Phenylnitrene Radical Cation and Its Isomers From Tetrazoles, Nitrile

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Phenylnitrene Radical Cation and Its Isomers From Tetrazoles, Nitrile Imines, Indazole and Benzimidazole Didier Bégué, Alain Dargelos, Carl Braybrook, and Curt Wentrup J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b11858 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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The Journal of Physical Chemistry

Phenylnitrene Radical Cation and Its Isomers From Tetrazoles, Nitrile Imines, Indazole and Benzimidazole

Didier Bégué,†* Alain Dargelos,† Carl Braybrook,‡ and Curt Wentrup¶*

†

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, 64000, Pau, France. E-mail: [email protected] ‡

Ian Wark Laboratory, CSIRO, Clayton, Victoria 3169, Australia

¶School

of Chemistry and Molecular Biosciences, The University of Queensland,

Brisbane, Queensland 4072, Australia. E-mail: [email protected]

Abstract: Phenylnitrene radical cations m/z 91, C6H5N, 8a.+ are observed in the mass spectra of 1-,

2-, and 5-phenyltetrazoles, even though no C-N bond is present

ACS Paragon Plus Environment

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in 5-phenyltetrazole. Calculations at the B3LYP/6-311G(d,p) level of theory indicate

that

initial

formation

of

the

C-phenylimidoylnitrene

13.+

and/or

benzonitrile imine radical cation 19.+ from 1H- and 2H-5-phenyltetrazoles 11 and 12 is followed by isomerizations of 13.+ to the phenylcyanamide ion 15.+ over a low barrier. A cyclization of imidoylnitrene ion 13.+ onto the benzene ring offers alternate,

very

facile

routes

to

the

phenylnitrene

ion

8a.+

and

the

phenylcarbodiimide ion 14.+ via the azabicyclooctadienimine 16.+. Eliminations of HNC or HCN from 14.+ and 15.+ again yield the phenylnitrene radical cation 8a.+. A direct 1,3-H shift isomerizing phenylcarbodiimide ion 14.+ to the phenylcyanamide ion 15.+ requires a very high activation energy of 114 kcal/mol, and this reaction needs not be involved. The benzonitrile imine – 3-phenyl-1Hdiazirine

–

phenylimidoylnitrene

-

phenylcarbodiimide/phenylcyanamide

rearrangement has parallels in thermal and photochemical processes, but the facile

cyclization

facilitated

by

the

of

imidoylnitrene

positive

charge

13.+

to

making

azabicyclooctadienimine the

nitrene

more

16.+

is

electrophilic.

Furthermore, the benzonitrile imine radical cation 19.+ can cyclize to indazole


2

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24.+, and a series of intramolecular rearrangements via hydrogen shifts, ring openings and ring closures allow the interconversion of numerous ions of composition C7H6N2.+, including 19.+, 24.+, the benzimidazole ion 38.+ and oaminobenzonitrile ion 40.+, all of which can eliminate either HCN or HNC to yield the C6H5N.+ ions of phenylnitrene, 8a.+, and/or iminocyclohexadienylidene, 34.+. Moreover, benzonitrile imine 19.+ can behave like a benzylic carbenium ion, undergoing a novel ring expansion to cycloheptatetraenyldiazene 45.+. The Nphenylnitrile imine ion 2d.+ derived from 2-phenyltetrazole 1d cleaves efficiently to the phenylnitrene ion 8a.+ but may also cyclize to the indazole ion 24.+. The N-phenylimidoylnitrene 59.+ derived from 1-phenyltetrazole 5d undergoes facile isomerization to the phenylcyanamide ion 15.+ and hence phenylnitrene radical cation 8a.+.

Introduction. Tetrazoles are versatile precursors of reactive intermediates, including nitrile imines, nitrenes and carbenes.1,2,3 Moreover, they are widely used in high-


3


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energy materials research4 and in biological chemistry, inter alia as bioisosteres of carboxylic acids.5 Nitrile imines 2 and several other species are formed on either flash vacuum pyrolysis (FVP) or matrix photolysis of 5- and/or 1-substituted tetrazoles 1 and 5 (Scheme 1).6,7,8 The nitrile imines 2 can exist in either allenic or propargylic or even carbenic forms,6,9,10 and they are known to rearrange to 1H-diazirines 3, imidoylnitrenes 4,11 and finally NH-carbodiimides 6, which can equilibrate with cyanamides 7 both thermally and photochemically.6,9 The structures of the nitrile imine radical cations are described in Section 2. In this paper we will generally use the propargylic structure to represent nitrile imines and their ions, bearing in mind that they may also adopt allenic or carbenic structures under the reaction conditions. The nitrile imines 2 can dissociate into nitriles and nitrenes 8,6,12 which can also be accessed by either pyrolysis or photolysis of azides 9 (Scheme 1).

