HeteroarylcarbeneâArylnitrene Radical Cation Isomerizations - The

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Heteroarylcarbene - Arylnitrene Radical Cation Isomerizations Didier Bégué, Alain Dargelos, Carl Braybrook, and Curt Wentrup J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00309 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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The Journal of Physical Chemistry

Heteroarylcarbene  Arylnitrene Radical Cation Isomerizations

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 3168, Australia

¶School

of Chemistry and Molecular Biosciences, The University of

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

Abstract: 5-Phenyltetrazole 1e is an important source of phenylnitrene or the phenylnitrene radical cation (m/z 91) under thermal, photochemical and electron impact conditions.

Similarly, 3- or 4-(5-tetrazolyl)pyridines 12b,c

yield pyridylnitrene radical cations 9a.+ (m/z 92) on electron impact. In

ACS Paragon Plus Environment

1

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contrast, 2-(5-tetrazolyl)pyridine 12a.+ generates 2-pyridyldiazomethane 24.+ and 2-pyridylcarbene 26.+ radical cations (m/z 119 and 91) upon electron impact. The 2-pyridylcarbene radical cation undergoes a carbene-nitrene rearrangement to yield the phenylnitrene radical cation. Calculations at the B3LYP/6-311G(d,p) level have revealed a facile H-transfer from the tetrazole to the pyridine ring in 2-(5-tetrazolyl)pyridine, 12a.+  21.+ taking place in the radical cations. Subsequent losses of N2 generate the pyridinium diazomethyl radical 22.+ or pyridinium-2-carbyne 23.+. These two

ions

can

isomerize

to

2-pyridyldiazomethane

24.+

and

2-

pyridylcarbene 26.+, the later rearranging to the phenylnitrene radical cations 9a.+.

13C-labeling

tetrazolyl)pyridine

12a

cations retaining the 4-pyridylnitrene

of the tetrazole rings confirmed that 2-(5-

generates

13C

18c.+,

2-pyridylcarbene/phenylnitrene

radical

label, but 4-(5-tetrazolyl)pyridine 12c generates which

has

lost

the

13C

label.

2-

Pyridylcarbene/phenylnitrene radical cations (m/z 91) also constitute the base peak in the mass spectrum of 1,2,3-triazolo[1,5-a]pyridine 34. Similarly, 4-pyridylnitrene radical cations 18c.+ or its isomers (m/z 92) are obtained

from

1,2,3-triazolo[1,5-a]pyrazine

36.

Several

other

-

heteroaryltetrazoles behave in the same way as 2-(5-tetrazolyl)pyridine, yielding

heteroarylcarbene/arylnitrene

radical

spectrometer, and this was confirmed by

cations

13C-labeling


in

the

mass

in the case of 1-(5-

2

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tetrazolyl)isoquinoline

42-13C.

In

general,

5-aryltetrazoles

generate

arylnitrene radical cations under electron impact, but -heteroaryltetrazoles generate -heteroarylcarbene radical cations.

Introduction Tetrazoles are of widespread use in organic, biological, and medicinal chemistry1,2 as well as the construction of energetic materials.3 They are also important precursors for the generation of nitrile imines, carbenes, and nitrenes and hence for investigation of the numerous rearrangements of these species under thermal and photochemical conditions.4 Nitrile imines 3 are formed on either flash vacuum pyrolysis (FVP) or matrix photolysis of 2,5-disubstituted tetrazoles 1.5 When these reactions are carried out in solution, the nitrile imines are widely used in 1,3-dipolar cycloaddition reactions,1,2 but in the absence of reaction partners they undergo facile thermal and photochemical rearrangement to 1H-diazirines 4 and carbodiimides 6 (Scheme 1).4,5


3


Scheme

1.

Formation

of

Nitrile

Imines

Page 4 of 28

3,

1H-Diazirines

4,

Imidoylnitrenes 5, Carbodiimides 6 and Cyanamides 7 from Tetrazoles.

