Pyrolysis of benzotriazoles. Relationships between 1- and 2-vinylbenzotriazoles, α- and β-azidostyrenes, N-phenylketenimine and indole. Pitfalls in the use of pyrolysis-mass spectrometry in mechanistic studies

Wentrup, Curt; Freiermuth, Beat

doi:10.1016/j.jaap.2016.07.003

Cited by 13 publications

(14 citation statements)

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“…At higher temperatures, methacrylonitrile 464 , 2-methylpropionitrile 463 , and benzene were formed, and these compounds were also formed on pyrolysis of PhNCCMe 2 461 itself (Scheme ). , Once again, the radical pair 462 is a likely intermediate…”

Section: Triazolesmentioning

confidence: 76%

“…If one product is obtained at one pyrolysis temperature and another at a higher temperature, it is natural to believe that the second product is formed from the first, e.g., by rearrangement. However, this is not necessarily true, and suitable control experiments are important . One way to demonstrate that two reactions are consecutive is to use the double pyrolysis technique, where two pyrolysis tubes at different temperatures are used in series. , Kinetic monitoring of all the pyrolysis products is also helpful.…”

Section: The Fvp Methodsmentioning

confidence: 99%

“…More recently, FVP of azide 29 was found to yield 2-phenyl-1-azirine ( 31 ) already at 350 °C, and the amount increased with the temperature. At 650 °C azirine 31 was obtained in 82% yield together with 9% of phenylacetonitrile 33 . On FVP at 750–900 °C, azirine 31 undergoes ring opening to regenerate the vinylnitrene 30 , migration of the phenyl group to form N -phenylketenimine 32 , and isomerization to phenylacetonitrile 33 .…”

Section: Azidesmentioning

confidence: 99%

“…CN-group migrations in aromatic nitriles are often observed at elevated temperatures. Similarly, phenylacetonitrile affords o -tolunitrile as the major products at 1000 °C. , …”

Section: Azidesmentioning

confidence: 99%

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Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

2017

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Flash vacuum pyrolysis (FVP) of azides is an extremely valuable method of generating nitrenes and studying their thermal rearrangements. The nitrenes can in many cases be isolated in low-temperature matrices and observed spectroscopically. NH and methyl, alkyl, aralkyl, vinyl, cyano, aryl and N-heteroaryl, acyl, carbamoyl, alkoxycarbonyl, imidoyl, boryl, silyl, phosphonyl, and sulfonyl nitrenes are included. FVP of triazoloazines generates diazomethylazines and azinylcarbenes, which often rearrange to the energetically more stable arylnitrenes. N elimination from monocyclic 1,2,3-triazoles can generate iminocarbenes, 1H-azirines, ketenimines, and cyclization products, and 1,2,4-triazoles are precursors of nitrile ylides. Benzotriazoles are preparatively useful precursors of cyanocyclopentadienes, carbazoles, and aza-analogues. FVP of 5-aryltetrazoles can result in double N elimination with formation of arylcarbenes or of heteroarylcarbenes, which again rearrange to arylnitrenes. Many 5-substituted and 2,5-disubstituted tetrazoles are excellent precursors of nitrile imines (propargylic, allenic, or carbenic), which are isolable at low temperatures in some cases (e.g., aryl- and silylnitrile imines) or rearrange to carbodiimides. 1,5-Disubstituted tetrazoles are precursors of imidoylnitrenes, which also rearrange to carbodiimides or add intramolecularly to aryl substituents to yield indazoles and related compounds. Where relevant for the mechanistic understanding, pyrolysis under flow conditions or in solution or the solid state will be mentioned. Results of photolysis reactions and computational chemistry complementing the FVP results will also be mentioned in several places.