Scheme 1. Thermal and Photochemical Formation and Rearrangement of Nitrile Imines 2


4

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1-Substituted tetrazoles 5 can give rise to imidoylnitrenes 4 by N2 elimination,7,11,13 but in addition, azides 9 and hence nitrenes 8 may be formed by cycloreversion with elimination of a nitrile (Scheme 2).11

Scheme 2. Possible Fragmentations in 1-Phenyltetrazoles


5


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In analogy with the thermal reactions of neutral tetrazoles, loss of N2 from the molecular ions of 2- and/or 5-substituted tetrazoles 1.+ yields abundant ions to which nitrile imine structures 2.+ are assigned (Scheme 3). These ions dissociate to yield nitrene molecular ions 8.+, which again is analogous to the corresponding photochemical fragmentations observed by IR spectroscopy in argon or PVC matrices.6,12 The collisionally induced dissociation (CID) mass spectrum of the m/z 91 ion formed by fragmentation of benzonitrile Nphenylimine ion 2a.+ is identical with that of “phenylnitrene” radical cations PhN. +,

8a.+, formed from the phenyl azide molecular ion 9a.+ (Scheme 3).14 This

ion is also formed from 2-phenyl-5-(4-pyridyl)tetrazole 1b.+. Similarly, the “ptolylnitrene” ion 8c.+ (m/z 105) formed from 2-(p-tolyl)-5-phenyltetrazole 1c via

N-(p-tolyl)nitrile imine 2c.+ is identical with m/z 105 formed from p-tolyl azide 9c.+.14 2-Phenyltetrazole radical cation 1d.+ also yields PhN.+, which is easily


6

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understood in terms of loss of HCN from 2d.+. Phenyl azide 9a.+ is of course also an efficient source of PhN.+, 8a.+.13

Scheme 3. Formation of Nitrile Imine 2.+ and Nitrene 8.+ Radical Cations from 2- and 5-Substituted Tetrazoles and Azides


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The appearance of arylnitrene radical cations 8.+ in the mass spectra of 5-aryltetrazoles 1e-h (Scheme 3) is more surprising. The question of their origin is closely related to the equally surprising observation that photolysis of 5phenyltetrazole 1e in a low-temperature matrix also generates phenylnitrene 8a.15 In this paper, we investigate the mechanisms by which nitrile imines derived from tetrazoles can lead to phenylnitrene radical cation PhN.+, 8a.+ and/or its isomers. The electronic structures and isomerization pathways of the phenylnitrene radical cations were described recently.13 Due to these rearrangements, in this paper, the term "phenylnitrene radical cation PhN.+" includes the potential rearranged ions of composition C6H5N.+, inter alia the azacycloheptatetraene ion 32.+, iminocyclohexadienylidene ion 34.+, and fulvenimine ion 36.+ (Eq. 1).

Experimental Section


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The mass spectra of tetrazoles shown in Figure 1 were measured at 70 eV electron ionization on a conventional Nier-Johnson geometry (EB) sector instrument AEI MS902 using direct insertion and warming the samples to maximally 100

oC.

Closely matching spectra were also recorded on a CEC-

21490 spectrometer, Thermo Fischer DFS inverse geometry (BE) double focusing, high-resolution spectrometer, and Thermo Scientific TSQ 8000 triple quad GC/MS instrument. High resolution mass measurements and linked scans were recorded on the Thermo Fischer DFS, where CID was performed using He as a collision gas in the first field-free region of the spectrometer and a 5 keV accelerating voltage.

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, which for related systems has proved to compare favorably with calculations at the CASPT2(7,8)/6-311++G(d,p) level.7,9,11,13 Transition state optimizations and IRC


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Page 10 of 54

calculations were carried out at the DFT level. Natural bond orbital (NBO) analyses were performed on the DFT-optimized structures at the CCSD(T)/ccpVTZ level of theory. All calculations were performed using the Gaussian 09 and Molpro program packages.16,17 Calculated energies are at 0 K. Results and Discussion 1. The Benzonitrile Imine Route from 5-Phenyltetrazole Low resolution mass spectra of 1-, 2-, and 5-phenyltetrazoles have been reported.18,19,20 The spectra shown in Figure 1 were recorded under identical conditions for ease of comparison. An abundant ion at m/z 91 (“phenylnitrene” radical cation, C6H5N.+) is seen in each spectrum. The processes m/z 146 

m/z 118 and m/z 118  m/z 91 are supported by metastable ion peaks at m/z 95.5 and 70, respectively. While the formation of arylnitrene radical cations 8.+ in the mass spectra of 1- and 2-aryltetrazoles 5 and 1 is easily understood, the formation of the same ion in the mass spectrum of 5-phenyltetrazole 1e (Figure 1a) requires an explanation. For 1e, the reactions m/z 146  118 (C7H6N2) , m/z 118  91 (C6H5N.+) , m/z 91 64 (C5H4.+), m/z 118  89


10

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(C7H5.+), and m/z 89  63 (C5H3.+) were confirmed by linked scans and high resolution mass measurements (Figures S1 and S2, Supporting Information). The corresponding linked scans and high resolution measurements for 1d, 5d, indazole and benzimidazole are reported in Figures S3-S10, Supporting Information.

