3-Nitrene-2-formylthiophene and 3-Nitrene-2-formylfuran: Matrix Isolation, Conformation, and Rearrangement Reactions

Article abstract of DOI:10.1021/acs.jpca.9b11638

Two new heteroarylnitrenes, 3-nitrene-2-formylthiophene (15/15′) and 3-nitrene-2-formylfuran (16/16′), in the triplet ground state have been generated in solid Ar (10.0 K) and N₂ (15.0 K) matrices by the 266 nm laser photolysis of 3-azido-2-formylthiophene (13) and 3-azido-2-formylfuran (14), respectively. According to the characterization with matrix-isolation IR spectroscopy and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level, both nitrenes exhibit two conformations depending on the orientation of the formyl groups. Upon subsequent green-light irradiation (532 nm), both the nitrenes 15/15′ and 16/16′ undergo ring closure to form 3,2-thienoisoxazole (17) and 3,2-furoisoxazole (18), respectively. Traces of 3-imino-4,5-dihydrothiophene-2-ketene (19), formally formed through the intramolecular 1,4-H shift in the corresponding nitrenes 15/15′, have been also identified among the laser photolysis products of the azide 13. In sharp contrast to the photochemistry, the high-vacuum flash pyrolysis (HVFP) of the azide 13 at ca. 1000 K mainly yields imino ketene in two conformations 19/19′ together with traces of isoxazole 17. In addition to the reversible conformational interconversion in the imino ketene 19 ? 19′, the photoisomerization from isoxazole 17 to imino ketene 19 has also been observed. The HVFP of the azide 14 at ca. 1000 K results in complete dissociation to HCN, C₂H₂, CO, CO₂, H₂O, and N₂. Unlike the recently disclosed hydrogen-atom tunneling (HAT) in the transformation from the structurally related 2-formyl phenylnitrene (2) to imino ketene 3 in a cryogenic Ar-matrix, the absence of HAT in nitrenes 15 and 16 can be reasonably explained by the higher barrier heights and also larger barrier widths in the isomerization reactions.

Full text of DOI:10.1021/acs.jpca.9b11638

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
3-Nitrene-2-Formylthiophene and 3-Nitrene-2-Formylfuran:
Matrix-Isolation, Conformation, and Rearrangement Reactions
Jie Liu, Zhuang Wu, Yang Yang, Weiyu Qian, Lina Wang, and Xiaoqing Zeng
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b11638 • Publication Date (Web): 20 Apr 2020
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3-Nitrene-2-Formylthiophene and 3-Nitrene-2-Formylfuran: Matrix-Isolation,
Conformation, and Rearrangement Reactions
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Jie Liu,† Zhuang Wu,† Yang Yang,† Weiyu Qian,† Lina Wang,§ and Xiaoqing Zeng†,§*
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†College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123 Suzhou, China.
E-mail: xqzeng@suda.edu.cn
§Department of Chemistry, Fudan University, Shanghai 200433, China. E-mail: xqzeng@fudan.edu.cn
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ABSTRACT: Two new heteroarylnitrenes, 3-nitrene-2-formylthiophene (15/15') and
3-nitrene-2-formylfuran (16/16'), in the triplet ground state have been generated in solid Ar (10.0
K) and N2 (15.0 K) matrices by the 266 nm laser photolysis of 3-azido-2-formylthiophene (13)
and 3-azido-2-formylfuran (14), respectively. According to the characterization with
matrix-isolation IR spectroscopy and quantum chemical calculations at the
B3LYP/6-311++G(3df,3pd) level, both nitrenes exhibit two conformations depending on the
orientation of the formyl groups. Upon subsequent green-light irradiation (532 nm), both nitrenes
15/15' and 16/16' undergo ring-closure to form 3,2-thienoisoxazole (17) and 3,2-furoisoxazole
(18), respectively. Traces of 3-imino-4,5-dihydrothiophene-2-ketene (19), formally formed
through intramolecular 1,4-H shift in the corresponding nitrenes 15/15', have been also identified
among the laser photolysis products of the azide 13. In sharp contrast to the photochemistry, the
high-vacuum flash pyrolysis (HVFP) of the azide 13 at ca. 1000 K mainly yields imino ketene in
two conformations 19/19' together with traces of isoxazole 17. In addition to the reversible
conformational interconversion in the imino ketene 19 ↔ 19', the photoisomerization from
isoxazole 17 to imino ketene 19 has also been observed. The HVFP of the azide 14 at ca. 1000 K
results in complete dissociation to HCN, C2H2, CO, CO2, H2O, and N2. Unlike the recently
disclosed hydrogen-atom tunneling (HAT) in the transformation from the structurally related
2-formyl phenylnitrene (2) to imino ketene 3 in cryogenic Ar-matrix, the absence of HAT in
nitrenes 15 and 16 can be reasonably explained by the higher barrier heights and also larger barrier
widths in the isomerization reactions.
