1.2.3.4- Thiatriazoles, mesoionic

An index of aromatic character based upon statistical evaluation of the deviations in peripheral bond orders has been devised and applied to five-membered ring heterocycles and their mesoionic derivatives including 1,2,3,4-thiatriazole <85TI409> and 5-phenyl-l,2,3,4-thiatriazol-3-oxide <93T8441>. See also <93QSAR146>. 

Electric dipole moment studies have provided excellent support for the formulation of certain heterocycles as mesoionic compounds. Hanley et al. <78JCS(P1)600> have measured dipole moments for mesoionic l,2,3,4-thiatriazol-5-imines (20), 5-ones (21) and 5-ylidenemalononitriles (22) using a vector analysis similar to that previously employed for sydnones and other mesoionic compounds assuming that mesoionic rings are regular pentagons. The dipole moment of 3-phenyl-1,2,3,4-thiatriazolium-5-anilide (20) was found to be 3.71 D and five derivatives of compounds (21) were measured and the ring moment calculated to be 3.8 D. In both cases the dipole moments are rather small but clearly consistent with the mesoionic formulation. 3-Phenyl-l,2,3,4-thiatriazolium-5-dicyanomethanide (22) was found to have a dipole moment of 8.84 D. 

Mesoionic 1,2,3,4-oxatriazolium-5-thiolates (172) are convenient starting materials for mesoionic thiatriazole 5-oxides. Thus arylhydrazines and carbon disulfide in ethanol yield arylhydrazinium dithiocarbamate salts, which are nitrosated with sodium nitrite in aqueous hydrochloric acid to give yellow l,2,3,4-oxatriazolium-5-thiolates (172). These heterocycles are isomerized by warming with ammonium hydroxide in ethanol to give colorless crystalline l,2,3,4-thiatriazolium-5-olates (173) (Scheme 36) <76CC306>. 

Heteroaromatic 1,2,3,5-thiatriazoles (1) have yet to be isolated and characterized but evidence for this apparently thermally unstable type of compound has been presented. 1,2,3,5-Thiatriazolines (2), (3), and (4), 1,2,3,5-thiatriazolidines (7), 1,2,3,5-thiatriazole heteropentalenes incorporating the ring system (5), and mesoionic 1,2,3,5-thiatriazoles (6) are known. Some of these types of heterocycle (2), (3), (5), and (7) have been reviewed in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) by Holm <84CHEC-I(6)579>. 

Mesoionic 4-amino-l,2,3,5-thiatriazoles constitute the only class of mesoionic 1,2,3,5-thiatriazoles known. They are prepared by the reaction of l-amino-l-methyl-3-phenylguanidine with approximately 2 equivalents of thionyl chloride with pyridine as solvent (88ACS(B)63>. They are obtained as the yellow 1 1 pyridine complexes (17). The dark-violet mesoionic 1,2,3,5-thiatriazole (18) was liberated on treatment with aqueous potassium carbonate (Scheme 3). The structure is established on the basis of elemental analysis and spectroscopic data. In particular, the IR spectrum is devoid of NH absorptions. Compound (18) exhibits a long-wavelength absorption at 463 nm in methanol. When mixed with an equivalent amount of pyridinium chloride, complex (17) is formed and the absorption shifts to 350 mn. The mesoionic thiatriazoles are sensitive towards mineral acids and aqueous base and although reaction takes place with 1,3-dipolarophiles such as dimethyl acetylene-dicarboxylate, a mixture of products were obtained which were not identified. 

Quaternization (see Section 6.09.5.3) significantly increases the stability of the 1,2,3,4-thiatriazole ring. Moreover, many mesoionic and imine derivatives are quite stable in comparison with the parent compounds. 

Only a few new works have been published on this subject that were not previously covered in the corresponding chapters of CHEC(1984) and CHEC-II(1996) <1984CHEC(6)579, 1996CHEC-II(4)691>. The most recent work reported the reactions of 1,2,3,4-thiatriazoles leading to mesoionic compounds <1998JMT27>. 