Figure 1. 70 eV Electron ionization mass spectra of phenyltetrazoles 1e (a), 5d (b), and 1d (c).


11


Page 12 of 54

5-Phenyltetrazole can exist in two tautomeric forms, 1H 11 and 2H 12, which can interconvert under unimolecular conditions with a free energy of activation barrier of the order of 50 kcal/mol (Scheme 4).21 In the condensed phase they can interconvert very easily by intermolecular hydrogen exchange. The 2H-tautomer dominates in the gas-phase, so that matrix isolation of the vapor of 5-phenyltetrazole yields almost exclusively the more stable 12.6,21 However, the 1H-tautomer 11 dominates in the solid,22 and both forms may be observed in solution. It is possible, therefore, that a mixture of ions 11.+ and 12.+ may be formed in a direct insertion mass spectrum of the crystalline tetrazole. Moreover, we find that the two ions 11.+ and 12.+ can also interconvert in the mass spectrometer via 1,5-H shifts with calculated activation energies of 54 and 65 kcal/mol, respectively (Scheme 4). Therefore, it is necessary to consider mechanisms for the formation of PhN.+, 8a.+, from both forms.


12

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Scheme 4. 1,5-H Shifts Interconverting 1H- and 2H-5-Phenyltetrazole Neutrals and Radical Cations (Activation Energies in kcal/mol)

In fact, several potential routes from the 1H- and 2H-5-phenyltetrazole radical cations 11.+ and 12.+ lead to the phenylcarbodiimide and phenylcyanamide radical cations 14.+ and 15.+ as shown in Scheme 5, where energies are reported relative to the nitrile imine radical cation 19.+ in order to allow comparison with related reactions for the neutrals.7 The most straightforward route from 11 leads directly to the C-phenylimidoylnitrene radical cation 13.+ via the imidoyl azide


13


Page 14 of 54

Scheme 5. Routes from 5-Phenyltetrazoles to Phenylnitrene Radical Cations. TS Energies in kcal/mol at the B3LYP/6-311G(d,p) Level Relative to 19.+ = 0. Numbers of the Type +xy.z Denote Actual Activation Energies

11A. The imidoylnitrene 13.+ can then rearrange to 15.+ in analogy with the known thermal and photochemical processes.6,7

In the case of the 2H-

tetrazole 12 the benzonitrile imine and 3-phenyl-1H-diazirine molecular ions 19.+


14

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(2e.+) and 20.+ are proposed intermediates before reaching imidoylnitrene 13.+ (Scheme 5). In addition, thanks to the presence of a positive charge, the nitrene 13.+ becomes electrophilic enough to cyclize onto the benzene ring over a very low barrier, yielding the azabicyclo[4.2.0]octadiene species 16.+ (Scheme 5). Ion 16.+ then rearranges to 18+. over a barrier of 19 kcal/mol via TS17A. Ion 18.+ can yield PhN.+ 8a.+ directly by elimination of HNC with a barrier of 24 kcal/mol (Schemes 5 and S4). Ion 16.+ can also rearrange to the phenylcarbodiimide ion 14.+ over a barrier of only 9.6 kcal/mol. Here, ring opening of 16.+ generates TS17B, which is set up for the 1,2-phenyl shift to 14.+. The pathway 13.+  16.+  14.+

is so facile that a direct 1,2-phenyl shift

13.+ 14.+ becomes uncompetitive; instead of the phenyl group completing a 1,2-migration, the nitrene adds to the ring. In the neutral series, the cyclization of singlet nitrene 13 onto the benzene ring, forming 16, has a higher but still readily accessible barrier of 1721 kcal/mol for the Z and E forms (Scheme 6). The neutral rearrangements of


15


Page 16 of 54

13 to diazirine 20 and carbodiimde 14 remain the lowest-energy reactions (Scheme 6).7 Scheme 6. Rearrangements of Neutral Singlet Imidoylnitrene 13 (Ground State and Activation Energies in kcal/mol relative to 19.+)