Comparatively little is known about the structures and potential rearrangements of carbene, nitrene, and nitrile imine radical ions formed


4

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by electron impact in the mass spectrometer. Here, we have shown recently that 2-pyridylnitrene radical cations undergo nitrogen scrambling completely analogous with the reversible ring expansion - ring contraction taking place under FVP and photolysis conditions.6 Furthermore, the mass spectra of 2,5-disubstituted tetrazoles 1.+ reveal the formation of nitrene

radical

cations,

thereby

suggesting

that

comparable

rearrangements take place under thermal and electron impact conditions. Thus, the chemically induced dissociation (CID) mass spectra of the m/z 91 ions formed by fragmentation of benzonitrile imines 3a-b (Scheme 2) are identical with the “phenylnitrene” ions R2N.+ (10a-b.+) formed from phenyl azide 11a,b.7 Likewise, the m/z 105 ion (p-tolylnitrene R2N.+ (10c.+) from N-(p-tolyl)nitrile imine 3c is identical with the one formed from p-tolyl azide 11c.

Scheme 2. Tetrazoles 1 and 2, Nitrile Imines 3 and Azides 8 and 11 as Sources of Arylnitrene Radical Cations R1N (9.+) and R2N (10.+).


5


Page 6 of 28

Similarly, 2-phenyltetrazole 1d undergoes loss of N2 to generate Nphenylnitrile imine Ph-N=N+=CH- 3d.+ on pyrolysis and photolysis. The base peak in the mass spectrum is m/z 91 (C6H5N.+) which results from loss of HCN.8 Very interestingly, m/z 91 is also an abundant ion in the mass spectrum of 5-phenyltetrazole 1e. We have recently shown that the energetically most favorable route to phenylnitrene radical cations is by rearrangement phenyldiazirine,

of

initially

formed

phenylcarbodiimide,

benzonitrile and

imine

radical

phenylcyanamide

ions

ions,

to Ph-

NHCN.+, 3.+  4.+  5.+  7.+, followed by loss of HCN to yield the nitrene ion R1N.+ (9.+) (Scheme 2).8 NH-carbodiimides 6 isomerize to cyanamides 7 under thermal and photochemical conditions, but in the


6

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radical cations this formal 1,3-H shift has too high activation barrier to be considered (Scheme 1).8 Notably, the mass spectrum of Ph-NH-CN 7a features a strong (90%) m/z 91 ion [C6H5N.+ (M-HCN)] in the EI mass spectrum (Figure S1, Supporting Information),8 and

photolysis of Ph-NH-

CN 7a in a matrix at 5 K generates phenylnitrene, PhN 9a.9 In this paper we will show that the radical cations of 5-tetrazolyl-azines behave very differently from the aryltetrazoles examined so far, generating radical cations of heteroaryl-carbenes, which, as usual, can rearrange to the corresponding arylnitrenes. The reason for this difference is examined computationally.

Experimental The mass spectra shown in Figures 2 and 4 were measured on a CEC21-490 sector instrument using direct insertion. The spectra in Figure 5 were recorded on a Hewlett Packard GC 5890-MS 5970 instrument. The linked scans, high resolution measurements and comparative scans of

13C-labeled

and unlabeled compounds were performed on a Thermo-

Fischer DFS inverse geometry (BE) double focusing, high resolution instrument.

Chemically

induced

dissociation

(CID)

experiments

were

performed using He as a collision gas in the first field-free region of the


7


Page 8 of 28

spectrometer and a 5 keV accelerating voltage. All mass spectra were recorded at 70 eV electron ionization.

Computational Methods Ground-state geometries and energies were determined at the DFT level using the UB3LYP exchange-correlation functional with the 6-311G(d,p) basis set. Transition state optimizations and internal reaction coordinate (IRC) calculations were carried out at this DFT level. In order to take long-range electron correlation into account, especially as regards the hydrogen bond in 21.+, calculations for 12a.+ and 21.+-24.+ were also performed at the CAM-B3LYP/6-311++G(3f,3pd) level.10,11 Energies were a little lower at this level but within 4 kcal/mol of the values at the UB3LYP/6-311G(d,p) level (see Table S7, Supporting Information). The activation

energies

were

within

2

kcal/mol.

All

calculations

were

performed using the Gaussian 09 program package.12 Calculated energies are at 0 K. Relative energies are referenced to the nitrile imine radical cations 13.+.

Results and Discussion The general structures of the 2-, 3-, and 4-(5-tetrazolyl)pyridines 12a-c are shown in Figure 1 and the mass spectra in Figure 2.


8

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Figure 1. The 2-, 3-, and 4-(5-tetrazolyl)pyridines

It is immediately obvious that the 2-isomer 12a affords m/z 91 [M – 2 N2].+ as the base peak, whereas the 3-and 4-isomers 12b-c instead yield medium intensity m/z 92 [M – N2 – HCN].+ peaks, and the m/z 91 ions are totally insignificant. Thus, the 3- and 4-pyridyl isomers behave in the same way as 5-phenyltetrazole8 giving [M-N2]+. (m/z 119, 17.+) and [M-N2HCN]+. “nitrene” ions (m/z 92, 18.+) (Figure 2 and Scheme 3). A linked scan confirmed that m/z 119 is the source of m/z 92 (18c.+) (Figure S3, Supporting

Information).

