Tautomeric equilibrium mesoionic compounds

A-2-Thiazoline-5-one may exist in three tautomeric forms (Scheme 108). The tautomeric equilibrium has been studied by H NMR (446. 453. 457. 464), infrared (453. 464-466). and ultraviolet (453, 464) spectroscopies 13C NMR data would be very instructive since model compounds for the three tautomers are now available. Polar solvents favor the enolic (211b) and the mesoionic (211c) forms (Table VII-12), the later being even more favored by high dilution (464). The percentage of the enol... 

Only a few examples of a Diels-Alder type reaction of the valence tautomeric ketene of mesoionic 1,3-dithiolones are known. Thus the reactions of differently substituted mesoionic compounds (2) with o-chloranil give the unusual adducts (142), derived via [4 + 2] cycloaddition to the non-detectable valence tautomeric ketene (141) which is in equilibrium with (2). The reaction rate depends upon the nature of the substituents, for example substituents with —I and/or —M effects lower the reaction rates (81ZN(B)609). 

Until very recently, it has not been recognized that the tautomerism of suitable mesomeric betaines is a source of NHCs. l,2,4-Triazolium-3-aminide 48 is a mesoionic compound and it has been used as analytical reagent for the detection of nitrate anions for a period of almost 100 years ( nitron, Busch s reagent) (1905CB861), before it was realized that nitron is in equilibrium with its NHC (Scheme 13). Numerous trapping reactions of the latter have been carried out (2012CC227). Rhodium complexes 49 were prepared, as well as thione formations and trapping reactions with carbon disulfide. 

Tautomeric Equilibrium of Mesoionic Compounds with a Normal N-Heterocyclic Carbene... 

Betaine—carbene interconversions can be achieved by decarboxylations or deprotonations from suitable betaines. Deprotonations of mesomeric betaines result in the formation of anionic N-heterocyclic carbenes. Some mesoionic compounds, for example, five-membered representatives of the conjugated mesomeric betaines, are in equilibrium with their tautomeric normal N-heterocyclic carbenes according to general Scheme 32 (2013ARK(i)424). 

Substituted derivatives. The 4-substituted analogues can in principle exist as two uncharged tautomeric forms 270 and 271 and as the type B mesoionic tautomer 272. All the evidence shows that these compounds exist predominantly as the NH2, OH, or SH tautomers 270. The equilibrium between the pyrazole 273 and the mesoionic tautomer 274 has been shown to favor the hydroxypyrazole 273 strongly (AG 6.15 kcal mol-1) <1972T463>. 

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