No new data were reported since the last reviews <1984CHEC(6)579, 1996CHEC-II(4)691>. We can only mention the use of the mesoionic thiatriazole derivative 25 to induce refractive index changes in polymer films upon... 

Fully conjugated 1,2,3,5-thiatriazoles have been described in the form of the mesoionic aminides 9 <1988ACB63>, as thiatriazolium salts 10 <1990J(P2)1619> and as compounds with hexavalent sulfur 11 <1997CJC1188>. The parent 1,2,3,5-thiatriazoles of type 12 are described as elusive intermediates in the reaction of amidrazones with thionyl chloride leading to benzonitrile formation <2004H01J833>. 

Substituted l,2,3,5-thiatriazolium+-aminides 9 constitute the only known class of mesoionic 1,2,3,5-thiatriazoles <1988ACB63>. They are formed in the reaction of l-amino-l,3-disubstituted guanidines with approximately 2 equiv of thionyl chloride in pyridine as the solvent. The mesoionic compounds 9 are sensitive toward acids and bases (Scheme 2) <1988ACB63>. 

Mesoionic 1,2,3,5-thiatriazoles 24 that are fused to an imidazole ring were successfully prepared from 1,2-diaminoimidazoles 23 and thionyl chloride (Scheme 3) <1981JOC4065>. The bcnzimidazo[l,2-t ]thiatriazole was prepared in an analogous manner. 

Mesoionic l,2,3,4-thiatriazol-5-ones, -5-thiones, -5-imines and -5-alkenes 605... 

Rate constants for the first-order decay of nitrile sulfides have been estimated by means of flash photolysis of thiatriazoles and mesoionic oxathiazoles and the results used to calculate activation parameters (79JCS(Pl)960). 

Alkylation of mesoionic 3-aryl-1,2,3,4-thiatriazol-5-ones (79 X = S, Y = 0, R = Ph, 4-MeC6H4, 4-ClC6H4) and of mesoionic 3-phenyl-l,2,3,4-oxatriazole-5-thione (79 X = 0, Y = S, R = Ph) with triethyloxonium tetrafluoroborate yields thiatriazolium salts (80 equation 51) <79JCS(Pl)732). 

The known systems comprise mesoionic 1,2,3,4-oxatriazoles and -thiatriazoles (see Chapter 4.01, Scheme 5) of the structures (85)—(91). Preparative aspects of the chemistry of five-membered mesoionic heterocycles have been extensively reviewed by Ollis and Ramsden covering the literature until 1974 (76AHC(l9)l). The review contains a detailed discussion of the mesoionic concept and the representation of mesoionic heterocycles and other systems. Since then a number of investigations of the chemistry of mesoionic 1,2,3,4-oxatriazoles and -thiatriazoles have been carried out, in particular by the authors mentioned and their coworkers. Mesoionic thiatriazoles were first described in 1976. 

Heterocycle (85) may be systematically described as 3-substituted anhydro-5-hydroxy-1,2,3,4-oxatriazolium hydroxide. However, this nomenclature is somewhat cumbersome and the terminology mesoionic has been used throughout this review. Thus compounds (85)-(87) and (91) are named mesoionic l,2,3,4-oxatriazol-5-ones (85), -5-thiones (86), -5-imines (87) and mesoionic l,2,3,4-thiatriazole-5-alkenes (91), respectively. Using IUPAC Rule. C-87 (85) is named 3-substituted l,2,3,4-oxatriazol-5-ylio oxide or 3-substituted l,2,3,4-oxathiazolylium-5-olate and (87 R = R1 = Ph) is named iV-[3-phenyl-5-(l,2,3,4-oxatriazolio)]anilide. 

HNMR chemical shifts for substituents in mesoionic oxatriazole-5-thiones (86) (79JCS(Pl)732), oxatriazol-5-imines (87) (79JCS(Pl)736), mesoionic thiatriazol-5-ones (88) (79JCS(P1)732), thiatriazole-5-thiones (89) (79JCS(P1)732), thiatriazol-5-imines (90)... 

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