The

calculated

ground

state

and

activation

energies (Scheme 3) are fully analogous to the data for 5-phenyltetrazole, and the meta-

or para-position of an N atom does not exert a large

influence. There is a ca. 10 kcal/mol difference in the energies of the ions

corresponding

to

the

closed

and

open

shell

states

of

the

imidoylnitrene, 15.+. Substantial energy differences were also found for the electronic states of phenylnitrene radical cation.8


9


Scheme

3.

3-

and

4-(5-Tetrazolyl)pyridine

Page 10 of 28

Radical

Cation

Fragmentations.a

a

Relative Energies in kcal/mol are reported for the c series (X = N, Y =

Z = CH). The corresponding energies for the b and b' series as well as additional, higher-energy paths from 16.+ to 15.+ are given in Figures S11-S13, Supporting Information. Numbers of the type TS 23.8 etc indicate the calculated activation energies.


10

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Figure 2. Mass spectra of 2-, 3-, and 4-(5-tetrazolyl)pyridines 12ac.


11


Page 12 of 28

A reason why the 5-tetrazolyl--azines behave so differently can be sought in the fact that, thanks to an intramolecular hydrogen bond, the 1H-tautomers of 5-tetrazolyl--azines are by far the most stable in the gas phase, accounting for 98-99% of the ensemble of tautomers,13 whereas for other 5-aryltetrazoles the 2H-tautomers (1) dominate in the gas phase.8 Thus, the intramolecularly H-bonded tetrazole 12a.+ (Scheme 4) lies ca. 10 kcal/mol below the non-H bonded isomer 20.+. A hydrogen shift in 12a.+ generates the still lower-energy isomer 21.+. Loss of N2 then yields the pyridinium diazomethyl radical derivative 22.+ over a very small barrier (see structures of 22.+ in Figure 2). This is a novel species, which has not been considered before. The species 22.+ is a direct source of the 2-pyridyldiazomethane ion 24.+, from which 2-pyridylcarbene ion 26.+ is readily obtained. The carbene-nitrene rearrangement8 then generates the phenylnitrene ion 9a.+ with barriers of no more than 25 kcal/mol (see Scheme 4). The species 22.+ may also with a slightly higher barrier eliminate a molecule of N2 to yield ion 23.+.

The relatively long exocyclic

C-C bond of 1.39 Å means that this ion is best described as a carbyne (23b.+) (Scheme 4 and Figure 3). This ion lies only ca. 11 kcal/mol above the 2-pyridylcarbene ion Z-26.+, to which it can rearrange over a barrier of 33 kcal/mol.


12

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Additional calculations at the CAM-B3LYP/6-311++G(3f,3pd) level were performed in order to address potential effects of long-range electron correlation. The energy of 12a.+ was 4 kcal/mol lower at this level, but the activation energy for 12a.+  21a.+ was unchanged (Table S7, Supporting Information). The energies of 21.+ and 22.+ were nearly unchanged, and that of 23.+ was 2 kcal/mol lower than at the UB3LYP/6-311G(d.p) level. The transition state for 22.+  24.+ was 2 kcal/mol higher. Thus, the energies obtained at the U-B3LYP/6-311G(d.p) level (Scheme 4) are considered to be satisfactory. We will report separately on calculations on the corresponding neutral molecules.

Scheme 4. Mechanism of Formation of the Radical Cations of 2Pyridylcarbene and its Isomers. Relative Energies in kcal/mola


13


a

Page 14 of 28

Numbers of the type TS 33.2 etc indicate the calculated activation

energies.

