Rearrangements of Nitrile Imines: Ring Expansion of Benzonitrile

Article Cite This: J. Org. Chem. 2019, 84, 8668−8673

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Rearrangements of Nitrile Imines: Ring Expansion of Benzonitrile Imines to Cycloheptatetraenes and Ring Closure to 3‑Phenyl‑3H‑diazirines Didier Bégué,*,† Alain Dargelos,† and Curt Wentrup*,‡ †

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CNRS/Université de Pau et des Pays de l’Adour/E2S UPPA, Institut des Sciences Analytiques et de Physicochimie pour l’Environnement et les Matériaux, UMR5254, Pau 64000, France ‡ School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: Nitrile imines are important intermediates in 1,3-dipolar cycloaddition reactions, and they are also known to undergo efficient, unimolecular rearrangements to carbodiimides via 1H-diazirines and imidoylnitrenes under both thermal and photochemical reaction conditions. We now report a competing rearrangement, revealed by CASPT2(14,12) and B3LYP calculations, in which C-phenylnitrile imines 8 undergo ring expansion to 1-diazenyl-1,2,4,6-cycloheptatetraenes 12 akin to the phenylcarbenecycloheptatetraene rearrangement. Amino-, hydroxy-, and thiolgroups in the meta positions of C-phenylnitrile imine lower the activation energies for this rearrangement so that it becomes potentially competitive with the cyclization to 1H-diazirines and hence rearrange to carbodiimides. The diazenylcycloheptatetraenes 12 thus formed can evolve further to cycloheptatetraene 30 and 2-diazenyl-phenylcarbene 16 over modest activation barriers, and the latter carbenes cyclize very easily to 2H- and 3H-indazoles, from which 6-methylenecyclohexadienylidene, phenylcarbene, fulvenallene, and their isomers are potentially obtainable. Moreover, another new rearrangement of benzonitrile imine forms 3-phenyl-3H-diazirine, which is a precursor of phenyldiazomethane and hence phenylcarbene. This reaction is competitive with the ring expansion. The new rearrangements predicted here should be experimentally observable, for example, under matrix photolysis or flash vacuum pyrolysis conditions.

1. INTRODUCTION Nitrile imines are widely used reactive intermediates, especially in 1,3-dipolar cycloaddition reactions in organic and bioorganic chemistry.1−4 Their electronic structures and reactivities have attracted intense interest.5−10 Six different structures are commonly considered: propargylic, allenic, carbenic, 1,3-dipolar and reverse dipolar forms, and 1,3diradical (Chart 1). Each individual structure may be described

density functional theory (DFT)-calculated spectra and structures. Nitrile imines 2 are formed on either flash vacuum pyrolysis (FVP) or matrix photolysis of 5-substituted tetrazoles 1 (Scheme 1).16 Although they are used extensively in 1,3Scheme 1. Thermal and Photochemical Formation and Rearrangement of Nitrile Imines 2

Chart 1. Six Fundamental Structures of Nitrile Imines

as a resonance hybrid, but for individual molecules, one particular mesomer may dominate, thereby making it the minimum energy structure. However, the energy differences as well as the activation barriers separating the different structures are generally small. Experimentally, nitrile imines can be divided into predominantly propargylic, allenic,11,12 and carbenic forms.13−15 They can be distinguished by the infrared spectra, which are generally in good agreement with the © 2019 American Chemical Society

Received: May 1, 2019 Published: June 7, 2019 8668

DOI: 10.1021/acs.joc.9b01183 J. Org. Chem. 2019, 84, 8668−8673

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The Journal of Organic Chemistry

Scheme 2. Rearrangements of Allenic Benzonitrile Imine 8aA to Diazenylcycloheptatetraene 12a, Diazenylphenylcarbene 16a, 3-Phenyl-1H-diazirine 18a, 3-Phenyl-3H-diazirine 22a, Phenylcarbodiimide 20a, and Phenyldiazomethane 24a

a

Energies in kcal/mol at the B3LYP/6-311g(d,p) level and in parentheses at the CASPT2(14,12)sp/6-311G(d,p) level. calculations were performed using the Gaussian 09 and Molpro program packages.20,21 Calculated energies are at 0 K.

