Formation and Reactivity of Triplet Vinylnitrenes as a Function of Ring Size

Article abstract of DOI:10.1021/acs.joc.9b01191

The photoreactivity of cyclic vinyl azides 1 (3-azido-2-methyl-cyclopenten-1-one) and 2 (3-azido-2-methyl-2-cyclohexen-1-one), which have five- and six-membered rings, respectively, was characterized at cryogenic temperature with electron paramagnetic resonance (EPR), IR, and UV spectroscopy. EPR spectroscopy revealed that irradiating (λ > 250 nm) vinyl azides 1 and 2 in 2-methyltetrahydrofuran at 10 K resulted in the corresponding triplet vinylnitrenes ³1N (D/hc = 0.611 cm^-1 and E/hc = 0.011 cm^-1) and ³2N (D/hc = 0.607 cm^-1 and E/hc = 0.006 cm^-1), which are thermally stable at cryogenic temperature. Irradiation of vinyl azides 1 (310 nm light-emitting diode at 12 K) and 2 (xenon arc lamp through a 310-350 nm filter at 8 K) in argon matrices showed that in competition with intersystem crossing to form vinylnitrenes ³1N and ³2N, vinyl azide 1 formed a small amount of ketenimine 3, whereas vinyl azide 2 formed significant amounts of azirine 7 and ketenimine 6. Thus, vinyl azide 1 undergoes intersystem crossing more efficiently than vinyl azide 2. Similarly, vinylnitrene ³1N is much more photoreactive than vinylnitrene ³2N. Quantum chemical calculations were used to support the mechanisms for forming vinylnitrenes ³1N and ³2N and their reactivity.

Full text of DOI:10.1021/acs.joc.9b01191

Article
pubs.acs.org/joc
Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
Formation and Reactivity of Triplet Vinylnitrenes as a Function of
Ring Size
DeVonna M. Gatlin,† William L. Karney,‡ Manabu Abe,§ Bruce S. Ault,†
and Anna D. Gudmundsdottir*,†
†Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States
‡Department of Chemistry, University of San Francisco, 2130 Fulton Street, San Francisco, California 94117, United States
§Department of Chemistry, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan
S
* Supporting Information
ABSTRACT: The photoreactivity of cyclic vinyl azides 1 (3-azido-2-
methyl-cyclopenten-1-one) and 2 (3-azido-2-methyl-2-cyclohexen-1-one),
which have five- and six-membered rings, respectively, was characterized at
cryogenic temperature with electron paramagnetic resonance (EPR), IR,
and UV spectroscopy. EPR spectroscopy revealed that irradiating (λ > 250
nm) vinyl azides 1 and 2 in 2-methyltetrahydrofuran at 10 K resulted in
the corresponding triplet vinylnitrenes 31N (D/hc = 0.611 cm−1 and E/hc
3
= 0.011 cm−1) and 2N (D/hc = 0.607 cm−1 and E/hc = 0.006 cm−1),
which are thermally stable at cryogenic temperature. Irradiation of vinyl
azides 1 (310 nm light-emitting diode at 12 K) and 2 (xenon arc lamp
through a 310−350 nm filter at 8 K) in argon matrices showed that in
competition with intersystem crossing to form vinylnitrenes 31N and 32N, vinyl azide 1 formed a small amount of ketenimine 3,
whereas vinyl azide 2 formed significant amounts of azirine 7 and ketenimine 6. Thus, vinyl azide 1 undergoes intersystem
3
3
crossing more efficiently than vinyl azide 2. Similarly, vinylnitrene 1N is much more photoreactive than vinylnitrene 2N.
3
3
Quantum chemical calculations were used to support the mechanisms for forming vinylnitrenes 1N and 2N and their
reactivity.
Scheme 1
INTRODUCTION
■
Vinyl azides and 2H-azirines are versatile functional groups
that can be used to form new C−N bonds in synthetic
applications.1 The dipolar character of vinyl azides makes them
suitable for cycloaddition to alkynes and olefins, which has
been used successfully in countless syntheses.2−8 Similarly, 2H-
azirine derivatives can undergo cycloaddition to alkenes, but
more importantly, they can react as both nucleophiles and
electrophiles. Because azirines are highly reactive owing to
their intrinsic ring strain, they are valuable building blocks in
synthesis.9,10 The quest for sustainable chemistry has attracted
attention to the use of sunlight, a natural resource, or energy-
efficient light-emitting diodes (LEDs) for synthetic applica-
tions.11,12 For example, photocatalytic sensitization of aryl and
vinyl azides has been successfully used for selective C−N bond
formation.13,14 However, the synthetic utility of the photo-
chemistry of vinyl azide and 2H-azirine derivatives is limited by
their complex photoreactivity, which depends on multiple
factors, such as molecular structure, irradiation wavelength,
and whether excitation occurs directly or with sensitizers.15−18
For example, direct irradiation of vinyl azide chromophores
can yield azirines in a concerted manner on the singlet excited
surface, whereas triplet sensitization yields triplet vinylnitrene
intermediates (Scheme 1).19,20 In comparison, direct irradi-
ation of 2H-azirines results in the formation of the
corresponding ylide, whereas triplet sensitization leads to C−
N bond cleavage to form vinylnitrenes.21−23 Thus, to use the
photoreactivity of vinyl azide or 2H-azirine derivatives in
synthetic applications for C−N bond formation, it is important
to be able to control their photochemistry, which can only be
achieved by understanding their reactivity. Evaluating whether
triplet vinylnitrenes are involved in a reaction is challenging
because they are generally short-lived intermediates that decay
efficiently by intersystem crossing to form products or to
reform the starting material, and, thus, it is necessary to rely on
Received: May 2, 2019
Published: July 2, 2019
© XXXX American Chemical Society
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spectroscopy for their identification.24 A further complication
in characterizing triplet vinylnitrenes is that they are not stable
at low temperatures, unless they are a part of a cyclic
structure.25,26
In this article, we compare the reactivity of two cyclic vinyl
azide derivatives, 3-azido-2-methyl-2-cyclopenten-1-one (1,
Scheme 2) and 3-azido-2-methyl-2-cyclohexen-1-one (2,
Scheme 2
Scheme 2), in cryogenic matrices. We specifically studied
these vinyl azides, which have built-in triplet sensitizer or
ketone chromophore, to form triplet vinylnitrenes, which are
not conjugated to aromatic rings. We demonstrate that
irradiation of both vinyl azides 1 and 2 with UV light results
in the formation of the corresponding triplet vinylnitrenes, 31N
3
and 32N. The ring size of vinylnitrenes 1N and 32N (Scheme
2) affects their formation and reactivity but not their biradical
character.
