Heteroarylcarbene–Arylnitrene Radical Cation Isomerizations

International audience5-Phenyltetrazole 1e is an important source of phenylnitrene or the phenylnitrene radical cation (m/z 91) under thermal, photochemical, and electron impact conditions. Similarly, 3- or 4-(5-tetrazolyl)pyridines 12b,c yield pyridylnitrene radical cations 9a•+ (m/z 92) upon electron impact. In contrast, 2-(5-tetrazolyl)pyridine 12a•+ generates 2-pyridyldiazomethane 24•+ and 2-pyridylcarbene 26•+ radical cations (m/z 119 and 91) upon electron impact. The 2-pyridylcarbene radical cation undergoes a carbene–nitrene rearrangement to yield the phenylnitrene radical cation. Calculations at the B3LYP/6-311G(d,p) level have revealed facile H-transfer from the tetrazole to the pyridine ring in 2-(5-tetrazolyl)pyridine, 12a•+ → 21•+, taking place in the radical cations. Subsequent losses of N2 generate the pyridinium diazomethyl radical 22•+ or pyridinium-2-carbyne 23•+. These two ions can isomerize to 2-pyridyldiazomethane 24•+ and 2-pyridylcarbene 26•+, the latter rearranging to the phenylnitrene radical cations 9a•+. 13C-labeling of the tetrazole rings confirmed that 2-(5-tetrazolyl)pyridine 12a generates 2-pyridylcarbene/phenylnitrene radical cations retaining the 13C label, but 4-(5-tetrazolyl)pyridine 12c generates 4-pyridylnitrene 18c•+, which has lost the 13C label. 2-Pyridylcarbene/phenylnitrene radical cations (m/z 91) also constitute the base peak in the mass spectrum of 1,2,3-triazolo[1,5-a]pyridine 34. Similarly, 4-pyridylnitrene radical cation 18c•+ or its isomers (m/z 92) is obtained from 1,2,3-triazolo[1,5-a]pyrazine 36. Several other α-heteroaryltetrazoles behave in the same way as 2-(5-tetrazolyl)pyridine, yielding heteroarylcarbene/arylnitrene radical cations in the mass spectrometer, and this was confirmed by 13C-labeling in the case of 1-(5-tetrazolyl)isoquinoline 42-13C. In general, 5-aryltetrazoles generate arylnitrene radical cations under electron impact, but α-heteroaryltetrazoles generate α-heteroarylcarbene radical cation

Phenylnitrene radical cation and its isomers from tetrazoles, nitrile imines, indazole, and benzimidazole

Phenylnitrene radical cations m/ z 91, CHN, 8a are observed in the mass spectra of 1-, 2-, and 5-phenyltetrazoles, even though no C-N bond is present in 5-phenyltetrazole. Calculations at the B3LYP/6-311G(d,p) level of theory indicate that initial formation of the C-phenylimidoylnitrene 13 and/or benzonitrile imine radical cation 19 from 1 H- and 2 H-5-phenyltetrazoles 11 and 12 is followed by isomerizations of 13 to the phenylcyanamide ion 15 over a low barrier. A cyclization of imidoylnitrene ion 13 onto the benzene ring offers alternate, very facile routes to the phenylnitrene ion 8a and the phenylcarbodiimide ion 14 via the azabicyclooctadienimine 16. Eliminations of HNC or HCN from 14 and 15 again yield the phenylnitrene radical cation 8a. A direct 1,3-H shift isomerizing phenylcarbodiimide ion 14 to the phenylcyanamide ion 15 requires a very high activation energy of 114 kcal/mol, and this reaction needs not be involved. The benzonitrile imine -3-phenyl-1 H-diazirine-phenylimidoylnitrene-phenylcarbodiimide/phenylcyanamide rearrangement has parallels in thermal and photochemical processes, but the facile cyclization of imidoylnitrene 13 to azabicyclooctadienimine 16 is facilitated by the positive charge making the nitrene more electrophilic. Furthermore, the benzonitrile imine radical cation 19 can cyclize to indazole 24, and a series of intramolecular rearrangements via hydrogen shifts, ring-openings and ring closures allow the interconversion of numerous ions of composition CHN, including 19, 24, the benzimidazole ion 38 and o-aminobenzonitrile ion 40, all of which can eliminate either HCN or HNC to yield the CHN ions of phenylnitrene, 8a, and/or iminocyclohexadienylidene, 34. Moreover, benzonitrile imine 19 can behave like a benzylic carbenium ion, undergoing a novel ring expansion to cycloheptatetraenyldiazene 45. The N-phenylnitrile imine ion 2d derived from 2-phenyltetrazole 1d cleaves efficiently to the phenylnitrene ion 8a but may also cyclize to the indazole ion 24. The N-phenylimidoylnitrene 59 derived from 1-phenyltetrazole 5d undergoes facile isomerization to the phenylcyanamide ion 15 and hence phenylnitrene radical cation 8a

A Unique Toxic Peptide from the Larvae of the South American Sawfly, Perreyia flavipes

A major toxic component of the South American sawfly larvae, Perreyia flavipes (Pergidae), has been shown to be a heptapeptide with the structure (1). The unique features of this peptide, which has been named pergidin, are the presence of five unnatural D amino acids and a phosphoseryl residue

Nitrene–nitrene rearrangement under thermal, photochemical, and electron-impact conditions: the 2-azidopyridines/tetrazolo[1,5-a]pyridines

N-15-Labeling demonstrates that the two nitrogen atoms in the 2-pyridylnitrene radical cation 2(+) become equivalent prior to fragmentation in the mass spectrometer. Furthermore, the mass spectra of 6- and 7-tetrazolo[1,4-a]pyridine are identical, as are those of 5- and 8-tetrazolo[1,5-a]pyridine, thereby again demonstrating interconversion of the nitrogen atoms in 2-pyridylnitrenes. These rearrangements parallel the reactions established under thermal (flash vacuum pyrolysis) and photochemical condition. Calculations of the energies of ground and transition states at the CASPT2(7,8) level support the notion that 2-pyridylnitrenes undergo very easy and exothermic ring expansion to 1,3-diazacycloheptatetraene 3, both in the neutrals and the radical cations. In addition, the ring opening to 4-cyanobutadienylnitrene 4 can take place in both the neutrals and the radical cations with modest activation barriers

Synthesis of symmetrical, substituted (alkane-α,ω-diyl)(bis[3,3′-allyl dithioethers]) monomers for photoplastic polymer networks

Novel symmetrical (alkane-α,ω-diyl)(bis[3,3′-allyl dithioethers]) compounds and their ether analogues, have been synthesised from (alkane-α,ω-diyl)bis([2-{chloromethyl}allyl]sulfane) precursors, for use in crosslinked polymers which exhibit photoplastic behaviour. Facile synthesis and purification of these monomers was achieved if the alkane-α,ω-diyl moiety had at least one oxygen atom in this linker. The number of sulfur atoms in these monomers was varied from four to two to zero to produce monomers which can be used to evaluate their importance on the photoplasticity behaviour