The phenylcarbodiimide ion 14.+ and the phenylcyanamide ion 15.+ can both generate PhN.+ 8a.+, by elimination of NHC and HCN, respectively (Scheme 5). Neutral phenylcyanamide 15 is a photochemical source of triplet phenylnitrene, and we have postulated that the triplet excited states of carbodiimides can generate triplet nitrenes and isocyanides on photolysis.15 The C6H5N.+ (m/z 91) ion is also formed in the mass spectral fragmentation of 2-


16

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phenyl-1,3,4-oxadiazol-5-one and its thio- and dithio-derivatives by elimination of CO2, COS, or CS2.23 It is most likely that, in all cases, the benzonitrile imine ion 19.+ is formed initially, and that formation of 8a.+ takes place as outlined in Scheme 5. It should be noted, however, that carbodiimides are not known to afford particularly strong “nitrene” ions (e.g. for diphenylcarbodiimide24 m/z 91, 25%;

for

dicyclohexylcarbodiimide25

m/z 97, 5%). In contrast, the mass

spectrum of phenylcyanamide yields a strong m/z 91 (90%) (Figure S11).13 The 1,3-hydrogen shift 14.+  15.+ also needs to be considered. This reaction is known for the neutrals, 14  15 taking place thermally in the gas phase, but this occurs by intermolecular H-exchange.6 It is also known to take place

photochemically,9

most

probably

via

excited

states.

The

ionic

isomerization 14.+  15.+ has a very high calculated barrier of 114 kcal/mol, which

makes

it

uncompetitive

in

comparison

with

the

other

processes

considered in Scheme 5. Similarly high barriers are known for other “forbidden” pericyclic 1,3-H shifts, e.g. the isomerization of ketenimine to acetonitrile.26 Thus, the

three

routes

to

phenylnitrene

8a.+

via


the

phenylcyanamide

15.+,

17


azabicyclo[4.2.0]octadiene

16.+,

and

Page 18 of 54

phenylcarbodiimide

14.+

ions

remain

preferred pathways. 2. Structures of Nitrile Imine Radical Cations The calculated structures of nitrile imine ions 19.+ and 2d.+ at B3LYP level are shown in Figure 2. At this level, they have clearly propargylic, nitrile-like structures with short CN bonds and a nearly linear R-CN moiety. We have shown previously that the propargylic structure is a general outcome of B3LYP calculations,

and

a

better

differentiation

between

propargylic,

allenic

and

carbenic structures is achieved at the CCSD(T) level.10


18

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Figure 2. Structures of C- and N-Phenylnitrile imines 19.+ and 2d.+ at the B3LYP/6-311G(d,p). Bond angles in degrees (on left) and bond lengths in Å (on right).

Accordingly, a natural bond orbital (NBO) analysis was carried out at the CCSD(T)/cc-pVTZ level with the results shown in Table 1.

Table 1. NBO Electron Occupancies for C- and N-Phenylnitrile Imines 19.+ and 2d.+ at the CCSD(T)/cc-pVTZ

Level.a

19.+ BD C5-C1

C1-N2

N2-N3

N3-H4

C1


LP N2

N3

H4

19


TOTAL

1.981 0.868

1.991 1.976 0.978 0.109*

1.995

C1-C6

C5-C6 1.970

C6-C7 1.984 1.715 0.130*

C5

C6

C7

C5-N3 1.980 0.895

N2-N3 1.989

N2-C1 1.992 1.990 1.960

N1-C6

C5-C6 1.973 0.874 0.180*

C6-C7 1.982

C5

C6

C7

TOTAL

1.959

Page 20 of 54

0.811

1.930 0.906 0.106*

C8-C9 1.986 0.845 0.117*

C9-C10 1.984 0.886

C10-C5 1.971 0.876 0.196*

C8 0.227*

C9

C10

C1-H4

C1

LP N2

BD C7-C8 1.983 0.865

C8-C9 1.983

C9-C10 1.981 1.774 0.123*

C9

C10

BD C7-C8 1.986 LP

TOTAL 2d.+ BD TOTAL

TOTAL

N3 1.888

H4

C10-C5 1.980

LP TOTAL a

C8 0.197*

BD = two‐center bond. LP = lone pair. * = (partially) vacant bonding p or LP

orbital. Total means sum of alpha and beta occupancies; for further details see the Supporting Information.

The NBO analysis reveals a partial delocalization into the aromatic ring, resulting in a ~1.5-order C1-C5 bond and a ~2.5-order C1=N2 bond but an N2N3 single bond, and for 19.+ the free radical electron resides largely on N3. In 2d.+ there is again a ~1.5-order C5-N3 bond but a CN triple bond, and the electron vacancy is largely delocalized over the aromatic ring.


