Imidoylnitrenes Râ²C( NR)âN, Nitrile Imines, 1H-Diazirines, and

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX-XXX

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Imidoylnitrenes R′C(NR)−N, Nitrile Imines, 1H‑Diazirines, and Carbodiimides: Interconversions and Rearrangements, Structures, and Energies at DFT and CASPT2 Levels of Theory Didier Bégué,*,† Hugo Santos-Silva,† Alain Dargelos,† and Curt Wentrup*,‡ †

Institut Pluridisciplinaire de Recherche sur l’Environnement et les Matériaux, Université de Pau et des Pays de l’Adour, 64000 Pau, France ‡ School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: The structures, energies, and rearrangements of imidoylnitrenes H−C(NH)−N, H2N−C(NH)−N, Ph−C(NH)−N, H−C(NPh)−N, and MeO−C(NCN)−N (10a−e) are investigated at DFT and CASPT2 levels of theory. Imidoylnitrenes are potentially formed by pyrolysis or photolysis of azides, tetrazoles (6, 6′), or sydnones. Unlike most acylnitrenes, the imidoylnitrenes 10 have triplet ground states. The first excited states are the open-shell singlets (OSSs), lying between ca. 4 and 20 kcal mol−1 above the triplets at the CASPT2 level. The second excited states are the closed-shell singlets (CSSs), lying >50 kcal mol−1 higher in energy. The OSS imidoylnitrenes can ring-close to 1H-diazirines 9 with very low activation energies (2−12 kcal mol−1), and the 1H-diazirines can then rearrange to nitrile imines 8 with activation energies of 37−48 kcal mol−1. Conversely, nitrile imines generated directly by pyrolysis or photolysis of 2,5-substituted tetrazoles 6 can rearrange to 1H-diazirines 9 and imidoylnitrenes 10 with activation energies of 37−60 kcal mol−1. Finally, the imidoylnitrenes 10 can rearrange to carbodiimides 11 with modest activation barriers of 12−20 kcal mol−1. Calculated vibrational data, UV−vis spectra, and spin densities in the triplet states are also reported, and zero-field splitting parameters |D/hc| in the range 0.9−1 cm−1 and nonzero |E/hc| values are predicted.

1. INTRODUCTION The chemistry of acylnitrenes R−CO−N 1 has been studied extensively, especially in relation to the synthetically important thermal and photochemical Curtius rearrangement of acyl azides to isocyanates, R−NCO 2.1,2 The thermal Curtius rearrangement is usually concerted, but it may be either concerted or stepwise via acylnitrenes in photolyses.3,4 Interestingly, many aroylnitrenes have closed-shell singlet (CSS) ground states due to partial bond formation between the acyl oxygen and the nitrene nitrogen, with the result that the singlets can be described as resonance hybrids of nitrenes 1 1a and oxazirines 11b (Scheme 1), but the corresponding triplet nitrenes31 have true nitrene structures. This is the case for cyanoformylnitrene, NC−C(O)N, and the aroylnitrenes.5,3,6−9 If the singlet−triplet splitting becomes small enough, then an

equilibrium between the triplet nitrene and the singlet oxazirine structure may be expected. Calculations indicate that formylnitrene, HC(O)N, has a very small singlet−triplet (S−T) splitting, and the ground state may therefore be either singlet or triplet,7,10−12 but the molecule has not yet been observed spectroscopically. Electron-withdrawing substituents can increase the stability of the triplet nitrenes and thereby cause triplet ground states in nitrenes such as FC(O)N,13−15 alkoxycarbonylnitrenes,16 and carbamoylnitrenes.17 In the case of cyanoformylnitrene, NC−C(O)N, the calculated S−T splitting was 19 kcal mol−1 at the MP2/6-31G* level,5 but a subsequent G3X(MP2) calculation reduced this to −0.7 kcal mol−1 with a triplet ground state. Thus this may be an example of a bistable system with an equilibrium between the singlet oxazirine and the triplet nitrene, and both may be observable. In contrast with the acylnitrenes, vinylnitrenes 3 have triplet ground states, examples of which have recently have been observed directly by ESR and UV−vis spectrocopy.18,19 The singlet vinylnitrenes undergo cyclization to 2H-azirines 520,21 and rearrangement to ketenimines 4 (Scheme 1).22 There is much less direct knowledge about the chemistry of imidoylnitrenes 10, but these species are postulated as the

