Iminocyclohexadienylidenes: carbenes or diradicals? the hetero-Wolff rearrangement of benzotriazoles to cyanocyclopentadienes and 1H-benzo[b]azirines

Begue, Didier, Santos-Silva, Hugo, Dargelos, Alain and Wentrup, Curt (2017) Iminocyclohexadienylidenes: carbenes or diradicals? the hetero-Wolff rearrangement of benzotriazoles to cyanocyclopentadienes and 1H-benzo[b]azirines. Journal of Physical Chemistry A, 121 32: 5998-6003. doi:10.1021/acs.jpca.7b05325


Author Begue, Didier
Santos-Silva, Hugo
Dargelos, Alain
Wentrup, Curt
Title Iminocyclohexadienylidenes: carbenes or diradicals? the hetero-Wolff rearrangement of benzotriazoles to cyanocyclopentadienes and 1H-benzo[b]azirines
Formatted title
Iminocyclohexadienylidenes: carbenes or diradicals? the hetero-Wolff rearrangement of benzotriazoles to cyanocyclopentadienes and 1H-benzo[b]azirines
Journal name Journal of Physical Chemistry A   Check publisher's open access policy
ISSN 1520-5215
1089-5639
Publication date 2017-08-17
Sub-type Article (original research)
DOI 10.1021/acs.jpca.7b05325
Open Access Status Not yet assessed
Volume 121
Issue 32
Start page 5998
End page 6003
Total pages 6
Place of publication Washington, DC, United States
Publisher American Chemical Society
Language eng
Abstract 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 N2 elimination affords Z- and E-2-iminocyclohexadienylidenes 3, which have triplet ground states (3A″). The open-shell singlet (OSS) (1A″) and closed-shell singlet (CSS) (1A') of 3 lie ∼15 and 40 kcal/mol higher in free energy, respectively. The OSS 3 (1A″) is best described as a 1,3-diradical, whereas the CSS (1A') 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 13C 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.
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: HERDC Pre-Audit
School of Chemistry and Molecular Biosciences
 
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