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INTRODUCTION
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Arylnitrenes (Ar‒N) are transient species that have been broadly used as highly reactive
intermediates in synthetic nitrogenation reactions,1 catalytic C‒H amination and aziridination of
olefins,2 fabrication of crystalline molecular flasks,3,4 and photolabeling in biology.5 Generally,
arylnitrenes can be facilely generated by the decomposition of the corresponding azide precursors
through thermally or photolytically initiated N2-elimination. However, rapid inter- and/or
intramolecular reactions occurring under the formation conditions render the studies on the
fundamental properties of this class of electron-deficient species challenging. As one of the most
frequently studied arylnitrenes, the prototypical phenylnitrene was first proposed by Huisgen and
co-workers in the decomposition of phenyl azide in the 1950s.6–8 The first-time spectroscopic
characterization of this triplet species was achieved when following the photolysis of the azide in
cryogenic glassy matrix by using EPR spectroscopy in 1962.9 Later on, the spectroscopy, structure,
and reactivity of pheylnitrene10 and diverse substituted derivatives11–13 have been the topics of
extensive experimental and computational studies in the past few decades. For instances, triplet
phenylnitrene is a thermally persistent species in the gas phase that can be produced by pyrolysis
of the corresponding azide.14 However, 2-formyl phenylnitrene (2), which can be generated by
photodecomposition of either 2-formyl phenylazide (1) or 2,1-benzisoxazole (4), isomerizes to
6-imino-2,4-cyclohexadien-1-ketene (3) through intramolecular 1,4-H shift (Scheme 1) even at
10.0 K via quantum mechanical tunneling (QMT).15 This triplet species 2 can also undergo
photo-induced isomerization to a benzazirine 5, and the latter spontaneously coverts to a cyclic
ketenimine by meanings of heavy-atom tunneling.16 More recently, competitive nitrogen and
carbon tunneling transformations have been observed during the transformation from an
amino-substituted benzazirine to p-aminophenylnitrene and ketenimine in solid argon (3–18 K).17
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Scheme 1. The generation and isomerization of 2-formyl phenylnitrene (2).15,16
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Similar to the rich chemistry of arylnitrenes, the fundamental properties of various
heteroarylnitrenes18 such as quinazolylnitrenes,19 pyridylnitrenes,20 and pyrrylnitrenes,21 have also
been intensively explored through the interplay of theory and experiment. In addition to the typical
ring-expansion of arylnitrenes to cyclic ketenimines (e.g., 2 → 6 in Scheme 1),16 most
heteroarylnitrenes can undergo ring-opening reactions to yield nitriles.22 For instances, triplet
2-pyridylnitrene prefers ring-expansion to cyclic carbodiimide as followed by further
isomerization to phenylcarbene via ring-contraction.23,24 Whereas, triplet 3-furylnitrene (8)25 and
3-thienylnitrene (11),26 generated in the photodecomposition of 3-furyl isocyanate (7) and
3-thienylsulfonyl nitrene (10) in cryogenic matrices, favors ring-opening to form nitriles 9 and 12
upon irradiation, no ring-expansion products could be identified.
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Scheme 2. The generation and isomerization of 3-furylnitrene (8)25 and 3-thienylnitrene (11).26
The distinct reactivity associated with different aryl- and heteroarylnitrenes stimulates us to
extend our study to 3-nitrene-2-formylthiophene (15) and 3-nitrene-2-formylfuran (16). Due to
their structural similarity with 2-formyl phenylnitrene (2) (Scheme 1), 3-furylnitrene (8), and
3-thienylnitrene (11) (Scheme 2), intramolecular rearrangement processes such as ring-expansion,
1,4-H shift through quantum mechanical tunneling, and ring-opening reactions might be expected.