RESULTS
■
Electron Paramagnetic Resonance (EPR) Spectrosco-
py. EPR spectroscopy verified that exposure of vinyl azides 1
and 2 to UV light in 2-methyltetrahydrofuran (mTHF) at 10 K
3
3
yields the corresponding vinylnitrenes, 1N and 2N. The
3
spectrum of 1N shows X2 and Y2 signals at 5534 and 5988
Gauss, respectively, at a resonance frequency of 9.4 GHz,
whereas the spectrum of 32N has X2 and Y2 signals at 5623 and
5873 Gauss, respectively (Figure 1). The zero-field splitting
(zfs) parameters for vinylnitrenes 31N and 32N were calculated
using the X2 and Y2 signals of each spectrum according to the
Wasserman equations.27 The calculated zfs parameters for
3
vinylnitrene 1N are D/hc = 0.611 cm−1 and E/hc = 0.011
Figure 1. (a) EPR spectrum obtained by irradiating (λ > 250 nm)
vinyl azide 1 in mTHF at 10 K. (b) EPR signals of 31N as a function
of temperature. (c) EPR spectrum obtained by irradiating (λ > 250
nm) vinyl azide 2 in mTHF at 10 K. (d) EPR signals of 2N as a
function of temperature. Signals due to the cavity of the EPR
cm−1, whereas those for 32N are D/hc = 0.607 cm−1 and E/hc
= 0.006 cm−1. The X2 and Y2 signals observed for vinylnitrenes
31N and 32N are characteristic of triplet vinylnitrenes, and the
3
3
3
D/hc values demonstrate that vinylnitrenes 1N and 2N have
spectrometer are labeled with *.
a significant 1,3-biradical character.25,26 It is noteworthy that
vinylnitrene 1N has a larger deviation from the cylindrical
symmetry of the spins than that of 2N, and, thus, the EPR
spectra of 1N and 2N have different shapes.
3
3
UV Absorption in Glassy mTHF Matrices. To further
3
3
3
3
characterize vinylnitrenes 1N and 2N, their UV−vis spectra
were obtained in glassy mTHF matrices at 77 K. Figure 2
displays the difference absorption spectra of vinyl azides 1 and
2 as a function of irradiation time. Vinyl azide 1 was irradiated
with an LED (∼310 nm), and vinyl azide 2 was irradiated
through a Pyrex filter (>300 nm). As vinyl azides 1 and 2 were
exposed to light, new broad absorption bands were formed
with λmax at ∼235 and 398 nm, and at ∼236 and ∼368 nm,
respectively. These bands, which increased throughout
Wentrup and co-workers have shown that most reported
triplet nitrenes exhibit a linear correlation between their
measured D values and the UB3LYP-calculated natural spin
densities on the nitrogen atoms.28−30 The zfs values for
3
3
vinylnitrenes 1N and 2N also fit this trend (vide infra).
The EPR signal intensities grew as a function of irradiation
time, and no new signals were observed upon further
3
irradiation. The EPR spectra of vinylnitrene 1N broadened
3
as the matrix was warmed to 80 K. However, upon recooling,
the intensity was recovered, thus, verifying that vinylnitrene
31N is stable up to at least 80 K (Figure 1b). Similarly,
irradiation (Figure 2), are assigned to vinylnitrenes 1N and
32N by comparison to their time-dependent density functional
theory (TDDFT)-calculated spectra. At the TD-B3LYP-
(IPCM,THF)/6-31+G* level, the major calculated electronic
transitions for vinylnitrene 31N are located at 410 (f = 0.002),
358 (f = 0.03), 281 (f = 0.02), 277 (f = 0.0039), and 238 (f =
3
vinylnitrene 2N is also stable up to 80 K (Figure 1d), as the
intensity of the spectra was recovered upon recooling.
However, at temperatures above 40 K, the signals from the
cavity of the EPR spectrometer become more intense than the
3
0.1471) nm, whereas the major bands for 2N are at located
3
signals due to vinylnitrene 2N.
402 (f = 0.0012), 344 (f = 0.0141), 282 (f = 0.0007), 276 (f =
B
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reason for this poor correspondence is that the accurate
prediction of electronic transitions with CASPT2 requires a
very large active space that includes many sigma molecular
orbitals (MOs), but such calculations exceeded our computa-
tional resources. Another possible explanation is that the
isodensity polarizable continuum model (IPCM) solvation
model used for the TDDFT calculations is not available for use
with the CASPT2 calculations.
The negative bands in Figure 2 are due to the depletion of
vinyl azides 1 and 2, as supported by the agreement between
the observed and calculated TDDFT absorption spectra.
Although the absorption observed at longer wavelengths in
the spectra in Figure 2 can be assigned to triplet vinylnitrenes
3
31N and 2N, it is not possible to exclude the possibility that
other photoproducts are also formed concurrently with the
vinylnitrenes or by secondary photolysis.
Matrix Isolation. As EPR and UV-vis absorption spectros-
3
3
copy show that vinylnitrenes 1N and 2N have remarkably
similar properties, we investigated whether their ring sizes
affected their formation and reactivity by using matrix isolation
and IR spectroscopy to identify any products formed
concurrently with the vinylnitrenes and their secondary
photoreactivity.