Scheme 1. Acyl- and Vinylnitrenes

Received: August 23, 2017 Revised: October 2, 2017 Published: October 6, 2017 © XXXX American Chemical Society

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DOI: 10.1021/acs.jpca.7b08445 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A crucial but so far unobserved links between nitrile imines 8 and carbodiimides 11 (Scheme 2). Nitrile imines 8 are generated by either flash vacuum pyrolysis (FVP) or photolysis of tetrazoles 623 or photolysis of sydnones 7.24 The synthesis, chemistry, and rearrangements of nitrile imines have been reviewed.23,25 Whereas some nitrile imines are isolable at room temperature,25

they are more frequently observed under matrix isolation conditions. They are known to rearrange very efficiently to carbodiimides 11 on both FVP and photolysis,4,23,24,26−28 and this is postulated to proceed via 1H-diazirines 9 and imidoylnitrenes 10. The latter may also be generated from 1-substituted tetrazoles 6′. A 1,2-shift29 in the imidoylnitrenes then affords the carbodiimides 11, which are very commonly observed as end products. Nitrile imines are the highest and carbodiimides (and the tautomeric cyanamides RNHCN) are the lowest energy isomers on this energy surface,23,30,31 so this rearrangement is extremely exothermic, by some 50 kcal mol−1.20 It is not surprising, therefore, that the high-lying imidoylnitrenes and 1H-diazirines have been difficult to observe. However, very recently, 1H-diazirines have been characterized by matrix isolation spectroscopy.26,27,31,32 There is very little direct spectroscopic evidence for the existence of imidoylnitrenes, but it is noted that weak, broad absorptions observed around 520 nm on laser flash photolysis of 5-phenyltetrazole and 1-methyl-5-phenyltetrazole may be due to the benzimidoylnitrenes.33 Despite the shortage of direct evidence, imidoylnitrenes are very frequently postulated as reactive intermediates in pyrolyses and photolyses of 1-aryltetrazoles 6′, where they typically cyclize onto the benzene rings to afford benzimidazoles and related compounds.34−38 Ring cleavage reactions with formation of carbodiimides or cyanamides have also been thoroughly investigated.39−41

Scheme 2. Nitrile Imines, 1H-Diazirines, Imidoylnitrenes, and Carbodiimides from Tetrazoles and Sydnones

Figure 1. Reaction path from nitrile imine 8a to carbodiimide 11a, passing through 1H-diazirine 9a and imidoylnitrene 10a. The red energy levels for 10 are for the triplets 10aZ(T) and 10aE(T). In all Figures, plain numbers are at the DFT level, and those in parentheses are at the CASPT2 level; * indicates open-shell calculations. All energies are in kcal mol−1. The open-shell CASPT2 energies are all relative to similar calculations on 8a.

Table 1. Relative Energies (kcal mol−1) of Triplet (T) and Open-Shell Singlet (OSS) States of the Z and E Conformers of Imidoylnitrenes 10a−ea ab

nitrene 10 Z E Z E

(OSS) (OSS) (T) (T)

10.9 7.7 3.0 0.0

(21.4) (18.3) (2.4) (0.0)

b 14.6 8.1 5.8 0.0

(38.7) (32.2) (4.6) (0.0)

c

d

13.7 8.0 5.5 0.0

5.9 3.9 1.3 0.0

e 5.7; 5.4; 0.3; 0.0;

13.3 (11.1; 22.2) 7.9 (10.9; 13.5) 0.7 (0.7; 1.0) 4.7 (0.0; 5.1)

a

T = triplet; OSS = open shell singlet; CSS = closed-shell singlet. Calculations at B3LYP/6-311G(d,p) (DFT) and CASPT2(9,8)/6-311+G(d,p) levels (values in parentheses). bE (CSS) 50.5 (51.2) kcal mol−1; Z (CSS) 54.5 (57.7) kcal mol−1. B


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The Journal of Physical Chemistry A To better understand the electronic nature of imidoylnitrenes, the energy barriers separating them from other isomers, and some of their spectroscopic properties, we report in this paper a computational investigation of several of these species as well as the isomeric nitrile imines, diazirines, and carbodiimides and the transition states separating them (compounds 8−11a-e, Scheme 2).

