Contrasting Photolytic and Thermal Decomposition of Phenyl Azidoformate: The Curtius Rearrangement Versus Intramolecular C-H Amination

Article abstract of DOI:10.1021/acs.jpca.7b07969

The decomposition of phenyl azidoformate, PhOC(O)N₃, was studied by combining matrix isolation spectroscopy and quantum chemical calculations. Upon UV laser photolysis (193 and 266 nm), the azide isolated in cryogenic noble gas matrices (Ne and Ar, 2.8 K) decomposes into N₂ and a novel oxycarbonylnitrene PhOC(O)N, which was identified by matrix-isolation IR spectroscopy (with ¹⁵N labeling) and EPR spectroscopy (|D/hc| = 1.620 cm^-1 and |E/hc| = 0.024 cm^-1). Subsequent visible-light irradiation (532 nm) causes rearrangement of the nitrene into phenoxy isocyanate PhONCO with complex secondary fragmentation (PhO· + ·NCO) and radical recombination species in matrices. The observation of PhONCO provides solid evidence for the Curtius rearrangement of phenyl azidoformate. In sharp contrast, flash vacuum pyrolysis (FVP) of PhOC(O)N₃ at 550 K yields N₂ and exclusively the intramolecular C-H amination product 3H-benzooxazol-2-one. FVP at higher temperature (700 K) leads to further dissociation into CO₂, HNCO, and ring-contraction products. To account for the very different photolytic and thermal decomposition products, the underlying mechanisms for the Curtius rearrangement (concerted and stepwise) of PhOC(O)N₃ and the intramolecular C-H amination of the nitrene in both singlet and triplet states are discussed with the aid of quantum chemical calculations using the B3LYP, CBS-QB3, and CASPT2 methods.

Full text of DOI:10.1021/acs.jpca.7b07969

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Article
Contrasting Photolytic and Thermal Decomposition of Phenyl Azidoformate:
the Curtius Rearrangement Versus Intramolecular C–H Amination
Huabin Wan, Jian Xu, Qian Liu, Hongmin Li, Yan Lu, Manabu Abe, and Xiaoqing Zeng
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07969 • Publication Date (Web): 25 Oct 2017
Downloaded from http://pubs.acs.org on October 30, 2017
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Contrasting Photolytic and Thermal Decomposition
of Phenyl Azidoformate: the Curtius Rearrangement
versus Intramolecular C–H Amination
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†
Huabin Wan, Jian Xu, Qian Liu, Hongmin Li, Yan Lu, Manabu Abe * and
†
†
†
†
‡
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Xiaoqing Zeng
*
†
College of Chemistry, Chemical Engineering and Materials Science, Soochow
University, 215123, Suzhou, P. R. China. E-mail: xqzeng@suda.edu.cn
‡
Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1
Kagamiyama,
Higashi-Hiroshima
Hiroshima
739-8526,
Japan.
E-mail:
mabe@hiroshima-u.ac.jp
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ABSTRACT
The decomposition of phenyl azidoformate, PhOC(O)N , has been studied by
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combining matrix isolation spectroscopy and quantum chemical calculations. Upon
UV laser photolysis (193 and 266 nm), the azide isolated in cryogenic noble gas
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matrices (Ne and Ar, 2.8 K) decomposes into N
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and a novel oxycarbonylnitrene
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PhOC(O)N, which has been identified by matrix-isolation IR spectroscopy (with N
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–1
–1
labeling) and EPR spectroscopy (|D/hc| = 1.620 cm and |E/hc| = 0.024 cm ).
Subsequent visible-light irradiation (532 nm) causes rearrangement of the nitrene into
phenoxy isocyanate PhONCO with complex secondary fragmentation (PhO• + •NCO)
and radical recombination species in matrices. The observation of PhONCO provides
solid evidence for the Curtius-rearrangement of phenyl azidoformate. In sharp
contrast, flash vacuum pyrolysis (FVP) of PhOC(O)N at 550 K yields N and
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exclusively the intramolecular C‒H amination product 3H-benzooxazol-2-one. FVP at
higher temperature (700 K) leads to further dissociation into CO , HNCO, and
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ring-contraction products. To account for the very different photolytic and thermal
decomposition products, the underlying mechanisms for the Curtius-rearrangement
(concerted and stepwise) of PhOC(O)N and the intramolecular C‒H amination of the
3
nitrene in both singlet and triplet states have been discussed with the aid of quantum
chemical calculations using the B3LYP, CBS-QB3, and CASPT2 methods.
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INTRODUCTION
Azidoformates RO-C(O)N are versatile reagents in synthetic organic chemistry that
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have been broadly used in metal-catalyzed C-H amination and aziridination via the
1
intermediacy of highly reactive oxycarbonylnitrenes RO-C(O)N. Therefore, the
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decomposition of azidoformates and the reactivity of oxycarbonylnitrenes have been
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the fundamentally important research topics in nitrene chemistry. For instance, the
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photolytic and thermal decomposition of the simplest alkyl azidoformate
MeO-C(O)N
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into the Curtius-rearrangement product MeONCO and N has attracted
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extensive experimental and theoretical interest in the past few decades. The nitrene
intermediate MeO-C(O)N in the triplet ground state has been only very recently
detected among the laser photolysis of the azide in cryogenic matrices, which can be
1
4
converted into MeONCO upon subsequent visible light irradiation. Both triplet
ethoxycarbonylnitrene and t-butyloxycarbonylnitrene, generated from laser photolysis
of non-azide sulfilimine precursors in solution, have also been directly detected by
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time-resolved IR spectroscopy.
Comparing to the extensively explored mechanisms for the decomposition of
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alkylcarbonyl azides
and alkoxycarbonyl azides,
the corresponding studies
of aryl-substituted azidoformates are limited, although their applications as nitrene
1
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transfer reagents have been frequently reported.