Vinyl azide 1 was irradiated with a 310 nm LED for a total of
∼115.5 min in an argon matrix at 12 K, and IR spectra were
recorded at various time intervals. The bands of vinyl azide 1 at
2109, 1720, 1653, 1385, 1335, 1269, 1139, 827, and 627 cm−1
were depleted upon irradiation (Figure 3a). Concurrently, new
bands with contrasting time profiles were formed, suggesting
primary and secondary photochemical reactions. The bands
that formed at 1659 and 1369 cm−1 increased for the first 55
min of irradiation and then decreased somewhat upon further
Figure 2. UV−vis difference spectra obtained by irradiating (λ > 250
nm) vinyl azides (a) 1 and (b) 2 in mTHF matrices at 77 K. TDDFT-
calculated electronic transitions are shown as bars for the S0 of 1 and
2 and for vinylnitrenes 1N and 2N, ketenimines 3 and 6, azirine 7,
ketene 4, and ketone 5. The 4 and 7 min traces have some overlaps.
3
3
3
irradiation. These bands were assigned to vinylnitrene 1N
based on comparison to its calculated IR spectrum. The
calculated frequencies of the CO and the coupled CC and
C−N stretching bands were located at 1630 and 1316 cm−1
after scaling by 0.9613,31 which is in good agreement with the
experimental IR bands (Figure 3b). In addition, the less
intense bands at 1026, 982, and 648 cm−1 were also assigned to
vinylnitrene 31N, as they exhibited the same time profile as the
bands at 1659 and 1369 cm−1, and they corresponded to the
calculated and scaled frequencies at 1046, 959, and 628 cm−1.
0.0011), 247 (f = 0.041), 244 (f = 0.0116), and 243 (f =
0.1427) nm. Additional support for these assignments was
provided by the electronic transitions computed by multistate
CASPT2 calculations (Table 1). The CASPT2 prediction of
relatively strong transitions around 360 nm for the triplet
nitrenes agrees reasonably well with the density functional
theory (DFT) results, although the agreement is not as good in
the remainder of the spectrum. In particular, CASPT2 does not
yield a strong excitation in the region of 240 nm. One possible
3
Concurrently with the formation of vinylnitrene 1N, bands
appeared at 1871, 1867, 1863, and 1662 cm−1. The intensities
of these bands increased with further irradiation, and there was
no indication of secondary photoreactions. We assigned these
bands to ketenimine 3 based on the comparison to its
calculated spectrum, in which the most significant bands were
calculated and scaled to be at 1899 and 1658 cm−1,
corresponding well with the observed bands. The CCN
bands at 1871, 1867, and 1863 cm−1 in the spectrum of 3 must
be due to different conformers of ketenimine 3 or different
sites in the argon matrices.
Table 1. CASPT2(12,11)/cc-pVDZ//B3LYP/6-31+G*
Computed Electronic Transitions and Oscillator Strengths
a
3
3
for Vinylnitrenes 1N and 2N
species
λ (nm)
f
31N
656
359
277
261
253
229
656
363
304
285
273
258
235
0.000025
0.012
0.0013
0.00051
0.000055
0.000057
0.000093
0.0045
After irradiation of vinyl azide 1 for ∼10 min, additional new
bands were formed at 2230, 2145, 1818, 1813, 1602, 1464,
1436, 1193, 993, 957, and 943 cm−1, which grew with further
irradiation. Because these bands continued to grow after the
32N
0.0010
3
intensity of the vinylnitrene 1N bands decreased, they are
0.00016
0.00054
0.000036
0.000071
3
assigned to secondary photoproducts from vinylnitrene 1N.
These bands were assigned to ketene 4 and ketone 5 by
comparison to their calculated spectra. The bands at 2145,
1602, 1193, 993, and 943 cm−1 assigned to ketene 4 fit well
with the calculated and scaled bands at 2106, 1564, 1217,
a
Using state-averaged CASSCF and multistate CASPT2 calculations.
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Figure 3. (a) IR spectra obtained before and after various periods of irradiation of vinyl azide 1 with a 310 nm LED in an argon matrix at 12 K. (b)
Difference spectrum obtained by subtracting the IR spectrum of vinyl azide 1 from the one obtained after 115.5 min of irradiation. (c) B3LYP/6-
31+G(d)-calculated IR spectra of vinylnitrene 31N, ketenimine 3, ketene 4, and ketone 5. Difference spectra obtained at various irradiation times,
and various expansions of the spectra and difference spectra can be found in the Supporting Information (SI) (Figures S1−S6). The formation of
azirine 8 from photolysis of vinyl azide 1 was ruled out based on the comparison to its calculated and scaled IR spectrum (see Figure S1 in the SI).
Figure 4. (a) IR spectra obtained before and after various periods of irradiation of vinyl azide 2 with a xenon arc lamp through a 310−350 nm glass
filter in an argon matrix at 8 K. (b) Difference spectrum obtained by subtracting the IR spectrum of vinyl azide 2 from the one obtained after 13.3 h
of irradiation. (c) B3LYP/6-31+G(d)-calculated IR spectra of vinylnitrene 32N, ketenimine 6, and azirine 7. Difference spectra obtained at various
irradiation times can be found in the SI (Figures S6 and S7).