2. COMPUTATIONAL METHODS Ground-state geometries and energies were determined at the DFT level using the B3LYP exchange-correlation functional with the 6-311G(d,p) basis set. The UB3LYP approach was used for systems capable of existing in open-shell states. To obtain reliable energies of the open-shell singlet (OSS) nitrenes and the transitions states connecting them to isomeric molecules, calculations were carried out at the CASPT2(9,8)/ 6-311+G(d,p) level. All open-shell calculations are denoted by * in the Figures (i.e., UB3LYP for the DFT calculations). Calculations were performed using the Gaussian 09 and the Molpro program packages.42,43 The transition-state optimizations and IRC calculations are carried out at the DFT level using Gaussian 09 and the GaussView 5 software package. Excited-state calculations were performed within the TDDFT U-B3LYP/6-311G(d,p) level of theory using the Orca 3.0.3 software.44 The 30 low-lying electronic excited states were calculated in a space dimension maximum set to 300. The UV−vis spectrum was obtained by summing the electronic transitions so-obtained, which were individually enveloped by 20 nm of fwhm real-space Gaussian functions in the 100−800 nm range. 3. RESULTS AND DISCUSSION The energy profiles connecting nitrile imines 8a−e, 1H-diazirines 9a−e, imidoylnitrenes 10a−e, and carbodiimides 11a−e (Scheme 2) are illustrated in Figures 1, 3, 5, and 6. The nitrile imines, 1H-diazirines, and carbodiimides have CSS ground states, which can therefore be described adequately by DFT methods. The imidoylnitrenes have triplet ground states and open-shell singlet states (OSSs) (see below). The molecular rearrangements are expected to take place on the singlet energy surface. Therefore, the open-shell nature of the singlet nitrenes makes CASPT2 calculations necessary. Each set of isomerization reactions will be described in turn. 3.1. Series a, R = H, R′ = H. The isomerization of nitrile imine 8a to carbodiimide 11a passes through two distinct intermediates, the diazirine 9a and the imidoylnitrene 10a, as shown in Figure 1. The nitrile imine 8a can rearrange to the 1H-diazirine 9a by accessing two different transition states (TS8−9), very close to each other with an energy difference of only 1.3 kcal mol−1 and lying on average 59.6 kcal mol−1 above 8a (full structural details are given in the Supporting Information). The 1H-diazirine can then undergo ring opening to the imidoylnitrene 10a passing through a single transition state (TS9−10) lying 9.6 kcal mol−1 above 9a. Examination of the CI vector in the CASPT2 calculation shows that the principal character of TS9−10 is monodeterminental and closed-shell. Thus the change of character happens between the TS and the open-shell 10a (see details on p 12 in the Supporting Information). The small energy barriers separating the singlet E- and Z-imidoylnitrenes 10aE and 10aZ from the 1H-diazirine 9a (3−6 kcal mol−1) will make it difficult to prevent this isomerization from occurring and thus observe

Figure 2. Structures of the triplet (T), open-shell (OSS) singlets, and closed-shell singlets (CSS) of imidoylnitrenes 10aE and 10aZ. The structures of all of the calculated nitrenes at DFT and CASSCF levels are available in the Supporting Information, Figures S6−S10, pp S218−S224. C


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The Journal of Physical Chemistry A

Figure 3. Reaction path from 8b to 11b, passing through diazirine 9b and imidoylnitene 10b. The red energy levels are for the triplet nitrenes. Plain numbers are at the DFT level, and those in parentheses are at the CASPT2 level; * indicates open-shell calculations. The open-shell CASPT2 energies are all relative to similar calculations on 8b.

