4-Pyridylnitrene and 2-pyrazinylcarbene

Article abstract of DOI:10.3762/bjoc.9.85

Both flash vacuum thermolysis (FVT) and matrix photolysis generate 2-diazomethylpyrazine (22) from 1,2,3-triazolo[1,5- a]pyrazine (24). FVT of 4-azidopyridine (18) as well as of 24 or 2-(5-tetrazolyl)pyrazine (23) affords the products expected from the nitrene, i.e., 4,4'-azopyridine and 2- and 3-cyanopyrroles. Matrix photolyses of both 18 and 24 result in ring expansion of 4-pyridylnitrene/2-pyrazinylcarbene to 1,5-diazacyclohepta-1,2,4,6-tetraene (20). Further photolysis causes ring opening to the ketenimine 27.

Full text of DOI:10.3762/bjoc.9.85

4-Pyridylnitrene and 2-pyrazinylcarbene
Curt Wentrup*, Ales Reisinger and David Kvaskoff
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 754–760.
School of Chemistry and Molecular Biosciences, The University of
Queensland, Brisbane, Qld 4072, Australia
doi:10.3762/bjoc.9.85
Received: 29 January 2013
Accepted: 22 March 2013
Published: 17 April 2013
Email:
Curt Wentrup* - wentrup@uq.edu.au
* Corresponding author
This article is part of the Thematic Series "New reactive intermediates in
organic chemistry".
Keywords:
carbene–nitrene interconversion; diazepines; flash vacuum
thermolysis; matrix photochemistry; nitrile ylides; reactive
intermediates
Guest Editor: G. Bucher
© 2013 Wentrup et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Both flash vacuum thermolysis (FVT) and matrix photolysis generate 2-diazomethylpyrazine (22) from 1,2,3-triazolo[1,5-
a]pyrazine (24). FVT of 4-azidopyridine (18) as well as of 24 or 2-(5-tetrazolyl)pyrazine (23) affords the products expected from
the nitrene, i.e., 4,4’-azopyridine and 2- and 3-cyanopyrroles. Matrix photolyses of both 18 and 24 result in ring expansion of
4-pyridylnitrene/2-pyrazinylcarbene to 1,5-diazacyclohepta-1,2,4,6-tetraene (20). Further photolysis causes ring opening to the
ketenimine 27.
Introduction
The carbene–nitrene interconversion exemplified with phenyl-
nitrene (1) and 2-pyridylcarbene (3) has been described in
considerable detail [1-3]. The three species 1–3 have all been
observed directly by IR or ESR spectroscopy in low-tempera-
ture matrices. The normal reaction products of phenylnitrene
are azobenzene (4) and aniline (5), but under forcing flash
vacuum thermolysis (FVT) conditions cyanocyclopentadiene 6
is formed (Scheme 1). Several other carbene–nitrene rearrange-
ments have been reported [3-5].
In addition to the ring expansion (1–2–3), two ring opening
reactions have been investigated in recent years. Type I ring
opening takes place in nitrenes possessing a 1,3-relationship
between a nitrene centre and a ring nitrogen atom and leads to
Scheme 1: Phenylnitrene–2-pyridylcarbene rearrangement.
nitrile ylides, such as 9 in the case of 3-pyridylnitrene (7),
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whereby the actual ring opening may take place in either the
nitrene itself or the ring-expanded ketenimine 8 (Scheme 2)
[6-8]. A 1,7-H shift finally converts the nitrile ylide 9 to the
open-chain ketenimine 10 (Scheme 2). 3-Pyridylcarbene under-
goes analogous Type I ylidic ring opening to an ethynylvinyl-
nitrile imine [6]. Type II ring opening is diradicaloid and
proceeds via an open-chain vinylnitrene or biradical 13 in
nitrenes such as 2-pyridylnitrene (11). This leads to minor
amounts of ketenimines 14 or glutacononitriles 15 beside the
main product, the cyclic carbodiimide 12, which is formed in a
degenerate rearrangement (11≡11). The major end products of
FVT are the cyanopyrroles 16 and 17 (Scheme 2) [3,9-11].