1058, and 960 cm−1, whereas the bands at 2230, 1818, 1813,
1464, 1436, and 957 cm−1 were assigned tentatively to ketone
5 based on its calculated and scaled bands at 2245, 1808, 1460,
1404, and 1018 cm−1. The CO bands at 1818 and 1813
cm−1 in the spectrum of ketone 5 must be due to different
conformers of 5 or different sites in the argon matrices. In
addition, the IR spectrum of neat ketone 5 has been reported
to have a CN stretch at 2235 cm−1 and a CO stretch at 1800
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Scheme 3. Proposed Mechanism for the Formation of Vinylnitrene 31N and Photoproducts 3−5 from the Photolysis of Vinyl
Azide 1
cm−1,32 which are shifted by 5 and 13 to 18 cm−1, respectively,
relative to the bands observed in matrices. However, these
shifts are likely due to the polar medium of neat ketone 5,
whereas the matrix spectrum shows better correspondence
with the calculated gas-phase IR spectrum.
calculated and scaled to be at 1756 and 1703 cm−1, which fits
well with the observed bands.
Irradiation of vinyl azide 1 in argon matrices resulted in the
formation of ketenimine 3 and vinylnitrene 31N as the primary
3
photoproducts. Further irradiation caused vinylnitrene 1N to
cleave to form secondary products 4 and 5. Because
Vinyl azide 2 was irradiated through a 310−350 nm glass
filter for ∼13 h (Figure 4) in argon matrices at 8 K. IR spectra
were obtained at various time intervals (5 min, 10 min, 1 h, 2
h, 2.5 h, 4.0 h, 4.8 h, and 13.3 h) to follow the progress of the
photolysis (the spectra are displayed in the SI). The IR bands
corresponding to vinyl azide 2 at 2138, 2109, 2101, 1680,
1634, 1356, 1289, and 1137 cm−1 were almost fully depleted
upon exposure to light for 13.3 h. As these bands were
depleted, new bands were concurrently formed, with the most
significant ones appearing at 1984, 1746, 1711, 1624, 1593,
1259, 1152, 1075, and 1008 cm−1. These bands were assigned
3
ketenimine 3 and vinylnitrene 1N were formed concurrently,
and EPR spectroscopy showed that vinylnitrene 31N is
thermally stable, ketenimine 3 is expected to originate from
the singlet excited state (S1) of vinyl azide 1, which also
intersystem crosses to form vinylnitrene 31N. It should also be
mentioned, it is possible that S1 of 1 forms singlet vinylnitrene
11N that yields ketenimine 3. Furthermore, it is possible that a
fraction of ketenimine 3 is produced by a secondary
3
photoreaction of vinylnitrene 1N (Scheme 3). The proposed
mechanism for the photoreactivity of vinyl azide 1 in matrices
is depicted in Scheme 3.
3
to vinylnitrene 2N, ketenimine 6, and azirine 7 based on the
comparison to their calculated spectra. The band at 1593 cm−1
In comparison, irradiation of vinyl azide 2 in matrices
resulted in ketenimine 6 and azirine 7, concurrent with the
3
was assigned to the CO band of vinylnitrene 2N, which is
calculated (after scaling) to be at 1601 cm−1. The bands
observed at 1259 and 1075 cm−1 were also assigned to
vinylnitrene 32N, matching well with the calculated and scaled
bands at 1282 and 1074 cm−1.
3
formation of vinylnitrene 2N. These results contrast with
those for vinyl azide 1, which did not produce an observable
azirine upon irradiation. Thus, we theorize that both
ketenimine 6 and azirine 7 are formed from the S1 of 2,
3
Furthermore, a strong band is calculated for ketenimine 6 at
1985 cm−1 (scaled), which matches very well with the strong,
broad adsorption observed at 1984 cm−1. This is a region
characteristic of the antisymmetric stretching of ketenimines.33
The band at 1624 cm−1 was assigned to the CO stretch of 6,
which was calculated and scaled to be at 1648 cm−1. In
addition, the bands at 1152 and 1008 cm−1 were assigned to
coupled CH3 stretching bands in ketenimine 6, which were
calculated and scaled to be at 1171 and 1050 cm−1.
which also intersystem crosses to vinylnitrene 2N. As before,
we cannot rule out that S1 of 2 forms a singlet vinylnitrene 12N
that yields ketenimine 6 and azirine 7. Scheme 4 displays the
proposed pathways for the product formation from vinyl azide
2 upon irradiation in argon matrices.
As mentioned earlier, the absorption spectra obtained by
irradiation of vinyl azide 1 in glassy mTHF matrices yielded
absorption bands that were assigned to vinylnitrene 31N based
on the comparison to its calculated TDDFT spectra.
Furthermore, as the TD-B3LYP(IPCM,THF)-calculated spec-
trum of ketenimine 3 had electronic transitions at 480 (f =
Finally, the bands at 1746 and 1711 cm−1 were assigned to
azirine 7. The CN and CO bands of azirine 7 were
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ketenimine 7 and azirine 6, which have major calculated
absorbance bands at 361 (f = 0.015), 302 (f = 0.0045), 251 (f
= 0.0184), 234 (f = 0.2674), 314 (f = 0.0046), 270 (f =
0.0249), 240 (f = 0.0188), and 213 (f = 0.256) nm,
respectively. Thus, the results obtained by irradiating vinyl
azides 1 and 2 in argon matrices and in glassy mTHF matrices
are in agreement.
Scheme 4. Proposed Mechanism for the Formation of
Vinylnitrene 2N and Photoproducts 6 and 7 from the
Photolysis of Vinyl Azide 2
3
Quantum Chemical Calculations. DFT calculations
(B3LYP/6-31+G(d)) were performed to better understand
why vinylnitrenes 31N and 32N react differently in argon
matrices. Optimization of the structure of vinyl azide 1 yielded
two minimal energy conformers, 1A and 1B, differing mainly in
the orientation of the azide group (Figure 5). The energies of
the S1 of 1 and its triplet excited state (T1) were located with
TDDFT calculations to be at 84 and 63 kcal/mol above its
ground state (S0) (Figure 6).