According to the early studies in
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solutions,
the parent phenyl azidoformate PhO-C(O)N splits off molecular N
3
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upon heating (250 °C) and yields aziridines as the formal intermolecular nitrene
trapping products. More importantly, the intramolecular cyclization product
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H-benzooxazol-2-one (Scheme 1) was also obtained, and it becomes the dominant
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product when the thermolysis of the azide was performed in the gas phase. Similar
intramolecular cyclization has also been observed in the sensitized photolysis (> 345
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nm) of 4-acetylphenoxycarbonyl azide in C F solution, both chemical trapping
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products and EPR results confirm a triplet multiplicity for the aryloxycarbonylnitrene.
The facile metal-free intramolecular C-H bond amination in the thermal
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decomposition of PhO-C(O)N resembles the intensively explored metal-catalyzed sp
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C-H bond insertions of arylnitrenes.
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Scheme 1. Decomposition of Ph-N , Ph-C(O)N ,
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21-22
and PhO-C(O)N3.
3
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The
experimentally
observed
intramolecular
cyclization
rather
than
Curtius-rearrangement in both thermal and photolytic decomposition of
aryloxycarbonyl azides is different from the intensively studied decomposition
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reactions of the closely related phenyl azide Ph-N3 and phenyl carbonyl azide
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13, 29-32
Ph-C(O)N
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,
in which the corresponding nitrene intermediates (Ph-N and
Ph-C(O)N have been directly detected by various spectroscopic methods, as followed
by ring-expansion and Curtius-rearrangement (Scheme 1), respectively. It should be
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noted that Ph-N has triplet ground state, whereas, Ph-C(O)N prefers closed-shell
singlet state. The missing nitrene intermediate (PhO-C(O)N) in the decomposition of
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PhO-C(O)N and the absence of the evidence for its Curtius-rearrangement to
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PhONCO in the previous studies requires a reinvestigation of the underlying
mechanism from both aspects of experiment and theory. Continuing our interest in the
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,
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,
decomposition of α-oxoazides (RC(O)N
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,
RS(O)N
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R
2
P(O)N
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and
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RS(O)
2
N
3
), herein, we report a comprehensive study of the decomposition of
PhO-C(O)N
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by combining laser photolysis, flash vacuum pyrolysis, matrix-isolation
IR and EPR spectroscopy, and quantum chemical calculations.
EXPERIMENTAL SECTION
Caution! Covalent azides are potentially hazardous and explosive. Although we have
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not experienced any incident during the work with PhOC(O)N , safety precautions
(face shields, leather gloves, and protective leather clothing) are recommended for
handing the azide.
Sample Preparation
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4
Phenyl azidoformate, PhOC(O)N
3
, was prepared according to published protocol.
1
The purity was checked by NMR spectroscopy ( H NMR (400 MHz, CDCl
3
, TMS) δ
1
3
=
7.18 (d, 2H), 7.26 (t, 1H) 7.39 (d, 2H) ppm, C NMR (100 MHz, CDCl , TMS) δ =
3
1
20.98, 126.70, 129.73, 150.72, 156.41 ppm) with a Bruker Avance III HD400
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spectrometer at 25 °C. 1- N sodium azide (98 atom % N, EURISO-TOP GmbH)
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was used for the preparation of N labeled sample. The commercially available
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H-benzooxazol-2-one (Energy Chemical, 98%) was purified through vacuum
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sublimation at 90 °C before the matrix isolation experiment.
Matrix–Isolation IR Spectroscopy
Matrix IR spectra were recorded on a FT-IR spectrometer (Bruker 70V) in a
reflectance mode by using a transfer optic. A KBr beam splitter and MCT detector
‒1
were used in the mid-IR region (4000-600 cm ). For each spectrum, 200 scans at a
‒1
resolution of 0.5 cm were co-added. The azide sample was mixed by passing a flow
of noble gas (Ne, Ar) though a U-trap (‒20 °C) containing ca.20 mg of the azide.
Then the mixture (sample/noble gas ≈ 1:1000) was passed though an aluminum oxide
furnace (o.d. 2.0 mm, i.d. 1.0 mm), which can be heated over a length of ca. 25 mm
‒1
by a tantalum wire (o.d. 0.4mm, resistance 0.4 ꢀ), and deposited (2 mmol h ) at a
high vacuum onto the Rh-plated Cu block matrix support (2.8 K) in a high vacuum
‒
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(~10 Pa). For 3H-benzooxazol-2-one, the solid sample was dispersed on the surface
of quartz wool and directly placed into a quartz inlet tube (o.d. 4.0 mm, i.d. 3.0 mm)
for co-deposition with noble gas at 70 °C. Photolysis was performed using an ArF
3+
excimer laser (Gamlaser EX5/250, 193 nm, 3 mJ, 3Hz), a Nd :YAG laser (266 nm,
532 nm, MPL-F-266, 10mW), and a high-power flashlight (Boyu T648, 440±20 nm,
2
0W).
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Matrix-Isolation EPR Spectroscopy
EPR spectra were recorded using a Bruker ELEXSYS E500 spectrometer operating at
the X-band equipped with a digital temperature controller. For the measurements
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‒
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under the organic glassy matrix conditions, the sample solution (20 mg mL ) in a
quartz tube (4.0 mm) was degassed by the freeze-pump-thaw cycle for three times.
After the sample deposition, the cryostat cold head was cooled to 5 K for photolysis
and measurement.