respectively. The two transition states lead to two isomers of imidoylnitrenes, 10bE and 10bZ. It is seen in Figure 4 that the overall structures of the imidoylnitrenes are “nitrene-like” as opposed to oxazirine-like. Because of steric interaction with the amino group hydrogens in the Z conformer, the E isomer 10bE is of lower energy. The two isomers are separated by an energy difference of 6.4 kcal mol−1. Notably, the energy barriers for ring closure of the imidoylnitrenes 10b to 1H-azirines 9b are extremely small (Table 2), so that it will be almost impossible to prevent the open-shell singlet nitrenes from rearranging. Therefore, it will be difficult to isolate and characterize these nitrenes experimentally, unless they are generated directly in the triplet ground states. It is not surprising, therefore, that we and others27,28 have not so far observed aminoimidoylnitrenes in matrix photolyses of 5-aminotetrazoles. As in the case of 8a, the reaction could also, in principle, proceed via another transition state, 85.7 kcal mol−1 above 8b, as shown in Figure S2 (Supporting Information). 3.3. Series c, R = H, R′ = Phenyl. The isomerization of nitrile imine 8c to carbodiimide 11c via 1H-diazirine 9c and imidoylnitrene 10c is shown in Figure 5. 1H-Diazirine 9c can be generated from the nitrile imine 8c via a transition state TS9−8 lying at 55.1 kcal mol−1. The diazirine lies 6.9 kcal mol−1 higher than 8c. As reported elsewhere, nitrile imine 8c can exist in both allenic and propargylic forms, and both forms ring close photochemically to 9c.21 The 1H-diazirine 9c can ring-open to the imidoylnitrene 10c via transition states TS9−10cZ and TS9−10cE lying 18.9 and 23.2 kcal mol−1 above 8c, respectively. Again, because of steric interaction with the phenyl group in the E conformer, the Z conformer 10cZ and the corresponding transition state are of the lower energy. 10cZ and 10cE are separated by an energy difference of 5.7 kcal mol−1. The very small energy barriers separating the imidoylnitrenes 10c from the 1H-diazirines 9c will make the OSS nitrenes short-lived, probably rearranging faster than the intersystem crossing to the triplet ground states and making it difficult to observe and characterize the triplet nitrenes experimentally. From imidoylnitrene 10cZ there is a barrier of only 12.5 kcal mol−1 (TS10c−11c) toward the carbodiimide 11c, the latter lying 45.6 kcal mol−1 below 8c. The full reaction path including

the imidoylnitrenes experimentally, unless they are generated directly in the more stable triplet ground states. The E and Z conformers of the imidoylnitrenes 10aE and 10aZ differ only by the orientation of the NH hydrogen atom. The carbodiimide 11a can be obtained by passing through the two corresponding E and Z transition states TS10a‑11a, one requiring 11.5 kcal mol−1 and the other 25.9 kcal mol−1. Another path >10 kcal mol−1 higher leads to the first stable excited state of 11, denoted 11* in Figure 1. This is a diradical state of symmetry A′′ (Cs) (see pp S25 and S29 in the Supporting Information). An alternate reaction path with a much higher transition state lying at 60−73 kcal mol−1 is shown in Figure S1 in the Supporting Information. The energies of the three relevant spin states of the E and Z isomers of the imidoylnitrene 10aE and 10aZ are indicated in Table 1 and the structures are in Figure 2. Unlike alkanoylnitrenes and aroylnitrenes, the imidoylnitrenes have triplet ground states. The open-shell singlet (OSS) nitrenes are the first excited states. The CASPT2 calculations indicate a singlet− triplet gap on the order of 18−20 kcal mol−1. The CSS nitrenes have over 50 kcal mol−1 higher energies and therefore are not expected to contribute to the observed chemistry under common experimental conditions. It is noted that the singlet−triplet splittings (S−T gaps) calculated at the CASPT2 level are significantly higher than at the DFT level. The reason can be found in the fact that the DFT-calculated open-shell singlets (OSS) are contaminated with the triplet states. This artificially lowers the energies of the OSS in the DFT calculations, thus giving S−T gaps that are too small. It is seen in Figure 2 that the overall structures are all quite similar, and unlike the acylnitrenes (Scheme 1), the singlet imidoylnitrenes do not show pronounced oxazirine-like structures. 3.2. Series b, R = H, R′ = NH2. The isomerization of nitrile imine 8b to carbodiimide 11b passes through the diazirine 9b and the nitrene 10b, as depicted in Figure 3. The 1H-diazirine 9b is accessed via a single transition states (TS8−9) at 45.0 kcal mol−1, and this step is almost thermoneutral. Diazirine 9b can convert to the E and Z forms of imidoylnitrene 10b via two transition states (TS9b‑10bE and TS9b‑10bZ), lying 11.1 and 16.8 kcal mol−1 higher than 8b, D