Results and Discussion
Here, we report details of the FVT as well as matrix photolysis
reactions of 4-azidopyridine (18) and the isomeric triazolo[1,5-
a]pyrazine (24) and its precursor, 2-(5-tetrazolyl)pyrazine (23).
Furthermore, we report evidence for ring expansion as well as
ring opening of 4-pyridylnitrene (19) and 2-pyrazinylcarbene
(21) (Scheme 3). Mild FVT of 18 at 400 °C, even in high
vacuum, affords 4,4’-azopyridine (25) in 54% yield [12]. This
kind of reaction is typically ascribed to the dimerization of
triplet nitrenes. Most importantly, FVT of the pyrazinylcarbene
precursor 23 also affords a small amount of 4,4’-azopyridine
(25), thus implying a rearrangement of carbene 21 to nitrene 19
(Scheme 3). Presumably, this takes place via the diazacyclo-
heptatetraene, 20, which was not directly observed under the
Scheme 2: Type I and type II ring opening and ring expansion in 3-
and 2-pyridylnitrenes, respectively.
FVT conditions. It was, however, readily observed under photo- diene 6 [12]. Similar pyrolysis of 4-azidopyridine results in ring
lysis conditions, as described below. Since nitrenes are intrin- contraction to a mixture of 2- and 3-cyanopyrroles 16 and 17.
sically more stable than the isomeric carbenes [3,12,13], a Because the ring contraction is highly exothermic, the product
rearrangement of 21 to 19 is perfectly reasonable.
will be chemically activated [14], and therefore interconversion
of the two cyanopyrroles cannot be avoided. Only a minute
In the case of phenylazide, mild FVT results in the formation of trace of 4,4’-azopyridine is formed under these conditions. The
azobenzene (4) (Scheme 1), but violent FVT, where the pres- total yield of the cyanopyrroles is low because of heavy char-
sure is allowed to rise to ca. 1 mbar due to a shock wave ring inside the pyrolysis tube. The exact mechanisms of ring
induced by rapid distillation of the azide into the furnace under contraction in arylnitrenes are under investigation, but in the
high vacuum, results in ring contraction to cyanocyclopenta- case of 2-pyridylnitrene (Scheme 2) three routes to the cyano-
Scheme 3: FVT reactions of 4-azidopyridine (18), 2-(5-tetrazolyl)pyrazine (23) and triazolo[1,5-a]pyrazine (24).
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Beilstein J. Org. Chem. 2013, 9, 754–760.
pyrroles have been described [10], as has their thermal intercon-
version [9,10]. The analogous, concerted ring contraction of
phenylnitrene to 5-cyanocyclopentadiene has a calculated
barrier of ca. 30 kcal/mol [15].
FVT of the tetrazole 23 at 400 °C/10−4 mbar causes loss of N2
and formation of 2-diazomethylpyrazine (22), which is easily
observed directly by its strong absorption at 2080 cm−1 when
the pyrolysate is isolated neat in liquid N2 at −196 °C, or at
2092 cm−1 upon isolation in an Ar matrix at 10 K. The diazo
compound is also formed on matrix photolysis of the triazole
(Figure 1). Compound 22 can exist as s-Z and s-E conformers.
One conformer dominates, absorbing strongly at 2092 cm−1; the
other, minor absorption is at 2076 cm−1 (Figure 1). Warming
the neat diazo compound to ca. −30 °C causes ring closure to
triazole 24. Compound 24 can be isolated in up to 20% yield in
preparative pyrolysis of 23 [16]. Further FVT of either 23 or 24
at ≥450 °C/10−3 mbar results in the formation of approximately
equal amounts of the cyanopyrroles 16 and 17 and azo com-
pound 25, but the overall yield is again poor, ca. 30%, due to
heavy charring in the pyrolysis tube. It is worth noting that
ethynylimidazoles, which could be thought of as ring-contrac-
tion products of carbene 21, were not detectable.