Quantum Chemical Calculation Methods
Structures and IR frequencies of stationary points were calculated using the DFT
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B3LYP method with the 6-311++G(d,p) basis set. Accurate relative energies of the
4
species were further calculated using the complete basis sets (CBS-QB3) and
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4
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CCSD(T) methods. The B3LYP/6-311++G(d,p) optimized structures were used for
the latter single-point energy calculations. Local minima were confirmed by
vibrational frequency analysis, and transition states were further confirmed by
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intrinsic reaction coordinate (IRC) calculation.
Time-dependent (TD)
B3LYP/6-311++G(d,p) calculations were carried out for the prediction of UV–vis
transitions. These calculations were performed using the Gaussian 09 software
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package.
The CASPT2 calculations were performed based on the
CASSCF(12,11)/6-311G** optimized structure with an active space of 12 electrons
53−54
and 11 active orbitals by using the MOLPRO program.
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RESULTS AND DISCUSSION
Photolysis of Phenyl Azidoformate with 193 nm Laser
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Given a strong absorption at about 190 nm for PhOC(O)N (Figure S1), the
photolysis of the azide was carried out first by using an ArF excimer laser (193 nm).
The IR difference spectrum showing the change of the matrix containing the azide in
solid Ne (1:1000 diluted) is depicted in Figure 1A.
Figure 1. A) Ne-matrix IR difference spectrum showing the decomposition of
3
PhOC(O)N upon an ArF laser irradiation (193 nm, 3 mJ, 3 Hz, 5 min); B) IR
difference spectrum showing the change of the matrix upon subsequent 532 nm light
irradiation (10 mW, 5 min); C) Calculated IR spectrum of syn-PhOC(O)N in the
triplet state at the B3LYP/6-311++G(d,p) level; D) Calculated IR spectrum of
syn-PhOC(O)N in the singlet state at the B3LYP/6-311++G(d,p) level; The IR bands
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of PhOC(O)N
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(a), PhOC(O)N (b), PhONCO (c), •NCO (d), Ph-O• (e),
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1
-isocyanate-2,4-cyclohexadienone (f), 4-isocyanate-2,5-cyclohexadienone (g),
-isocyanate-butadienyl-4-ketene (h) and H O (*) are labeled. Spectrum B is
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expanded in intensity by 5 times for clarity. Bands for the depleted species point
downward and those of the formed species point upward.
Upon irradiation, new species were formed as evidenced by the appearance of
several new IR bands with the concomitant depletion of the azide, including the ones
in the region of 2300–2100 cm‒ for characteristic vibrations of NCO asymmetric
stretching modes. More importantly, there are a few new bands at 1711.0 (f), 1695.0
(g), and 1649.1 cm‒ (b), which are likely to be associated with C=O stretching
vibrations. Given the theoretically predicted (TD-B3LYP/6-311++G(d,p))
visible-light absorptions for the nitrene in the triplet state (syn: 545 nm, anti: 545 nm,
Table S2), the matrix was subsequently irradiated with visible light (532 nm). The
resulting IR difference spectrum (Figure 1B) shows the predominant depletion of the
1
1
‒1
carrier of the band at 1649.1 cm (b). Additionally, traces of the azide precursor (a)
were also depleted by the visible light. As the result, the aforementioned new species
‒1
‒1
bearing IR bands at around 2300–2100 cm and 1711.0 (f) and 1695.0 cm (g) were
produced.
To aid the assignment, DFT calculations on the IR spectrum of the most likely
candidate species nitrene PhOC(O)N in the singlet and triplet states and the
Curtius-rearrangement product PhONCO were performed. Similar to the very
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recently disclosed conformational properties of MeOC(O)N, two conformers of the
nitrene PhOC(O)N, depending on the syn and anti configuration between PhO and
C=O, were calculated to be true minima in both electronic states, and their calculated
IR frequencies are listed in Table 1. For clarity, the spectra for the triplet (Figure 1C)
and singlet states (Figure 1D) in the lower-energy syn-conformation are compared
with the observed IR bands (Figure 1B, pointing downward).
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Table 1. Observed and calculated vibrational frequencies (> 600 cm ) of
PhOC(O)N.
a
b
Calculated
Observed
syn-triplet
syn-singlet
3206 (<1)
3204 (2)
anti-triplet
3202 (2)
3197 (6)
3190 (10)
3182 (5)
3173 (<1)
1648 (85)
1635 (26)
1618 (188)
1516 (72)
1486 (3)
1347 (2)
1328 (1)
1214 (393)
1210 (42)
1185 (12)
1181 (9)
1097 (7)
1043 (9)
1018 (4)
995 (<1)
978 (<1)
944 (30)
924 (4)
anti-singlet
3205 (1)
3201 (1)
3194 (10)
3185 (7)
3175 (<1)
1764 (468)
1643 (16)
1629 (56)
1521 (67)
1489 (6)
1413 (114)
1350 (1)
1331 (3)
1209 (128)
1188 (19)
1182 (19)
1101 (8)
Ne-matrix
Ar-matrix
3
254 (4)
203 (4)
3
3193 (13)
3194 (12)
3185 (7)
3
3
1
182 (9)
173 (<1)
3174 (<1)
1777 (428)
1642 (12)
1628 (53)
1518 (81)
1488 (2)
664 (313)
1649.1
1496.8
1644.6
1494.8
1636 (4)
1
1
1
620 (1)
514 (27)
487 (6)
1353 (2)
1402 (65)
1348 (1)
1
1
333 (1)
213 (33)
1330 (1)
1227.8
1195.6
1223.8
1191.8
1
193 (24)
1184 (8)
168 (92)
106 (9)
048 (9)
1017 (<1)
1205 (125)
1186 (44)
1182 (40)
1099 (7)
c
1
1162.8
1071.2
1028.6
-
1
1070.7
1028.7
1
1047 (32)
1028 (21)
1017 (20)
994 (<1)
978 (<1)
924 (4)
1049 (28)
1031 (19)
1017 (11)
997 (<1)
980 (<1)
924 (4)
1
9
9
003 (<1)
87 (<1)
78 (39)
953.5
953.6
930 (7)
45 (<1)
8
839 (<1)
840 (<1)
840 (1)
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801 (10)
816 (16)
764 (46)
695 (32)
653 (2)
815 (26)
764 (24)
698 (40)
664 (26)
640 (8)
820 (19)
767 (56)
696 (29)
649 (4)
7
6
72 (73)
99 (34)
752.4
689.2
752.6
687.6
682 (4)
c
6
77 (14)
25 (1)
638 (17)
627 (4)
641 (16)
627 (2)
673.3
-
6
628 (1)
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a
‒1
Calculated harmonic frequencies (unscaled, cm ) for the singlet and triplet states at
‒
1
the B3LYP/6-311++G(d,p) level of theory, IR intensities (km mol ) in parentheses.