dipolar cycloadditions, under unimolecular reaction conditions, they rearrange to 1H-diazirines 3, imidoylnitrenes 4, and NH-carbodiimides 6.17,18 Both the allenic and the propargylic forms of benzonitrile imine, PhCNNH (2, R1 = Ph, R2 = H) were observed to cyclize to the 1H-diazirine 3, which then isomerized nearly quantitatively to carbodiimide 6 on photolysis in an Ar matrix.12 Moreover, the NH-carbodiimides can equilibrate with cyanamides 7 both thermally and photochemically.11,12 Imidoylnitrenes 4 can be generated directly from 2-substituted tetrazoles 5.17,18 The rearrangements 2 → 3 have computed activation energies in the range 37−60 kcal/mol, but the subsequent rearrangements 3 → 4 and 4 → 6 have low activation energies (2−20 kcal/mol), which are easily overcome under the usual reaction conditions.17 A competing fragmentation of (formally propargylic) nitrile imines to nitriles and nitrenes can occur on photolysis.11,19 The purpose of the present study is to investigate whether additional, not previously reported rearrangements of nitrile imines are possible.

3. RESULTS AND DISCUSSION In this paper, we will describe and compare three intramolecular rearrangement routes of aromatic nitrile imines, as shown in Scheme 2. Section 1 describes route (i), which is the new ring expansion of benzonitrile imine 8a to 1diazenylcycloheptatetraene 12a and subsequent ring contraction to 2-diazenylphenylcarbene 16a. Route (ii) is the previously investigated ring closure to 3-phenyl-1H-diazirine 18a and further to phenylcarbodiimide 20a.12,17 Section 2 describes route (iii), which is the new ring closure to 3-phenyl3H-diazirine 22a, resulting in the easy formation of phenyldiazomethane 24 and hence phenylcarbene 26. Section 3 describes substituent effects on the rearrangement barriers. Section 4 describes further reactions arising from the ring expansion reaction. Energies of ground and transition states were calculated at the B3LYP and CASPT2 levels of theory (Scheme 2). Compounds are designated 8a, 10a, and so on for the parent compounds, and the transition states are TS 9a and so on. A and P designate allenic and propargylic structures of nitrile imines, respectively. Many of these substituted compounds 8b, 8c, and so forth can exist in different conformers. In the main manuscript, we consider only the lower energy conformers, and all others can be found in Tables S1 and S2 (Supporting Information) using descriptors such as 12aa, 12ab, and so on. As the allenic and propargylic forms of the nitrile imines 8 are very close energetically12 (see the Supporting Information), it makes no difference whether the reactions are initiated from the propargylic or the allenic form. They are formulated in Scheme 2 using the allenic form, 8aA, in which formal positive

2. COMPUTATIONAL METHODS Ground-state geometries and energies were determined at the DFT level using the B3LYP exchange−correlation functional with the 6311G(d,p) basis set. This or similar procedures have been used successfully in many previous studies of related molecules.3,6,11−15 In order to take any potential multiconfigurational effects into account, calculations were also carried out at CASSCF and CASPT2(14,12)sp/6-311G(d,p) levels, which allow direct comparison with previous calculations on the benzonitrile imines.12 Transition-state optimizations and intrinsic reaction coordinate (IRC) calculations were carried out at the DFT level. Natural bond orbital (NBO) analyses were performed at the CASSCF(14,12)sp/6-311G(d,p) level of theory. All 8669


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Figure 1. Structures of ground and transition states in the transformation of 8aA to 12a and 16a at the CASPT2(14,12) level (bond lengths in Å and bond angles in degree).