20

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3. Cyclization of Nitrile Imine Radical Cations and Ensuing Rearrangements A cyclization of the neutral nitrile imine 19 to the 3-dehydro-7aH-indazole tautomer 21 (Eq. 2) seems a priori unlikely, and indeed it has a high calculated barrier of 61 kcal/mol.

Nonetheless,

such

a

cyclization

becomes

possible

in

the

mass

spectrometer, where much higher energies are available. The cyclization of the radical cation 19.+ to 21.+ has a calculated barrier of 58 kcal/mol (Scheme 7), and the structure of the transition state is highly bent at the

nitrile imine

carbon atom with a CCN angle of 118.4o like in a carbene (Figure 3). The NBO analyses for these species at the CCSD(T) level are summarized in Table 2.


21


Figure 3. Structure of TS19-21.+ and

Page 22 of 54

21.+ at the DFT level.

Table 2. NBO Electron Occupancies for C-Phenylnitrile imine 19.+, Transition State TS19-21.+, and

21.+ at the CCSD(T)/cc-pVTZ

Level.a

19.+ BD TOTAL

C5-C1 1.981 0.868

C1-N2 1.991 1.976 0.978 0.109*

N2-N3 1.995

C1-C6

C5-C6 1.970

C6-C7 1.984 1.715 0.130*

C5

C6

C7

C5-C1 1.977

C1-N2 1.988 0.791 0.137*

N2-N3 1.986 0.885 0.105*

C1-C6

C5-C6 1.978 0.780 0.232*

C6-C7 1.984 0.846

C5 0.528

C6

C7 0.365

TOTAL

N3-H4 1.959

BD C7-C8 1.986

C1

LP N2 0.811

C8-C9 1.986 0.845 0.117*

C9-C10 1.984 0.886

C8 0.227*

C9

C10

N3-H4 1.967

C1 0.967 0.370*

N3 1.930 0.906 0.106*

H4

C10-C5 1.971 0.876 0.196*

LP TOTAL TS19-21.+ BD TOTAL

TOTAL

DB C7-C8 1.986

LP N2 1.859

N3 1.873

C8-C9 1.985 1.691

C9-C10 1.983

C10-C5 1.972

C9

C10 0.401*

BD N3-C10 0.751 0.140*

LP TOTAL

C8


22


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21.+ BD TOTAL

C5-C1 1.985 0.832

C1-N2 1.988 0.899

N2-N3 1.985

C1-C6

C5-C6 1.985 0.759 0.145*

C6-C7 1.986 0.854

TOTAL

a

N3-H4 0.992 DB C7-C8 1.985

C1 0.974 0.384* C8-C9 1.985 1.803

LP N2 1.937 0.349*

N3 1.701 0.717

C9-C10 1.972

C10-C5 1.966

BD N3-C10 1.979

See definitions of notations in Table 1.

In the transition state TS19-21.+ there is a singly occupied p orbital (LP) on C1 and also a vacant orbital delocalized over C1 and N2. In 21.+ there is partial double-bond character of the C1-C5 and C1-N2 bonds, indicating that this ion has allenic rather than carbenic structure, but a single electron is residing in a p orbital (LP) at C1 in keeping with the structure shown in Scheme 7. Hydrogen shifts in 21.+ lead to indazole 24.+ and its isomer 25.+ (Scheme 7), from where ring opening leads to 33.+ and the iminocyclohexadienylidene ion 34.+, and therefore potentially also phenylnitrene ion 8a.+ , benzazirine 35.+ and the fulvenimine ion 36.+ in close analogy with the rearrangements described for the neutrals.27 The relationship between 8a.+, 34.+ and 36.+ was


23


Page 24 of 54

described earlier.13 These rearrangements can also lead to benzimidazole 38.+, and both the indazole and benzimidazole molecular ions can rearrange to that of o-aminobenzonitrile, 40.+. A further route from the ring-opened ion 29.+ leads into the familiar13 ionic azacycloheptatetraene - phenylnitrene rearrangement (29.+  30.+  31.+  32.+  8a.+). The ring-opened species 37.+ can be reached from both the indazole and benzimidazole molecular ions and provides a third

route to the iminocyclohexadienylidene and phenylnitrene ions 34.+ and

8a.+.

Scheme 7. Connections Between the Radical Cations of Benzonitrile Imine 19, Indazole 24, Benzimidazole 38, o-Aminobenzonitrile 40, Phenylnitrene 8a, and Iminocyclohexadienylidene 34. Energies in kcal/mol Relative to 19.+


24

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25


13C-labeling

Page 26 of 54

revealed that C2 as well as another carbon atom are

involved in the elimination of NCN/HNC from the benzimidazole molecular ion, 28

and the two nitrogen atoms in the indazole molecular ion are both involved

in

elimination

of

HCN/HNC.