b
c
Observed band positions for the most intense matrix sites in matrices at 2.8 K. Not
identified due to band overlap.
Generally, the experimental observations show better agreement with the
calculations for the energetically more favorable triplet state than those of the singlet
state (Table 1). Particularly, the observed band at 1649.1 cm‒1 coincides with the
theoretical prediction for the C=O stretching vibration at 1664 cm‒1 (syn). The
frequency is also close to those observed for other carbonyl nitrenes such as
‒1
‒1
14
2
MeOC(O)N (syn: 1639.9 cm ; anti: 1597.7 cm , Ar-matrix) and H NC(O)N
‒1
55
(1638.0 cm , Ar-matrix). Its assignment to the C=O stretching vibration is also
15
supported by the N-labeling experiment, and consistent with the theoretical
1
calculation no noticeable N isotopic shift was observed for this band when a 1:1
5
15
mixture of PhO-C(O) N
15
α β γ α β γ
N N and PhO-C(O)N N N was used as the nitrene
precursor (Figure S2). The calculated strongest IR bands for PhOC(O)N in the
‒1
‒1
singlet state (syn: 1777 cm ; anti: 1764 cm ) mainly belong to the C-N stretching
15
‒1
‒1
vibration with large N isotopic shifts (syn: 14 cm ; anti: 12 cm ). In contrast, the
C-N stretching vibration in the triplet nitrene is symmetrically coupled with the C-O
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‒
1
stretching and appears at 953.5 cm with a well-resolved N isotopic shift of 5.6
15
‒1
‒1
cm . The asymmetrically coupled stretching mode locates at 1195.6 cm .
According to the calculated IR spectra of the triplet nitrene in the syn and anti
conformations (Table 1), the latter is expected to have a strong IR band at around
0
1
2
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8
9
0
1
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0
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4
5
6
7
8
9
0
‒
1
618 cm with a large IR intensity of 188 km mol . The absence of this band in the
‒1
1
experimental spectrum demonstrates that the generated triplet nitrene prefers the
energetically more favorable syn conformation.
Photolysis of Phenyl Azidoformate with 266 nm Laser
As shown in Figure 1B, visible light irradiation at 532 nm causes efficient depletion
of PhOC(O)N and formation of several NCO-containing species. To distinguish the
IR bands of the photoproducts, further irradiation of the matrix with 266 nm laser
was applied. For comparison, an independent experiment on photolysis of
PhOC(O)N by using a 266 nm laser was performed. The IR difference spectra
3
reflecting the changes of the respective matrices are shown in Figure 2.
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60
Figure 2. A) Ne-matrix IR difference spectrum showing the decomposition of
PhOC(O)N
spectrum showing the change of the matrix containing the photolysis products of
PhOC(O)N (successive irradiations with 193 and 532 nm) upon further 266 nm
light irradiation (10 mW, 20 min). The IR bands of PhOC(O)N (a), PhONCO (c),
-isocyanate-2,4-cyclohexadienone (f), 4-isocyanate-2,5-cyclohexadienone (g),
3
upon 266 nm (10 mW 60 min) laser irradiation; B) IR difference
3
3
6
1-isocyanate-butadienyl-4-ketene (h) and CO (i) are labeled. Bands for the depleted
species point downward and those of the formed species point upward.
Comparing to the 193 nm laser irradiation (Figure 1A), less products were formed
3
after the 266 nm laser photolysis of PhOC(O)N (Figure 2A). One of the main
differences is the absence of the IR bands (PhONCO, c) in the latter experiment,
which can be reasonably explained by the depletion of these bands upon the 266 nm
irradiation (Figure 2B). Considering its formation from PhOC(O)N and the
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6
‒
1
associated IR band at 2200.6 cm , the carrier should belong to the
Curtius-rearrangement product PhONCO. This assignment is confirmed by the
agreement with the calculated IR spectrum (Table S3), especially for the IR band at
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
1
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0
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9
0
1
2
3
4
5
6
7
8
9
0
‒
1
‒1
9
09.3 cm (ν : 928 cm ), which associates with the characteristic O–N stretching
calcd
15
‒1
vibration with an observed N isotopic shift of 7.1 cm (Figure S2). Similarly, the
‒1
15
band at 2200.6 cm exhibits a N isotopic shift of 7.8 cm .