and negative charges are indicated in line with conventional custom.1,2 However, it may be more correct to consider the negative charge as delocalized over the whole system (see the molecular orbitals in Figure S2, Supporting Information). The ring closure of 8aA to 3H-3-phenyldiazirine 22a is described in Section 3 and further rearrangements resulting from the ring expansion in Section 4. 3.1. Ring Expansion of Benzonitrile Imine to 1Diazenylcyclohepta-1,2,4,6-tetraene. The ring expansion of 8aA to 12a requires an activation energy of 59.5 kcal/mol at the CASPT2 level (Scheme 2) and proceeds via bicyclo[4.1.0]hepta-2,4,7-triene 10a. Once the cycloheptatetraene 12a has been reached, it may undergo the usual phenylcarbenecycloheptatetraene rearrangement16,22,23 to the diazenylphenylcarbene 16a, but 12a is of significantly lower energy. Structures of ground and transition states during the reaction 8aA → 12a → 16a at the CASPT2 level are shown in Figure 1 (the corresponding data at the DFT level are presented in Figure S1, Supporting Information). Stages of the ring expansion reaction of 8A to 12a along the internal reaction coordinate at the B3LYP level are illustrated in Figure 2 (structures of further intermediate stages are shown in Figure S3, Supporting Information). An NBO analysis of the electron distributions during these transformations at the CASPT2(14,12)sp/6-311G(d,p) level is presented in Tables S2 and S3. The five highest occupied molecular orbitals for 8aA, the two highest occupied MOs for TS9a−16a, and the two lowest unoccupied MOs 8aA-16a are depicted in Figure S2, where it is seen that the allenic nitrile imine carbon C1 has a small

Figure 2. Stages of the ring expansion of 8aA to 12a calculated at the B3LYP/6-311G(d,p) level (see further intermediate structures in Figure S3).

degree of carbene character in the HOMO − 1 (a carbenic σorbital) and a high degree of carbene character in the LUMO. The ring expansion described in Figures 1 and 2 is formulated in terms of the allenic nitrile imine 8aA. However, as the allenic and propargylic forms are virtually isoergic,12 the reaction can be initiated from either. Already at the stage V1 (Figure 2), the CNNH moiety is bent and allenic with CN and NN double bonds, regardless of whether the allenic or the propargylic nitrile imine is used as the starting geometry. The NNH moiety becomes more twisted at V2 and V3 in 8670


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tautomerizations may occur by wall catalysis in FVP reactions,16 and for reactions in solution, they can occur by intermolecular H-transfer. In the absence of such bimolecular reactions and wall effects, route (iii), Scheme 2 now emerges as the most likely route to aryldiazomethanes from aryltetrazoles. 3.3. Substituent Effects. In order to examine possible substituent effects on the reaction pathways, the activation energies for rearrangement of para- and meta-substituted derivatives of 8 were investigated at the DFT level (Figure 3).

