Mesoionic structures 1,2,4-triazoles

Most of the reported 1,2,5,6-tetrazocine systems probably do not exist in the monocyclic form but rather as the tetraazapentalene betaine structure (see Introduction) 2 3 7"13 for the tetra-phenyl-substituted system, the compound does actually exist as the mesoionic tetraphenyl-[1,2.3]triazolo[l,2-h][l,2,3]triazole, as determined by H and 13C NMR spectroscopy.15 The following syntheses arc therefore questionable. 

The novel mesoionic 1,2,4-thiadiazole 78 was reported to be the unexpected byproduct in the reaction of the triazole 76 with ferric chloride (the bicyclic compound 77 also gave the same result). Besides spectroscopic and X-ray diffraction evidence, a preparative proof for the structure of 78 was also provided <00JHC261>. 

Potts and McKeough81 have recently obtained several interesting mesoionic azapentalenes 83, 84 by reaction of the appropriate 1,4-dicarbonyl compound with phosphorus pentasulfide. Compounds 83 and 84 can be written as dipolar structures, but participation of sulfur d-orbitals allows nonpolar forms to be envisaged. MO calculations neglecting rf-orbital participation predict the thieno[3,4-c]pyrrole system (84) to be very unstable.430 A related system 86 has been prepared by Japanese workers63 from the diamino-r-triazole (85) and sulfur dichloride. 

Numerous structures containing the thiocarbonyl ylide dipole are conceivable. Incorporation of the thiocarbonyl ylide dipole into a bicyclic heterocyclic system is possible by the conversion of the cyclic thione (203) into the ring-fused mesoionic system (204). The thiocarbonyl ylide dipole (205) undergoes cycloaddition with both alkenic and alkynic electron-poor dipolarophiles in refluxing benzene or xylene so that, after extrusion of hydrogen sulfide or sulfur, respectively, from the initial 1 1 cycloadducts (206) and (207), a ring-fused pyridinone is formed. The method has been used for the annelation of pyridinones to the imidazole, 1,2,4-triazole, thiazole and 1,3,4-thiadiazole systems... 

The first syntheses of mesoionic compounds included 1,2,4-triazoles and their oxa and thia analogues. The structure postulated at first as (20) was rejected on stereochemical grounds and on consideration of the large dipole moment, to be replaced by a charged monocyclic structure, defined as mesoionic and considerably extended to a great variety of theoretically and practically important examples (57QR15, 76ahc(19)3). 

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