‒1
Once the presence of PhONCO is ascertained, its photochemistry at 266 nm should
be taken into account for the assignment of the remaining IR bands observed in the
1
93 and 266 nm laser photolysis of the azide. As shown in Figure 1B, traces of
‒1
14/15
‒1 56–57
radicals •NCO (d, 1920.1 cm , ꢁν(
N) = 7.3 cm )
and PhO• (e, 1552.4,
‒
1 58
1486.5, 787.4, and 637.6 cm ) were simultaneously consumed upon the visible
light irradiation. Given the known photochemistry of phenol (→ PhO• + H•, 275
5
9
60
nm) and anisole (→ PhO• + •CH
, 276 nm) in solid Ar matrices, the formation of
3
PhO• and •NCO can be readily understood as the fragmentation products of
PhONCO under the laser irradiations. As observed for these two closely related
phenoxy compounds, recombination of the pair of radicals (PhO• and •NCO) in
same matrix cages in the favorable ortho- and para-positions affords
cyclohexadienones as the secondary photoproducts. Therefore, DFT calculations on
cyclohexadienones (f and g) and ring-opening derivative (h, Scheme 2) were
performed, the IR spectral results are collected in Table S4 and the relative energies
are shown in Scheme S1.
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60
3
Scheme 2. Schematic description of the photolytic decomposition of PhOC(O)N .
The identification of the two lowest-energy ortho-NCO-cyclohexadienone (f, 2240.3,
‒1
1
711.2, 1259.1, 705.9, and 687.3 cm ) and para-NCO-cyclohexadienone (g, 2256.0,
‒1
1
694.6, 1282.4, 894.8 cm ) isomers is mainly based on the distinct photobehaviors
towards the aforementioned irradiations (532 and 266 nm) and also the calculated
spectra (Table S4). For example, both the ortho- (f) and para-isomers (g) were
formed upon the 532 nm irradiation (Figure 1B). However, the latter (g) was formed
with the consumption of the former (f) upon the 266 nm irradiation (Figure 2B).
Although, the para-isomer (g) was obtained as the dominant photoproduct of the
azide upon 266 nm irradiation, no ortho-isomer (f) was observed (Figure 2A). More
importantly, the shift of the C=O stretching vibration in the ortho-isomer (νexptl
:
‒1
‒1
‒1
1
711.2 cm , νcalcd: 1749 cm ) and the para-isomer (νexptl: 1694.6 cm , νcalcd: 1734
cm ) coincides with the similar shift in the ortho-isomer (νexptl: 1728.6 cm‒1,
‒1
6
1
‒1
59
Ar-matrix) and the para-isomer (νexptl: 1682 cm , Ar-matrix) of the parent
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6 6
cyclohexadienone C H O.
Upon the 266 nm laser irradiation (Figure 2), depletion of
ortho-NCO-cyclohexadienone (f) occurs with concomitant formation of CO (i,
0
1
2
3
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5
6
7
8
9
0
1
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9
0
‒1
2
139.1 cm ) and another new species (h, 2273.8, 2127.5, 1648.1, 1395.5, 1035.5,
‒
1
and 891.3 cm ), which was also observed among the 193 nm photolysis of the azide
PhOC(O)N (Figure 1A) and the 532 nm laser photolysis of nitrene PhOC(O)N
Figure 1B). The assignments of the first two bands at 2273.8 and 2127.5 cm to the
3
‒
1
(
‒1
‒1
asymmetric N=C=O (νcalcd: 2360 cm ) and C=C=O stretches (νcalcd: 2199 cm ) to
1
-isocyanate-butadienyl-4-ketene (h) as the photo-induced (266 nm) ring-opening
product of ortho-NCO-cyclohexadienone (f) is further supported by the observation
of analogous photo-induced (> 280 nm) ring-opening of ortho-cyclohexadienone
6 6
C H O into butadienylketene, for which the asymmetric C=C=O stretching vibration
‒1
61
locates as 2124.7 cm in Ar-matrix. It is frequently observed that UV photolysis of
62–63
ketenes
64–66
and isocyanates
leads to CO-elimination and formation of carbenes
and nitrenes, respectively, however, only CO but no counterpart (j–l, Scheme 2)
could be observed.
In order to prove a triplet multiplicity for PhOC(O)N and also capture the possible
triplet nitrene and carbene intermediates in the laser photolysis of the azide, the
photochemistry (266 nm) of PhOC(O)N in organic glassy toluene at 5 K was
3
investigated by using EPR spectroscopy (Figure 3). After 30 minutes of irradiation,
typical triplet nitrene signals appear in the region of 7000–10000 G at 9.399367 GHz
resonance frequency. The derived zero-field splitting parameters (ZFSP) |D/hc| =
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–
1
–1
.620 cm and |E/hc| = 0.024 cm are close to those of 4-acetylphenoxycarbonyl
1
–1
–1 23
nitrene (|D/hc| = 1.65 cm and |E/hc| = 0.024 cm ) and other carbonyl nitrenes
–1
–1 14
3
like CH OC(O)N (|D/hc| = 1.66 cm and |E/hc| = 0.020 cm ), EtOC(O)N (|D/hc| =
10
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53
54
55
56
57
58
59
60
–
1
–1 67
–1
.603 cm and |E/hc| = 0.0215 cm ), and FC(O)N (|D/hc| = 1.623 cm and |E/hc|
1
–1 68
=
0.031 cm ). Therefore, they are reasonably assigned to the expected nitrene
intermediate PhOC(O)N in the triplet ground state.
Figure 3. A) EPR spectrum (ν
nm, 30 min) of PhOC(O)N
prolonged laser irradiation (266 nm, 13 hours) of PhOC(O)N
0
= 9.399367 GHz) obtained by laser irradiation (266
3
in toluene at 5 K; B) EPR spectrum obtained by
3
in toluene at 5 K.
