied. The oxodiazolium salt (231) reacts with aniline to afford the acylamidrazone (232) (71JCS(C)409)... 

The reaction of iminophosphorane (264) with acyl chlorides followed by fluoroboric acid affords 2-substituted 5-phenylthiazolo[2,3-ft][l,3,4]thiadiazolium salts (269) (Equation (73)) <92T1285>. 

Thiazolo[2,3-h]-l,3,4-thiadiazoles [C2N2S-C3ISS]. The reaction between l,3,4-thiadiazole-2(3//)-thiones and phenacyl bromide gives sulphides, e.g. (122 R = NHAc), which may be cyclized to the thiazolo[2,3-/>]-1,3,4-thiadiazolium salts (123 R = NHAc), whilst with PhCHBrCOCl the anhydro-5-hydroxy-2-methyl-6-phenylthiazolo[2,3-6]-l,3,4-thiadiazolium hydroxide (124 X = N) is readily obtained. ... 

The optimum yield of the chloropropyl 1,2,5-thiadiazole 97 was produced by treatment of the enamino-ester 95 with trithiazyl trichloride (92) in boiling CCI4. It was postulated that the reaction proceeded via the thiadiazolium salt 96 which could be then dealkylated by chloride ion to give 97 <01JCS(P1)662>. 

In contrast to the isocyanate reaction with l-aza-2-azoniaallene salts, isothiocyanates react across their C=S bond in the [3+2] cycloaddition reaction to give 1,3,4-thiadiazolium salts. The initially formed [3+2] cycloadducts 53 undergo a 1,2-shift to give the isomeric adducts 54 °. 

In the [3-1-2] cycloaddition reactions of isothiocyanates with l-aza-2-azoniaallene salts the reaction also proceeds across the C>=S bond of the isothiocyanate to give 1,3,4-thiadiazolium salts in high yields 2. ... 

The action of nucleophilic reagents on the thiadiazolium salts (683) and (685) has been investigated. Hard nucleophiles generally attack a carbon atom, whereas soft nucleophiles react at sulphur. Thus sodium hydroxide reacts with the two salts to yield mainly the benzamidines (684) and (686), respectively, via initial attack at C-5 and subsequent ring opening and loss of sulphur whereas treatment of compound (685) with sodium sulphide, sodium thiosulphate, sodium thio-phenate, or sodium borohydride gives the thiobenzoyl derivative (687) by reaction at the sulphur atom (see Scheme 6). ... 

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