Mesoionic structures

Reactivity involving the mesoionic protomer has been investigated very recently (562). It was known that fixed mesoionic structures undergo cycloaddition with various dipolarophiles (473) and that such a reactivity Ph. 

Oxazolium hydroxide, anhydro-2-m-bromophenyl-5-hydroxy-3-methyl-4-trifluoroacetyl-X-ray structure, 6, 180 Oxazolium hydroxide, anhydro-4-hydroxy-cycloaddition reactions, 6, 208 IR spectra, 6, 186 mesoionic structures, 6, 179 tautomerism, 6, 185 reactions, 6, 206-211 synthesis, 6, 225... 

Zwitterion structures (206 207 etc.) as well as the mesoionic structure 208 have been proposed for these compounds.The exact significance of structure 208 and the term mesoionic is unclear in... 

The first synthesis of a l,2,3,4-oxatriazolium-5-amenate derivative (83, R=Ri=C, H5) was reported by Busch et al. as early as 1896 [144]. The product was obtained by the nitrosation of the diphenylthiosemicarbazide precursor 119. The real structure remained unclear until 1971, when Christophersen et al. [145] assigned the correct mesoionic structure to the compound. These authors prepared a series of mesoionic... 

I, 2,4-triazine (310) and several isocyanates affords l,3,4-thiadiazolo[2,3-c][l,2,4]triazinium-7-imide (311) possessing a mesoionic structure (91T-6747 94S1197). 

All the studies on the elucidation of the structure of diazoazoles lead unequivocally to a mesoionic structure in which the negative charge is localized in the azole ring. Of primary importance were C-NMR and IR spectra, as well as X-Ray crystal diffraction data. With regard to the stability in the solid state of diazoazoles and in solution in the pH range where they are not protonated, the presence of ortho ring nitrogens destabilizes the diazo compounds. 

In the same year, for the first time, a mesoionic structure of type 19 was proposed for a diazoazole (S3AG442). Since then, several studies were carried out with the aim of proving the structure of the diazoazoles by... 

The x-ray structure of 1,2,3-benzothiadiazole complexed with AsFj (9) shows that the arsenic binds at N3 <86CJC849>. When Fe2(CO)9 reacted with (10) one of the products was (11), for which x-ray diffraction revealed the unusual feature of the nitrogen and sulfur joined by an iron atom (Equation (2)) <890M296l>. The mesoionic structure (13) is formed by methylation of 1,2,3-thiadiazole (12). It can best be described as a resonance hybrid of structures (13a) and (13b) and this was corroborated by the x-ray data (Scheme 1) <91jhC477>. 

Using a combination of techniques 1,3,4-thiadiazole was shown to have a dipole moment of 3.0 D directed from the sulfur atom toward the center of the N—N bond. Dipole moment measurements were useful for proving the mesoionic structure and the large negative charge of an exocyclic sulfur atom for certain 1,3,4-thiadiazole derivatives <84CheC-I(4)545>. 

When (40) is irradiated by Hg lamp at room temperature in the presence of pentamethyl-cyclopentadienyl dicarbonyl cobalt(III), Co(III) dithiolato complexes (45) and (46) are formed implying involvement of benzonitrile sulfide as an intermediate <92CL243). Thermolysis of (40) (and its phenyl ring substituted derivatives (40a)) at 110-140 °C in aromatic solvents results in formation of another heterocyclic mesoionic structure (47) and appears to proceed as a radical process (Scheme 2) (91TL4023). The reaction is inhibited by radical scavengers. 

Proton NMR data are reported for substituted mesoionic l,2,3,4-thiatriazolium-5-olates and 5-thiolates. Although the spectra are of little diagnostic value they are consistent with the mesoionic structures <79JCS(P1)732>. Representative C shifts for C(5) of 5-substituted thiadiazoles are 5-Ph, 178.46 ppm 5-PhNH, 173.8 ppm 5-BzS, 179.8 ppm 5-PhCOS, 171.5 ppm and for the thiatriazolium salt (9) 186.42 ppm <84CHEC-I(6)579>. 

Upon methylation (163) yields a thermally stable product, which on the basis of spectral data is assigned the mesoionic structure (165) <84BSB405). 

Analogues of imidazolylidene bearing two adjacent boron atoms in the heterocycle (3) have been synthesized and their coordination chemistry has been studied.16 They were shown to be better er-donors than their classical counterparts. The imidazo[l,5-ajpyridine has been shown to be a modular base for new stable NHCs (4).17 The same platform also allowed the isolation of abnormal NHCs (5) with a mesoionic structure and a strong a-donor character. 

The formulation of a mesoionic structure for these types of compounds is strongly supported by spectral data, dipole moments and X-ray crystallography. 

Mesoionic structures and other internal salts may display a molecular ion of an ambiguous structure which is isobaric with the original molecule. Since the fragmentation patterns of... 

In view of the extreme instability of these mesoionic structures it appears unlikely that the corresponding 1,3-dioxoIium species will be isolable. 

The mesoionic 1,3-dithiolones of type (2) show substituent-dependent IR carbonyl stretching vibrations between 1612 and 1558 cm-1 (Table 7). These low frequencies are characteristic for this class of compound and are in agreement with the mesoionic structure. Most of the mesoionic 1,3-dithiolones containing p-substituted phenyl groups show split carbonyl absorption bands, presumably as the result of Fermi resonances (76CB740). 

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>. 

Figure 3 Structures of seven sets of 7t-electron sextet six-, seven-, eight- and nine-membered rings with no Z-Z adjacencies mesoionic structures are indicated by an asterisk.

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. 

Oxazoles 1, benzoxazoles 2, oxazolium salts 3, and oxazole A -oxides 4 are fully conjugated compounds (Figure 1). In addition, the two mesoionic structures l,3-oxazolium-5-olates 5 and (l,3-oxazolium-4-olates) 6, commonly known as miinchnones and isomiinchnones, respectively, are also considered to be conjugated rings. There are five systems of hydroxyl-substituted oxazoles and they exist in their oxo forms the 2(3H)-, 2(5H)-, 4(5//)-, 5(2//)-, 5(4//)-oxazolones 7-11. Three forms of dihydrooxazoles are known 2,3-, 2,5-, and 4,5-dihydrooxazoles respectively 12-14. The fully saturated ring is called oxazoline 15. The monooxo derivatives are 2-oxazolidinone 16,4-oxazolidinone 17, and 5-oxazolidinone 18. The three variants of oxazolidinediones are 19-21 and the fully oxidized oxazolidi-netrione is 22. 

Cyclodehydration of a-(thioaroylthio)- or a-(thiocarbamoylthio)carboxylic acids (96) with acetic anhydride in the presence of catalytic amounts of boron trifluoride etherate, or with a mixture of acetic anhydride and triethylamine (1 1), leads to the 2-aryl- or 2-amino-l,3-dithiolium-4-olates (97) listed in Table VIII.Chemical properties, dipole moments, NMR, IR, and UV spectra, and charge distribution calculations are consistent with a mesoionic structure. " ... 

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