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

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 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.

Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry

“…Thef irst excited singlet state is the open-shell 11 OSS,w hich lies about 8kcal mol À1 above the triplet, and the closed-shell singlet 11 CSS lies about 35 kcal mol À1 higher. [15] Thep reference for the OSS can be understood in terms of preservation of aromatic character (Scheme 3). Thec alculated CÀNb ond lengths of 1.31 indicate partial double-bond character in all three electronic states ( Figure 1; standard bond lengths:C = N1 .25 ;C À N 1.48 ).…”

“…[17] Flash vacuum pyrolysis (FVP) of 12 leads to the product 1-cyanocyclopentadiene (16), which is formed in quantitative yield by tautomerization of 15.I mportantly,h owever, calculations indicate that 11 will ring-close to benzazirine (14) faster than the ring contraction to 15,i na greement with labeling experiments on isatin. [15]…”

“…(12)],which was characterized by both IR and ESR spectroscopy at 5K (ESR in 2-methyltetrahydtrofuran (MTHF): j D/hc j= 1.42, j E/hc j= 0.038 cm À1 ). [57] Then itrene rearranged to the isocyanateC F 3 NCO 59S with ahalf-life of less than 10 min and alarge 14 N/ 15 Nkinetic isotope effect (1.18-1.33), even at 2.8 K. Thec alculated activation barrier is > 10 kcal mol À1 .T unneling was slower in the toluene and MTHF matrices,w here the ESR signal intensities increased steadily but decreased again upon warming the matrix, almost disappearing completely at 35 K. Approximately 90 %w ere recovered on re-cooling to 5K,thus indicating thermally induced intersystem crossing to the singlet 58S. Three things have to happen:I SC,C urtius rearrangement, and tunneling to facilitate the whole process.…”

“…(13) and (14)], [59][60][61] alkoxy-and aryloxycarbonylnitrenes [Eq. (15)], [62] and carbamoylnitrenes R 2 NÀCOÀN [Eq. (16)] [63] have been prepared by either FVP or matrix photolysis of azide or isocyanate precursors and characterized by IR and/or ESR spectroscopy.…”

“…The carbene/diradical 11 is an important intermediate in the pyrolysis and photolysis of benzotriazoles . The ground state is the triplet 11 T. The first excited singlet state is the open‐shell 11 OSS, which lies about 8 kcal mol −1 above the triplet, and the closed‐shell singlet 11 CSS lies about 35 kcal mol −1 higher . The preference for the OSS can be understood in terms of preservation of aromatic character (Scheme ).…”