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Figure 5, but the energy barriers protecting the open-shell singlet nitrene 10d from rearrangement to the diazirine 9d or the carbodiimide 11d are a little higher (ca. 10 and 15 kcal mol−1, respectively) (Figure 6). A longer lifetime of the OSS nitrene will increase the likelihood of intersystem crossing to the triplet, which might be observable experimentally. 3.5. Cyclization Reactions. As mentioned in the Introduction, N-arylimidoylnitrenes are frequently found to cyclize to benzimidazoles and related compounds.34−38 Thus flash vacuum pyrolysis of 1-phenyltetrazoles yields benzimidazoles (Scheme 3). The DFT calculations confirm cyclization of the OSS imidoylnitrene 10d to 3aH-benzimidazole 12d. A 1,5-H shift then generates benzimidazole 13d. Suitably substituted nitrile imines also cyclize, in this case to form indazoles (Scheme 4). For example, FVP of 1,5-diphenyltetrazole generates diphenylnitrile imine, which cyclizes to 3-phenylindazole.23,45 Therefore, nitrile imine 8d has another escape route, generating indazole 18d and thereby hindering the rearrangement to imidoylnitrene 10d and carbodiimide 11d. The initially formed 3aH-indazole 15d can rearrange to indazole in a series of 1,5-H shifts (Scheme 4). 3.6. Series e, R = CN, R′ = OMe. Lwowski and coworkers have reported that strongly electron-withdrawing groups (CN or SO3CF3) on the imine nitrogen together with alkoxy or aryloxy substituents on carbon stabilize the azide forms of 1-substituted tetrazoles 6’, and addition and insertion reactions observed on either thermolysis or photolysis of N-cyano-Caryloxyimidoyl azides in solution were ascribed to the corresponding imidoylnitrenes.46−48 Therefore, we also examined system e, R = CN, R′ = OMe with the results shown in Figure 7. Here a relatively low barrier or 37 kcal mol−1 separates the nitrile imine 8e from the diazirine 9e (Figure 8). The openshell singlet imidoylnitrenes 10e can exist in several isomeric and conformeric forms, which are protected by energy barriers of up to 12 and 25 kcal mol−1 against rearrangement to the diazirines and carbodiimides, respectively. Therefore, again, these nitrenes might be experimentally observable, provided they can be generated without excess energy.

4. SPECTROSCOPY Vibrational data for all calculated species are listed in the Supporting Information. UV−vis absorption spectra of the imidoylnitrenes were calculated for the triplet states at the UB3LYP/6-311G** level, and the simulated spectra of 10a−e are shown in the Supporting Information. Apart from the unsubstituted compound 10a, the nitrenes are predicted to have weak absorptions in the range of 400−600 nm. For nitrene 10c the weak, longwavelength absorptions are predicted at 440, 616, and 534 nm, which is not inconsistent with the observation of a weak, broad absorption between 400 and 660 nm with a maximum at 510−520 nm.33 The natural spin densities for the triplet states were calculated at the EPR III level (Gaussian 09) and are also listed in the Supporting Information. High spin densities of 1.5 to 1.6 on the nitrene-N atoms indicate that high values of the zero-field splitting parameter |D/hc| on the order of 0.9 to 1.0 cm−1 may be expected in the ESR spectra. (See the linear correlation of spin densities with D values.49,50) There is significant spin density on the order of 0.7 on the imine nitrogen atom. This suggests significant delocalization of spin over the NCN moiety, with the consequence that the expected D/hc values are smaller than for acylnitrenes such as FC(O)N,13 alkoxycarbonylnitrenes,51,52 and

Figure 4. Structures of imidoylnitrenes 10b (T), 10b (OSS), 10c (T), 10c (OSS), 10d (T), and 10d (OSS) at the DFT level. All calculated structures are available in the Supporting Information (structural data in Table 3).

alternate, higher-lying transition states, is shown in Figure S3 (Supporting Information). 3.4. Series d, R = Phenyl, R′ = H. As expected, this energy profile is very similar to that of the isomeric system shown in E


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The Journal of Physical Chemistry A Table 2. Barriers between 10 (OSS) and 9 and between 10 (OSS) and 11 in kcal mol−1 nitrene

a

b

c

d

e

Δ(10(OSS)−9) Δ(10(OSS)−11)

2.9−6.1 11.5−25.9 (2.4−12.0)

0.3−1.1 19.3−20.9

0.9−2.3 12.5−20.9

8.2−10.2 15.7−21.8

4.2−12.1 17.2−25.1 (12.1−23.4)

a T = triplet; OSS = open shell singlet. Calculations at B3LYP/6-311G(d,p) (DFT) and CASPT2(9,8)/6-311+G(d,p) levels (values in parentheses).

