Interconversion of nitrenes, carbenes, and nitrile ylides by ring expansion, ring opening, ring contraction, and ring closure: 3-Quinolylnitrene, 2-quinoxalylcarbene, and 3-quinolylcarbene

Article abstract of DOI:10.1071/CH08523

Photolysis of 3-azidoquinoline 6 in an Ar matrix generates 3-quinolylnitrene 7, which is characterized by its electron spin resonance (ESR), UV, and IR spectra in Ar matrices. Nitrene 7 undergoes ring opening to a nitrile ylide 19, also characterized by i

Full text of DOI:10.1071/CH08523

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 275–286
Interconversion of Nitrenes, Carbenes, and Nitrile Ylides
by Ring Expansion, Ring Opening, Ring Contraction, and
Ring Closure: 3-Quinolylnitrene, 2-Quinoxalylcarbene,
and 3-Quinolylcarbene
David Kvaskoff,A Ullrich Mitschke,A Chris Addicott,A Justin Finnerty,A
Pawel Bednarek,A and Curt WentrupA,B
ASchool of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, Qld 4072, Australia.
BCorresponding author. Email: wentrup@uq.edu.au
Photolysis of 3-azidoquinoline 6 in an Ar matrix generates 3-quinolylnitrene 7, which is characterized by its electron spin
resonance (ESR), UV, and IR spectra in Ar matrices. Nitrene 7 undergoes ring opening to a nitrile ylide 19, also charac-
terized by its UV and IR spectra. A subsequent 1,7-hydrogen shift in the ylide 19 affords 3-(2-isocyanophenyl)ketenimine
20. Matrix photolysis of 1,2,3-triazolo[1,5-c]quinoxaline 26 generates 4-diazomethylquinazoline 27, followed by
4-quinazolylcarbene 28, which is characterized by ESR and IR spectroscopy. Further photolysis of carbene 28 slowly
generates ketenimine 20, thus suggesting that ylide 19 is formed initially. Flash vacuum thermolysis (FVT) of both 6 and
26 affords 3-cyanoindole 22 in high yield, thereby indicating that carbene 28 and nitrene 7 enter the same energy sur-
face. Matrix photolysis of 3-quinolyldiazomethane 30 generates 3-quinolylcarbene 31, which on photolysis at >500 nm
reacts with N2 to regenerate diazo compound 30. Photolysis of 30 in the presence of CO generates a ketene (34).
3-Quinolylcarbene 31 cyclizes on photolysis at >500 nm to 5-aza-2,3-benzobicyclo[4.1.0]hepta-2,4,7-triene 32. Both
31 and 32 are characterized by their IR and UV spectra. FVT of 30 yields a mixture of 2- and 3-cyanoindenes via a
carbene–carbene–nitrene rearrangement 31 → 2-quinolylcarbene 39 → 1-naphthylnitrene 43. The reaction mechanisms
are supported by density functional theory calculations of the energies and spectra of all relevant ground and transition
state structures at the B3LYP/6–31G* level.
Manuscript received: 27 November 2008.
Final version: 19 January 2009.
C
XN2
x
C
Introduction
X
H
XH
hν
hν
ꢁ
ꢁ
N
5
ꢀ
C
The chemistry of nitrenes and carbenes is of significant theo-
retical and practical importance. For example, they can be
employed in photoaffinity labelling,[1] and the ring expansion of
arylnitrenes in syntheses of azepines and diazepines.[2] More-
over, several rearrangement reactions have been discovered.
Photolyses of 3-pyridyldiazomethane and 3-pyridyl azide (1a
and 1b) in low temperature Ar matrices lead to ring opening
of the corresponding carbene and nitrene 2a and 2b and/or the
seven-membered cyclic cumulene 3 to the nitrile ylides 4. The
ylides 4 were directly observed by IR and UV spectroscopy
(Scheme 1).[3] Further photolysis led to the conversion of 4
to an allene or ketenimine 5 by means of a 1,7-H shift. The
involvement of open-chain nitrile ylides has also been postulated
as an important aspect of the rearrangements of 2-pyrazinyl-
and 4-pyrimidylnitrenes,[4] and these reaction paths have been
supported by theoretical calculations.
ꢀN2
∼1,7-H
ꢀ
C
N
N
N
1
2
4
X
a
b
X ꢂ CH
X ꢂ N
N
3
Scheme 1. 3-Pyridylcarbene and 3-pyridylnitrene ring-opening to nitrile
ylide 4.
(Scheme 2).[5] We found that the photolysis of 6 in 1,4-dioxan
(high pressure Hg–Xe lamp, 0◦C, quartz) yielded the same
products, which are perfectly consistent with formation of
3-quinolylnitrene 7 in its triplet ground state T0 (7T), which
dimerizes or abstracts hydrogen.
We have now investigated 3-quinolylnitrene and report fur-
ther decisive evidence for ylidic ring opening as a dominant
photochemical reaction pathway. A previous investigation of
the thermal decomposition of 3-azidoquinoline 6 in bromoben-
zene revealed the formation of a mixture of 3-aminoquinoline
8, 3,3ꢀ-azoquinoline 9, and pyrazino[2,3-c:5,6-cꢀ]diquinoline 10
Early work by Reiser et al.[6] described the characterization of
nitrenes formed by photolysis of aromatic azides, including 6, by
UV-visible spectroscopy in organic matrices at 77 K. The quan-
tum yield (0.87) and mechanism of photodecomposition were
investigated, anditwasconcludedthatN2 eliminationtakesplace
in a hot ground state.[6b] It is undoubtedly the triplet ground state
© CSIRO 2009
10.1071/CH08523
0004-9425/09/030275