preparation for cyclization. At V4, the C5−C1−N1 moiety is strongly bent like in a carbene, the single bond C1−C6 is halfway formed, and the C1−C5 bond is beginning to become a double bond (see NBO data, Table S5 and MOs in Table S4). The bicyclo[4.1.0]hepta-2,4,7-triene 10a exists in a very shallow minimum and undergoes exothermic opening to the diazenylcycloheptatetraene 12a, which is the global minimum among the structures 10a−16a. 3.2. Ring Closure of Benzonitrile Imine to 3-Phenyl3H-diazirine. The new and competing ring closure of benzonitrile imine 8a to 3H-3-phenyldiazirine 22a is described in route (iii), Scheme 2. It takes place through a single transition state, TS21a, which conceptually can be viewed as the nitrile imine carbon acting as a phenylcarbene adding to the N−H bond (see the BNO data and the transition-state structure in the Supporting Information, Tables S3 and S7). The orientation of the hydrogen atom in the CNNH moiety in the transition state determines the reaction outcome: if this hydrogen atom is trans with respect to the C atom, the 1Hdiazirine 18a is formed; if it is cis, the 3H-diazirine 22a is formed. At the CASPT2 level, the formation of the 3Hdiazirine has an activation energy of 55 kcal/mol, that is, it is lower than that of the ring expansion (route (i)) by ∼4 kcal/ mol, and higher than that of the 1H-diazirine (route (ii)) by ∼8 kcal/mol. This means that all three reactions are potentially possible under photochemical conditions and under hightemperature FVP conditions. Once 22a is formed, phenyldizomethane 24 is readily formed via a modest activation barrier (TS23). Hence, this route will give access to phenylcarbene 26 and all rearrangement products derived from it. Indeed, it was shown that FVP of both 5-phenyltetrazole and 5-phenyl-1,3,4-oxadiazol-2(3H)one produce phenyldiazomethane 24 and fulvenallenea rearrangement product of phenylcarbeneas well as benzonitrile imine (and its rearrangement product phenylcarbodiimide 20a). The phenyldiazomethane was the first product to be observed at an FVP temperature of 550 °C.11 Aryldiazo compounds have, in many cases, been observed IR-spectroscopically or isolated directly from FVP of 5aryltetrazoles,16 and it has usually been assumed that this is due to a tautomerization to the 5H-tetrazole isomer such as 27c (eq 1)

Figure 3. Substituent effects on the ring expansion and ring closure barriers in 8x. Energy values for TS9x, TS11x, TS17x, and TS21x in kcal/mol at the B3LYP/6-311G(d,p) level relative to the ground states of 8x.

All compounds 8 can exist in almost isoergic allenic and propargylic forms, of which only the allenic forms are shown in Figure 3. The structures and energies are listed in Figure 3 and Table S10 (Supporting Information). The cyclopropane intermediates 10 always exist in relatively shallow energy minima protected by barriers of ca. 2−10 kcal/mol. An amino group in the para position has no significant effect on the ring expansion reaction (route (i)), but the meta-amino group in 8c lowers the barriers (TS9c and TS11c) significantly, by ca. 5 kcal/mol each. Nitro and fluoro groups have hardly any effect, but OH and SH groups lower the barrier by 2−3 kcal/mol (8g and 8i). Amino groups in the two meta (3,5) positions (8j) lower the barrier by 6−8 kcal/mol and two m-dimethylamino groups (8m) by 7−11 kcal/mol. Thus, for the 3,5-bis-dimethylamino derivative 8m, the ring expansion and the cyclization (routes (i) and (ii)) become energetically competitive, and it can be expected that ring expansions to diazenylcycloheptatetraenes 12 should be observable under conditions of high temperature FVP or matrix photolysis.

The calculated energies of the tautomers and the activation energies between them at the DFT level are indicted in eq 1, where it is seen that the highest activation barrier is 65 kcal/ mol. This is about 6 kcal/mol higher than route (iii), Scheme 2. Furthermore, the reaction in eq 1 may also be slower because it involves two sequential, reversible, endothermic processes. One reaction that can be excluded is the unimolecular 1,3-H shift from benzonitrile imine 8a to phenyldiazomethane 24, which, like many other direct 1,3-H shifts,24 has a high calculated barrier of ∼91 kcal/mol (Scheme 2). Nonetheless, it must be kept in mind that such 8671


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The Journal of Organic Chemistry Scheme 3. Further Reactions Arising from the Nitrile ImineCycloheptatetraene Rearrangementa

a

Energies of ground and transition states in kcal/mol relative to 8aA at the B3LYP/6-311g(d,p) level.