1.390 1.397 116.7 1.385

1.380 1.404

123.8 1.344

1.419 123.4 1.381

1.425

1.407

122.7

118.6

1.294 1.141

1.380

122.3

1.391 123.9 1.377

1.349

Figure 3. Calculated structures of the Z and E isomers of the pyridinium diazomethyl radical 22.+ (-19.1 and -15.2 kcal/mol, respectively) and the pyridinium carbyne species 23.+


14

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Thus, aas shown in Scheme 4, there are two facile routes for the formation of the 2-pyridylcarbene ion and hence the phenylnitrene ion and its isomers. Moreover, the species 22.+ and 23.+ can also rearrange to the ring-expanded ion 30.+, which constitutes another entry to the C6H5N.+ energy surface leading to the stable fulvenimine radical ion 33.+ previously investigated as a rearrangement product of phenylnitrene and iminocyclohexadienylidene ions8 (Scheme 4). The species 30.+ is a cyclic acetylene, but there is a second, higher-energy conformer of this structure and three separate transition states, all with energies between 45.0 and 56.6 kcal/mol, leading to the conformers of 30.+ (see Supporting Information).

Further possible species on this energy surface are shown

in Figure S14, Supporting Information. The validity of the mechanism in Scheme 4 was supported by labeling.

13C-

In the mass spectrum of 5-13C tetrazole 12a-13C, m/z 92 is the

base peak (100%), proving that

13C

is retained, i.e. H13CN has not been

eliminated (Scheme 5 and Table S1). The resulting phenylnitrene ion 9a13C.+

is expected to be labeled primarily in the ortho position following

the

previously

established

mechanisms

of

rearrangement

of

2-

pyridylcarbene4 and its radical cation8 (see Scheme 6).


15


Scheme 5. Generation of

13C12C H N 5 5

and

Page 16 of 28

12C H N 5 4 2

Ions from 5-13C-

Tetrazoles

In contrast, in the spectrum of the similarly labeled 4-pyridyl derivative

12c-13C,

N2

+

H13CN

are

eliminated,

yielding

the

4-

pyridylnitrene ion 18c.+ (Scheme 5 and Figures S1 - S3, Supporting Information). Although 9a-13C.+ and 18c.+ have the same mass (m/z 92), high resolution measurements of the unlabeled ions confirmed their different compositions, C6H5N and C5H4N2, respectively.

2-Pyridylcarbene radical cation 26.+ is also obtained by electron impact on 1,2,3-triazolo[1,5-a]pyridine 34 (Scheme 6),14 which generates a mass spectrum virtually identical with that of phenyl azide (several ions of the composition C6H5N.+ are assumed to interconvert to afford nearly identical spectra).8 The m/z 91 ion formed from 34 is the base peak


16

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(Figure 4). All the calculated activation barriers shown in Scheme 6 are very

modest,

easily

overcome

under

the

70

eV

electron

impact

conditions.

Scheme 6. Alternate Generation of 2-Pyridylcarbene Radical Cation 26.+ from 1,2,3-Triazolo[1,5-a]pyridine 34. Relative Energies in kcal/mola

a

Numbers of the type TS -17.3 etc indicate the calculated activation

energies.


17


Page 18 of 28

Figure 4. Mass spectrum of 1,2,3-triazolo[1,5-a]pyridine 34.

As indicated in Scheme 3, the pyridylnitrenes 18b.+ and 18c.+ (m/z 92) are also obtained from the corresponding azides 19. Their mass spectra are very similar, and again a nearly identical mass spectrum is also

obtained

pyrazinylcarbene

from

1,2,3-triazolo[1,5-a]pyrazine

radical

cation

39.+

can

36.

interconvert

Here, with

the

2-

the

4-

pyridylnitrene ion 18c.+ via the diazacycloheptatetraene 40.+ (Scheme 7 and Figure 5). Here, too, the calculated activation barriers are very modest and easily overcome under mass spectrometry conditions.

Scheme 7. Alternate Generation of C5H4N2.+ Radical Cations from 1,2,3-Triazolo[1,5a]pyrazine 36. Relative Energies in kcal/mol


18

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Figure 5. Mass spectra of 4- and 3-pyridyl azides 19c and 19b and 1,2,3-triazolo[1,5-a]pyrazine 36.


19


Page 20 of 28

The fragmentation pattern described for tetrazole 12a with [M-2N2]+. as the base peak is also shown by several other 5-tetrazolyl--azines, including the 2-quinoline, 14-quinazoline derivatives

and 3-isoquinoline, 2- and 4-pyrimidine, and

41-50 (Chart 1 and Figures S4-S7, Supporting

Information). Here too, the 3- and 4-(5-tetrazolyl)quinolines 47 and 48 are different, eliminating N2 + HCN instead of 2 N2 (Table S2).

Chart 1. -Tetrazolylazines Eliminating 2 N2 in the Mass Spectrometer to Formally Generate the -Azinylcarbene Radical Ions.