276
D. Kvaskoff et al.
NH2
N3
N:
g ꢂ 2
N
N
N
6
7
8
N
N
N
N
N
ꢁ
N
ꢁ
N
N
9
10
N
NH
N3
hν
or
OMe
O
NaOMe/MeOH
N
N
N
N
OMe
H
6
6
11
12
N
NH2
N
NEt2
N3
hν
1500 2500 3500 4500 5500 6500 7500 8500
Magnetic field [G]
HNEt2
N
13
Fig. 1. Electron spin resonance spectrum of triplet 3-quinolylnitrene 7T in
Ar matrix at ∼15 K, obtained by photolysis of 3-azidoquinoline 6 at 308 nm
for 2 min. ZFS parameters D = 0.9306 and E = 0.0007 cm−1. X2 = 6810 G,
Y2 = 6844 G, H0 = 3471.3 G, microwave frequency 9.72832 GHz. The sig-
nal at g = 2 is due to doublet radicals. See also Fig. S1 for the results of
further photolysis at λ > 600 nm (no effect) and > 395 nm (nitrene vanishes
in 5 min).
Scheme 2. Solution chemistry of 3-quinolylnitrene.
N
N:
Trapping
products
N
N
N
N
7
15TS (141.6)
14 (60.0)
T0 0; S1 59.8
o-quinoidal, it is a less likely intermediate (calculated
133 kJ mol−1 above triplet 7T). The zwitterionic valence isomer
17 – a cyclic nitrile ylide – is of lower energy (97 kJ mol−1 above
triplet 7T), but, owing to its polarity, it is not likely to afford the
observed products. The alternative cyclic ketenimine 18 retains
one aromatic ring, which makes it more stable than 16 by a cal-
culated 111 kJ mol−1 and a potential intermediate in the matrix
photochemistry. However, 18 is not the species being trapped
in solution, for no corresponding diazepine has been isolated
(Scheme 3).
Nꢁ
N
N
N
N
N
18 (22.1)
17 (97.4)
16 (133.0)
Scheme 3. Possible rearrangements of 3-quinolylnitrene (relative energies
in kJ mol−1, calculated at the B3LYP/6–31G* level).
nitrenes (T0) that are observed under such conditions. Energies
of open-shell singlet nitrenes (S1) cannot be calculated using
density functional theory (DFT) methods, but they can be esti-
mated using Cramer’s method.[7] In this way, we estimate that the
open-shell nitrene 7S (S1) lies ∼60 kJ mol−1 above the triplet,
7T (T0) (Scheme 3).
The solution photolysis of 3-azidoquinoline in the presence
of trapping reagents was found to afford benzo[e]-1,4-diazepine
derivatives 11 and 12.[8,9] Thus, irradiation through Pyrex in
the presence of sodium methoxide resulted in compound 11,[8]
whereas irradiation through quartz and acid workup led to the
diazepinone 12 (43%) together with a substantial amount (24%)
of the hydrogen-abstraction product, 3-aminoquinoline 8.[9]
However, 3-diethylamino-4-aminoquinoline 13 was obtained in
the presence of diethylamine, and no diazepine product was
isolated (Scheme 2).[9]
These results suggest the involvement of azirene 14 rather
than the alternative 15 as precursor of the azepines (Scheme 3).
Being o-quinoidal, azirene 15 is significantly higher in energy
than 14;[10,11] in fact, 15 is not an energy minimum but a tran-
sition state at the B3LYP/6–31G* level (14 and 15 lying 60 and
142 kJ mol−1 above the triplet nitrene 7T, respectively) (all ener-
gies are corrected for zero-point vibrational energies (ZPVEs)
throughout the current paper)). Azirene 14 is the likely source
of the solution-trapping products 11–13. The ring-expanded
cyclic ketenimine 16 could also afford these products, but being
Results and Discussion
3-Quinolylnitrene 7
Photolysis at 308 nm of 3-azidoquinoline 6 isolated in an Ar
matrix at ∼15 K in a cryostat for electron spin resonance
(ESR) spectroscopy afforded a strong signal, typical of aromatic
nitrenes, with zero-field splitting (ZFS) parameters D = 0.9306
and E = 0.0007 cm−1, and assigned to the triplet 7T (Fig. 1).
We have previously reported linear correlations between exper-
imental D values and calculated spin densities ρ on the carbene
or nitrene centre in carbenes[12] and nitrenes.[13] The D value
of 0.9306 cm−1 for 7 fits the nitrene correlation very well
(ρ = 1.5112 at the UB3LYP/EPR-III level).
3-Azidoquinoline 6 absorbs in the UV at λmax = 216, 250,
275, 320, and 335 nm, and in the IR, the strongest band is
due to the N3 antisymmetric stretching vibration at 2117 cm−1
(Figs S2, S3 and Table S1, Accessory Publication). Photolysis
of Ar-matrix isolated azide 6 at λ > 324 nm in a cryostat
for IR and UV spectroscopy was very clean, yielding 2-(2-
iminovinyl)isocyanobenzene 20 in high yield (Scheme 4, Fig. 2).
IR absorptions at 3304 and 3294, 2025 and 2047, and 2125
and 2129 cm−1 correspond to the NH, ketenimine and isocyano
functions in 20. The appearance of multiplets can be ascribed
to the presence of different conformers and sites, and it is typ-
ical of ketenimines to exhibit multiple peaks.[14] There is very
good agreement with a calculated spectrum of a mixture of the

Interconversion of Nitrenes, Carbenes, and Nitrile Ylides
277
hν
0.0
ꢀ0.1
ꢀ0.2
ꢀ0.3
ꢀ0.4
N3
N:
C
λ ꢃ 308 nm
N
H
ꢁ
Ar, 7 K
ꢀ
C
N
N
N
19
6
7
λ ꢄ 600nm
∼1,7-H
689
383
369
A
C
NH
ꢁ
N
ꢀ
C
628
20
545
CN
C
581
N
ꢁ
Scheme 4. Reactions of 3-azidoquinoline on photolysis.
H
ꢀ
300
400
500
600
700
800
H
λ [nm]
C
C
NH
Fig. 3. UV-visible difference spectrum. Photolysis of a 10 K Ar matrix of
3-azidoquinoline 6 at 308 nm for 20 min to generate ylide 19 + nitrene 7
followed by λ > 600 nm for 7 min to deplete ylide 19. Negative spectrum
due to ylide 19.
N
C
ꢁ
ꢀ
0.0
565
526
491
(a)
462
2025
ꢀ0.2
ꢀ0.4
ꢀ0.6
425
A
359
369
N
2129
N
375
400
3304
(b)
300
500
600
700
800
λ [nm]
N3
Fig. 4. UV-visible difference spectrum. Photolysis of the Ar matrix of
3-azidoquinoline 6 at 308 nm for 20 min to generate ylide 19 + nitrene 7
followed by λ > 600 nm for 7 min to deplete ylide 19 and λ > 395 nm for
1 min to deplete the nitrene. Negative spectrum due to nitrene 7.
N
3300 2900 2500 2100 1700 1300
ν [cmꢀ1
900
500
]
and 43 km mol−1, respectively, B3LYP/6–31G*; wavenumbers
scaled by a factor of 0.9613).
Fig. 2. (a) Calculated IR spectrum of a 3:1 mixture of ketenimine s-E-20
and s-Z-20 (maximum absorption at 2043 cm−1; B3LYP/6–31G*; wavenum-
bers scaled by a factor 0.9613). (b) Difference-IR spectrum of argon
matrix-isolated 3-azidoquinoline 6 at ∼10 K (negative peaks; the N3 asym-
metrical stretch at 2117 cm−1 has been attenuated by a factor of 1.5) and
the product of photolysis (positive peaks, spectrum after irradiation at
λ > 324 nm for 4 min).
Further experiments revealed that both the nitrene 7 and the
nitrile ylide 19 can be observed by UV-visible (Figs 3 and 4) and
IRspectroscopy(Figs5and6)byusingshort-wavelengthphotol-
ysis of 6 (308 nm). Both the nitrene 7 and the ylide 19 absorb in
the visible, up to ∼600 and 700 nm, respectively. Subsequent
photolysis at λ > 600 nm selectively removes the ylide, thus
allowing the assignment of its absorptions in both the visible and
the IR spectra (Figs 3 and 5). Renewed photolysis at λ > 395 nm
removes the nitrene 7, thus again permitting the assignment of
its UV and IR absorptions (Figs 4 and 6). The nitrene is formed
first in the 308-nm photolysis, appearing within seconds and
dominating the UV-visible spectrum (Fig. S4a, Accessory Pub-
lication).Asmallamountofthenitrileylide19appearswithinthe
first 30 s of photolysis but dominates the UV-visible spectrum
after another 40 s of photolysis. The intensities of the nitrene
and azide peaks decrease concomitantly with the increased
absorbance of the ylide (Fig. S4b, Accessory Publication). We
s-E and s-Z conformers of 20 as shown in Fig. 2a. Tables of
observed and calculated wavenumbers are provided in Table S2
in the Accessory Publication.
Prolonged irradiation of matrices of ketenimine 20, e.g. the
matrix in Fig. 2b for 2–11 h at 254 nm, caused decreased inten-
sities of the peaks due to this compound, including the NH
peaks, and development of new, intense absorptions at 2128s
and 763m cm−1 ascribed to photoisomerization of 20 to (2-
isocyanophenyl)acetonitrile. The only strong calculated peaks
for this compound are at 2114 and 750 cm−1 (absorbances 171