As expected, prolonged 266 nm laser irradiation of the azide in frozen toluene (5 K)
leads to the disappearance of PhOC(O)N, whereas, another two new EPR signals
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2
2
2
2
2
2
2
3
3
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3
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3
3
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3
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5
5
5
5
5
6
–
1
–1
with ZFSP of |D/hc| = 0.978 cm and |E/hc| = 0.016 cm and |D/hc| = 0.395 cm
–1
–1
and |E/hc| = 0.03 cm appear. The former is very likely to be associated with
4
0
-hydroxylphenylnitrene (l, Scheme 2) by referring to its reported ZFSP of |D/hc| =
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
–1
–1 69
.974 cm and |E/hc| = 0 cm . The latter might belong to a dienylnitrene that was
generated through the prolonged 266 nm laser-induced CO-elimination from the
NCO group in either h or j (Scheme 2), since conjugated vinylnitrenes were always
–1 70–71
found to have significantly lower |D/hc| values 0.40–0.55 cm .
Flash Vacuum Pyrolysis of Phenyl Azidoformate
The photochemistry of PhOC(O)N
3
in matrix clearly suggests a stepwise
Curtius-rearrangement into PhONCO and N
2
via the intermediacy of the nitrene
PhOC(O)N, for which a triplet multiplicity was observed due to fast intersystem
crossing (ISC) from the initially generated singlet state. Similar ISC has been
8
–1
disclosed for oxycarbonylnitrene C
2 5
H OC(O)N (kISC = (1-5)×10 s ) in the 266 nm
7
.
2
laser photolysis of the C H OC(O)N
5
2
3
in CF
2
ClCFCl
2
Despite the formation of
complex secondary photoproducts of the nitrene, no evidence for the involvement of
the intramolecular C-H bond insertion product 3H-benzooxazol-2-one (Scheme 1)
could be obtained, which is dramatically different from the previously observed
21-22
thermal decomposition of PhOC(O)N
3
.
To probe into the mechanism, the thermal decomposition of PhOC(O)N
3
in Ne
dilution (1:1000) was explored by combining flash vacuum pyrolysis (FVP) and
matrix-isolation IR spectroscopy. The IR spectrum of the FVP products of the azide
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(
Figure 4B) at 550 K shows almost complete dissociation of the azide (a), no
Curtius-rearrangement product PhONCO but 3H-benzooxazol-2-one (m) was
–1 73
obtained. Among the pyrolysis products, HNCO (n, 2268.9 and 773.6 cm ) and
10
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52
53
54
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58
59
60
–
1
CO (q, 2347.8 cm ) can be readily identified.
2
Figure 4. A) Ne-matrix IR spectrum of the authentic sample of
3
H-benzooxazol-2-one; B) Ne-matrix IR spectrum of the flash vacuum pyrolysis
products of PhOC(O)N at 550 K; C) Ne-matrix IR spectrum of the flash vacuum
3
pyrolysis products of PhOC(O)N at 700 K. The IR bands of PhOC(O)N (a),
3
3
3
2
H-benzooxazol-2-one (m), HNCO (n), carbonylcyclopentadiene (p), CO (q),
1
2
-cyanocyclopentadiene (t), H O (*) and unknown species (#) are labeled.
To aid the assignment, the IR spectrum of an authentic sample of
3
4
H-benzooxazol-2-one (m) in solid Ne-matrix was recorded, this is shown in Figure
A. Obviously, most of the newly appeared IR bands (m) in Figure 4B are identical
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5
6
to those of 3H-benzooxazol-2-one. Therefore, this is consistent with the early studies
that thermal decomposition of PhOC(O)N
3
exclusively yields intramolecular C-H
bond insertion product. The simultaneous formation of HNCO (n) and CO
2
(q)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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0
1
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
implies further fragmentation of 3H-benzooxazol-2-one. This assumption is
supported by the observation of relatively strong IR bands for the secondary
fragments at elevated pyrolysis temperature (Figure 4C, 700 K).
Given the dominant unimolecular decomposition in the Ne dilution (1:1000), two
distinct routes for the decomposition of 3H-benzooxazol-2-one (m) (Scheme 3)
might be involved. One is the elimination of HNCO and formation of
6
-oxocyclohexa-2,4-dienylidene
(o,
not
observed)
as
followed
by
Wolff-rearrangement to the more stable carbonylcyclopentadiene (p). Very similar
74
decomposition has been recently reported for 6-carbonylcyclohexa-2,4-dienone.
The tentative identification of carbonylcyclopentadiene (p) among the FVP products
of PhOC(O)N relies on the IR band at 2129.6 cm–1 in Ne-matrix (2135 cm‒1 in
3
7
4
Ar-matrix) for the most characteristic C=C=O asymmetric stretching vibration, the
remaining bands of this species are too weak to be observed. The alternative route
involves the initial formation of cyclohexa-2,4-dienylideneamine (r, not observed)
2
upon CO -elimination, followed by successive ring-contraction and 1,4-H migration
–1
into the thermally persistent cyclopenta-1,3-dienecarbonitrile (t, 2230.5 cm ). The
same intramolecular rearrangement has been observed in the decomposition of isatin
7
5
(1H-indole-2,3-dione).
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60
Scheme 3. Schematic description of the decomposition of PhOC(O)N3.
Photolysis of 3H-Benzooxazol-2-one with 193 nm Laser
The absence of 3H-benzooxazol-2-one (m) in the laser photolysis of PhOC(O)N
3
might be caused by secondary decomposition upon the laser irradiation conditions.
Thus, the photochemistry of 3H-benzooxazol-2-one in solid Ne-matrix was also
investigated for the first time by using the 193 nm laser. The corresponding IR
difference spectrum (Figure 5A) shows efficient depletion of m with the formation of
CO (i), CO
2
(q), 1-cyanocyclopentadiene (t), and other new species.
