Iminocyclohexadienylidenes: Carbenes or Diradicals? The Hetero

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Iminocyclohexadienylidenes – Carbenes or Diradicals? The Hetero-Wolff Rearrangement of Benzotriazoles to Cyanocyclopentadienes and 1H-Benzo[b]azirines BEGUE Didier, Hugo Santos-Silva, Alain Dargelos, and Curt Wentrup J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05325 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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The Journal of Physical Chemistry

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

Didier Bégué,†* Hugo Santos-Silva,† Alain Dargelos,† and Curt Wentrup‡* †

CNRS/Université de Pau et des Pays de l’Adour, Institut des Sciences Analytiques et de

Physico-Chimie pour l’Environnement et les Matériaux – IPREM, UMR 5254, 64000, Pau, France ‡

School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane,

Queensland 4072, Australia

Abstract: The thermal rearrangements of benzotriazole 1 to fulvenimine 4 and 1H-benzazirine 6 are investigated at DFT and CASPT2 levels of theory. Ring opening of benzotriazole 1 to 2-diazocyclohexadienimine

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 ca. 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 6 is only ca. 13 kcal/mol. Therefore, reaction paths involving the singlet iminocyclohexadienylidene diradicals 3

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will necessarily cause equilibration with 1H-benzazirine 6 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-acetyldiazocyclohexadienimines

8,

N-acetyliminocyclohexadienylidene

diradicals

9,

and

N-

acetylfulvenimine 10. The E-N-acetyliminocyclohexadienylidene E9 ring closes to the N-acetyl1H-benzazirine 11 prior to ring contraction to N-acetylfulvenimine 10, and the Z-N-acetyl-2diazocyclohexadienimine Z8 ring closes to 2-methylbenzoxazole 12. 1H-benzazirines are predicted to be spectroscopically observable species.

1. INTRODUCTION Benzotriazoles undergo a variety of intriguing molecular rearrangements.1 For example, they are important starting materials for the Graebe-Ulmann synthesis of carbazoles (eq 1).2,3,4 Iminocyclohexadienylidene carbenes or diradicals are the assumed intermediates in these reactions.1

Benzotriazoles are also the starting materials for the synthesis of cyanocyclopentadienes, cyanopyrroles, and condensed analogs by flash vacuum pyrolysis (FVP) (eq 2).5,6


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The iminocyclohexadienylidenes 3 are the postulated intermediates formed by N2 elimination from the diazo valence isomer 2 of benzotriazole 1. The question whether they are better described as diradicals 3a or carbenes 3b (eq 3) will be examined in this paper.

A hetero-Wolff 7 rearrangement of 3 generates fulvenimine 4, which finally tautomerizes to cyanocyclopentadiene 5, which is the isolated product (eq 2). Compounds 2 and 4 have been observed by IR spectroscopy in matrix photolysis reactions,8,9 and the N-acyl derivatives of 4 have been observed as products of FVP of 1-acylbenzotriazoles.10 The ground state of 3 is the triplet diradical, which has been characterized by ESR spectroscopy at 77-200 K.11 The spectrum has the characteristics of a diradical rather than a carbene with a strong half-field signal near 1500 Gauss and a small value of the zero-field splitting parameter D/hc (3: |D/hc| = 0.17 cm-1; |E/hc| = 0.0025 cm-1). FVP of 7a-13C-labeled isatin 6 revealed a further complication, viz. the complete interconversion of the regioisomeric carbenes/diradicals 3a* and 3c* via 1H-benzazirine 7 prior to ring contraction to 4 and then 5 (eq 4).12 Therefore, the nature and involvement of benzazirine 7 (7-aza-bicyclo[4.1.0]hepta-1,3,5-triene) is also included in our computational study.


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It is pertinent to note the analogy with the Wolff rearrangement of 2oxocyclohexadienylidene to 6-fulvenone (eq 5), which can be achieved by photolysis of 2diazocyclohexadienone13,14,15,16 or by FVP of 2-carbonylcyclohexadienone.17

The reaction is usually formulated as a carbene reaction, but the closed-shell singlet carbene is a high energy species, and it is necessary to consider the open-shell diradical nature of the 2oxocyclohexadienylidene.17 The same is true for 4-oxocyclohexadienylidene.18 The ring contractions shown in eq 5 may also take place as concerted reactions without a discrete carbene/diradical intermediate.13-17

2. COMPUTATIONAL DETAILS Ground-state geometries and energies of closed-shell molecules were determined at the DFT level using the B3LYP exchange-correlation functional with the 6-311+G** basis set as implemented in the ORCA program package.19 In order to obtain reliable energies of the diradical and carbene species and the transition states connecting them to isomeric molecules, calculations were carried out at the CASPT2(8,8)/6-311+G** level using the MOLPRO package.20 Test calculations with other basis sets (def2-TZVPP, def2-TZVPPD, ma-def2-TZVPP and aug-cc-pVTZ) at the B3LYP level, and aug-cc-pVTZ at the CASPT2 level) showed very minimal energy variations of the order of 1 kcal/mol.