278
D. Kvaskoff et al.
(a)
(a)
H
C
H
C
C
C
NH
NH
N
N
C
ꢁ
ꢁ
C
ꢀ
ꢀ
(b)
(b)
CN
C
N
N
ꢁ
H
ꢀ
N
(c)
3300
(c)
3300
2900
2500
2100
1700
1300
900
500
2900
2500
2100
ν [cmꢀ1
1700
1300
900
500
ν [cmꢀ1
]
]
Fig. 5. (a) Calculated IR spectrum of ketenimine 20 (3:1 mixture of s-E-
and s-Z-20; strongest band at 2043 cm−1; absorbance 670.2 km mol−1).
(b) IR difference spectrum. Photolysis of an Ar matrix of 3-azidoquino-
line 6 at 308 nm for 20 min to generate ylide 19 + nitrene 7 followed by
λ > 600 nm for 7 min to convert nitrile ylide 19 to ketenimine 20 (posi-
tive peaks; 2025, 2129 cm−1). Negative spectrum due to ylide 19 (1974,
1985, 2183 cm−1). (c) Calculated IR spectrum of ylide 19 (strongest band
at 1973 cm−1; absorbance 398.3 km mol−1. The peak at 706 cm−1 has been
attenuated by a factor of 1.4). All calculations at the BLYP/6–31G* level;
wavenumbers scaled by a factor of 0.9613. The absorbances in spectrum (a)
have been attenuated by a factor of 1.6 relative to spectrum (c).
Fig. 6. (a) Calculated IR spectrum of ketenimine 20 (3:1 mixture of s-E-
and s-Z-20; strongest band at 2043 cm−1; absorbance 670.2 km mol−1).
(b) IR difference spectrum. Photolysis of theAr matrix of 3-azidoquinoline 6
at 308 nm for 20 min to generate ylide 19 + nitrene 7 followed by λ > 600 nm
for 7 min to convert ylide 19 and λ > 395 nm for 1 min to convert nitrene
7 to ketenimine 20. Negative spectrum due to nitrene 7. (c) Calculated IR
spectrum of triplet nitrene 7T (strongest band at 1544 cm−1; absorbance
33.3 km mol−1). All calculations at the B3LYP/6–31G* level; wavenumbers
scaled by a factor 0.9613. The absorbances in spectrum (a) are attenuated by
a factor 18 relative to spectrum (c).
assume it is the ground state triplet nitrene 7T that is observed in
the UV-visible and IR spectra, and the calculated electronic tran-
sitions for 7T agree very well with the experimental spectrum
(Fig. S5, Accessory Publication). The nitrene visible spectrum
also features a vibrational progression (Fig. 4), typical of triplet
aromatic nitrenes.[10,13] The photolyses of both the ylide and
the nitrene lead to the ketenimine 20 (see Figs 5 and 6). These
results establish the reaction sequence 7 → 19 → 20 (Scheme 4).
A transition state for a possible direct path 7 → 20 could not be
located computationally, but the reactions 7 → 19 and 19 → 20
are very facile (see Mechanism section below).
Confirmation of the assignments of the UV-visible and IR
spectra to the nitrene 7 and ylide 19 was obtained by ESR
spectroscopy. The nitrene XY2 signal shown in Fig. 1 reached
a maximum after 5 min and then decreased slowly on continued
photolysis at 308 nm, presumably owing to ring-opening to ylide
19. The intensity of the nitrene signal was unaffected by long-
wavelength irradiation (>600 nm), but it decreased rapidly on
irradiation at λ > 395 nm, just as observed in the UV-visible and
IR spectra (Fig. S1, Accessory Publication).
In addition to the IR signals described above, weaker peaks
grew at 1910 and 1695–1700 cm−1 on photolysis of the azide 6
at 308 nm (Fig. 7).The 1910 cm−1 signal disappeared on bleach-
ing of ylide 19 at λ > 515 nm and is tentatively assigned to
the cyclic ketenimine 18. Several other cyclic ketenimines of
similar structure give rise to absorptions near 1900 cm−1 [10–12]
.
The involvement of 18 as a key intermediate is discussed below
and indicated in Schemes 6 and 7. The 1695–1700 cm−1 sig-
nal disappeared concomitantly with the bleaching of the nitrene
7 at λ > 324 nm. All the calculated vibrational absorptions of
azirene 14 have matching bands in the experimental IR spec-
trum (Figs 7b and e). The calculated wavenumber for the C=N
vibration in azirene 14 is 1721 cm−1 (B3LYP/6–31G*, scaled by
a factor of 0.9613). This kind of difference between observed
and calculated wavenumbers is normal for azirenes.[10,11,15] The
additional presence of the cyclic nitrile imine 17 (see Schemes 3
and 6) is very likely but cannot be ascertained owing to the
weakness of many of the calculated bands. Compound 17 is
calculated to have its strongest absorption at 1705 cm−1 (see
Fig. 7f). Thus, the multiplet at 1695–1700 cm−1 (Fig. 7d) may

Interconversion of Nitrenes, Carbenes, and Nitrile Ylides
279
N:
C
FVT
N
(a)
ꢁ
N
N
N
ꢀ
C
N
7
19
H
FVT
CN
H
(c)
(d)
CN
N
N
H
22
21
1910
1700
1695
Scheme 5. 3-Quinolylnitrene–3-cyanoindole rearrangement on flash vac-
(b)
uum photolysis (FVT).
formed at any temperature below 600◦C. It is proposed that both
photolysis and thermolysis proceed via the same nitrile ylide 19
(Scheme 5), although the products are different. Further details
of the rearrangement of 3-quinolylnitrene to 3-cyanoindole are
discussed below with reference to Schemes 6 and 7.
N
Mechanism
(e)
N
N
The nitrile ylide intermediate 19 is analogous to the nitrile ylide
4b in Scheme 1.There are several possible states of this interme-
diate. 23T and 23S are triplet and open-shell singlet diradicals,
respectively. The ylide 19 is a canonical structure of the corre-
sponding closed shell diradical. The calculated energies of 23T,
23S, and 19 are 128, 131, and 94 kJ mol−1, respectively, relative
to the triplet nitrene 7T.
Nꢁ
ꢀ
(f)
The activation barriers for 7S → 19 and 7T → 23T are 140
and 145 kJ mol−1, respectively, at the UB3LYP/6–31G* level,
but for 7S → 19 there is a ∼5% spin contamination by the triplet
state (S2 = 0.0966). The barrier for 7S → 23S cannot be calcu-
lated at this level of theory, because the attempted transition state
converges to that for 7S → 19. A small singlet-triplet splitting
(ꢀST) of 3.8 kJ mol−1 between 23T and 23S was calculated.
This is similar to the value for the diradical 24, where ꢀST
∼5 kJ mol−1 was obtained at the CASSCF level.[13] The closed-
shell ylide 19 is clearly the species observed by IR and UV
spectroscopy, and it is also significantly lower in energy than the
diradicals 23T and 23S. In other words, the nitrile ylide structure
stabilizes the molecule significantly in spite of the partial loss
of the benzene resonance energy. At the B3LYP/6–31G* level,
ylide 19 is non-planar, with the ylidic H atom out-of-plane. The
structure of 19 is o-quinoidal at this level, with a high degree
of localized single and double bonds, rather than a delocalized
‘aromatic’ structure 19ꢀ, which would have a larger charge sep-
aration. In contrast, the species 24[13] and 25[15] formed by ring
openingof2-quinazolyl-and3-isoquinolylnitrenes, respectively,
are triplet diradicals, observable by ESR spectroscopy; here,
nitrile ylide structures cannot be formulated.
The alternative path to ylide 19 via the azirene-type transition
state 15 and diazacycloheptatetraene 18 has an overall barrier of
142 kJ mol−1 relative to the triplet nitrene, or ∼82 kJ mol−1 rel-
ative to the open-shell singlet nitrene S1 (Scheme 6). In other
words, the barriers for the two paths to 19 are virtually isoergic,
and both may be involved, i.e. the ring-opening reaction may take
place either in the nitrene 7 or in the diazacycloheptatetraene
18 (Scheme 6). It is stressed that the reactions are photo-
chemical, whereas the calculations are necessarily constrained
to the ground state energy surface.
2400
2100
1800
1500
ν [cmꢀ1
1200
900
600
]
Fig. 7. (a) Calculated IR spectrum of cyclic ketenimine 18 (strongest
absorption at 1917 cm−1; absorbance 261.1 km mol−1). (b) Difference-IR
spectrum from the photolysis of 3-azidoquinoline 6 in Ar matrix at 308 nm
for 3 min (initial absorbance of 1.1 decreases to ∼0.35), followed by photo-
lysis >515 nm for 5 min to bleach ylide 19, followed by photolysis >324 nm
for 10 min to bleach nitrene 7. Positive peaks: ketenimine 20 formed. Neg-
ative peaks are due to azide 6 (cf. Figs 2 and S3) as well as two further
speciesabsorbingat1910and1695–1700 cm−1. (c)Insetof1910 cm−1 peak.
(d) Inset of 1695–1700 cm−1 peak. (e) Calculated IR spectrum of azirene 14
(C=N absorption at 1721 cm−1; absorbance 43.3 km mol−1). (f) Calculated
IR spectrum of the cyclic nitrile ylide 17 (strongest absorption at 1705 cm−1
;
absorbance 164.0 km mol−1). All calculations at the B3LYP/6–31G* level;
wavenumbers scaled by a factor of 0.9613.
be due to a mixture of 14 and 17, but it is noted that it is not
uncommon for azirenes to feature multiplet shapes.[10,15] The
calculated absolute absorbances of the most intense peaks in
cyclic ketenimine 18, the azirene 14, and the cyclic nitrile ylide
17 are 261.1, 43.3, and 164.0 km mol−1, respectively. There-
fore, it is clear that a substantial amount of the azirene 14 is
present.
Flash Vacuum Thermolysis
In contrast to the photolysis, flash vacuum thermolysis (FVT)
of 6 at 450–600◦C afforded 3-cyanoindole 22 cleanly. This is
the thermodynamically most stable compound among all the
calculated structures. The IR spectra of the thermolysates at dif-
ferent temperatures are in excellent agreement with those of an
authentic sample of 22 (Fig. 8) as well as with the calculated
IR spectrum (Table S3, Accessory Publication). There was no
spectroscopic evidence for any other product besides 22 being
Comparison of Schemes 3 and 6 could suggest a contradic-
tion: azirene 14 is likely to be the species trapped in solution,