Table 3. Key Structural Data for All of the Nitrenes 10a−e at the DFT Levela R,R′ CN (Å)

C−Nnitrene (Å)

∠NCN (deg)

E Z E Z E Z E Z E Z E Z

1

A A 3 A 3 A 1 A 1 A 3 A 3 A 1 A 1 A 3 A 3 A 1

H,H

H,NH2

H,Ph

Ph,H

1.357 1.348 1.332 1.322 1.286 1.291 1.309 1.318 123.2 122.5 122.6 121.7

1.355 1.343 1.325 1.309 1.303 1.313 1.336 1.354 116.8 117.5 118.0 115.9

1.364 1.351 1.336 1.322 1.295 1.303 1.322 1.335 117.7 118.0 117.4 116.9

1.393 1.383 1.373 1.354 1.262 1.264 1.278 1.285 117.9 116.5 117.0 115.4

CN,OMe 1.325 1.341 1.335 1.337 1.261 1.286 1.302 1.314 130.6 118.4 124.4 117.3

1.332 1.337 1.328 1.332 1.297 1.315 1.333 1.351 120.1 114.5 117.3 114.7

a

Structures of all of the calculated nitrenes at DFT and CASSCF levels are available in the Supporting Information, Figures S6−S10, pp S218−S224.

Figure 5. Reaction path from 8c to 11c, passing through diazirine 9c and imidoylnitrene 10c. The red energy levels are for the triplet nitrenes. Plain numbers are at the DFT level. * indicates open-shell calculations.

carbamoylnitrenes53 (in all cases |D/hc| ≈ 1.6) but much higher than for vinylnitrenes (|D/hc| = 0.4−0.7 cm−1).4,18,19,49 There is very little delocalization of spin density to the amino group in 10b, which is expected to have a D/hc value close to 1 cm−1. The delocalization over the NCN moiety may also be expected to result in significantly nonzero values of E/hc. Experience has shown that the ESR-spectroscopic observation of triplet imidoylnitrens in matrices at cryogenic temperatures

following photolysis of tetrazoles is far from straightforward. A likely reason may be the low activation energies for rearrangement of the singlet imidoylnitrenes calculated above. Consequently, it may be necessary to generate the triplet imidoylnitrenes directly in low-temperature matrices, either by adding a triplet sensitizer or by incorporating a triplet sensitizer directly in the imidoylnitrene and hence in the tetrazole or azide precursor. F


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Figure 6. Reaction path from 8d to 11d, passing through diazirine 9d and imidoylnitrene 10d. The red energy levels are for the triplet nitrenes. Plain numbers are at the DFT level. * indicates open-shell calculations.

Scheme 3. Cyclization of N-Phenylimidoylnitrene to Benzimidazolea

Energies of ground and transition states in kcal mol−1 relative to 10d (OSS) at the DFT level. a

Scheme 4. Cyclization of N-Phenylnitrile Imine to Indazolea

a Energies of ground and transition states in kcal mol−1 relative to 8d at the DFT level.

5. CONCLUSIONS Imidoylnitrenes R′−C(NR)−N have triplet ground states. The OSSs are between 4 and 20 kcal mol−1 higher in energy at the CASPT2 level. The CSSs are the second excited states lying >50 kcal mol−1 higher in energy. The OSS imidoylnitrenes undergo ring-closure to 1H-diazirines with very low activation energies (2−12 kcal mol−1), and they can rearrange to carbodiimides with modest activation barriers of 12−20 kcal mol−1. These two reactions will make it difficult to observe the imidoylnitrenes experimentally unless they are generated directly in the more stable triplet ground states. Nitrile imines generated