Figure 6 shows the DFT-computed relative energies of the
different electronic states of each azide, along with the
corresponding nitrenes and related isomers. The optimized
structure of vinylnitrene 31N and a N2 molecule was located 3
3
kcal/mol below the S0 of 1. Because vinylnitrene 1N is more
stable than ketenimine 3 and azirine 8 (Figure 7), we theorize
that it does not intersystem cross to form 3 or 8 at cryogenic
temperature. This finding further supports the idea that azirine
8 and ketenimine 3 arise from the excited singlet reactivity of
vinyl azide 1. We were not able to optimize the T1 of 1 or the
3
transition state for forming vinylnitrene 1N from the T1 of 1,
indicating that the energy barrier for forming vinylnitrene 31N
must be very small.
0.0075), 333 (f = 0.017), 255 (f = 0.0134), and 256 (f =
0.03878) nm, it must be formed with vinylnitrene 31N.
However, it was difficult to identify the secondary photo-
products ketene 4 and ketone 5 from the absorption spectra
(Figure 2a), because the irradiation was stopped before the
new absorption bands were depleted. In comparison,
irradiation of azide 2 yielded absorption spectra that could
Similarly, two minimum energy conformers, 2A and 2B,
were optimized for vinyl azide 2, and TDDFT calculations
located its S1 and T1 to be at 82 and 63 kcal/mol above its S0,
respectively (Figure 6). Similar to vinyl azide 1, we could not
optimize the T1 of 2 or the transition state connecting the T1
3
3
of 2 to vinylnitrene 2N. Interestingly, vinylnitrene 2N is less
stable than ketenimine 6 and azirine 7 (Figure 7); however, as
3
be attributed to vinylnitrene 2N, as stated earlier, and also
Figure 5. B3LYP/6-31+G(d)-optimized structures of (a) vinyl azide 1A, 1B and vinylnitrene 31N and (b) vinyl azide 2A, 2B and vinylnitrene 32N.
Selected bond lengths (black) are in angstroms. Numbers in red are the calculated spin densities.
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predicted to lie 12.0 kcal/mol above 31N, and vinylnitrene 12N
3
is computed to be 11.2 kcal/mol higher in energy than 2N.
Thus, the computed singlet−triplet energy gaps of the
vinylnitrenes are more consistent for the different ring sizes
using CASPT2 than using B3LYP. One surprising finding was
1
that 2N is not a potential minimum but rather a transition
state (vide infra).
The calculated stationary points on the potential energy
3
3
surface connections between vinylnitrenes 1N and 2N and
the corresponding secondary photoproducts are displayed in
3
Figure 7. The barriers for vinylnitrene 1N to cleave and form
ketene 4 and ketone 5 are less than those connecting
3
vinylnitrene 2N to ketene 9 and ketone 10, which fits with
3
vinylnitrene 1N being more photochemically reactive than
vinylnitrene 32N. Diradicals 5BR and 10BR are ∼22 kcal/mol
more stable than their counterparts 4BR and 9BR because in
5BR and 10BR, the unpaired electron on N resides in an
orbital that overlaps with the π-system of the enone moiety
rather than in an in-plane p-orbital. In contrast, in 4BR and
9BR, an unpaired electron is localized in an in-plane p-orbital
on N. For diradicals 4BR, 5BR, 9BR, and 10BR, the singlet
and triplet states are nearly degenerate. In only one case were
1
we unable to locate a local minimum, that of 5BR, because
upon geometry optimization, the structure collapsed to a cyclic
product (ketone 5).
Figure 7 also shows relevant singlet species. Of particular
1
note is our finding that 2N is not a minimum but rather a
transition state for interconversion of two isomeric azirines, 7a
and 7b. The nitrogen can bend to either face of the six-
membered ring, giving rise to two azirines that differ in the
conformation of the six-membered ring. This finding is
consistent with the results of Banert et al., who were unable
Figure 6. Calculated (B3LYP/6-31+G(d)) stationary points on the
potential energy surface of vinyl azides (a) 1 and (b) 2. Energies for
S1 and T1 states are computed vertical excitation energies from the S0
state of the azide. The calculated energies are expressed in kcal/mol.
Singlet species are indicated in blue and triplet species in red.
36
1
to locate a minimum for 2N. Moreover, it is similar to the
situation for the parent singlet vinylnitrene (H2CCH−N),
which is a transition state for the formation of two mirror-
image azirine products. The ca. 1 kcal/mol barrier for the
vinylnitrene 32N is stable at cryogenic temperature, we
theorize that intersystem crossing to ketenimine 6 and azirine
7 must be restricted. As in similar systems, we propose that
1
3
transformation of vinylnitrene 1N to azirine 8 is consistent
intersystem crossing from vinylnitrene 2N requires rotation
with the shallow nature of the potential surface in the vicinity
of these singlet nitrenes, and the pattern that the nature of the
stationary point can change depending on substituents or ring
size.
around the vinyl bond to achieve a conformation that makes
intersystem crossing possible, but the ring structure restricts
this rotation.
Spin density calculations (B3LYP/6-31+G(d)) showed that
3
3
vinylnitrenes 1N and 2N have similar spin density patterns,
with the N- and the α-C atoms exhibiting spin densities of
∼1.3 and 0.6, respectively (Scheme 5). Further, the oxygen
atom only has a small spin density of ∼0.2. Thus, the unpaired
electron density is mainly localized on the vinylnitrene
chromophore, and the nitrenes have significant 1,3-biradical
character.
DISCUSSION
■
From the experimental results, we conclude that intersystem
crossing, S1 to T1, is more effective for vinyl azide 1 than for
vinyl azide 2 based on the following considerations. Vinyl azide
2 forms significant amounts of azirine 7 and ketenimine 6 from
its singlet excited state in competition with intersystem
crossing to vinylnitrene 2N. In comparison, vinyl azide 1
forms only a small amount of ketenimine 3 concurrently with
1
3
The geometries of the open-shell singlet vinylnitrenes 1N
and 12N were optimized using DFT with the broken symmetry
method, as described in the literature.34,35 The broken
symmetry calculations, which were achieved using guess =
mix as a keyword in Gaussian09, resulted in singlet
3
intersystem crossing to vinylnitrene 1N. Thus, the decreased
ring size of vinyl azide 1 must enhance its intersystem crossing
in comparison to that of vinyl azide 2. Similar phenomena have
been observed for aliphatic and aromatic ketone derivatives, as
intersystem crossing is enhanced in strained cyclic com-
pounds.37,38
1
1
vinylnitrenes 1N and 2N that are located 15 and 5 kcal/
mol, respectively, above the corresponding triplet configu-
rations. Furthermore, the total spin ⟨S2⟩ values of 1.005 and
1.014, respectively, suggested significant spin contamination
from the triplet state.