Herein, we report a first-time study on the photochemistry of 3-azido-2-formylthiophene (13) and
3-azido-2-formylfuran (14) in solid Ar (10.0 K) and N2 (15.0 K) matrices by combining
matrix-isolation IR spectroscopy and quantum chemical calculations (Scheme 3). In addition to
the identification of the two novel heteroarylnitrenes 15/15' and 16/16' in syn/anti conformations,
the photo-induced ring-closure of nitrenes 15 and 16 to elusive isoxazoles 17 and 18 and the 1,4-H
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shift of nitrene 15 to imino ketene 19 bypassing hydrogen-atom tunneling have been observed.
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Scheme
3.
Proposed
photochemistry
of
3-azido-2-formylthiophene
(13)
and
3-azido-2-formylfuran (14).
EXPERIMENTAL SECTION
Sample Preparation. 3-Azido-2-formylthiophene (13) was synthesized and purified according to
the literature.27 Briefly, a suspension of 3-bromo-2-formylthiophene (0.191 g, 1 mmol) and
sodium azide (0.195 g, 3 mmol) in dimethyl sulfoxide (1.5 mL) was stirred at 65 ºC for 24 h. Then,
the mixture was poured into water and extracted with ether. The combined extracts were
evaporated, and the vapor of the yellow crude product was passed through two successive cold
U-traps (–20 and –196 ºC). Pure azide 13 was retained in the first trap as solid. The quality of the
sample was checked by NMR spectroscopy (Figure S1). 1H NMR (600 MHz, DMSO-d6): δ = 9.82
(s), 8.15 (d, J = 5.4 Hz), 7.34 (d, J = 5.4 Hz) ppm and 13C NMR (150 MHz, DMSO-d6): δ =
180.73, 145.68, 137.16, 127.15, 122.86 ppm.
3-Azido-2-formylfuran (14) was synthesized and purified in
a similar manner.
3-Bromo-2-formylfuran (0.175 g, 1 mmol), sodium azide (0.195 g, 3 mmol), and dimethyl
sulfoxide (1.5 mL) were used for the reaction. The solvent was poured into water and extracted
with ether, and the organic layers were evaporated under reduced pressure. Then, the vapor of the
residue was passed through two successive cold U-traps (–20 and –196 ºC), and pure azide 14 was
retained in the first trap. The quality of the sample was checked by NMR spectroscopy (Figure S2).
1H NMR (600 MHz, CDCl3): δ = 9.71 (s), 7.62 (d, J = 1.8 Hz), 6.65 (d, J = 1.8 Hz) ppm and 13
NMR (150 MHz, CDCl3): δ = 176.42, 147.88, 116.85, 112.66, 106.70 ppm.
C
Matrix-isolation IR Spectroscopy. Matrix IR spectra were recorded on a FT-IR spectrometer
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(Bruker 70V) in a reflectance mode using a transfer optic. A KBr beam splitter and liquid-nitrogen
cooled MCT detector were used in the mid-IR region (4000–600 cm–1). For each spectrum, 200
scans at a resolution of 0.5 cm–1 were coadded.
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The solid sample was mixed by passing a flow of Ar or N2 gas through a U-trap containing ca. 20
mg of the azide (13: 19 °C and 14: 33 °C). Then the mixture (azide/dilution gas ≈ 1 : 1000
estimated) was passed through 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 by a tantalum wire (o.d. 0.4 mm, resistance 0.4 Ω) and
immediately deposited (2 mmol h–1) at a high vacuum onto the Rh-plated copper block matrix
support (Ar: 10.0 K; N2: 15.0 K) in high vacuum (10–6 Pa), using a closed-cycle helium
cryostat (Sumitomo Heavy Industries, SRDK-408D2-F50H) inside the vacuum chamber. The
electric power (voltage/current) used in pyrolysis experiments was 5.4 V/3.6 A. Photolysis
experiments were performed by using a Nd3+:YAG laser (266 nm, 532 nm, MPL-F-266, 10 mW),
and a high-power flashlight (Boyu T648, 365 nm, 20 W).
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Computational Details. Structural optimizations were performed using density functional theory
(DFT) methods of B3LYP,28 mPW1PW929 and M06-2X30 combined with the 6-311++G(3df,3pd)
basis set. Accurate relative energies of the species were further calculated by using the complete
basis set CBS-QB331 and CCSD(T)32 methods. For the CCSD(T) single-point calculations, the
6-311++G(3df,3pd) basis set and the B3LYP/6-311++G(3df,3pd) optimized structures were used.