Figure 1: Difference-IR spectrum of 2-diazomethylpyrazine (22)
(positive peaks) in Ar matrix at 7 K, obtained by photolysis of triazolo-
pyrazine 24 at 290 nm for 10 min (negative peaks). Spectrum assigned
to 22: 795, 835, 918, 1009, 1056, 1150, 1299, 1434, 1476, 1574, 2076
and 2092 cm−1; the latter two absorptions are assigned to the two s-E
and s-Z conformations of the diazo group. The peak at 1872 cm−1 is
due to the formation of a small amount of the photoproduct 20. See the
Supporting Information File 1, Figure S1 for a full spectrum of 24 with
peak listing.
seven-membered ring ketenimine 20 as observed by IR spec-
Photolysis of either 4-azidopyridine (18) or triazole 24 in an Ar troscopy (Figure 2). In the case of triazole 24, efficient ring
matrix at 7–10 K causes efficient ring expansion to the cyclic opening to the diazo compound 22 happens first (Figure 1), so
Figure 2: Ar matrix IR-difference spectra showing the products of broadband UV photolysis of 4-azidopyridine (18) (bottom, bands labelled A) and
1,2,3-triazolo[1,5-a]pyrazine (24) (top, bands labelled T). In the photolysis of 18 the negative peaks (A) are due to the subtracted spectrum of azide
18; the positive peaks are due to the products 20 (B) and 27 (C). In the photolysis of 24 the negative peaks are the products 20 and 27; the positive
peaks are due to the subtracted spectrum of the triazole 24 (T). A small peak labelled X at 2340 cm−1 is due to CO2. Ordinate in arbitrary absorbance
units. See Figure S1–S3 in Supporting Information File 1 for full spectra of 18, 20 and 24 with peak listings.
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Beilstein J. Org. Chem. 2013, 9, 754–760.
the latter is the immediate precursor of 20. The IR spectrum of the the pyridyl azide photolysis in Figure 2 because of masking
the triazole 24 is shown in Figure S1, Supporting Information by the strong, negative azide bands. However, the 2120 cm−1
File 1.
band is clearly seen later in the photolysis when most of the
azide has been consumed (see Figure 4 and Supporting Infor-
It is clearly seen in Figure 2 that the two starting materials 18 mation File 1, Figure S5). The temporal evolution of the azide
and 24 afford nearly identical product mixtures, and the absorp- photolysis reveals that the cyclic ketenimine 20 is formed first,
tions of ketenimine 20 account for the majority of the spectrum, followed by the open chain ketenimine 27 (Figure S3,
including the very strong cumulenic absorption at 1872 cm−1. Supporting Information File 1). It should be noted that keten-
This cumulene 20 is obtained nearly pure after short photolysis imine 27 can exist as four different s-E and s-Z conformers,
times (Figure 3).
with slightly different IR absorptions (the calculated spectra are
shown in Figure 4 and Figure S6 and Figure S7, Supporting In-
Further photolysis, especially at higher energy (254 nm), causes formation File 1). The experimental spectra indicate that all four
formation of a second product with prominent signals in the IR conformers of 27 are formed, giving rise to bands at
spectrum at 2028, 2038, 2049, 2060 and 2120 cm−1 (Figure 4 2028–2060 cm−1, i.e., there is enough energy available to effect
and Supporting Information File 1, Figure S4 and Figure S5). E–Z and s-E–s-Z isomerization in the matrix. Also the iso-
The group of bands at 2028–2060 cm−1 is ascribed to cyanide band at 2120 cm−1 shows splitting, but this is less well
the ketenimine moiety (N=C=C), and the band at 2120 cm−1 resolved (Figure 4). All this makes it difficult to assign other,
to the isocyanide (NC) function in the ring-opened N-(isocyano- much weaker, peaks in the IR spectra to compound 27. How-
vinyl)ketenimine (27) (Scheme 4). The latter band is not seen in ever, the following bands, which are not ascribed to the cyclic
ketenimine 20, can be assigned to 27: Observed wavenumbers
and, in parentheses, the calculated harmonic frequencies of the
conformers of 27 at the B3LYP/6-31G* level, scaled by 0.9613:
711 (691–703), 1300 (1256–1290), 1380 (1373–1377),
1608–1624 (1612–1634), 2028, 2038, 2049, 2061 (2038–2063),
2119–2124 (2112–2117) cm−1 (see Supporting Information
File 1 for computational details).