29

The

mass

spectra

of

indazole

24

and

benzimidazole 38 are very similar; both eliminate HCN/HNC to yield m/z 91 (C6H5N.+) (see Figures S7 – S10, Supporting Information). These spectra are also very close to that of o-aminobenzonitrile 40, and it has been suggested that the molecular ions of 24 and 38 rearrange to that of 40 prior to elimination of HCN/HNC, but mechanisms for these rearrangements and eliminations were not

proposed.28,30

The

reactions

described

in

Scheme

8

provide

such

mechanisms. Indeed, CID mass spectrometry has indicated that the m/z 91 ions

(C5H5N.+)

derived

from

indazole

24.+,

benzimidazole

38.+;

and

o-

aminobenzonitrile all possess the same structure or mixture of structures, assigned as 34.+ and/or 36.+.31

For clarity, the overall paths developed in

Scheme 7 are summarized in Scheme 8.


26

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It is noteworthy that the ring opening of benzimidazole 38.+ to 37.+ and recyclization to 27.+ provide for elimination of

HCN/HNC using two different

carbon atoms in the formation of the C6H5N.+ ions, m/z 91 in agreement with the observations. Similarly, rearrangements of the indazole molecular ion 24.+ to 27.+ and 34.+ provide for involvement of both of the nitrogen atoms in the expulsion of HCN/HNC. It is interesting to note that the vinylnitrene ion 26.+, the spiroazirine ion 27.+, the nitrilium ion 37.+ and the isocyanide ion 39.+ have analogies

in

intermediates

invoked

in

the

photochemical

indazole

-

benzimidazole and pyrazole - imidazole rearrangements.32,33

Scheme 8. Summary of the Principal Routes from the Radical Cations of Benzonitrile

Imine

19,

Indazole

24,

Benzimidazole

38,

and

o-

Aminobenzonitrile 40 (all m/z 118) to those of iminocyclohexadienylidene 34, fulvenimine 36, and Phenylnitrene 8a, (all m/z 91). Energies in kcal/mol


27


4.

Ring

Expansion

of

Benzonitrile

Imine

Page 28 of 54

Radical

Cation

to

Cycloheptatetraenyldiazene A further interesting fragmentation of 19.+ produces .NNH and the cation C7H5+ (m/z 89) with a significant abundance (see Figure 1a and Figures S1-S2). Loss of N2 from 19.+ generates a smaller amount of the radical ion C7H6.+ (m/z 90). A previously unrecognized reaction involves the ring expansion of 19.+ to cycloheptatetraenyldiazene 44.+ over a barrier of ca. 69 kcal/mol (Scheme 9). Here, the nitrile imine behaves like an azocarbene10 or, because of the positive


28

Page 29 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


charge,

a

benzyl

carbenium

ion,

undergoing

a

phenylcarbene-

cycloheptatetraene-type rearrangement34 via cyclization to the cyclopropene intermediate 42.+ (see details below). Ion 44.+ can undergo a further carbenecarbene rearrangement to the phenylcarbene derivative 46.+. This compound cyclizes with a barrier of less than 2 kcal/mol to the 2H-indazole ion 47.+, which can further rearrange to indazole 24.+, but unlike the situation for the neutrals the 2H-indazole ion is actually of lower energy than the indazole ion. Elimination

of the

.NNH

radical from the cycloheptatetraenyldiazene

radical cation 44.+ generates the cation 48+, which is formally a cycloalkyne, over a barrier of ca. 24 kcal/mol. This ion or its rearrangement products are likely sources of the C7H5+ (m/z 89) ion in the mass spectrum (Figure 1a and Figures S1-S2). The direct elimination of

.NNH

from 19.+ to generate the

phenylcarbyne ion 49+ is much less favorable with a calculated energy barrier of 96 kcal/mol. The EZ conformer of ion 44.+ can also transfer an H atom and eliminate N2 to generate the cycloheptatetraene radical cation 50.+ (C7H6.+) and hence the cyclopropene and phenylcarbene ions 51.+ and 52.+.


29


Page 30 of 54

The phenylcarbene derivative 46.+ can also undergo a very facile insertion into the N-H bond to form the 3H-indazole ion 53.+, from which the diazo compound 54.+ and hence 2-methylenecyclohexadienylidene 55.+ are readily formed via small activation barriers. As in the case of the neutrals,35 55.+ can interconvert with the benzocyclopropene ion 56.+ before undergoing ring contraction to the fulvenallene ion 57.+. Both 46.+ and 55.+ can in principle isomerize to the phenylcarbene ion

52.+ and hence 51.+ and 50.+.

