Mesoionic compounds tautomeric

Keywords Desmotropy Heterocyclic carbenes Heterocyclic ring systems Heterocyclic ylides Mesoionic compounds Tautomerism... 

Thiadiazoles were first described in 1882 by Fischer and further developed by Busch and his coworkers. The advent of sulfur drugs and the later discovery of mesoionic compounds greatly accelerated the rate of progress in the field of thiadiazoles. Thiadiazoles carrying mercapto, hydroxy and amino substituents can exist in many tautomeric forms and this property is being intensively studied using modern instrumental methods. 

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). 

Measured dipole moments for a series of substituted tetrazoles gave interesting comparisons (56JA4197). The moments for 1-ethyltetrazole (5.46 D) and 2-ethyl tetrazole (2.65 D) were at either side of the mesoionic compound (7) (4.02 D) and the dipole moments could not be used to identify mesoionic structures. The dipole moments for the 1,5-disubstituted tetrazole structure were consistently high, >5.3 D, while those of the 2,5-disubstituted structure were low, <2.65 D theoretical calculations support this (61T237). Recently, dipole moment measurements were made on 5-(p-tolyl)tetrazole in dioxane (fio 4.99), its 1-methyl isomer 6.03) and its 2-methyl isomer (jUd 2.41) (80JHC1374). These results, when applied to the annular tautomerism of 5-( p-tolyl)tetrazole, suggested 60 10% of the 1-NH form in this medium. 

Substituents have little effect on ring aromaticity. In hydroxy- and thiol-imidazoles the /a values are almost identical with that of the parent. These 7a values are obtained from x-ray data on mesoionic compounds which can be accepted as models for the anticipated zwitterions which play important roles in the tautomeric equilibria (see also Section 3.02.4.3.1) <94H(37)249>. 

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... 

Desmotropy of some five-, six-, and seven-membered heterocyclic rings and of some important pharmaceutical compounds and tautomeric equilibria of heterocyclic carbenes with mesoionic compounds and ylides are described. 

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). 

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... 

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>. 

Type B mesoionic heterocycles have the capacity to tautomerize to an acyclic isomer <1976AHC(19)1>. For the 1,2-dithiolium-4-olates, IR studies indicate that the cyclic structure 316 is favored over the acyclic structure 317, except where R is an amino substituent <1987PS(31)109>. A crystal structure of dehydrodithizone 318 has confirmed the cyclic mesoionic structure <1970JA1965> but most of the reactions of this compound can be interpreted in terms of initial valence tautomerism to the bisphenylazo isomer 319 <1979COC(4)1221>. 

There are only a few examples of Dewar isomers of aromatic compounds defined in a classical sense nonionic six n electron systems. Therefore, this review includes some mesoionic, tautomeric, and even dihydro aromatic compounds. As shown, this field of chemistry is rather young it will develop further in both synthetic and theoretical aspects. 

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