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3. RESULTS & DISCUSSION 3.1. Benzotriazole and Iminocyclohexadienylidene. In order to examine the open-shell nature of the singlet carbene/diradical, calculations of 3 and the transitions states connected to it were carried out at the CASPT2(8,8)/6-311+G** level of theory. Closed shell singlet molecules are adequately described by DFT methods, and they were calculated at the B3LYP/6-311+G** level. All energies are given as Gibbs free energies and include the eliminated molecule of N2 where relevant. The structures of diazo compounds 2 and the three spin states of 3 are shown in Figures 1 and 2. Of the two possible conformers of 2 (Figure 1 and Scheme 1) the E form E2 is 2 kcal/mol lower in energy than Z2 at the B3LYP level. The Z and E forms of the triplet ground state of 3 T (3A”) are only 0.2 kcal/mol apart at the CASPT2 level. The open shell singlets (OSS) 3 (1A”) lie 7-9 kcal/mol above the triplets and are best described as diradicals with the two unpaired electrons located at N and C1 (Z3 (1A”) has the lower energy). The closed shell singlets (CSS) 3 (1A’) are much higher in energy, lying 32-39 kcal/mol above the triplets and are best described as carbenes with two electrons in a σ-orbital on C1. The structural data (Figure 2c and f) show reduced angles ∠C1-C2-C3 of 112 and 113o for the closed shell singlet carbenes E3 (1A’) and Z10 (1A’), and increased lengths of the C-C bonds to the carbene center.


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Figure 1. The diazo compounds E2 and Z2, triplet 3 T (3A”), open-shell singlet 3 OSS (1A”) and closed-shell singlet 3 CSS (1A’) states of E- and Z-iminocyclohexadienylidenes 3.

The calculated spin densities in triplet 3 (3A”) indicate that the unpaired electron on N is in a p orbital conjugated with the ring, so that about 0.41 electron is delocalized over the ring like in a benzyl radical (see Supporting Information). Thereby it also adds π electron density at C1. Similarly, in the OSS carbene 3a (1A”), the p electron on N can delocalize over the whole system (see resonance structures in Figure 1). Consequently, the reduced electron density on N and increased electron density on C1 induce about 20% carbene character on C1 in the open-shell states of 3.


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Figure 2. Calculated structures at the CASPT2(8,8)/6-311+G** level. (a) E3 (3A”) (T); (b) E3 (1A”) (OSS); (c) E3 (1A’) (CSS); (d) Z10 (3A”) (T); (e) 10 (1A”) (OSS), γ is ∠2,7-8,10 = 53o; (f) Z10 (1A’) (CSS), γ is ∠2.7-8,10 = 83o (∠2,3-7,8 = 173o); (g) Z9, γ is ∠ 2,7-8,10 = 16o; (h) 7 (∠4-3-2-7 = -119o, ∠6,1-2,7 = 159o, ∠4,5-6,1 = -7o); (i) 12, γ is ∠1,2-7,8 = 105o (∠4,3-2,7 = -124o, ∠6,1-2,7 =164o, ∠4,5-6,1 = -5o, ∠2,7-8,10 = -109o, ∠2,7-8,9 = 74o, ∠7,8-9,10 = 177o).


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Scheme 1. Benzotriazole - Iminocyclohexadienylidene Rearrangements. Free energies ∆G at the CASPT2(8,8)/6-311+G** (B3LYP/6-311+G**) levels in kcal/mol.

There are potentially two paths for the singlet-state rearrangement of benzotriazole 1 to fulvenimine 4, one concerted and one via the open-shell diradical 3 (Scheme 1). The first step is the ring opening to the E and Z forms of 2-diazocyclohexadienimine 2 with an activation barrier of 28 kcal/mol at the DFT level. The elimination of N2 from diazo compounds have experimental activation energies of 38-40 kcal/mol.1 We calculate 35 and 37 kcal/mol for Z2 and E2, respectively, and 33 and 40 kcal/mol for the diazo compounds Z9 and E9 described in Section 2 at the CASPT2(8,8) level (Supporting Information pp 40 and 92). The resulting open-shell diradicals Z3 and E3 can undergo the hetero-Wolff rearrangement to 4 with a barrier of 38 kcal/mol at the CASPT2 level (transition state energies 53 and 52 kcal/mol, respectively, relative to the diazo compound Z2).