280
D. Kvaskoff et al.
N
N
N
N
C
N
N
N
N
N
24
25
H
C
C
N
N
N:
100.4
N
N
N
N
7 (T0 0)
14 (60.0)
(S1 59.8)
15TS (141.6)
(S2 137.9)
(a)
134.6
N
139.7
108.5
C
N
N
N
ꢁ
N ꢀ
N
C
16 (133.0)
18 (22.1)
H
19 (94.1)
114.7
159.5
H
ꢁ
CN
121.7 ∼1,7-H
N
ꢀ
N
N
C
NH
ꢁ
N
21 (ꢀ117.6)
17 (97.4)
ꢀ
C
15.0
20 (s-Z 26.5)
(s-E 21.9)
CN
44.5
(b)
N
(ꢀ78.8)
4000
3500
3000
2500
2000
ν [cmꢀ1
1500
1000
500
27.1
CN
]
Fig. 8. (a) Argon-matrix isolated product from flash vacuum thermolysis
of 3-azidoquinoline 6 at 600◦C; (b) authentic sample of 3-cyanoindole 22 in
argon matrix at 8 K. Major peaks are at 3506, 2236, 1418, 1247, 1244, and
N
H
22 (ꢀ190.1)
742 cm−1
.
Scheme 6. 3-Quinolylnitrene reactions. Formation of 14, 19, and 20 on
photolysis; 22 on thermolysis. Calculated energies for ground and transition
states relative to triplet nitrene 7T = 0 (numbers in normal font) in kJ mol−1
at the B3LYP/6–31G* level.
is ∼7 kJ mol−1 higher than that yielding 21. This is not a large
energy difference under FVT conditions. The absence of keten-
imine 20 in the FVT reactions may be due to the 1,7-H shift being
reversible (Scheme 6). These 1,7-H shifts involve migrations
to atoms possessing lone pairs or triple-bonded carbon and are
therefore in principle pseudopericyclic and not restricted by the
Woodward–Hoffmann rules of orbital symmetry.[16] The energy
required for the reversion 20 → 19, some 100 kJ mol−1, would
certainly be available under FVT conditions. Moreover, the small
calculated energy differences between 19, 23T, and 23S would
facilitate thermal population of the diradical states under FVT
conditions. The open-shell singlet diradical 23S may cyclize to
21, thus again leading to 22 as the final product.
Arylnitrenes can also undergo a concerted ring con-
traction to five-membered-ring nitriles; an activation bar-
rier of 130 kJ mol−1 was calculated for this process in
the open-shell singlet phenylnitrene (S1; 1Aꢀꢀ) at the
CASPT2//CASSCF(8,8)/cc-pVDZ level.[12] Such a process
would also in principle be possible for 3-quinolylnitrene 7, but it
is not competitive with the much lower barrier for the route via
nitrile ylide 19. A single point calculation at the B3LYP/6–31G*
level afforded an energy of 169.6 kJ mol−1 for the transition
but it appears to play a minor role as a side product in the
matrix photochemistry. However, there is no contradiction, and
there are other examples of this phenomenon, e.g. the case of
4-quinolylnitrene.[11] If the formation of diazepines in solution
is due to kinetic control, and azirene 14 has a lower energy and is
formed faster than the azirene-type transition state 15, then 14
is the species being trapped. Under matrix isolation conditions,
in contrast, the photochemical reaction rates, surface crossings,
and relative stabilities of products are determining factors.
The ring closure of ylide 19 to 3-cyano-3H-indole 21 is a
very facile process with a calculated activation barrier of only
21 kJ mol−1 at the B3LYP/6–31G* level (Scheme 6). There is
a strong thermodynamic driving force for cyclization of ylide
19 to 21, which is exothermic by 212 kJ mol−1 (Scheme 6). The
isomerization of the unobserved 21 to 3-cyanoindole 22 can take
place easily by means of two consecutive thermal 1,5-H shifts
(Scheme 6). The calculated energy of the transition state for the
1,7-H shift in ylide 19 yielding ketenimine 20 (122 kJ mol−1
)