The D/hc values for vinylnitrenes 31N and 32N are
remarkably similar, indicating that these species are delocalized
vinylnitrenes, as predicted by their calculated spin densities
Owing to the high spin contamination in the DFT
calculations, the singlet and triplet vinylnitrenes were also
computed using the CASSCF and CASPT2 methods (for more
details, see the Experimental Section). At the CASPT2(6,6)/
3
3
(Scheme 5). The E/hc values for vinylnitrenes 1N and 2N
are less alike; presumably, the lower E/hc value for 1N is a
3
reflection of it having less symmetry of the spin magnetic
1
3
cc-pVDZ//CASSCF(6,6)/cc-pVDZ level, vinylnitrene 1N is
moments than 2N. A comparison of the zfs parameters of
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Figure 7. Calculated (B3LYP/6-31+G(d)) stationary points on the potential energy surface of vinylnitrenes (a) 1N and (b) 2N. The calculated
energies are expressed in kcal/mol. Singlet species are indicated in blue and triplet species in red. Black structures are shown for species with both
singlet and triplet states. No stationary point was located for 15BR, as it collapses to cyclic products upon optimization; thus, the energy indicated
1
for 5BR is the singlet energy at the triplet geometry.
Scheme 5. zfs Values and Calculated Spin Densities (Shown
in Red) of Cyclic Vinylnitrenes
spin densities to vinylnitrene 311, highlighting that the
a
unpaired electrons are not localized on the aromatic ring. In
3
vinylnitrene 12, the spin density on the nitrogen atom is less
3
3
3
prominent than for vinylnitrenes 1N, 2N, and 11; however,
3
the spin density in vinylnitrene 12 is not localized on the
aromatic ring but delocalized on the α-carbon atom with a
small contribution on the oxygen atom. The spin densities of
3
3
3
3
triplet vinylnitrenes 1N, 2N, 11, and 12 are delocalized on
the vinylnitrene chromophore, and the aromatic rings have a
little effect on the vinylnitrene character of 311 and 312. Thus,
both simple vinylnitrenes and vinylnitrenes with extended
conjugated systems have similar unpaired electron densities.
Our results support those of Banert and co-workers,36,39 who
reported that direct photolysis of vinyl azide 1 at −50 °C yields
products 13 and 14 (Scheme 6) and theorized that they were
3
formed by the reaction of vinylnitrene 1N with other triplet
a
Only spin densities greater than 0.15 are included; for more details,
vinylnitrene intermediates to form new C−N and N−N bonds.
In contrast, they demonstrated that direct irradiation of vinyl
azide 2 at −50 °C resulted in azirine 7, whereas triplet
sensitization with benzophenone resulted in products 15 and
16, which are attributed to the dimerization of vinylnitrene
32N. Our studies support the notion that upon direct
photolysis, vinyl azide 2 will mainly exhibit singlet reactivity
see the SI.
3
3
vinylnitrenes 1N and 2N to those of other cyclic vinyl-
nitrenes such as 311 and 312 (Scheme 5) shows that
vinylnitrenes 1N and 2N have more significant 1,3-biradical
3
3
3
3
character than vinylnitrene 11 but less than vinylnitrene 12.
3
Even though the unpaired electrons of vinylnitrenes 1N and
3
32N are delocalized on the vinylnitrene chromophore with only
in competition with intersystem crossing to vinylnitrene 2N,
a small contribution on the carbonyl oxygen, they have similar
whereas vinyl azide 1 will react mainly on its triplet surface.
H
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averaging 48 scans at 1 cm−1 resolution, using a PerkinElmer
Spectrum One Fourier transform infrared (FT-IR) spectrometer.
Spectra were recorded during and after matrix deposition.
Scheme 6
Quantum Chemical Calculations. Geometries of all species
were initially optimized at the B3LYP level of theory with the 6-
31+G(d) basis set using the Gaussian09 program.41−43 Calculated
UV−vis spectra were obtained using TDDFT calculations with the
incorporation of solvation using the IPCM model.44−46 The
calculated transition states were confirmed to contain one imaginary
vibrational frequency by analytical determination of the second
derivative of the energy with respect to the internal coordinates.
Intrinsic reaction coordinate calculations were used to verify that the
transition states were correlated with the proper reactants and
products.47,48 All DFT calculations were performed at the Ohio
Supercomputer Center.