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Section: Triazolesmentioning

confidence: 76%

Section: The Fvp Methodsmentioning

confidence: 99%

Section: Azidesmentioning

confidence: 99%

“…CN-group migrations in aromatic nitriles are often observed at elevated temperatures. Similarly, phenylacetonitrile affords o -tolunitrile as the major products at 1000 °C. , …”

Section: Azidesmentioning

confidence: 99%

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Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

2017

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“…In this case, the ring closure route (i) generates the stable 2‐phenyl‐3 H ‐azirine 17 a , which can ring‐open to the α‐styrylnitrene 18 a or rearrange in a concerted manner to N ‐phenylketenimine 19 a . The azirine 17 a has been obtained previously by FVP of α‐styryl azide at 350 °C, which may react either concertedly or via the nitrene 18 a [28] . Similarly, the azirine 17 a may rearrange to ketenimine 19 a concertedly or via the vinylnitrene 18 a , although the concerted reaction has the lower activation energy (Scheme 6 a).…”

Section: Resultsmentioning

confidence: 99%

Rearrangements of Aromatic Nitrile Oxides and Nitrile Ylides: Potential Ring Expansion to Cycloheptatetraene Derivatives Mimicking Arylcarbenes

Marchais

Bégué

Dargelos

et al. 2020

Chemistry A European J

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Nitrile imines, nitrile oxides and nitrile ylides are widely used in 1,3‐dipolar cycloaddition reactions. They also undergo thermal and photochemical rearrangements to carbodiimides, isocyanates, and ketenimines, respectively. Calculations at DFT and CASPT2 levels of theory reveal novel, potential rearrangements, in which the aromatic 1,3‐dipoles mimic phenylcarbene and undergo ring expansion to cycloheptatetraene derivatives. These rearrangements can potentially take place in both the singlet ground states and the triplet excited states, and they are accelerated by m,m’‐bis(dimethylamino) substitution on the phenyl moieties. The new rearrangement becomes the energetically preferred path for m,m’‐bis(dimethylamino)benzonitrile oxide in the triplet state. In the m,m’‐bis(dimethylamino)benzo nitrile ylide, the cyclization to the 2‐phenyl‐1‐azirine is favored over the ring expansion to a cycloheptatetraene by ca. 5 kcal mol−1 in the singlet state. In the bent triplet states, 1,3‐hydrogen shifts interconverting nitrile ylides are potentially possible.

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Flash Vacuum Pyrolysis

Wentrup

2018

Kirk-Othmer Encyclopedia of Chemical Technology

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Flash vacuum pyrolysis ( FVP ) allows the generation, direct observation, and in many cases isolation of reactive intermediates and unusual molecules such as carbenes, nitrenes, free radicals, and cumulenes. Detailed characterization of unstable species is achieved in conjunction with matrix isolation or gas‐phase spectroscopic techniques. In addition, FVP has many important applications in organic synthesis, where reactions are often initiated by extrusion of small molecules (eg, N 2 , CO , CO 2 , or SO 2 ), or by elimination (eg, HCl , CH 3 COOH , HNCO , or isobutene), or by (retro)pericyclic reaction. These reactions are chemoselective, stereospecific, and usually proceed via the lowest activation energy paths to the products, which are often formed in high yields. Photochemistry is often complementary to pyrolysis, and the two methods in combination are powerful tools in investigations of chemical reactions.

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Pyrolysis of benzotriazoles. Relationships between 1- and 2-vinylbenzotriazoles, α- and β-azidostyrenes, N-phenylketenimine and indole. Pitfalls in the use of pyrolysis-mass spectrometry in mechanistic studies

Abstract: Pyrolysis of benzotriazoles. Relationships between 1-and 2-vinylbenzotriazoles, and -azidostyrenes, N-phenylketenimine and indole. Pitfalls in the use of pyrolysis-mass spectrometry in mechanistic studies

Cited by 13 publications

References 41 publications

Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

Rearrangements of Aromatic Nitrile Oxides and Nitrile Ylides: Potential Ring Expansion to Cycloheptatetraene Derivatives Mimicking Arylcarbenes

Flash Vacuum Pyrolysis

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