Interconversion of Nitrenes, Carbenes, and Nitrile Ylides
281
CH:
N
CHN2
N
structure for the concerted ring contraction of the open-shell
singlet nitrene, 7S → 21, i.e. 110 kJ mol−1 above 7S, but this
transitionstructurehasa12%spincontamination(S2 = 0.24964)
(see Accessory Publication).
N
N
N
N
N
N
27 (s-Z ꢀ22.1)
(s-E ꢀ9.2)
26 (ꢀ57.2)
28 (E 1A 168.8)
(Z 1A 154.0)
(E 3Aꢅ 106.4)
(Z 3Aꢅ 97.7)
4-Quinazolylcarbene 28
1,2,3-Triazolo[1,5-c]quinazoline 26 was obtained by chromato-
graphy of the tosylhydrazone of quinazoline-4-carboxaldehyde
on alumina. Compound 26 was matrix-isolated in Ar at 10 K
and photolyzed at λ = 254 nm using a low-pressure Hg lamp
or at 265–270 nm using a high-pressure Hg–Xe lamp. This
caused rapid development of a new, strong band at 2104 cm−1
together with several new bands in the fingerprint region (1568w,
1548w, 1496m, 1434m, and 760w cm−1). Comparison with
the calculated IR spectrum (B3LYP/6–31G*) readily identifies
this photoproduct as the s-E conformer of diazo compound
27 (Scheme 7; Fig. S6, Accessory Publication). The thermal
triazole–diazo equilibrium lies far to the side of the triazole 26:
deposition of 26 through the FVT oven at 300–390◦C did not
produce any diazo absorption at 2104 cm−1 in the resulting Ar
matrix. Warming of a matrix of diazo compound 27 to allow
the argon to evaporate demonstrated that the diazo compound
(absorbing at 2100 cm−1 in the neat solid state) is stable till
at least 100 K. Broadband UV irradiation of an argon matrix of
triazole 26 for 5 min using the high-pressure lamp caused the for-
mation of both diazo compound 27 and carbene s-Z-28, as seen
in the excellent agreement with the calculated spectra (Fig. S7,
Accessory Publication). Further photolysis of the carbene was
very slow and afforded only weak, new bands at 2129, 2047, and
2025 cm−1 ascribed to rearrangement to ketenimine 20, i.e. the
samespeciesasformedfrom3-quinolylazide6. Inthephotolysis
at 265–270 nm, only a trace of the triazole was still present after
1 h; the diazo compound and carbene were still present after 4 h;
the diazo compound had essentially disappeared after 6 h, but
the carbene was still present. Weak signals due to ketenimine 20
appeared after 40 min but remained very weak after 6 h. When
triazole 26 was photolyzed at 254 nm using the low-pressure
lamp, the triazole had essentially vanished after 4 h, and strong
signals for both diazo compound 27 and carbene 28 were present.
Weak signals due to ketenimine 20 were present after 2 h. The
carbene and a trace of the diazo compound were still present after
13 h. Owing to the prolonged irradiation times needed to convert
the carbene 28, the ketenimine 20 so formed isomerized in part
N
N
29TS (195.3)
N:
N
N
N
N
N
15TS (141.6)
7 (S1 59.8)
18 (22.1)
139.7
(T0 0)
H
108.5
CN
C
114.7
N
ꢁ
N
N
ꢀ
C
21 (ꢀ117.6)
H
19 (94.1)
15.0
121.7
CN
C
NH
ꢁ
N
ꢀ
C
N
(ꢀ78.8)
20 (s-Z 26.5)
(s-E 21.9)
44.5
27.1
CN
N
H
22 (ꢀ190.1)
Scheme 7. 1,2,3-Triazolo[1,5-c]quinazoline reactions (photolysis to 27,
28, and 20; flash vacuum thermolysis to 22). Numbers in normal font: cal-
culated energies in kJ mol−1 at the B3LYP/6–31G* level relative to triplet
nitrene 7T = 0. The energy of a molecule of N2 has been subtracted from
the energies of 26 and 27 to bring them all on the same scale.
This is a typical triplet carbene spectrum with ZFS parameters
D = 0.5253 cm−1 and E = 0.0251 cm−1. As mentioned above, a
linear correlation exists between experimental D values and spin
densities ρ for carbenes[12] and nitrenes.[13] The correlation for
carbenes[12] is presented in Fig. S8 in the Accessory Publica-
tion. The D value for 28 fits excellently on the D–ρ correlation
(ρ = 1.64411 for E-28 at the UB3LYP/EPR-III level).
with formation of the new absorptions at 2128 and 763 cm−1
,
as were also observed in the case of azide 6, and assigned to
(2-isocyanophenyl)acetonitrile. Thus, there is little doubt that
4-quinazolylcarbene 28 enters the same energy surface as does
3-quinolylnitrene 7, and the most rational way to the product
is 28 → 18 → 19 → 20 or less likely 28 → 18 → 7 → 19 → 20
(Scheme 7). The absence of nitrene 7 in the ESR experiments
described below under both photolysis and FVT conditions
indicates that the ‘direct’route 28 → 18 → 19 → 20 is being fol-
lowed. The calculated energies make this scheme very feasible,
with the cyclopropene-type structure[12,17] 29 as a key transition
state, in principle easily accessible from the singlet carbene 28.
The formation of 3-cyanoindole 22 on thermolysis is described
below.
Direct evidence for the photochemical formation of carbene
28 was also obtained by ESR spectroscopy. To this end, tria-
zole 26 was vapourized through an FVT oven held at 250◦C,
deposited with Ar on a Cu rod at 15 K, and photolyzed at
λ = 308 nm to produce the ESR spectrum shown in Fig. 9.
The ESR investigation confirmed that the carbene 28 is very
resistant towards further irradiation in agreement with the poor
yield of photolysis product 20 described above.The carbene ESR
spectrum was essentially unchanged after 10 h of additional irra-
diation of the matrix at 222 nm, and it also survived annealing at
45 K. Detectableamountsofnitrene7werenotformedduringthe
prolonged irradiation. FVT of 26 at 580◦C with isolation of the
product in Ar matrix at 15 K for ESR spectroscopy also did not
produce the ESR signals of either the carbene 28 or the nitrene 7,
but 3-cyanoindole 22 was isolated after warm-up of the matrix
(see below). ESR spectroscopy has been successfully employed
in other cases to detect nitrenes produced by rearrangement of
carbenes in FVT reactions.[12]
FVT of triazole 26 at 600◦C with Ar matrix isolation
of the thermolysate at 26 K resulted in the clean formation

282
D. Kvaskoff et al.
hν
ꢀN2
g ꢂ 2
CHN2
CH
N
hν
N
N
N
hν, N2
N
30
33
31
hν
32
CH
Z1
CO
X2
H
C
O
C
C
C
H
ꢁ
N
ꢀ
N
C
H
34
Scheme 8. 3-Quinolylcarbene photolysis reactions (31, 32, and 34
observed; 33 not observed).
Z2
Y2
(a)
H
0.06
N
0.04
500
2000
3500
5000
6500
8000
9500
Magnetic field [G]
A
0.02
0.00
Fig. 9. Electron spin resonance spectrum of triplet 4-quinazolinylcar-
bene 28 obtained by photolysis of triazole 26 (Ar, 15 K) at 308 nm for
45 min (the spectrum appears already within 5 min). ZFS parameters:
(b)
D = 0.5253 cm−1
,
E = 0.0251 cm−1 (H0 = 3471.1 G, Z1 = 2142.6 G,
X2 = 5091.1 G, Y2 = 6127.7 G, Z2 = 9073.3 G). Microwave frequency
9.727753 GHz. The signal at g = 2 is due to doublet radicals.
ꢀ0.02
2200 2000 1800 1600 1400 1200 1000 800 600
of 3-cyanoindole 22, the spectrum being identical with the
one shown in Fig. 8a. This is readily explained by thermal
ring expansion of carbene 28 to the unobserved cyclic keten-
imine 18, which undergoes ring contraction to 22 as described
above (Schemes 6 and 7). Cyclopropene-type transition struc-
tures analogous to 29 have been invoked before in thermal
carbene–nitrene and carbene–carbene rearrangements.[17] The
corresponding fused cyclopropene transition structures formed
from 6-phenanthridylcarbene[12] and 1-naphthylcarbene[18] are
calculated to lie ∼60–100 kJ mol−1 above the singlet car-
benes. These energies are readily accessible under FVT
conditions.
ν [cmꢀ1
]
Fig. 10. (a) Calculated IR spectrum of triplet 3-quinolylcarbene 31
(s-Z-conformer; B3LYP/6–31G*). (b) Difference spectrum from photolysis
of 30 at 222 nm, Ar matrix, ∼10 K; negative peaks diazo compound 30;
positive peaks 3-quinolylcarbene 31.
values fit on the D–ρ correlation[12] (see Fig. S8, Accessory
Publication).
Broadband photolysis of 3-quinolylcarbene 31 at λ > 500 nm
afforded a medium-sized band at 1734 cm−1 assigned to the
tricyclic cyclopropene 32 (Fig. 12) in analogy with the known
interconversion of 2-naphthylcarbene and benzobicyclo[4.1.0]
hepta-2,4,7-triene.[19,21] In experiments where the diazo com-
pound 30 was completely depleted by photolysis at 365 nm,
further photolysis of carbene 31 so formed at λ > 500 nm caused
regeneration of the diazo compound, most notably seen in a rapid
growth of the diazo stretching peak at 2069 cm−1, thereby imply-
ing that the carbene reacted with nitrogen in the matrix cavity
(Scheme 8). There is previous evidence for the regeneration of
diazomethane by reaction of methylene (H2C:) with N2.[22] Fur-
ther confirmation of the presence of the carbene was obtained by
photolysis of 30 in an Ar matrix containing 10% CO. Photolysis
at 365 nm caused disappearance of the diazo compound and con-
comitant appearance of a new peak at 2121 cm−1, a typical value
for a C=C=O stretching vibration, assigned to 3-quinolylketene
34 (Scheme 8). This technique of carbene trapping has been
applied in other research.[23]
3-Quinolylcarbene 31
Based on the known ring opening of 3-pyridylcarbene 2a
(Scheme 1), and that of 3-quinolylnitrene 7 reported above
(Schemes 4 and 6), a similar ring opening of 3-quinolylcarbene
31 to a nitrile ylide 33 could be expected (Scheme 8). It is possi-
ble that this does happen in a reversible reaction, but we did not
observe it directly. Matrix photolysis of 3-diazomethylquinoline
30 at several wavelengths between 222 and 390 nm led to the
disappearance of the strong diazo stretch at 2069 cm−1 and
formation of a new species with an IR spectrum otherwise
very similar to that of 30. It is ascribed to 3-quinolylcarbene
31 because of the very good agreement with the calculated IR
spectrum (Fig. 10).
The carbene absorbs in the 360–554 nm range in the UV-
visible (Fig. 11) and features two pronounced vibrational
progressions with spacings of ∼502 and 1367 cm−1
.
Although the matrix photolyses did not progress beyond
the carbene 31 and cyclopropene 32, FVT of 30 afforded a
mixture of 2- and 3-cyanoindenes 44 and 45, thereby suggest-
ing the occurrence of a deep-seated carbene–carbene–nitrene
rearrangement, where 3-quinolylcarbene 31 enters the energy
surface already known to connect 2-quinolylcarbene 39 and 1-
naphthylnitrene 43 (Scheme 9).[24] This energy surface and the
The UV-visible spectrum of 31 has an appearance very simi-
lar to that reported for 2-naphthylcarbene in an Ar matrix,[19]
as well as that of the isomeric 2-naphthylnitrene,[10] except
that the nitrene absorptions are red-shifted relative to the car-
bene. The ESR spectrum of the s-Z and s-E conformers of
3-quinolylcarbene has been reported previously,[20] and the D