Figure 4: Bottom: IR spectrum from the matrix photolysis of azide 18
after the azide has been depleted completely (10 min broadband UV,
Figure 3: Top: calculated IR spectrum of 20 at the B3LYP/6-31G* level
(wavenumbers scaled by 0.9613): ν’ (relative intensity) 697 (9), 782
(16), 815 (22), 876 (9), 905 (12), 980 (5), 1021 (14), 1108 (8), 1208
(3), 1265 (8), 1325 (13), 1511 (3), 1550 (11), 1866 (100) cm−1.
Ordinate in arbitrary absorbance units. Bottom: Ar matrix IR-difference
spectrum of diazacycloheptatetraene 20 (ν’ 710, 794, 826, 887, 1028,
1109, 1272, 1332, 1545, 1872 cm−1), formed by photolysis of
4-azidopyridine (18) (3 min broadband UV; positive bands due to 20;
negative bands due to 18). See Figures S2 and S3 in Supporting Infor-
mation File 1 for full spectra of 18 and 20 with peak listings.
followed by 30 min low pressure Hg lamp at 254 nm). Top: the calcu-
lated absorptions of the four isomers of the open chain ketenimine 27
(1:1:1:1 ratio of the s-E,E; s-E,Z; s-Z,E; and s-Z,Z isomers). The calcu-
lated peak at 2039 cm−1 is due to the overlapping peaks of two
isomers at 2039 and 2040 cm−1. See Figure S6 and Figure S7 in
Supporting Information File 1 for structures and spectra of the indi-
vidual isomers. Calculated spectra are at the B3LYP/6-31G* level
(wavenumbers scaled by 0.9613). The 254 nm photolysis causes a
faster rearrangement of 20 to 27; broadband UV photolysis achieves a
similar change in ca. 60 min.
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Beilstein J. Org. Chem. 2013, 9, 754–760.
Scheme 4: Photolysis reactions of azide 18 and triazole 24 in Ar matrix.
Experimental
We postulate that the rearrangement to 27 occurs via the unob-
served nitrile ylide intermediate 26 (Scheme 4). The calculated General
IR spectra of the four s-E and s-Z conformers of 26 are shown The apparatus and procedures for preparative FVT [18] and for
in Figure S8 and Figure S9 in Supporting Information File 1. Ar matrix isolation [11,19,20] were as previously described.
Related nitrile ylides are known to undergo very facile 1,7-H KBr and CsI windows were used for IR spectroscopy. FVT
shifts (see, e.g., Scheme 2) [3,6]. Both the nitrile ylide and the products were isolated in liquid nitrogen (77 K) in the prepara-
ring-expanded ketenimine were observed in the corresponding tive thermolyses, and at 22–25 K in Ar matrices for IR experi-
benzannelated systems, viz 4-azidoquinoline and 2-diazo- ments. IR spectra of the Ar matrices were measured at 7–10 K
methylquinoxaline [17], and the ylidic ring opening is very with a resolution of 1 cm−1. Photolyses were done through
facile in 3-pyridylcarbene [6]. Thus, there is good precedence quartz by using a 75 W low-pressure Hg lamp (254 nm) or a
for transformations of the type 26→27 (Scheme 4).
1000 W high pressure Hg/Xe lamp equipped with a monochro-
mator and appropriate filters. A water filter was used to remove
infrared radiation.
