Sydnone aromaticity

The sydnones may be represented by structures (123a-d), of which the zwitterionic structure (123a) most clearly implies an aromatic sextet. The diamagnetic susceptibility exaltation for Af-phenylsydnone of ll.Ox 10 cm moP is comparable with the corresponding value for pyrrole (10.2x10 ). 3-p-Bromophenylsydnone (123 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. 

An interesting variation of this quinoxaline synthesis is outlined by the synthesis of sydnoquinoxalines shown in Scheme 103. The starting material is phenylsydnone 288 with an iminophosphorane group in an o-position. With isocyanate or isothiocyanate carbodiimide intermediates 289 are formed by an electrophilic aromatic substitution at the sydnone ring (4 position), the 4-(arylamino)sydno[3,4-a]quinoxalines (290) are obtained (91S745). 

The final products derived from 1 1 addition of sydnones to alkenes are usually pyrazoles or dihydropyrazoles. The pyrazoles are formed by oxidation of intermediate dihydropyrazoles. An unusual example of aromatization by elimination of an alkane (toluene) in prefence to hydrogen is illustrated in Scheme 7 <89TL4625>. 

Laser flash photolysis (LFP) studies have been carried out on the generation of N,C-diaryl nitrile imines from sydnones and from tetrazoles in solution at 77 K. They were found to have hfetimes of milliseconds and were quenched by dimethyl acetylenedicarboxylate (DMAD) (A q=5-9 x 10 M s ) and by carboxyhc acids (feq= lO -lO s ) (13). The strong dependence of v ax on the nature of the aromatic substituents in N,C-diaryl nitrile imines was interpreted by a hnear free energy relationship as due to intramolecular charge transfer (14). 

Magnetic criteria have received wide application mainly as a qualitative test for aromaticity and antiaromaticity. The values of the exaltation of diamagnetic susceptibility (in 10-6A cm-3 mol-1), and therefore aromaticity, decrease in the sequence thiazole (17.0) > pyrazole (15.5) > sydnone (14.1). The relative aromaticity of heterocycles with a similar type of heteroatom can be judged from values of the chemical shifts of ring protons. The latter reveals paramagnetic shifts when Tr-electron delocalization is weakened. For example, in the series of isomeric naphthoimidazoles aromaticity decreases in the sequence naphthof 1,2-djimidazole (8 = 7.7-8.7 ppm) > naphtho[2,3- perimidine (8 = 6.1-7.2 ppm). This sequence agrees with other estimates, in particular with energetic criteria. 

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-... 

Oxadiazoles are unknown. The importance of the 1,2,3-oxadiaza ring system lies in the stabilization offered in the derivatives of higher oxidation state known as sydnones (1) and sydnonimines (2). Fusion with an aromatic ring does not stabilize the five-membered... 

As shown in Table 4 the 3-alkyl protons in the NMR spectra of sydnones are considerably further downfield than those of the 4-alkyl protons. The strong deshielding effect of the positively charged N-3 atom accounts for this shift. Protons in other five-membered rings, e.g. thiophene or 1,2,4-oxadiazole, have chemical shifts near those of six-membered aromatic rings (57-8 p.p.m.) but the sydnone protons are upheld (5 6.2-6.8 p.p.m.) (63CI(L)1926) (see also Chapter 4.01, Tables 9 and 17). 

Mesoionic Compounds. These compounds cannot be satisfactorily represented by Lewis structures not involving charge separation. Most of them contain five-membered rings. The most common are the sydnones, stable aromatic compounds that undergo aromatic substitution when R is hydrogen. 

The mesoionic compounds, such as sydnones, are not pure aromatic compounds of a classical sense, but their photoreactions give bicyclic intermediates having four-membered rings which are very useful for the syntheses of new compounds. Thus, some of these reactions will be presented. 

Perhaps the most studied of oxadiazoles are the sydnones and some views on their aromatic character are summarized here. For detailed reviews the reader is referred to Ohta and Kato,244 Bloor et al.Ub Stewart,246 and Baker and Ollis.247 The sydnones may be represented by structures 57a-d, of which the mesoionic structure 57a most clearly implies an aromatic sextet. 

The stability of sydnones, due to their aromatic character, and that of trans-aldonitrones with respect to their cis isomers should be important factors in keeping the reactivity of these compounds low in 1,3-cycloadditions. 

Another striking property of sydnones that is consistent with the view of aromaticity is that they undergo electrophilic substitution reactions, as with bromine in Scheme 7.28. 

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