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These barriers are ca. 7 and 14 kcal/mol above the energies of the closed-shell singlets. This indicates that a surface crossing will take place, and that the major part of the activation energies for ring contraction in the OSS of 3 is actually due to excitation to the CSS states, which then undergo the ring contraction over a small barrier, and this is confirmed by inspection of the Cartesian displacements of the imaginary frequency. The excitation from OSS 3 to CSS 3 of symmetry Cs (1A') allows the direct connection to 4 of symmetry Cs (1A'). Alternatively, the diazo compounds E2 and Z2 may undergo concerted elimination of N2 with ring contraction to 4 with activation energies of 27 and 37 kcal/mol, respectively at the DFT level, but this reaction was only located at the DFT level. Evidence that the diradicals 3 also undergo intersystem crossing to the triplet ground states is given by the observation that aniline is always formed as a by-product.5,21 This is ascribed to hydrogen abstraction resulting from collisions of triplet 3 with other molecules. As mentioned in the Introduction, labeling experiments demonstrated an interconversion of the iminocyclohexadienylidenes 3 with 1H-benzazirine 7 under FVP conditions (eq 4). It is seen in Scheme 1 that the transition state for the interconversion between E3 and 7 lies at 28 kcal/mol relative to Z2 at the CASPT2 level, i.e. lower than the barrier for ring contraction to 4 by 25 kcal/mol. Therefore, on reaction paths where E3 is formed, interconversion with 7 will necessarily take place. The tautomerization of fulvenimine 4 to cyanocyclopentadiene 5 is not included in Scheme 1 because this is a bimolecular proton exchange process, taking place either by collision with the hot wall of the pyrolysis tube, or in the liquid phase during work-up.6


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3.2. 1-Acetylbenzotriazole. The pyrolysis of 1-acylbenzotriazoles results in N2 elimination and cyclization to benzoxazoles,22,23,24,25 but the yields of these compounds are low. Online mass spectrometry and low-temperature IR spectroscopy revealed that the FVP of 1acetylbenzotriazole 8 (Scheme 2) 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.10 In an effort to better understand these reactions, the calculated energies of relevant compounds and transition states were computed with the results shown in Scheme 2. Similarly to the case of the unsubstituted iminocyclohexadienylidenes 3, the lowest-energy singlet species are the E and Z forms of the open-shell diradicals 10 (1A”), and the closed shell singlets 10 (1A’) are considerably higher in energy (Figure 2 and Scheme 2).

Scheme 2. 1-Acetylbenzotriazole – Iminocyclohexadienylidene Rearrangements. Free energies ∆G at the CASPT2(8,8)/6-311+G** (B3LYP/6-311+G**) levels in kcal/mol.


10

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At the DFT level only, there is a direct path from the diazo compound E9 to the Nacetylfulvenimine 11 with an activation barrier of only 26 kcal/mol, similar to the corresponding path E2 4 described above, but this path could not be located at the CASPT2 level. There is also a concerted path from the diazo compound Z9 to 2-methylbenzoxazole 13, and this was located at both DFT and CASPT2 levels with activation barriers of 27 and 35 kcal/mol, respectively (Scheme 2). The transition state for this reaction (see Supporting Information p. 83) looks very much like the incipient carbene/diradical Z10 with the N2 molecule departing, but by the time the N2 molecule has departed completely, the new C-O bond in 13 has formed. The structure of Z9 is shown in Figure 2g, where it is seen that the N-acetyl moiety is twisted out of plane with a dihedral angle of 16o (structures of all the diazo compounds are shown in the


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Supporting Information). It is also seen in Figure 2f that the N-acetyl group in the closed-shell carbene Z10 is twisted out of plane by 83o, which can be understood in terms of avoidance of interaction with the in-plane carbenic pair of electrons at C1. The two reactions E9→11 and Z9→13 have similar activation energies in agreement with the experimental observation that both are occurring concurrently in FVP reactions. Surprisingly, a path from the carbene/diradical Z10 to 13 could not be located, but this could be explained by the above-mentioned avoidance of lone pair – lone pair interaction. The triplet and OSS of Z10 also show avoidance of overlap with dihedral angles of 56o and 53o, respectively (Figures 2d and 2e). When the carbene/diradical E10 is formed, it too can undergo ring contraction to fulvenimine 11, but with a significantly higher barrier of 37 kcal/mol (53-16 kcal/mol) at the CASPT2 level. The interconversion of E10 with the benzazirine 12 requires a lower barrier of only 24 kcal/mol (41 – 16.6 kcal/mol) at the CASPT2 level (Scheme 2). Thus, although 12 has not been observed experimentally, it can be expected to be formed reversibly in reactions involving iminocarbene/diradical E10.