Interconversion of Nitrenes, Carbenes, and Nitrile Ylides
283
0.15
C
C
s-Z 166.0
379
C
H
CH2
ꢁ
N
H
539
ꢀ
0.10
0.05
NC
393
417
481
C
515
502
554
33
35 (s-Z ꢀ29.4)
(s-E ꢀ39.1)
(s-Z 158.5)
(s-E 149.6)
159.8
0.00
A
ꢀ0.05
ꢀ0.10
ꢀ0.15
ꢀ0.20
ꢀ0.25
N
36 (ꢀ44.8)
365
N
(TS 147.2)
200
300
400
λ [nm]
500
600
700
CH
49.6
76.2
N
Fig. 11. Difference UV-visible spectrum from the photolysis of
3-diazomethylquinoline 30 in Ar matrix at 222 nm. Negative peaks:
3-diazomethylquinoline 30. Positive peaks: photolysis product, 3-quinolyl-
carbene 31.
N
N
31 (s-E 1Aꢆ 35.2)
(s-Z 1Aꢆ 32.2)
(s-E 3Aꢅ 1.4)
(s-Z 3Aꢅ 0)
32 (5.4)
37 (32.9)
94.7
0.05
(a)
96.0
CH
N
N
N
0.04
N
38TS (101.8)
39 (s-E 1A 62.7)
(s-Z 1A 50.1)
(s-E 3Aꢅ 8.2)
(s-Z 3Aꢅ 3.6)
40 (86.6)
0.03
A
0.02
N
N
N:
0.01
0.00
(1Aꢆ 20.3)
(1Aꢅ ꢀ58.8)
(3Aꢅ ꢀ110.6)
1734
41 (ꢀ72.1)
42TS (44.9)
43
(b)
99.0
CN
2200 2000 1800 1600 1400 1200 1000 800 600
ν [cmꢀ1
CN
CN
]
ꢀ92.1
ꢀ74.1
ꢀ59.7
Fig. 12. (a) Calculated IR spectrum of benzobicycloheptatriene 32
(B3LYP/6–31G*). (b) Positive peaks due to photolysis product formed at
λ > 500 nm; negative peaks due to both diazo compound 30 and carbene 31.
44 (ꢀ242.5)
(ꢀ169.4)
(ꢀ213.5)
ꢀ20.7
CN
CN
formation of cyanoindenes have been described in consider-
able detail previously.[10,11,24] It is possible that ring opening
to 33 and 35 takes place, but because the cyanoindenes are the
lowest-energy species on this energy surface, they are always
the end products of FVT reactions. They interconvert thermally
through consecutive 1,5-H and 1,5-CN shifts.[24] When the FVT
product of 30 was isolated inAr matrix at 20 K, weak bands were
observed in the range 1909–1922 cm−1 and are assigned to aza-
cycloheptatetraene 41. This species has been observed before
and features a characteristic multiplet for the ketenimine stretch
in the 1909–1922 cm−1 region,[10,24a] presumably due to matrix
site effects. Further weak bands at 1957 and 2149/2135 cm−1
could possibly be due to the allene and isocyanide groups in 35.
If the nitrile ylide 33 were formed, it would have a very low
computed barrier of ∼1 kJ mol−1 for cyclization to the cyclic
allene 36 (Scheme 9), which is calculated to be a relatively stable
intermediate. Such allenes have very weak IR absorptions in the
1800 cm−1 range[23c] and are not, therefore, readily detectable.
45 (ꢀ249.3)
(ꢀ120.5)
Scheme 9. Flash vacuum thermolysis of 3-quinolyldiazomethane 30 to
cyanindoles 44 and 45. Calculated ground and transition state energies in
kJ mol−1 at the B3LYP/6–31G* level relative to triplet s-Z-31 (3Aꢀꢀ) = 0
(numbers in normal font). The tricyclic cyclopropene and azirene structures
38 and 42 are calculated transition states.
carbene 31 to ylide 33 (Scheme 9) is much higher than the cor-
responding difference between nitrene 7 (S1) and ylide 19 in
Scheme 6. In contrast, the cyclization of singlet carbene 31 to
cyclopropene 32 is exothermic by ∼27 kJ mol−1, and the bar-
rier is ∼44 kJ mol−1 (Scheme 9). The carbene–carbene–nitrene
rearrangement 31 → 39 → 43 is relatively facile, the maximum
barrier being ∼70 kJ mol−1 above the singlet carbene s-Z-31
(Scheme 9). Such barriers are very easily accessed under FVT
conditions.The transition state for the concerted ring contraction
of the open-shell singlet nitrene 43 (1Aꢀꢀ) to 1-cyano-1H-indene
1
The energy difference for the ring opening of the singlet Aꢀ