In addition, the energies of the nitrenes in the closed-shell singlet (CCS) state, open-shell singlet
(OSS) state, and triplet state were also calculated by applying the second-order
multiconfigurational perturbation theory (CASPT2) method (referred to as “RS2C” in Molpro),33
which accounts for dynamic correlation, using the complete active space SCF (CASSCF) wave
functions as references in the RS2C calculation. The active space includes 8 electrons and 8 active
orbitals, namely, CASPT2(8,8). All the CASPT2 calculations were carried out with the
CASSCF(8,8)/cc-pVTZ optimized geometries. Time-dependent (TD)34,35 calculations were
performed at the B3LYP/6-311++G(3df,3pd) level for the prediction of UV/Vis transitions. Local
minima were confirmed by vibrational frequency analyses, and transition states by additional
intrinsic reaction coordinate (IRC) calculations.36,37 For tunneling computations on the triplet
manifold we used the calculated potential energy data along the intrinsic reaction path at the
B3LYP/6-311++G(3df,3pd) level. The intrinsic reaction coordinate (IRC) was computed in
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nonmass-weighted Cartesian coordinates, by using the “IRC=Cartesian” option, expressed in units
of Bohr. The DFT and CBS-QB3 calculations were performed using the Gaussian 09 software
package.38 The CCSD(T), CASPT2, and CASSCF calculations were performed with the
MOLPRO 2012 program.39
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RESULTS AND DISCUSSION
Photochemistry of 3-azido-2-formylthiophene (13)
The photolysis of 3-azido-2-formylthiophene (13) in solid Ar-matrix at 10.0 K was
performed by using a 266 nm laser. After 11 s of irradiation, half of the azide decomposes and
yields new species with strong IR bands at 2140.4, 2129.1, 1663.9, 1649.4, 1243.4, 1186.1, 740.6,
and 678.2 cm−1. The first two bands are close to the frequencies for the characteristic asymmetric
CCO stretching vibration modes νasym(C=C=O) found in ketenes such as 3 (2149 cm−1,
Ar-matrix).15 Therefore, it is very likely that the 1,4-H shift product imino ketene 19 of the nitrene
intermediate 15 (Scheme 3) forms during the laser photolysis of the azide 13. The formation of
imino ketene 19 is also supported by the observation of two weak but distinguishable IR bands at
3310.2 and 3292.5 cm−1 for the N‒H stretching vibrations in the two conformers (vide infra).
Figure 1. (A) IR difference spectrum showing the decomposition of 3-azido-2-formylthiophene (13) in Ar-matrix
(10.0 K) upon 266 nm laser irradiation (11 s). (B) IR difference spectrum showing the rearrangement of nitrenes
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15 (syn) and 15' (anti) to isoxazole 17 upon subsequent visible light irradiation (532 nm, 15 min). (C, D)
B3LYP/6-311++G(3df,3pd) calculated IR spectra (scaled by a factor of 0.98) for nitrenes 15 (syn) and 15' (anti) in
the triplet state. Bands of imino ketenes 19 (syn), 19' (anti), H2O (&), and unknown species (*) are also marked.
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Considering the TD-B3LYP/6-311++G(3df,3pd) calculated absorptions at 493 nm (15:
oscillator strength f = 0.0106) and 480 nm (15': f = 0.0104) for the nitrene intermediate 15 in the
triplet ground state (Table S1), the matrix containing the 266 nm laser photolysis of the azide 13
was further irradiated with green light (532 nm). As a result, all the IR bands associated with the
nitrenes 15 and 15' vanish completely and another set of IR bands (17) forms concomitantly
(Figure 1B). Simultaneously, conversion occurs to the two conformers of imino ketene 19 (syn →
anti) under the irradiation conditions. Among the depleted IR bands (Figure 1B, bands pointing
downward), the two strongest ones at 1663.9 and 1649.4 cm−1 can be reasonably assigned to the
C=O stretching vibrational modes ν(C=O) in the two conformers of the expected nitrene 15, which
differ mainly in the orientation of the two C=O groups (Scheme 3). The frequencies are slightly
red-shifted in comparison with the ν(C=O) mode in the azide precursor 13 (1689.1/1678.6 cm−1 in
Ar-matrix). This assignment is also supported by the good agreement of the observed band
positions (1663.9 and 1649.4 cm−1) with the B3LYP/6-311++G(3df,3pd) calculations at 1683 and
1672 cm−1 for the anti and syn conformers of nitrene 15 in the triplet state (Table 1). In contrast,
the calculated ν(C=O) modes in the two conformers of the nitrene 15 in the higher-energy
closed-shell singlet state locate at 1713 (syn) and 1748 cm−1 (anti, Table S2). Interestingly, there
are two weak IR bands at 1720.7 and 1689.3 cm−1 (bands marked with asterisks in Figure 1B)
appear after the depletion of the triplet nitrene, their intensities keep increasing even after further
266 nm laser irradiation. According to the TD-B3LYP/6-311++G(3df,3pd) calculations (Table S1),
the two conformers of the singlet nitrene 15 display intense absorptions at 270 (syn) and 278 nm
(anti), the unexpected photobehavior renders the assignment of these two bands to the nitrene in
the singlet state unlikely. It should be noted that magnetically bistable nitrenes25,40 and closely
related carbene analogues41 have been recently observed in cryogenic matrices.