Stages of the ring expansion reaction of 19.+ to 44.+ along the internal reaction coordinate at the DFT level are illustrated in Figure 4. An NBO analysis of the bonding and lone pair distribution is presented in Table 3, and pictures of the two highest occupied and the two lowest unoccupied MOs are shown in Figure S13-S18, Supporting Information.

Scheme 9. Ring expansion of Benzonitrile Imine Radical Cation to Cycloheptatetraenyldiazene and Subsequent Reactions


30

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31



Page 32 of 54

32

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Figure 4. Stages of the ring expansion of 19.+ to 44.+ along the internal reaction coordinate and bond lengths (Å, on the left) and angles (o, on the right).

Table 3. NBO Electron Occupancies for the Ring Expansion of CPhenylnitrile Imine 19.+ at the CCSD(T) Level.a

V5.+ BD TOTAL

LP

C5-C1

C1-N2

N2-N3

N3-H

C1

N2

N3

1.978

1.992

1.986

1.969

0.326*

0.795

1.928


H4

33


0.990

Page 34 of 54

0.275*

0.942

0.983

0.106* DB

C1-C6 TOTAL

C5-C6

C6-C7

C7-C8

C8-C9

C9-C10

C10-C5

1.938

1.984

1.986

1.986

1.982

1.978

0.108*

1.624

1.573

1.651

0.134*

0.138*

0.435*

LP C5

C6

C7

C8

C9

C10

TOTAL

V4.+ BD TOTAL

LP

C5-C1

C1-N2

N2-N3

N3-H4

C1

N2

N3

1.971

1.989

1.983

1.973

0.630*

1.633

1.957

0.978

0.750

0.110*

H4

0.920 0 .110*

DB TOTAL

C1-C6

C5-C6

C6-C7

C7-C8

C8-C9

C9-C10

C10-C5

0.701

1.880

1.96864

1.986

1.986

1.984

1.976

0.111*

0.727

0.739

0.830

1.666

0.149* LP TOTAL

C5

C6

0.432*

0.314*

C7

C8

C9

C10

V3.+ BD TOTAL

LP

C5-C1

C1-N2

N2-N3

N3-H4

1.953

1.986

1.988

1.973

C1

N2

N3

1.791

1.977

0.811

1.777

0.836

0.152*

0.309*

0.130*

H4

DB TOTAL

C1-C6

C5-C6

C6-C7

C7-C8

C8-C9

C9-C10

C10-C5

0.919

1.860

1.983

1.987

1.986

1.982

1.978

0.855

0.904

0.110*

0.853 0.127*

LP


34

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C5

C6

C7

TOTAL

C8

C9

C10

0.242*

0.238*

V2.+ BD TOTAL

LP

C5-C1

C1-N2

N2-N3

N3-H4

1.975

1.984

1.982

1.977

1.671

C1

1.919

N2

N3

H4

0.958

1.982

0.919* DB

TOTAL

C1-C6

C5-C6

C6-C7

C7-C8

C8-C9

C9-C10

C10-C5

0.952

1.685

1.980

1.988

1.986

1.978

1.981

0.693

0.915

0.833

0.901

C8

C9

C10

0.661*

LP C5

C6

C7

TOTAL

0.216*

TS.+ BD TOTAL

LP

C5-C1

C1-N2

N2-N3

N3-H4

1.966

1.980

1.990

1.975

1.758

C1

N2

N3

H4

1.916

1.984

1.936 DB

TOTAL

C1-C6

C5-C6

C6-C7

C7-C8

C8-C9

C9-C10

C10-C5

1.825

0.801

1.981

1.987

1.986

1.972

1.986

0.854

0.821

0.886

0.110* LP C5 TOTAL a

C6

C7

0.214*

C8

C9

C10

0.217*

0.241*

See definition of notations in Table 1. The atom numbering in Figure 2 is

used.

The

ring

expansion

reaction

takes

place

via

the

formation

of

a

cyclopropabenzene derivative V4-V2 in Figure 4 before reaching the transition state, which itself has cyclopropene character. In V5 the initially propargylic


35


Page 36 of 54

CNN moiety of 19.+ has become more allenic, with a 2.5-order CN bond and half a lone pair on the middle N. In V4 there is a C=N double bond and a “vacant lone pair” on C1 as in a carbene or benzylic cation. A long C1-C6 cyclopropene bond is forming (1.495 Å). In V3 there is no longer a lone pair on C1, and the C1-C6 bond has been further formed. In V2 the C1-C6 cyclopropene bond is fully formed, and the “vacant lone pair” has moved to N2.

In addition to the reactions described above, the benzonitrile imine radical cation 19.+ (C7H6N2, m/z 118) also undergoes further fragmentations to NH + C6H5CN.+ (m/z 103), and HNNC. +

C6H5+ (m/z 77) (Eq. 3, Figure 1a and

Figure S1). Since these fragmentations involve breaking of single bonds, the energies are high (129 and 104 kcal/mol, respectively).

