Flash vacuum thermolysis, a versatile method in organic chemistry. Part II, fragmentation patterns in specific classes

Wiersum, U. E.

doi:10.1002/recl.19821011101

Cited by 39 publications

(4 citation statements)

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“…Wiersum confirmed this reaction under FVP conditions. 21 No yield was given, but it was mentioned that polymeric material was also formed.…”

Section: 23-triazolesmentioning

confidence: 84%

“…In his classic monograph, Hurd lists many examples of pyrolysis of organic compounds by passing vapors through red-hot tubes at atmospheric pressure . In contrast, in FVP, a sample is volatilized into a resistively heated pyrolysis tube, usually quartz, in high vacuum, usually 10 –3 –10 –5 hPa, and the products are collected in a liquid nitrogen cold trap or condensed with Ar at 10–22 K to form a matrix. ,− Detailed descriptions of several types of apparatus have been published, including the coupling of a FVP apparatus with matrix isolation equipment. Many examples of FVP–matrix isolation with IR, UV–vis, or electron spin resonance (ESR) spectroscopic characterization of the products will be given in this review.…”

Section: The Fvp Methodsmentioning

confidence: 99%

“…Druliner reported the formation of 2-phenylbenzoxazole 473 in modest yield (11% in 6 h) following heating of 1-benzoylbenzotriazole 471 in diphenyl ether at 250 °C, presumably taking place via the carbene/diradical 472 (eq ). Wiersum confirmed this reaction under FVP conditions . No yield was given, but it was mentioned that polymeric material was also formed. …”

Section: Triazolesmentioning

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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“…Wiersum confirmed this reaction under FVP conditions. 21 No yield was given, but it was mentioned that polymeric material was also formed.…”

Section: 23-triazolesmentioning

confidence: 84%

Section: The Fvp Methodsmentioning

confidence: 99%

Section: Triazolesmentioning

confidence: 99%

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Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

2017

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“…The pyrolysis of 1-acylbenzotriazoles results in N 2 elimination and cyclization to benzoxazoles, − but the yields of these compounds are low. Online mass spectrometry and low-temperature IR spectroscopy revealed that the FVP of 1-acetylbenzotriazole 8 (Scheme ) caused ring contraction to N -acetylfulvenimine 11 in addition to the formation of 2-methylbenzoxazole 13 , thus explaining why the yield of the latter is low .…”

Section: Resultsmentioning

confidence: 99%

Iminocyclohexadienylidenes: Carbenes or Diradicals? The Hetero-Wolff Rearrangement of Benzotriazoles to Cyanocyclopentadienes and 1H-Benzo[b]azirines

et al. 2017

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The thermal rearrangements of benzotriazole 1 to fulvenimine 4 and 1H-benzazirine 7 are investigated at DFT and CASPT2 levels of theory. Ring opening of benzotriazole 1 to 2-diazo-cyclohexadienimine 2 followed by N elimination affords Z- and E-2-iminocyclohexadienylidenes 3, which have triplet ground states (A″). The open-shell singlet (OSS) (A″) and closed-shell singlet (CSS) (A') of 3 lie ∼15 and 40 kcal/mol higher in free energy, respectively. The OSS 3 (A″) is best described as a 1,3-diradical, whereas the CSS (A') has the character of a carbene. A hetero-Wolff rearrangement of OSS 3 yields fulvenimine 4, which is a precursor of cyanocyclopentadiene 5, with a calculated activation barrier of 38 kcal/mol at the CASPT2(8,8) level, whereby there is a surface crossing from the OSS to the CSS near the transition state. The barrier for cyclization to 1H-benzo[b]azirine 7 is only ∼13 kcal/mol. Therefore, reaction paths involving the singlet iminocyclohexadienylidene diradicals 3 will necessarily cause equilibration with 1H-benzazirine 7 prior to ring contraction to iminofulvene 4 and cyanocyclopentadiene 5, in agreement with experimental observations based on C labeling. The thermolysis of 1-acetylbenzotriazole 7 leads to the analogous N-acetyl-diazocyclohexadienimines 8, N-acetyliminocyclohexadienylidene diradicals 9, and N-acetylfulvenimine 10. The E-N-acetyliminocyclohexadienylidene E9 ring closes to the N-acetyl-1H-benzazirine 11 prior to ring contraction to N-acetylfulvenimine 10, and the Z-N-acetyl-2-diazocyclohexadienimine Z8 ring closes to 2-methylbenzoxazole 12. 1H-benzazirines are predicted to be spectroscopically observable species.

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