hexadiene 34 with a barrier of ca. 40 kcal/mol. Loss of N2 from 34 readily yields 2-methylenecyclohexadienylidene 35. The known rearrangements on the C7H6 energy surface25 now yield benzocyclopropene 36 and fulvenallene 39 over modest energy barriers. The EZ conformer of the diazenylcycloheptatetraene 12a can also undergo a H-shift with concomitant N2 loss to form the cycloheptatetraene 30 over a barrier of only 12 kcal/mol. This therefore provides a second entry to the C7H6 energy surface of phenylcarbene 32, bicyclo[4.1.0]hepta-2,4,7-triene 31, bicyclo[3.2.0]heptatriene 37, spiro[4.2]cycloheptatriene 38, and fulvenallene 39, which has been described in detail elsewhere25 (Scheme 3). Fulvenallene is in fact a product of FVP of 5-phenyltetrazole 27a at 900 °C.11 As discussed above, route (iii) in Scheme 2 is the preferred route to phenyldiazomethane, and this together with the reaction 12a → 30 → 32 in Scheme 3 are the potential routes to phenylcarbene and hence fulvenallene.

The observed substituent effect suggests that the nitrile imine carbon (C1) is electron-deficient. This would be in agreement with the partial carbenic nature of this carbon in the ring expansion process, but further, higher level calculations and experiments are expected to shed more light on this. The barriers for the formation of the diazirines 18 (TS17) and 22 (TS21) (routes (ii) and (iii)) are reduced by para-amino groups, most notably for the p-dimethylamino derivative 8n, where both barriers are lowered by 3 kcal/mol. It could have been thought that an ortho-substituent like in 8o might hinder the cyclization to the cyclopropane derivative 10o, but there is hardly any effect on these barriers. The barriers for the formation of diazirines are reduced, however, with route (ii) being favored. The energy data relating to Figure 3 as well as those for 10a−o at the B3LYP level are available in Table S10 (Supporting Information). In order to evaluate the potential importance of long-range effects, calculations were also carried out at the CAM-B3LYP level for 8j and 8m, which were stabilized by a further ∼1 kcal/mol (Table S10). In order to aid the experimental identification, calculated UV−vis and IR spectral data for the ring expansion products 12a, 12g, 12i, 12j, and 12m at the B3LYP/6-311G(d.p) level are listed in Tables S6 and S11 (Supporting Information). 3.4. Further Rearrangements Arising from the Ring Expansion. The cycloheptatetraene and phenylcarbene derivatives, 12a and 16a, obtained as described in Scheme 2, can undergo further rearrangements and N2 eliminations as summarized in Scheme 3. Additional, higher-energy transformations are detailed in Scheme S3 (Supporting Information). The phenylcarbene 16a can cyclize to 2H- and 3Hindazoles 28 and 33 with very small activation energies, and the 3H-indazole is a precursor of 2-diazo-1-methylenecyclo-

4. CONCLUSIONS The ring expansion of benzonitrile imine 8a to 1-diazenyl1,2,4,6-cycloheptatetraene 12a is predicted to be competitive with the well-established ring closure to 3-phenyl-3H-diazirine 18a, which rearranges further to phenylcarbodiimide 20a. Substitution with electron-donating substituents in the meta positions lower the activation energies appreciably to 49−53 kcal/mol for the 3,5-bis(dimethylamino) derivative 8m. Such ring expansions are expected to be observable under appropriate experimental conditions such as matrix photolysis or FVP. A previously unrecognized ring closure of 8a to 3phenyl-3H-diazirine 22a is also predicted. The choice between forming 18a and 22a depends on the conformation of the CNNH moiety in 8a, the anti-isomer forming 18a, and the syn8672