284
D. Kvaskoff et al.
is calculated to lie at 99 kJ mol−1, i.e. 158 kJ mol−1 above 43
(1Aꢀꢀ), but there is a 13% spin contamination in this TS structure
(S2 = 0.2669). The mechanism of ring contraction and the inter-
conversionsofthecyanoindenes(Scheme9)havebeendescribed
before,[24] and were in accord with the interpretation that at least
a portion of the nitrenes underwent direct ring contraction.
Further calculations on this system, e.g. the ring opening of
carbene 31 to ylide 33 and potential ring closure of 33 to 3-
ethynyl-3H-indole are described in the Accessory Publication.
Experimental
General Methods
Closed-cycle liquid helium cryostats (Air Products) were used
for matrix experiments in conjunction with a Lakeshore 330
temperature controller. Procedures for IR, UV, and ESR spec-
troscopy have been published.[12,15] ESR spectra were recorded
at 15–20 K, and IR spectra at ∼8 K with a resolution of 1 cm−1
.
In FVT experiments, a mixture of argon and sample was led
through a quartz tube (10 cm length, 0.8 cm internal diameter
(ID), equipped with heating wire and a thermocouple), housed
in a vacuum casing flanged directly onto the cryostat cold head
with a flight distance of ∼2 cm from the exit of the quartz tube
to the cold target. The product was isolated with argon on a
KBr or CsI window at 20–26 K. For photolysis experiments, the
same FVT oven was used as a sublimation device, and ∼5 mg of
Conclusions
3-Quinolylnitrene 7 is generated by matrix photolysis of 3-
azidoquinoline 6 and characterized by its ESR, UV-visible,
and IR spectra. Nitrene 7 undergoes facile ring opening to
the nitrile ylide 19, which is characterized by its UV-visible
and IR spectra. Ylide 19 in turn undergoes a facile photo-
chemical 1,7-H shift to generate o-isocyanophenylketenimine
20. Ketenimine 20 is formed in poorer yield by matrix photo-
lysis of 4-quinazolylcarbene 28, itself obtained by photolysis
of 4-diazomethylquinazoline 27 and characterized by IR and
ESR spectroscopy. FVT of 3-azidoquinoline 6 as well as
1,2,3-triazolo[3,4-c]quinazoline 26/4-diazomethylquinazoline
27 affords 3-cyanoindole 22 in high yield. These results suggest
that 4-quinazolylcarbene 28 and 3-quinolylnitrene 7 enter the
same energy surface under both photolysis and FVT conditions.
Matrix photolysis of 3-quinolyldiazomethane 30 generates
3-quinolylcarbene 31, which is characterized by its IR and
UV-visible spectra. Carbene 31 reacts with N2 to regenerate
diazo compound 30, and with CO to generate a ketene, to
which structure 34 is assigned. Further photolysis of carbene
31 at λ > 500 nm affords the tricyclic cyclopropene, 5-aza-
2,3-benzobicyclo[4.1.0]hepta-2,4,7-triene 32, characterized by
its IR spectrum. The FVT of 30 to yield a mixture of 2-
and 3-cyanoindenes is suggested to take place via a ther-
mal carbene–carbene–nitrene rearrangement 3-quinolylcarbene
31 → 2-quinolylcarbene 39 → 1-naphthylnitrene 43.
substance was deposited with Ar (99.9999%; 0.2–0.4 mL s−1
)
on a CsI target at 20–22 K over a 30 min period. This usu-
ally afforded an IR absorbance of 1–2.5. As the nitrenes are
themselves reactive under photolysis conditions, their observa-
tion requires short photolysis times. Photolyses were carried out
using either a high pressure Hg–Xe lamp (1000 W; Hanovia) in
combination with cutoff and bandpass filters, a low-pressure Hg
arc lamp (254 nm; 75W; Gräntzel), or excimer lamps operating
at 222 nm (25 mW cm−2) and 308 nm (50 mW cm−2) (Ushio).
Materials
3-Azidoquinoline[5,9] 6 and 3-diazomethylquinoline[20] 30 were
prepared according to literature procedures and purified by
column chromatography. Compound 6 was chromatographed
on silica gel 60, eluting with dichloromethane/hexane 3:1 and
subsequent sublimation; mp 80–82◦C (lit.[9b] 79–80◦C). Com-
pound 30 was chromatographed on deactivated, neutral alumina
grade III (8% H2O), with 2:1 hexane/diethyl ether as eluent,
as an orange solid, mp 78.5◦C. 3-Cyanoindole 22 was obtained
commercially and purified by sublimation before use.
Matrix Isolation of 3-Azidoquinoline 6
Azide 6 was sublimed at 55◦C (10−3 Pa) and codeposited with
Ar at 22 K as described above, and the resulting matrix was
cooled to 8 K for IR spectroscopy. ν(Ar, 8 K)/cm−1 3097vw,
3079vw, 3038vw, 3015vw, 2117vs, 2099m, 1601w, 1509vw,
1502vw, 1494w, 1471vw, 1466vw, 1426m, 1401vw, 1378vw,
1349m, 1345m, 1298w, 1284m, 1258w, 1248m, 1238vw, 1130w,
1115w, 982vw, 979vw, 972vw, 956vw, 929vw, 909vw, 895vw,
885w, 856w, 783w, 751m, 604w (see Table S1 for comparison
with the calculated IR spectra of the s-Z and s-E conformers
of 6).
Computational Method
DFT calculations[25] were carried out using the Gaussian suite
of programs.[26] Geometry optimizations were performed at
the B3LYP/6–31G* level.[26,27] This computational level has
been shown to produce energies very close to those obtained
by CASPT2 and CASSCF(12,12)/6–31G* calculations.[10] The
energies of triplet nitrenes 7T (T0, 3Aꢀ) and 43 (3Aꢀꢀ) and dirad-
ical 23T were calculated at the UB3LYP/6–31G* level, and the
energies of the open-shell singlets 7S (S1, Aꢀ) 23S and 43
1
(1Aꢀꢀ) were estimated using the Cramer–Ziegler method.[7] Spin
contaminations by 5–13% of the triplet states were found in tran-
Photolysis of Matrix-Isolated 3-Azidoquinoline 6
sition states derived from the Aꢀꢀ singlet nitrenes. The B3LYP
A sample of 6 was codeposited with argon at 22 K and then
cooled to 8 K. Broadband photolysis at λ > 305 nm for 10 min
yielded 3-(2-isocyanophenyl)ketenimine 20. ν(Ar, 8 K)/cm−1
3314vw, 3304w, 3294w, 3125vw, 3085vw, 3046vw, 2129m,
2125m, 2045m, 2032m, 2023s, 1593w, 1495m, 1464w, 1303w,
1262w, 1203w, 1113w, 1103w, 1087vw, 1040w, 1017vw, 982vw,
955m, 944m, 940m, 936m, 927m, 909vw, 904vw, 829m, 813w,
784w, 762m, 758m, 728w, 653w, 599w, 527vw, 452vw, 417w.
Further photolysis at λ = 254 nm for up to 11 h caused progres-
sive decrease of the intensities of the aforementioned bands and
development of new absorptions 2128s and 763m cm−1 ascribed
to (2-isocyanophenyl)acetonitrile.
1
formulation of density functional theory corresponds to Becke’s
three-parameter exchange functional[27a] in combination with
the Lee–Yang–Parr correctional functional.[27b] Wave function
stability, harmonic vibrations and infrared intensities were calcu-
lated at this level in order to characterize the stationary points as
minima and to evaluate ZPVEs.The calculated frequencies were
scaled by a factor of 0.9613 to account for the overestimation
of vibrational frequencies at this level of theory.[28] Energies,
IR spectra and standard orientations of all optimized structures
and imaginary frequencies for transition states are summarized
in the Accessory Publication.