Conclusion
2-Pyrazinylcarbene (21) rearranges to 4-pyridylnitrene (19) on
FVT as evidenced by the isolation of 4,4’-azopyridine (25). Materials
Both 19 and 21 undergo ring expansion to the diazacyclohepta- 4-Azidopyridine (18) was prepared from 4-bromopyridine and
tetraene 20 on UV photolysis. Compound 20 is clearly identi- NaN3 in 10% ethanol under reflux for 8 h by adaptation of a
fied by its matrix IR spectrum, particularly the strong cumu- literature method [21]. 1,2,3-Triazolo[1,5-a]pyrazine (24) was
lenic stretch at 1872 cm−1. Further photolysis causes the gradual prepared according to a literature procedure [16].
depletion of 20 and formation of the ring opened ketenimine 27.
The latter reaction indicates ring opening of either cumulene 20 FVT of 4-azidopyridine (18)
or carbene 21 to the nitrile ylide 26 followed by a rapid 1,7-H The azide (0.50 g) was distilled into the FVT apparatus from
shift to 27.
a sample flask held at 35 °C and thermolysed at
400 °C/10−2–10−3 mbar in the course of 1 h. 4,4’-Azopyridine
In summary, 4-pyridylnitrene 19 behaves much like phenyl- (25) (54% yield; red solid, mp 107–108 °C) was identified by
nitrene 1. The thermal chemistry of both the nitrene 19 and the comparison with a sample prepared according to den Hertog et
carbene 21 is dominated by nitrene reactions (giving azo- al. (mp 107.5 °C) [22]. Traces of unreacted azide 18 and 2- and
pyridine and cyanopyrroles), because the carbene rearranges to 3-cyanopyrroles 16 and 17were detected by GC–MS and IR
the lower energy nitrene. However, the 2-pyrazinylcarbene 21 spectroscopy.
possesses a 1,3-relationship between the carbene centre and a
ring nitrogen atom like in 3-pyridylcarbene [6] and therefore “Violent pyrolysis” was achieved by holding the sample flask at
undergoes ylidic ring opening. Thanks to the reversible 100 °C. The rapid distillation causes a shock wave and a pres-
carbene–nitrene rearrangement (19
20
21) formation of sure increase to 1 mbar under continuous pumping with a two-
the ylide 26 and hence the ketenimine 27 becomes the dominant stage oil pump capable of a base vacuum of 10−4 mbar. The
reaction under the conditions of matrix photolysis.
whole reaction is finished in a matter of seconds. The 2- and
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Beilstein J. Org. Chem. 2013, 9, 754–760.
3-cyanopyrroles 16 and 17 were isolated in a ratio of ca. 1:1 in a
total yield of 10% as analysed by GC. Only a trace of 4,4’-azo-
pyridine (25) was formed under these conditions. There was
heavy charring inside the pyrolysis tube.
Supporting Information
Supporting Information File 1
Additional matrix IR spectra of 18, 24, and their photolysis
products, calculated IR spectra of 20, 26 and 27, and
computational details.
Photolysis of 4-azidopyridine (18)
The azide (5–10 mg) was evaporated from a reservoir at −30 to
−40 °C and codeposited with Ar at 25 K. After cooling to 7 K,
the spectrum was recorded (Figure S2, Supporting Information
File 1): IR (Ar, 7 K) 813, 1271, 1303, 1586, 2100, 2119, 2145,
2280 cm−1.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-85-S1.pdf]
Acknowledgements
This work was aided by the National Computing Merit
Allocation Scheme, project g01, supported by the Australian
Government.
The azide matrix was irradiated with broadband UV light from
the high-pressure Xe/Hg lamp, at 290 nm using the monochro-
mator, or at 254 nm using the low-pressure Hg lamp. The
resulting product-difference IR spectra are shown in Figure 2,
Figure 3 and Figure 4. The temporal evolution of the spectrum
under continuous broadband photolysis at 7 K is shown in
Figure S3, Figure S4 and Figure S5, Supporting Information
File 1.
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