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(11) Bégué, D.; Qiao, G. G.; Wentrup, C. Nitrile Imines: Matrix Isolation, IR Spectra, Structures and Rearrangement to Carbodiimides. J. Am. Chem. Soc. 2012, 134, 5339−5350. (12) Nunes, C. M.; Reva, I.; Fausto, R.; Bégué, D.; Wentrup, C. Bond-shift isomers: the co-existence of allenic and propargylic phenylnitrile imines. Chem. Commun. 2015, 51, 14712−14715. (13) Bégué, D.; Wentrup, C. Carbenic Nitrile Imines: Properties and Reactivity. J. Org. Chem. 2014, 79, 1418−1426. (14) Baskir, E. G.; Platonov, D. N.; Tomilov, Y. V.; Nefedov, O. M. Infrared-Spectroscopic Study of Amino-Substituted Nitrilimines and Their Photochemical Transformations in an Argon Matrix. Mendeleev Commun. 2014, 24, 197−200. (15) Nunes, C. M.; Reva, I.; Rosado, M. T. S.; Fausto, R. The Quest for Carbenic Nitrile Imines: Experimental and Computational Characterization ofC-Amino Nitrile Imine. Eur. J. Org. Chem. 2015, 2015, 7484−7493. (16) Wentrup, C. Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles. Chem. Rev. 2017, 117, 4562−4623. (17) Bégué, D.; Santos-Silva, H.; Dargelos, A.; Wentrup, C. Imidoylnitrenes R′C(=NR)-N, Nitrile Imines, 1H-Diazirines, and Carbodiimides: Interconversions and Rearrangements, Structures, and Energies at DFT and CASPT2 Levels of Theory. J. Phys. Chem. A 2017, 121, 8227−8235. (18) Abe, M.; Bégué, D.; Santos-Silva, H.; Dargelos, A.; Wentrup, C. Direct Observation of an Imidoylnitrene: Photochemical Formation of PhC(=NMe)−N and Me−N from 1-Methyl-5-phenyltetrazole. Angew. Chem., Int. Ed. 2018, 57, 3212−3216. (19) Toubro, N. H.; Holm, A. Nitrilimines. J. Am. Chem. Soc. 1980, 102, 2093−2094. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.2; Gaussian, Inc.: Wallingford, CT, 2009. (21) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; Celani, P.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G.; et al. MOLPRO, Version 2012.1, A Package of Ab Initio Programs, 2012; http://www.molpro.net, (retrieved February 18, 2019). (22) (a) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J. The C7H6 Potential Energy Surface Revisited: Relative Energies and IR Assignment. J. Am. Chem. Soc. 1996, 118, 1535−1542. (b) Schreiner, P. R.; Karney, W. L.; von Ragué Schleyer, P.; Borden, W. T.; Hamilton, T. P.; Schaefer, H. F., III. Carbene Rearrangements Unsurpassed: Details of the C7H6Potential Energy Surface Revealed. J. Org. Chem. 1996, 61, 7030−7039. (c) Wong, M. W.; Wentrup, C. Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6Energy Surface. J. Org. Chem. 1996, 61, 7022−7029. (23) Reactive Intermediates Chemistry; Moss, R. A.; Platz, M. S.; Jones, M., Jr., Eds.; Wiley: Hoboken, NJ, 2004. (24) Wentrup, C.; Bégué, D.; Leung-Toung, R. Ethynamine− Ketenimine−Acetonitrile Rearrangements: A Computational Study of Flash Vacuum Pyrolysis Processes. ChemRxiv. https://doi.org/ 10.26434/chemrxiv. 5373964.v1, 2017, (retrieved February 1, 2019). (25) Kvaskoff, D.; Lüerssen, H.; Bednarek, P.; Wentrup, C. Phenylnitrene, Phenylcarbene, and Pyridylcarbenes. Rearrangements to Cyanocyclopentadiene and Fulvenallene. J. Am. Chem. Soc. 2014, 136, 15203−15214.

isomer forming 22a. The cyclization to 22a constitutes a favorable route to phenyldiazomethane and hence phenylcarbene and fulvenallene from benzonitrile imine 8a. Another route to phenylcarbene is via elimination of N2 from the EZ form of 12a to form cycloheptatetraene 30.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.9b01183. Computational methods, Cartesian coordinates, absolute energies, vibrational analysis, and imaginary frequencies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.B.). *E-mail: [email protected] (C.W.). ORCID

Didier Bégué: 0000-0002-4553-0166 Curt Wentrup: 0000-0003-0874-7144 Notes

The authors declare no competing financial interest.

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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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REFERENCES

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Rearrangements of Nitrile Imines: Ring Expansion of Benzonitrile

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