5. 2-Phenyltetrazole


36

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We now turn to 2-phenyltetrazole 1d. The mass spectrum of 1d.+ is described in the Introduction, in Scheme 3, and in Figure 1c and Figures S3-S4. The energetics of the fragmentation to the nitrile imine radical ion 2d.+ are indicated in Eq. 4, which clearly represents a very facile route to the formonitrile Nphenylimine (N-phenylnitrile imine) 2d.+ (C7H6N2, m/z 118) and PhN.+ 8a.+ (C6H5N, m/z 91).

The calculated barriers for the simple cleavages to a phenyl cation, m/z 77, 1d.+  C6H5+ and 2d.+  C6H5+, are much higher in accord with the fact that

m/z 77 has a very low intensity (see Figure 1c and Figure S3), but it is noted that this is also very much lower than that of m/z 77 derived from 19.+ (Eq. 3). In contrast to the potential cyclization of neutral C-arylnitrile imines described in Eq. 2, N-arylnitrile imines are frequently observed to cyclize to


37


Page 38 of 54

indazoles.3,7 In the parent system, this reaction (Scheme 10) has a modest activation barrier of 27 kcal/mol.7

Scheme 10. Formation of Neutral Indazole 24 from N-Phenylnitrile Imine (2d, R1 = H, R2 = Ph). Relative Energies in kcal/mol

Similarly, ion 2d.+ can cyclize to the indazole molecular ion 24.+ as described in Scheme 11.

Scheme 11. Cyclization of Nitrile Imine Radical Cation HCNNPh.+ 2d.+ to Indazole.+ 24.+ (Energies in kcal/mol relative to 19.+ = 0)


38

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The first step of this reaction has the same calculated barrier as the dissociation to HCN and PhN.+ 8a.+ (Eq. 4). Thus, it can be expected that some of the ions of m/z 118 retain the nitrile imine structure 2d.+, and some may rearrange to indazole tautomers. The very weak m/z 77 (C6H5.+) ion (~5% abundance) in the mass spectrum of 1d (Figure 1c and Figure S3) would be in agreement with a partial rearrangement to the indazole molecular ion, which itself yields only a very weak m/z 77 (~1%; see Figures S7-S8, Supporting Information). It is known that photolysis of 2-phenyltetrazole yields Nphenylnitrile imine 2d,12 and pyrolysis of 2-aryltetrazoles results in cyclization of the nitrile imines to indazoles.6

6. 1-Phenyltetrazole


39


Page 40 of 54

As seen in Scheme 1, 1-aryltetrazoles 5 are direct precursors of Narylimidoylnitrenes 4 in thermolysis and photolysis reactions.7,11 Our calculations indicate that, in the mass spectrometer, 1-phenyltetrazole 5d follows an analogous route via the N-phenylimidoylnitrene ion 59.+ and then phenylcyanamide 15.+ (Scheme 12). The imidoylnitrene ion 59.+ may also eliminate HCN directly via a low-lying transition state to yield PhN.+ , 8a.+. The cycloreversion reaction 5d.+  9a.+ + HCN provides an alternate but energetically less favorable route to 8a.+, and there is no evidence that this takes place in the mass spectrometer. The spectra (Figure 1b and Figures S5S6) show that M - N2 - HCN is clearly the preferred route to m/z 91 (C6H5N.+).

Scheme 12. Fragmentations of the 1-Phenyltetrazole Radical Cation 5d.+. Energies in kcal/mol


40

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The Z-form of imidoylnitrene 59.+ can also cyclize onto the benzene ring to form the 3aH-benzimidazole species 60.+ with a very low barrier of only 4 kcal/mol (Scheme 13).

A further 1,5-H shift affords the benzimidazole

molecular ion 38.+. The corresponding cyclization of neutral Nphenylimidoylnitrene 59 to neutral benzimidazole 38 is well documented3,7,11 and has a calculated barrier of 22 kcal/mol.7

Scheme 13. Imidoylnitrene - Benzimidazole Rearrangement (taking Place in Radical Cations and Neutrals). Energies are given for the Ionic Rearrangement in kcal/mol


41


Page 42 of 54

Therefore, in this case too, the m/z 118 ions derived from 5d may undergo cyclization to the benzimidazole ion in competition with fragmentation to PhN.+, 8a.+.

However, this cannot be the only route, since the mass spectrum of 1-

phenyltetrazole shows a strong m/z 77 (Figure 1b and Figure S5), whereas it is very weak (

Phenylnitrene Radical Cation and Its Isomers From Tetrazoles, Nitrile

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