Figure 7. Structures of transition states. G


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Figure 8. Reaction path from 8e to 11e, passing through diazirine 9e and imidoylnitrene 10e. The red energy levels are for the triplet nitrenes. Plain numbers are at the DFT level, those in parentheses at the CASPT2 level; * indicates open-shell calculations. The open-shell CASPT2 energies are all relative to similar calculations on 8e. and Trimers of Cyanogen N-Oxide NCC≡N→O. An X-ray, FVT-MS/ IR and Theoretical Investigation. J. Chem. Soc., Perkin Trans. 2 2000, 2, 473−478. (6) Gritsan, N. P.; Pritchina, E. A. Are Aroylnitrenes Species with a Singlet Ground State? Mendeleev Commun. 2001, 11, 94−96. (7) Pritchina, E. A.; Gritsan, N. P.; Maltsev, A.; Bally, T.; Autrey, T.; Liu, Y.; Wang, Y.; Toscano, J. P. Matrix Isolation, Time-Resolved IR, and Computational Study of the Photochemistry of Benzoyl Azide. Phys. Chem. Chem. Phys. 2003, 5, 1010−1018. (8) Wentrup, C.; Bornemann, H. The Curtius Rearrangement of Acyl Azides Revisited - Formation of Cyanate (R-O-CN). Eur. J. Org. Chem. 2005, 2005, 4521−4524. (9) Kubicki, J.; Zhang, Y.; Vyas, S. A.; Burdzinski, G.; Luk, H. L.; Wang, J.; Xue, J.; Peng, H.-L.; Pritchina, E. A.; Sliwa, M.; Buntinx, G.; Gritsan, N. P.; Hadad, C. M.; Platz, M. S. Photochemistry of 2Naphthoyl Azide. An Ultrafast Time-Resolved UV-Vis and IR Spectroscopic and Computational Study. J. Am. Chem. Soc. 2011, 133, 9751−9761. (10) Shapley, W. A.; Bacskay, G. B. Ab Initio Quantum Chemical Studies of the Formaldiminoxy (CH2NO) Radical: 2. Dissociation Reactions. J. Phys. Chem. A 1999, 103, 4514−4524. (11) Shapley, W. A.; Bacskay, G. B. A Gaussian-2 Quantum Chemical Study of CHNO: Isomerization and Molecular Dissociation Reactions. J. Phys. Chem. A 1999, 103, 6624−6631. (12) Mebel, A. M.; Luna, A.; Lin, M. C.; Morokuma, K. A. Density Functional Study of the Global Potential Energy Surfaces of the [H, C,N,O] System in Singlet and Triplet States. J. Chem. Phys. 1996, 105, 6439. (13) Zeng, X. Q.; Beckers, H.; Willner, H.; Grote, D.; Sander, W. The Missing Link: Triplet Fluorocarbonyl Nitrene FC(O)N. Chem. - Eur. J. 2011, 17, 3977−3984. (14) Sherman, M. P.; Jenks, W. S. Computational Rationalization for the Observed Ground-State Multiplicities of Fluorinated Acylnitrenes. J. Org. Chem. 2014, 79, 8977−8983. (15) Sun, H.; Zhu, B.; Wu, Z.; Zeng, X. Q.; Beckers, H.; Jenks, W. S. Thermally Persistent Carbonyl Nitrene: FC(O)N. J. Org. Chem. 2015, 80, 2006−2009. (16) Chavez, T. A.; Liu, Y.; Toscano, J. P. Time-Resolved Infrared (TRIR) Studies of Oxycarbonylnitrenes. J. Org. Chem. 2016, 81, 6320−6328. (17) Li, H.; Wan, H.; Wu, Z.; Li, D.; Bégué, D.; Wentrup, C.; Zeng, X. Q. Direct Observation of Carbamoylnitrenes. Chem. - Eur. J. 2016, 22, 7856−7862. (18) Sarkar, S. K.; Sawai, A.; Kanahara, K.; Wentrup, C.; Abe, M.; Gudmundsdottir, A. D. Direct Detection of a Triplet Vinylnitrene, 1,4Naphthoquinone-2-ylnitrene, in Solution and Cryogenic Matrices. J. Am. Chem. Soc. 2015, 137, 4207−4214.

by pyrolysis or photolysis of 2,5-substituted tetrazoles or sydnones can ring-close to the 1H-diazirines with activation energies of 37−60 kcal mol−1 and then rearrange to imidoylnitrenes and carbodiimides with the modest barriers indicated above.

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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.jpca.7b08445. Computational details, Cartesian coordinates, absolute energies, vibrational analysis, imaginary frequencies, complete energy profiles, and UV−vis and ESR calculations. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Didier Bégué: 0000-0002-4553-0166 Hugo Santos-Silva: 0000-0002-7075-0101 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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