The anion of N-phenylsydnone, which can be generated on treatment of N-phenylsydnone with cyanomethyllithium without decomposition, can be represented as tripolar zwitterionic and as anionic N-heterocyclic carbene resonance forms. Its palladium complex was prepared from 4-bromo-3-phenylsydnone and tetrakis(triphenylphosphine)palladium and proved to be active as catalyst in Suzuki-Miyaura reactions. Thus, 2,5-dibromo-3,4-dinitrothiophene was effectively converted into 2,5-diaryl-3,4-dinitrothiophenes with 1-naphthyl, (4-trifluoromethoxy)phenyl, [4-(methylsulfanyl)phenyl], and biphenyl-4-yl boronic acid. 3-(Phenanthren-9-yl)quinoline was prepared by Suzuki-Miyaura reaction starting from 3-bromoquinoline. 1-Chloro-2,4-dinitrobenzene cross-coupled with phenyl boronic acid, 1-naphthyl boronic acid, 9-phenanthryl boronic acid. 4-Bromobenzylic alcohol gave (4-isopropylphenyl)methanol on sydnone-palladium complex-catalyzed reaction with isopropyl boronic acid.
1. Of the carcinogenic aromatic nitrogenous compounds investigated, 10 out of 11 were found to be biotransformed in rabbit liver microsomal preparations to nitroxide radicals, as detected by ESR spectroscopy; with non-carcinogenic nitrogenous compounds only 2 out of 8 formed nitroxide radicals. 2. The radicals are stable mixed nitroxides formed by covalent binding of the aromatic nitroxide to endogenous constituents of the microsomal membrane with preservation of the nitroxide radical group (spin-trapping). 3. From the high chemical stability and the observed ESR spectra, spin-label-like structures can be deduced. 4. From this and the known one-electron redox properties of spin-labels, a redox cycle catalysing the production of reactive oxygen species and involved in tumour initiation and/or promotion by aromatic nitrogenous compounds is proposed.
Sydnone imines were deprotonated at the C4 position to give lithium‐stabilized anions, which proved to be stable for several months in solution. These anions are elements of the intersection of the substance classes of N‐heterocyclic carbenes and mesomeric betaines. They can be formulated as anionic N‐heterocyclic carbenes. The negative charge translates into high‐field shifts of the 13C NMR resonance frequencies of C4 in comparison to other NHCs. Similar to the chemistry of N‐heterocyclic carbenes, reactions with isocyanates and isothiocyanates gave 4‐(N‐carbamoyl)‐benzoylsydnone imines in trapping reactions. As a result of ring transformations involving the characteristic π‐architecture of the mesoionic framework, imidazolidine‐2,4‐dithiones, imidazolin‐2,4‐diones and 1,2,4‐triazol‐5‐ones were formed depending on the substitution pattern of the starting materials and the reaction conditions.
The catalyst system consisting of lithium N-phenylsydnone-4-carboxylate/Pd(PPh3)4 has proven to be an effective catalyst for the Suzuki–Miyaura reaction of 2,4-dinitrochlorobenzene with boronic acids in acetic acid at pH 5.7, whereas the N-phenylsydnone carbene palladium complex [sydPd(PPh3)2Br] required pH 8.0.
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