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Figure 5. A) Ne-matrix IR difference spectrum showing the decomposition of
3
H-benzooxazol-2-one (m) upon an ArF laser irradiation (193 nm, 3 mJ, 3 Hz, 6
min); B) Ne-matrix IR difference spectrum showing the conversion of
(E)-6-imino-2,4-cyclohexadiene-1-one (u') to (Z)-6-imino-2,4-cyclohexadiene-1-one
(u) upon 532 nm light irradiation (10 mW, 10 min); C) Ne-matrix IR difference
spectrum showing the reverse isomerization of u to u' upon visible light irradiation
440±20 nm, 20 W, 10 min). The IR bands of CO (i), 3H-benzooxazol-2-one (m),
CO2 (q), 1-cyanocyclopentadiene (t), (Z)-6-imino-2,4-cyclohexadien-1-one (u),
E)-6-imino-2,4-c cyclohexadien-1-one (u'), (Z)-6-imino-1,3,5-hexatrien-1-one (v)
and H O (*) are labeled. Bands for the depleted species point downward and those of
(
(
2
the formed species point upward.
The formation of CO (q) suggests that the remaining fragmentation species
2
(E)-6-imino-2,4-cyclohexadiene-1-one (u') and/or (Z)-6-imino-2,4-cyclohexadiene
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-
1-one (u) should also be formed (Scheme 3). Considering the known photo-induced
7
6
interconversion between these two conformers, the matrix containing the 193 nm
laser photolysis of 3H-benzooxazol-2-one (m) was subjected to successive visible
light irradiations of 532 and 440±20 nm, which enabled the unambiguous
identification of the IR bands for both species among the laser photolysis of m
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(Figure 5A). Consistent with the previously observed ring-opening of u to
(Z)-6-imino-1,3,5-hexatrien-1-one (v), the latter was also observed in this study as
–1
evidenced by the IR bands at 2118.7 and 2026.3 cm in Ne-matrix (2114 and 2032
‒1
76
cm in Ar-matrix) for the characteristic C=C=O and C=C=N stretching vibrations,
respectively. Therefore, the absence these fragmentation species among the laser
3
photolysis of PhOC(O)N unambiguously confirms that the amination product of the
azide was not involved in the photochemistry of the azide and the nitrene in
cryogenic matrices.
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6
Quantum Chemical Calculations
The observation of distinct Curtius-rearrangement and cyclization products in the
thermal and photolytic decomposition of PhOC(O)N implies the involvement of
different reaction pathways for the initially generated singlet nitrene PhOC(O)N. To
uncover the mechanism for the decomposition PhOC(O)N , the potential energy
3
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3
profile for its conformational interconversion were first explored at the CBS-QB3 and
B3LYP/6-311++G(d,p) levels (Figure S4). Four conformers are true minima on the
PES, and the one (syn-syn) with both PhO and N
3
groups being in syn-conformation
to the C=O group is the global minimum. The syn-anti conformer is slightly higher
–
1
energy by 1 kcal mol at the CBS-QB3 level. According to the calculated IR spectra
for the four conformers (Table S1), the frequency for the C=O stretching vibration in
–1
the syn-anti conformer (1830 cm , B3LYP/6-311++G(d,p)) is higher than that in the
–1
–1
syn-syn conformer (1789 cm ). Indeed, a weak IR band at 1793.0 cm exists in the
–
IR spectrum of the azide (Figure 1A), which is blue-shifted by 32.5 cm comparing
1
–
1
to that of the syn-synconformer (1760.5 cm ). The coexistence of two conformers
7
,
7–78
73,79
have been frequently observed in carbonyl azides such as OC(N
3
)
2
3
HC(O)N ,
1
4
and CH
3 3
OC(O)N .
The calculated PESs for the decomposition of PhOC(O)N
3
are summarized in Figure
6
. The syn-syn conformer may either undergo concerted dissociation into PhONCO
2
and N via TS3 or convert to the less stable syn-anti conformer as followed by
successive N
2
-elimination (TS1) and PhO-migration (TS2) into PhONCO. Since TS1
–1
is lower in energy than TS3 by 3 kcal mol (CBS-QB3), the stepwise route via the
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nitrene intermediate is slightly more favorable.
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58
59
60
–
1
Figure 6. Calculated potential energy profile (ꢁG, kcal mol ) for the
Curtius-rearrangement of PhOC(O)N at the CBS-QB3 and B3LYP/6-311++G(d,p)
in parentheses) levels.
3
(
The initially generated nitrene PhOC(O)N is unstable, not only because of its facile
exothermic rearrangement into the isocyanate PhONCO with an estimated barrier of
–1
merely 11 kcal mol (CBS-QB3), but also due to relaxation to the triplet ground state.
Consistent with the IR and EPR analysis, the triplet state was calculated to be lower in
energy than the closed-shell singlet, and the energy gap (ꢁE ) in the syn-conformer is
ST
–1
1
3 kcal mol at the CASPT2(12,11)/6-311G**//CASSCF(12,11)/6-311G** level
(
Table 2). The open-shell singlet of PhOC(O)N lies significantly higher than the
–1
triplet by 32 and 35 kcal mol for the syn and anti conformers.
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‒
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Table 2. Calculated relative energies (kcal mol ) for the conformers of PhOC(O)N at
different levels of theory.
syn
syn
singlet
14
anti
anti
methods
b
b
triplet
triplet
singlet
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B3LYP/6-311++G(d,p)
CBS-QB3
0
0
0
0
0
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1
2
3
5
14
4
5
a
CCSD(T)/cc-pVTZ
6
6
CASSCF(12,11)/6-311G**
11
8
c
CASPT2(12,11)/6-311G**
13
10
a
Single-point energy calculation based on the B3LYP/6-311++G(d,p) optimized
b
c
Closed-shell state. Single-point energy calculation based on the
structures.