Interconversion of Nitrenes, Carbenes, and Nitrile Ylides
285
FVT of 3-Azidoquinoline 6
signals, but 3-cyanoindole was recovered (identified by GC-MS
comparison with a sample described above).
Thermolyses of samples of 6 at 450–600◦C followed by argon
matrixisolationofthepyrolysatesgave3-cyanoindole22. ν(FVT
at 600◦C, Ar, 8 K)/cm−1 3506vs, 3501vs, 3141vw, 3096vw,
3078w, 3057vw, 2236vs, 1636w, 1537m, 1497w, 1459s, 1437w,
1422m, 1418s, 1354w, 1340w, 1335m, 1332w, 1305vw, 1247s,
1244s, 1232vw, 1177vw, 1152vw, 1133w, 1116vw, 1105m,
1086vw, 1060w, 1014w, 957vw, 930vw, 918vw, 876vw, 820w,
768m, 742vs, 612vw, 600vw, 581w, 576w, 522w, 504m, 472vw,
458w, 453w, 420m.
Matrix Photolysis of 1,2,3-Triazolo[1,5-c]quinazoline 26
The triazole 26 was sublimed at 44◦C in a stream of Ar for
1 h, and the material was deposited onto a KBr window at
26 K. The matrix was cooled to 10 K, and the infrared spec-
trum was recorded: ν(Ar, 10 K)/cm−1 1963w, 1933w, 1819w,
1636w, 1618w, 1601w, 1568w, 1531w, 1483s, 1479m, 1466w,
1417w, 1415w, 1410w, 1401s, 1379w, 1375w, 1370w, 1328w,
1313w, 1302w, 1294w, 1281w, 1263w, 1242m, 1227m, 1216w,
1195w, 1162w, 1158w, 1144w, 1116w, 1108w, 1096w, 1093m,
1037w, 1034w, 1021w, 1010w, 964m, 953w, 912w, 892m, 825w,
813m, 787w, 774w, 765s, 764s, 698w, 684w, 627w, 524w, 472w,
427w. Alternative deposition through the FVT oven at 300◦ or
390◦C did not cause any change (no diazo compound formed).
Irradiation of the matrix of 26 (λ = 254 nm, 10 min) caused the
matrix to turn pink, and the following new bands ascribed to
diazo compound 27 were recorded (Fig. S6): ν(Ar, 10 K)/cm−1
2104s, 1568w, 1548w, 1497m, 1434m, 760w. Further photolysis
of the same matrix using the broadband irradiation from the
high pressure Hg–Xe lamp for 5 min afforded the following new
bands ascribed to carbene 28 (Fig. S7): 1475, 1405, 1290, 906,
766, 666, 571 cm−1. Photolysis of a matrix of 26 at 265–270 nm
for 1 h afforded weak peaks assigned to ketenimine 20: ν/cm−1
2129w, 2045w, 2032w, 2023w, 1495w, 955w, 944w, 758w. Only
a trace of triazole 26 was still present after 1 h. Only a trace
of diazo compound 27 was still present after 6 h. Photolysis
at 265–270 nm for more than 1 h caused decreased absorp-
tions due to 20 and increased absorptions of bands at 2128s
and 763m cm−1 ascribed to (2-isocyanophenyl)acetonitrile. In
photolyses at 254 nm, signals due to carbene 28 were still present
after 19 h.
3-Cyanoindole 22 (commercial sample). ν(Ar, 8 K)/cm−1
3508vs, 3142vw, 3096vw, 3078w, 3057vw, 2236vs, 1635w,
1590vw, 1545w, 1537m, 1532m, 1497w, 1459m, 1436w, 1421m,
1417s, 1354w, 1340w, 1335m, 1331w, 1304vw, 1246s, 1244s,
1232w, 1176vw, 1152vw, 1135w, 1116w, 1105m, 1086vw,
1059w, 1013w, 957vw, 930vw, 918vw, 876vw, 819vw, 768m,
742vs, 682vw, 663vw, 656vw, 611vw, 604vw, 575w, 515vw,
504m, 470vw, 457vw, 452w, 420m.
1,2,3-Triazolo[1,5-c]quinazoline 26
4-Methylquinazoline (0.265 g, 1.84 mmol), selenium dioxide
(0.186 g, 1.68 mmol) and 1,4-dioxan (2 mL) were heated in an
oil bath at 98◦C for 26 min. The solvent was evaporated, and
the resulting brown solid subjected to Kugelrhor distillation
(76◦C, 1 Pa).The distillate, a yellow solid (0.041 g) was immedi-
ately dissolved in methanol (5 mL), p-toluenesulfonylhydrazine
(0.061 g, 0.328 mmol) was added, and the mixture was stirred
for 10 min. The residue obtained after concentration was
chromatographed on alumina with diethyl ether/hexane 1:1.
1,2,3-Triazolo[1,5-c]quinazoline 26 (0.019 g, 6% based on 4-
methylquinazoline) was obtained as a white solid. Recrystal-
lization from diethyl ether afforded white needles, mp 187◦C.
(Calc. for C9H6N4: C 63.5, H 3.6, N 32.9. Found: C 63.4, H
3.6, N 33.1%.) m/z 170 (M+•), 142 (–N2), 115 (–N2, –HCN),
88, 62. ν(KBr)/cm−1 3129w, 3060m, 3039w, 1622s, 1612m,
1560w, 1479s, 1465w, 1460w, 1396s, 1316w, 1302w, 1244s,
1217s, 1188m, 1148w, 1097m, 1035m, 1023m, 1011w, 969s,
913w, 895s, 882w, 835m, 787w, 778s, 766s, 686m, 625w, 586w,
525w, 508w, 476w, 438m, 428m. δH (CD3COCD3, 400 MHz)
FVT of 1,2,3-Triazolo[1,5-c]quinazoline 26
Triazole 26 was co-sublimed (60◦C) with Ar through a quartz
tube held at 600◦C, and the resulting product was condensed
on a KBr deposition window (26 K) in the course of 50 min.
The window was then cooled to 10 K. Comparison of the IR
spectrum with that of an authentic sample of 3-cyanoindole
22 demonstrated that this was the near-exclusive product. ν(Ar,
10 K)/cm−1 3509s, 3501s, 3143w, 3096w, 3077w, 3056w, 2236s,
1537m, 1499w, 1459m, 1427w, 1421w, 1417m, 1354w, 1340w,
1335w, 1332w, 1304w, 1264w, 1247m, 1232w, 1176w, 1159w,
1152w, 1133w, 1116w, 1105m, 1086w, 1059w, 1014w, 957w,
930w, 924w, 917w, 875w, 819w, 768w, 744s, 742s, 610w, 600w,
581w, 576w, 522w, 504m, 458w, 453m, 420m.
3
4
3
7.79 (td, 2 × J 7.52, J 1.36, 1H, H8 or H9), 7.84 (td, 2 × J
7.92, 4J 1.64, 1H, H9 or H8), 8.05 (dd, 3J 7.92, 4J 1.44, 1H, H7 or
H10), 8.36 (ddd 3J 7.64, 4J 1.48, 5J 0.52, 1H, H10 or H7), 8.65 (d,
3J 0.92, 1H, H1), 9.63 (s, 1H, H5). δC (CD3COCD3, 100 MHz)
119.4 (C10a), 125.1, 126.5, 129.7, 130.2, 131.7, 133.5, 137.6
(C5), 141.0.
ESR Spectrum of 4-Quinazolylcarbene 28
Triazole 26 was sublimed at 60◦C, the vapour was passed
through the FVT oven held at 250◦C, and deposited with Ar
on a Cu rod at 15 K for ESR spectroscopy. Photolysis of the
matrix at 308 nm gave the carbene within 5 min, and the maxi-
mum concentration was reached within 30–45 min of photolysis
(Fig. 9). ZFS parameters: D = 0.5253 cm−1, E = 0.0251 cm−1
(H0 = 3471.1 G, Z1 = 2142.6 G, X2 = 5091.1 G, Y2 = 6127.7 G,
Z2 = 9073.3 G). Microwave frequency 9.727753 GHz. Further
photolysis at 222 nm for 10 h left the carbene practically
unchanged, except for the growth of signals due to H radicals
and a ꢀms = 2 signal at 1695 G probably due to a diradical. The
carbene survived annealing of the matrix to 45 K and was recov-
ered unchanged on renewed cooling to 15 K. Pyrolysis of the
triazole with ESR detection was attempted. FVT at 500◦C left
the triazole unchanged. FVT at 580◦C did not produce any ESR
Matrix Photolysis of 3-Quinolyldiazomethane 30
The diazo compound was isolated in Ar (diazo stretch at
2069 cm−1 in the IR) and photolysed at 10–15 K using λ 222,
310–390, or 365 nm. In each case, the carbene 31 was formed
(Figs 10 and 11). Further irradiation of the matrix at λ > 500 nm
using a cutoff filter resulted in the disappearance of the carbene
and formation of the cyclopropene 32 (1734 cm−1) (Fig. 12). A
new sample of 30 was photolyzed in Ar matrix at λ 365 nm
for 40 min until the diazo peak at 2069 cm−1 had vanished
completely. Subsequent photolysis at λ > 500 nm caused the
regeneration of the diazo compound 30 (2069 cm−1). Photo-
lysis of a new sample of 30 in an Ar matrix containing 10%
CO at λ 365 nm resulted in the disappearance of the diazo

286
D. Kvaskoff et al.
peak and development of a new peak at 2121 cm−1 ascribed to
ketene 34.
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1482 (0.33), 1349 (0.25), 1287 (0.21), 1248 (0.12), 1230 (0.12),
1138 (0.23), 1120 (0.19), 1020 (0.28), 963 (0.20), 951 (0.20),
873 (0.38), 845 (0.60), 810 (0.21), 791 (0.43), 775 (0.56), 746
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(0.24), 1420 (0.16), 1374 (0.16), 1110 (0.26), 1034 (0.19), 1017
(0.38), 1010 (0.25), 944 (0.20), 889 (0.26), 791 (1.00), 769
(0.63), 712 (0.46), 670 (0.37), 598 (0.24), 527 (0.37).
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Accessory Publication
Electronic supplementary information is available showing ESR
spectra of 7T at various photolysis wavelengths (Fig. S1), matrix
UV and IR spectra of azide 6, UV-visible spectrum of the forma-
tion of nitrene 7 and ylide 19, calculated UV-visible transitions
of nitrene 7T (Figs S2–S5), Ar matrix IR spectrum of 27 and 28
(Figs S6, S7), triplet carbene spin density–zero field splitting
parameter (ρ–D) correlation (Fig. S8), tabulations of experi-
mental and calculated IR absorptions of 6, 20, and 22, tables
of relative energies of all species relating to Schemes 6, 7, and 9,
graphic representation of all energies relating to Schemes 6 and
7, and tables of standard orientations, energies and vibrational
frequencies of all optimized ground and transition structures cal-
culated at the (U)B3LYP/6–31G* level.This material is available
from the journal’s website.
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Acknowledgements
The present work was supported by the Australian Research Council.
We thank the Alexander-von-Humboldt Foundation, Germany, for a
Feodor-Lynen Fellowship for UM, Dr Lisa George for assistance with
some preliminary experiments, and the Centre for Computational Molec-
ular Science at the University of Queensland for generous allocation of
supercomputer time.
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