To obtain more accurate singlet−triplet energy gaps, the energies
of the vinylnitrenes were computed at the CASPT2(6,6)/cc-pVDZ
level,49,50 using CASSCF(6,6)/cc-pVDZ-optimized geometries. The
active space for these calculations consisted of the five π/π* MOs plus
the in-plane nonbonding MO (NBMO) on nitrogen. The electronic
transitions of the triplet vinylnitrenes were computed using a
combination of state-averaged CASSCF and multistate CASPT2
calculations on the B3LYP/6-31+G(d) structures. The 12-electron,
11-orbital active space in these CASPT2(12,11)/cc-pVDZ calcu-
lations consisted of all of the π/π* MOs, the in-plane NBMO on
nitrogen, plus three σ/σ* pairs (combinations of the C−O σ bond,
the Cα−Cβ σ bond, and the σ bond between the carbonyl carbon and
the α carbon). Using a slightly smaller active space of 10 electrons and
10 orbitals did not significantly affect the results. The excited-state
CASPT2 calculations employed level shift parameter of 0.2 to
eliminate intruder states. The CASSCF and CASPT2 calculations
were performed with Molcas 8.2.51 The MOs were visualized with
Molden.52
CONCLUSIONS
■
We have shown that direct irradiation of vinyl azides 1 and 2 at
cryogenic temperature results in the formation of vinylnitrenes
31N and 32N, respectively, which are stable at cryogenic
temperature. These triplet vinylnitrenes have significant 1,3-
biradical character, as predicted by DFT calculations and
verified with EPR spectroscopy. The strain of the five-
membered ring in vinyl azide 1 enhances intersystem crossing
to form T1 of 1 and subsequently vinylnitrene 31N. In
comparison, irradiation of vinyl azide 2 results in significant
yields of singlet products in competition with intersystem
crossing to form vinylnitrene 32N, which is photostable. Thus,
the photoreactivities of vinyl azides 1 and 2 are different,
because intersystem crossing is efficient in vinyl azide 1 but not
in vinyl azide 2. Thus, to successfully apply simple cyclic vinyl
azides in synthetic applications, it is necessary to establish
whether they react on their singlet or triplet surface.
Preparation of Vinyl Azides 1 and 2.53 Synthesis of 3-Chloro-
2-methylcyclopent-2-en-1-one (Step 1). In a round-bottom flask, 2-
methylcyclopentane-1,3-dione (1.0 g, 8.9 mmol) was dissolved in 25
mL of chloroform. While stirring, trichlorophosphane (PCl3; 1.3 mL,
15 mmol) was added dropwise, and then the mixture was refluxed for
approximately 6 h and allowed to stir with no heat for an additional
13 h. The reaction mixture was placed in an ice bath to cool, then
poured into 30 mL of cold deionized water and extracted with cold
diethyl ether (2 × 50 mL). The extracted layers were washed with
20% aqueous NaOH and brine, and then dried over MgSO4. The
filtrate was dried under an air stream to give an oil of 3-chloro-2-
methylcyclopent-2-en-1-one (0.896 g, 6.76 mmol, 75% yield). The oil
was used for the next reaction step without any further purification.
EXPERIMENTAL SECTION
■
EPR Spectroscopy. Dilute solutions of vinyl azides 1 and 2 in
anhydrous mTHF were prepared (100 and 80 mM, respectively) and
then degassed via the freeze−pump−thaw method three times before
recording EPR spectra using a Bruker X-band EPR E500
spectrometer. All measurements were performed at 10 K, unless
otherwise stated. Temperature-dependence measurements were also
collected after 10 min irradiation. The cyclic vinyl azides were
irradiated using a mercury arc lamp (>250 nm).
UV−Vis Absorption Spectroscopy. Dilute solutions of vinyl
azides 1 and 2 were prepared in anhydrous mTHF. All measurements
were performed at 77 K. Vinyl azide 1 was irradiated with a xenon arc
lamp with a borosilicate/Pyrex glass filter (>300 nm). Vinyl azide 2
was irradiated with an LED (∼310 nm).
Matrix IR Spectroscopy. Matrix isolation of vinyl azide 1 was
performed on a conventional equipment.40 Argon was deposited at 22
K on a CsI window, and then argon and the sample were deposited at
room temperature for 46 min under vacuum. The system was then
cooled to 12 K before starting the measurement. The sample was
irradiated through the quartz window with an LED (∼310 nm). IR
spectra were collected periodically at various irradiation times. Vinyl
azide 2 was sublimed at 50 °C and deposited with argon at 22 K in
the vacuum of a diffusion pump. The cryostat cold head was cooled to
8 K before irradiation. The matrix was irradiated with a xenon arc
lamp through a 310−350 nm glass filter, and IR spectra were collected
at various times. IR spectra were recorded from 450 to 4000 cm−1,
1
Chloro-2-methylcyclopent-2-en-1-one was characterized by H NMR
and FT-IR spectroscopy, and the obtained spectra agreed with those
published in the literature.53 1H NMR (400 MHz, CDCl3) δ 2.79−
2.71 (m, 3H), 2.56−2.51 (m, 2H), 1.65 (t, J = 1.9 Hz, 3H) ppm. IR
(neat): 1706, 1641 cm−1.
Synthesis of 3-Azido-2-methylcyclopent-2-en-1-one (Step 2). 3-
Chloro-2-methylcyclopent-2-en-1-one (0.65 g, 4.9 mmol) was
dissolved in 6 mL of methanol and 2 mL of deionized water while
stirring in a 250 mL round-bottom flask. NaN3 (0.49 g, 7.5 mmol)
was added, and the mixture was stirred for ∼41.5 h. The reaction
mixture was extracted with diethyl ether (3 × 25 mL), washed with
deionized water, and then dried over MgSO4. The filtrate was dried
under high vacuum to obtain a brown oil of 3-azido-2-
methylcyclopent-2-en-1-one (1; 0.30 g, 2.2 mmol, 44% yield). The
oil was used for cryogenic experiments without purification. Vinyl
1
azide 1 was characterized by H NMR and IR spectroscopy, and the
observed spectra agreed with those published in the literature.53 1H
NMR (400 MHz, CDCl3) δ 2.79−2.70 (m, 2H), 2.58−2.49 (m, 2H),
1.65 (q, J = 1.6 Hz, 3H) ppm. IR (neat): 2012, 1690, 1629 cm−1.