CASSCF(12,11)/6-311G** optimized structures.
Molecular structures of the nitrene PhOC(O)N in the triplet, closed-shell singlet
(
CSS), and open-shell singlet (OSS) states are calculated at the
CASSCF(12,11)/6-311G** level (Figure S5). Similar to the theoretical studies of
15, 29–38, 80–81
other α-oxo nitrenes,
the O=C–N angles in the closed-shell singlet
PhOC(O)N (syn: 97.0° and anti: 97.3°) are much smaller than those in the open-shell
singlet (syn: 118.0° and anti: 118.4°) and triplet state (syn: 120.7°, anti: 118.3°), these
dramatic structural differences have been rationalized in terms of the stabilizing
intramolecular NꢂꢂꢂO interactions in the oxazirine-like closed-shell singlet state.
The stability of triplet PhOC(O)N was evaluated by calculating the PESs for the
Curtius-rearrangement and the intramolecular cyclization (Figure 7). To facilitate the
transformation, the lower-energy syn-conformer of the nitrene needs first to rotate to
–1
the anti-conformer via TS4 by overcoming a small barrier (6 kcal mol ). The latter is
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thermally unstable due to either fragmentation (TS5) into •NCO (e) and PhO• (d) or
the more favorable intramolecular cyclization (TS6) into an elusive diradical
intermediate (diradical-I, w). Unlike the nitrene in the singlet state (Figure 6, TS2),
calculations to localize the transition state for the Curtius-rearrangement product
PhONCO in the triplet always end up with the dissociation into •NCO and PhO•,
which is consistent with the result that triplet PhONCO is not a true minimum.
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‒
1
Figure 7. Calculated potential energy profile (ꢁG, kcal mol ) for the formation of
3
H-benzooxazol-2-one (m) from both anti-singlet (blue line) and anti-triplet (black
line) PhOC(O)N at the CBS-QB3 and B3LYP/6-311++G(d,p) (in parentheses) levels.
As for the putative triplet diradical intermediate (w), the barrier (TS7) for the 1,2-H
‒1
shift into 3H-benzooxazol-2-one (triplet m) amounts to 39 kcal mol . The subsequent
spin-relaxation from triplet to the singlet state is highly exothermic due to an
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extremely large singlet-triplet gap (–82 kcal mol , CBS-QB3). Further optimization
of the diradical (w) in the closed-shell singlet state gives rise to the formation of
ortho-NCO-cyclohexadienone (f) through C–O bond breaking.
0
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Alternatively, a lower-energy pathway for the cyclization of triplet nitrene PhOC(O)N
to triplet m through the initial formation of diradical-II (x, Figure 7) via H-abstraction
‒1
was also found. The barrier (TS8) for the H-abstraction is 25 kcal mol , higher than
‒1
that for the C–N bond formation (TS6, 16 kcal mol ). However, the subsequent C–N
‒1
bond formation in diradical-II needs to overcome a small barrier (7 kcal mol , TS9),
‒1
thus lowering the overall barrier for the cyclization of the nitrene to 31 kcal mol .
The route for the cyclization in singlet PhOC(O)N was also calculated and depicted in
Figure 7. Similar to the intensively explored mechanisms for the C-H amination of
82–84
arenes via a nitrene insertion,
the initial step involves the formation of an
aziridine intermediate (y, Figure 7) which needs to surmount a barrier (TS10) of about
kcal mol‒1 according to the relaxed energy scan of the N‒C2 bond length at the
2
B3LYP/6-311++G(d,p) levels. However, subsequent cleavage of the N‒C1 bond in
the three-membered ring (y) leads to a concerted breakage of the C–O bond and yields
ortho-NCO-cyclohexadienone (f) via TS11. This is not unexpected since the diradical
intermediate formed from the initial N‒C1 bond cleavage in y is not a true minimum
in the closed-shell singlet state, its structural optimization results in the formation of f.
Nevertheless, the transition state (TS12) for the isomerization from f to m was located,
which involves C–O bond formation and concerted H-migration. The large barrier (39
‒1
kcal mol ) renders this pathway unlikely.
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CONCLUSIONS
3
The photolytic and thermal decomposition of phenyl azidoformate PhO-C(O)N has
been studied by combining matrix-isolation spectroscopy, laser photolysis, flash
vacuum pyrolysis, and quantum chemical calculations. Upon laser photolysis (193
and 266 nm), the azide isolated in cryogenic noble gas matrices (Ne and Ar)
decomposes and furnishes the elusive nitrene intermediate PhO-C(O)N in the triplet
ground state. This is characterized for the first time in this work with IR and EPR
spectroscopy. Subsequent visible light irradiation (532 nm) of the triplet nitrene
mainly causes isomerization into PhONCO and complex secondary derivatives, thus
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3
completing the stepwise Curtius-rearrangement of the azide PhO-C(O)N . This
experimental observation is completely different from the early solution studies in
which no Curtius-rearrangement but only intramolecular C–H amination product
3
H-benzooxazol-2-one was isolated as the decomposition product of the azide. In
contrast to the photochemistry, flash vacuum pyrolysis of PhO-C(O)N at 550 K
3
2
yields N and the cyclization product 3H-benzooxazol-2-one, no evidence for the
involvement of the Curtius-rearrangement product PhONCO was obtained.
To probe the underlying mechanism, the pathways for the decomposition of
3
PhO-C(O)N and subsequent rearrangement reactions of the nitrene PhO-C(O)N in
both singlet and triplet states have been computationally explored. Pathways for the
stepwise Curtius-rearrangement of the azide have been located. However, no plausible
3
route for the catalysis-free intramolecular C–H amination of PhO-C(O)N via the
intermediacy of the nitrene PhO-C(O)N could be found. Further studies such as
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