3.3. Stabilities of 1H-Benzazirine, Benzoxirene, Benzothiirene and Benzoselenirene. As shown in Schemes 1 and 2, the benzazirines 7 and 12 are protected by significant energy barriers of ~9-24 kcal/mol. Therefore, they should be observable by matrix isolation or gas-phase spectroscopies when generated in FVP or pulsed pyrolysis experiments. If the instability of 1Hbenzazirines is due to antiaromaticity of the three-membered ring,26,27 the substantial stability of N-acetylbenzazirine 12 toward ring opening may be ascribed to the N-acetyl group removing electron density from the nitrogen, thereby making the ring less antiaromatic. The calculated bond lengths in 7 and 12 (Figure 2) are very similar to those in benzocyclopropene.28 It should be


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noted, however, that the benzazirines are not planar (see dihedral angles in Figure 2 and full details in the Supporting Information). In contrast to the benzazirines, there is no experimental evidence for formation of benzoxirene 18 by ring closure of 2-oxocyclohexadienylidene 16 (eq 6).

13

C-labeling clearly

demonstrated absence of 18 in the FVP of methyl salicylate 14, which generates the ketenes 15 and 17.17

Absence of benzoxirene in the thermolysis of sodium o-halogenophenoxides was reported earlier.29

13

C-labeling also demonstrated absence of involvement of either naphtho[a]oxirene or

naphtho[b]oxirene in the photolysis of naphthoquinonediazides,30,31 and naphtho[a]oxirene was excluded in the FVP of methyl hydroxynaphthalenecarboxylates.32 This can be ascribed to the fact that the energy barriers for Wolff rearrangement of the oxocarbenes are lower than the barriers for the potential interconversion with the benzoxirenes.17 However, due to the larger size and polarizability of the heteroatoms, benzothiirene 19 and benzoselenirene 20 are expected to be more stable, and the IR spectrum resulting from both matrix photolysis and FVP of benzothiadiazole have in fact been attributed to benzothiirene.33,34 IR and photoelectron spectroscopic evidence for the analogous formation of benzoselenirene on photolysis34 and FVP34,35 of benzoselenadiazole was also reported. Furthermore, chemical evidence for benzothiirenes in the thermolysis of sodium o-bromobenzenethiolates was adduced.29


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Benzocyclopropene 21 is of course a stable compound, which undergoes ring opening and Wolff-type ring contraction to fulvenallene on FVP at 800 oC.12,36

Figure 3. Benzothiirene, benzoselenirene and benzocyclopropene.

4. CONCLUSION The lowest singlet states of iminocyclohexadienylidenes 3 and 10 are open-shell diradicals (1A”) rather than closed-shell carbenes (1A’) at the CASPT2(8,8) computational level. In agreement with experimental observations, these diradicals undergo facile, reversible cyclisation to 1Hbenzazirines 7 and 12 prior to hetero-Wolff rearrangement to fulvenimines 4 and 11 and then cyanocyclopentadienes. In addition, Z-N-acetyl-2-diazocyclohexadienimine Z8 undergoes competitive ring closure to 2-methylbenzoxazole 13 in agreement with experimental observations. The 1H-benzazirines 7 and 12 are predicted to be protected by energy barriers of 9 and 24 kcal/mol, respectively, toward ring opening. Therefore, they are potential targets for direct observation by matrix isolation and/or online spectroscopic methods.

ASSOCIATED CONTENT Supporting Information Computational details, Cartesian coordinates, structural information, absolute energies, vibrational analysis, imaginary frequencies and spin populations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca. 05325g


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AUTHOR INFORMATION Corresponding Authors *D. Bégué. E-mail: [email protected] *C. Wentrup. E-mail: [email protected] ORCID Didier Bégué: 0000-0002-4553-0166 Curt Wentrup: 0000-0003-0874-7144 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Queensland Cyber Infrastructure Foundation at The University of Queensland and the Mésocentre de Calcul Intensif Aquitain of the Université de Bordeaux and the Université de Pau et des Pays de l’Adour.

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1-Acylbenzotriazole:

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dienylidenemethaneimines by Tandem Mass Spectrometry and Low-Temperature Infrared Spectrometry. Org. Mass Spectrom. 1990, 25, 197-203. (11)

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Iminocyclohexadienylidenes: Carbenes or Diradicals? The Hetero

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