20

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The fragmentation pattern of the 1-isoquinolyl derivative 42 was confirmed by

13C-labeling

(Scheme 8 and Figures S5 and S6).

Scheme 8. Fragmentation Patterns of 1-(5-Tetrazolyl)isoquinoline and 1(5-13C-Tetrazolyl)isoquinoline 42 and 42-13C.


21


Page 22 of 28

The 2-(5-tetrazolyl)-pyrazine and -quinoxaline derivatives 49 and 50 (Chart 1) exhibit both modes of fragmentation, i.e. [M-2N2]+. and [M-N2HCN]+. (Table S3 and Figure S10); one could say that these compounds have the structural elements and fragmentation patterns of both 2-pyridyl and 3-pyridyl analogues.

Conclusion It

was

shown

recently

that

phenylnitrene

is

generated

on

matrix

photolysis of 5-phenyltetrazole 1e.9 Similarly, the phenylnitrene radical cation and/or its isomers of composition C6H5N (m/z 91) are obtained upon electron impact.8 It has now been shown that 3- and 4-(5tetrazolyl)pyridines 12b,c yield 3- and 4-pyridylnitrene radical cations 18b,c.+ (m/z 92) on electron impact, but 2-pyridyldiazomethane 24.+ and


22

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2-pyridylcarbene 26.+ radical cations (m/z 119 and 91) are obtained in the mass spectrum of 2-(5-tetrazolyl)pyridine 12a.+. The 2-pyridylcarbene radical

cation

then

undergoes

the

well-established

carbene-nitrene

rearrangement3 to yield the phenylnitrene radical cation 9a.+ or isomers of this ion. Thanks to a strong hydrogen bond between the tetrazole and pyridine moieties, 2-(5-tetrazolyl)pyridines prefer to exist in the 1H-form 12a, where a full transfer of hydrogen to the pyridine nitrogen generates the isomer 21. This allows facile generation of the radical cations of pyridinium diazomethyl radical 22.+, pyridinium-2-carbyne 23.+, and 2pyridyldiazomethane 24.+ from which 2-pyridylcarbene 26.+, phenylnitrene 9a.+ or their isomers are readily formed. The different behaviors of the 2and 4-(5-tetrazolyl)pyridines was confirmed by rings,

whereby

2-(5-tetrazolyl)pyridine

13C-labeling

12a

of the tetrazole

generates

pyridylcarbene/phenylnitrene radical cations, which retain the

2-

13C

label,

whereas 4-(5-tetrazolyl)pyridine 12c generates pyridylnitrene 18c.+ or its isomers, in which the

13C

label has been eliminated in the form of

H13CN. The carbene route was also confirmed by of

1-(5-tetrazolyl)isoquinoline

42-13C.

The

13C-labeling

radical

in the case

cations

of

2-

pyridylcarbene, phenylnitrene, or their isomers (m/z 91) are also formed on

electron

impact

of

1,2,3-triazolo[1,5-a]pyridine

34,

and

the

pyridylnitrene radical cations 18c.+ or its isomers (m/z 92) are generated


23


Page 24 of 28

similarly from 1,2,3-triazolo[1,5-a]pyrazine 36. -Heteroaryltetrazoles in the quinoline, isoquinoline, pyrimidine, quinazoline, pyrazine and quinoxaline series (41-50) behave in the same way as 2-(5-tetrazolyl)pyridine, yielding heteroarylcarbene/arylnitrene radical cations and/or their isomers in the mass

spectrometer.

aryltetrazoles

The

(yielding

cause nitrene

of

the radical

different

behaviors

cations)

and

of

5-

5-(-

heteroaryl)tetrazoles (yielding -heteroarylcarbene radical cations) can be traced to the hydrogen bonds between tetrazole N-H and pyridine-N, leading to the formation of pyridinium tetrazoles 21.+ and pyridinium diazomethyl radicals 22.+.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca. Additional mass spectra (Figures S1-S10 and Tables S1-S3). Computational details, Cartesian coordinates, absolute energies, vibrational analysis, and imaginary frequencies (Figures S11-S16 and Tables S4-S10.

AUTHOR INFORMATION


24

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Corresponding Authors *D. Bégué. E-mail: [email protected] *C. Wentrup. E-mail: [email protected] ORCID: Curt Wentrup: 0000-0003-0874-7144 Didier Bégué: 0000-0002-4553-0166 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.

References (1) Synthetic Applications of 1,3-Dipolar Cycloaddition Chemistry Toward

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