Sydnones resonating structures

Mesoionic compounds have been known for many years and have been extensively utilized as substrates in 1,3-dipolar cycloadditions.158-160 Of the known mesoionic heterocycles, munchnones and sydnones have generated the most interest in recent years. These heterocyclic dipoles contain a mesoionic aromatic system i.e. 206) which can only be depicted with polar resonance structures.158 Although sydnones were extensively investigated after their initial discoveiy in 1935,160 their 1,3-dipolar character was not recognized until the azomethine imine system was spotted in the middle structure of (206). C-Methyl-N-phenylsydnone (206) combines with ethyl phenylpropiolate to give the tetrasub-... 

The best known mesoionic compounds have five-membered rings, and initially it was advocated by Baker, Ollis, Ramsden and other authors that only five-membered heterocycles which cannot be satisfactorily represented by any one covalent or ionic structure possessing a sextet of TT-electrons in association with the five atoms comprising the ring may be called "mesoionic". Here, following Katritzky, mesoionic means a mesomeric betaine. The first such compounds to be discovered were sydnones, followed by miinchnones and then by diazolones. In all these compounds the Z-type atom is part of a carbonyl group, and two Y-type atom chains separate two odd-numbered chains of X- and Z-atom chains. Only the main resonance structures are displayed in formulas (Figure 11). 

Structure of a sydnone resonance contributors (mesomeric structures)... 

Sydnones. A term originally applied to the compd obtained by action of acetic anhydride on N-nitrOsophenyl -glycine Eade, Esrl, J. Chem. Soc. 1946, 591 Earl, Chem. A Ind. (London) 1953, 746 Rec. Trav. Chim. 75, 346 (1956). Now used to describe 5-membered mesoionic compounds that are best represented by resonance structures Y. Noel. Bull Soc. Chim. France 1964, 173. Photochromic properties S. Nespurek, M. Sorm, Colt Czech. Chem. Common. 42, 811 (1977). t+N-NMR study L. Scefaniak, Tetrahedron 33,... 

The mesoionic compounds are represented by structures that cannot be properly described by Lewis forms not involving charge separation. Typical examples are the sydnones 80 the first example was obtained at the University of Sydney by the action of acetic anhydride on N-nitrosophenylglycine [75]. They were consequently named after the Australian town [76]. These structures are best approximated as resonance hybrids. They can be represented by any contributing mesomeric structure a, b, c or by the general structure d. 

The sydnones may be represented by structures (160a-d), of which the zwitterionic structures (160a,b) most clearly imply an aromatic sextet. p-Bromophenylsydnone (160 R-H, R -p-bromophenyl) is essentially planar. However, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. Contributions of structures (160c,d) to the resonance hybrid are important, and the structural index of sydnone reveals its a rather low aromaticity (AN= 17%). 

The 13C spectra of several sydnones and sydnonimines have been determined (74JCS(P2)875). In 3-methylsydnone the IV-methyl carbon resonance is at 39.8 p.p.m. and the ring carbons 4 and 5 resonate at 96.8 and 169.2 p.p.m., respectively. These data and those for other sydnones have been correlated with ring structure. 

NH)-l,2,3-oxadiazoles. The l4N spectra of sydnones show a sharp signal which is superimposed on a broad resonance (111). The sharp signal at about +100 ppm may be assigned to the N -R moiety (Table XX), since the line-width is typical for such structures and because the spectrum of 3,4-diphenylsydnone-2-15N indicates (112) that the N-2 shift is +42 ppm. 

There are a substantial number of heterocyclic substances for which no plausible, unpolarised canonical structure can be written such systems are termed mesoionic . Despite the presence of a nominal positive and negative charge in all resonance contributors to such compounds, they are not salt-like, are of course overall neutral, and behave like organic substances, dissolving in the usual solvents. Examples of mesoionic structures occur throughout the text. Amongst the earliest mesoionic substances to be studied w ere the sydnones, for which several contributing structures can be drawn. 

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