Synthesis of 3-Chloro-2-methylcyclohex-2-en-1-one (Step 1). In
a round-bottom flask, 2-methylcyclohexane-1,3-dione (5.0 g, 40
mmol) was dissolved in 50 mL of chloroform. While stirring, PCl3 (10
mL, 114 mmol) was added dropwise, and the mixture was refluxed for
approximately 4 h. The reaction mixture was placed in an ice bath to
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Article
cool and then poured into 30 mL of cold deionized water and
extracted with cold diethyl ether (25 mL). The extracted layers were
washed with 10% aqueous NaOH and brine, dried over MgSO4, and
then the filtrate was concentrated in a rotary vacuum evaporator to
give an oil of 3-chloro-2-methylcyclohex-2-en-1-one (2.5 g, 18 mmol,
44% yield). The oil was used for the next reaction step without further
purification. 3-Chloro-2-methylcyclohex-2-en-1-one was characterized
by 1H NMR and FT-IR spectroscopy, and the obtained spectra agreed
with those published in the literature.53 1H NMR (400 MHz, CDCl3):
δ 2.75 (tq, J = 6.1, 2.0 Hz, 2H), 2.50−2.41 (m, 2H), 2.04 (dq, J = 8.0,
6.4 Hz, 2H), 1.91 (t, J = 2.0 Hz, 3H) ppm. IR (neat): 1674, 1626,
1431, 986 cm−1.
Synthesis of 3-Azido-2-methylcyclohex-2-en-1-one (Step 2). 3-
Chloro-2-methylcyclohex-2-en-1-one (1.8 g, 12 mmol) was dissolved
in 10 mL of methanol and 5 mL of deionized water by stirring. NaN3
(1.3 g, 20 mmol) was added, and the mixture was stirred for ∼4 days
(95 h). The reaction mixture was extracted with diethyl ether (25
mL), washed with deionized water, and dried over MgSO4. The
solvent was removed under vacuum to yield an oil of 3-azido-2-
methylcyclohex-2-en-1-one (2; 1.7 g, 11 mmol, 94% yield). This oil
was used for cryogenic experiments without any further purification.
Vinyl azide 2 was characterized by 1H NMR and FT-IR spectroscopy,
and the obtained spectra agreed with those published in the
literature.53 1H NMR (400 MHz, CDCl3): δ 2.62 (tq, J = 6.0, 1.8
Hz, 2H), 2.41 (dd, J = 7.4, 6.0 Hz, 2H), 2.08 (d, J = 6.4 Hz, 2H), 1.73
(t, J = 1.8 Hz, 3H) ppm. IR (neat): 2092, 1652, 1615 cm−1.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.9b01191.
1
IR and H NMR spectra of vinyl azides 1, 2 and their
1
3
(14) Farney, E. P.; Yoon, T. P. Visible-Light Sensitization of Vinyl
Azides by Transition-Metal Photocatalysis. Angew. Chem., Int. Ed.
2014, 53, 793−797.
chloro-precursors; optimized structures of 1, 1N, 1N,
1
3
2, 2N, 2N, and 3−9; CASSCF and CASPT2 absolute
1
3
1
3
energies of nitrenes 1N, 1N, 2N, and 2N (PDF)
(15) Scriven, E. F. V.; Turnbull, K. Azides: Their Preparation and
Synthetic Uses. Chem. Rev. 1988, 88, 297−368.
AUTHOR INFORMATION
Corresponding Author
*E-mail: anna.gudmundsdottir@uc.edu.
■
(16) Scriven, E. F. V., Ed.; Azides and Nitrenes: Reactivity and Utility;
Academic Press, Inc.: Orlando, FL, 1984.
(17) Platz, M. S. Nitrenes. In Reactive Intermediate Chemistry; Moss,
R. A., Platz, M. S., Jones, M., Jr., Eds.; John Wiley & Sons, Inc.:
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(18) Bucher, G. Photochemical Reactivity of Azides. In CRC
Handbook of Organic Photochemistry and Photobiology, 2nd ed.;
Horspool, W. M., Lenci, F., Eds.; CRC Press: Boca Raton, FL, 2004;
pp 44/1−44/31.
ORCID
William L. Karney: 0000-0003-0976-742X
Manabu Abe: 0000-0002-2013-4394
Bruce S. Ault: 0000-0003-3355-1960
Anna D. Gudmundsdottir: 0000-0002-5420-4098
(19) Osisioma, O.; Chakraborty, M.; Ault, B. S.; Gudmundsdottir, A.
D. Wavelength-Dependent Photochemistry of 2-Azidovinylbenzene
and 2-Phenyl-2H-azirine. J. Mol. Struct. 2018, 1172, 94−101.
(20) Rajam, S.; Jadhav, A. V.; Li, Q.; Sarkar, S. K.; Singh, P. N. D.;
Rohr, A.; Pace, T. C. S.; Li, R.; Krause, J. A.; Bohne, C.; Ault, B. S.;
Gudmundsdottir, A. D. Triplet Sensitized Photolysis of a Vinyl Azide:
Direct Detection of a Triplet Vinyl Azide and Nitrene. J. Org. Chem.
2014, 79, 9325−9334.
(21) Weragoda, G. K.; Das, A.; Sarkar, S. K.; Sriyarathne, H. D. M.;
Zhang, X.; Ault, B. S.; Gudmundsdottir, A. D. Singlet Photoreactivity
of 3-Methyl-2-phenyl-2H-azirine. Aust. J. Chem. 2017, 70, 413−420.
(22) Gamage, D. W.; Li, Q.; Ranaweera, R. A. A. U.; Sarkar, S. K.;
Weragoda, G. K.; Carr, P. L.; Gudmundsdottir, A. D. Vinylnitrene
Formation from 3,5-Diphenyl-isoxazole and 3-Benzoyl-2-phenyl-
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was generously supported by the NSF (CHE-
1464694, CHE-1565793, and CHE-1800140) and the Ohio
Supercomputer Center. D.M.G. is grateful for the Ann P.
Villalobos Fellowship. M.A. gratefully acknowledges financial
support by JSPS KAKENHI (Grant No. JP17H03022).
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DOI: 10.1021/acs.joc.9b01191
J. Org. Chem. XXXX, XXX, XXX−XXX