Table 1. Calculated and experimentally observed IR frequencies of nitrenes 15 (syn) and 15'
(anti).
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calculateda
CSS
observedb
sym approximate
assignmentc
triplet
Ar-matrix
N2-matrix
syn
anti
syn
anti
syn
anti
syn
anti
3182 (<1)
3159 (1)
3184 (<1)
3160 (1)
3179 (<1)
3161 (1)
2895 (39)
1672 (275)
1493 (4)
1393 (13)
1358 (19)
1290 (9)
1241 (97)
1162 (45)
1086 (4)
997 (7)
3181 (<1)
3161 (1)
2847 (78)
1683 (198)
1491 (6)
1402 (8)
1349 (22)
1294 (15)
1266 (144)
1109 (29)
1073 (4)
1013 (22)
975 (<1)
907 (<1)
809 (7)
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A''
A''
A'
A''
A'
A'
A''
A'
A'
A''
A'
A''
A''
A'
A''
ν(CH)
ν(CH)
2964 (11)
2829 (77)
2858.9 w
1649.4 s
2860.2 vw
1651.1 s
ν(OC–H)
ν(C=O)
ν(C=C)
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1713 (190) 1748 (166)
1663.9 s
1662.6 m
1494 (8)
1417 (90)
1349 (9)
1282 (33)
1243 (65)
1137 (21)
1064 (15)
1025 (<1)
969 (22)
940 (<1)
771 (2)
1487 (15)
1422 (18)
1391 (61)
1275 (12)
1230 (106)
1096 (3)
1031 (36)
1007 (29)
991 (<1)
942 (1)
1389.7
1354.8
1289.2
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1186.1
1088.0 w
1391.8 m
1355.0 m
1291.2 m
1249.2 m
ρ(OC–H)
ν(CN) + ρ(CH)
ρ(CH)
1345.1 w
1293.8 w
1256.4 m
1348.7 vw
1295.7 vw
1260.7 m
1132.5 w
ν(C=C)
1143.6 vw 1190.3 m
1090.4 w
ν(C–Cring
)
ρ(CH)
1002.4 w 1022.3 vw 1005.1 w
1024.1 vw
ρ(CH)
982 (<1)
907 (<1)
800 (2)
ω(OC–H)
τ(CH)
786 (3)
ν(CS)
761 (42)
685 (72)
653 (7)
759 (43)
677 (56)
649 (4)
748 (49)
670 (44)
648 (5)
748 (49)
735 (22)
644 (5)
740.6 s
740.6 s
751.6 m
681.2 s
751.6 m
745.3 w
ω(CH)
678.2 s
737.2 w
ν(CS)
655.9 w
625.4 vw
656.0 w
626.0 vw
ring distortion
ring distortion
ring breathing
ring distortion
ring distortion
ρ(CN)
642 (8)
646 (7)
631 (4)
634 (4)
591 (10)
495 (<1)
486 (<1)
337 (2)
543 (5)
592 (6)
533 (3)
472 (<1)
406 (15)
398 (2)
465 (4)
438 (1)
442 (2)
411 (9)
319 (<1)
287 (3)
375 (7)
276 (8)
242 (<1)
186 (16)
184 (<1)
−69 (<1)
262 (<1)
206 (11)
160 (3)
ring distortion
ω(CN) + ω(OC–H)
ρ(OC–C)
244 (12)
170 (12)
101 (4)
189 (14)
158 (9)
123 (<1)
111 (<1)
ω(C=O)
aCalculated harmonic IR frequencies (cm–1, scaled by a factor of 0.98)42 and intensities (km mol–1, in parentheses)
for the closed-shell singlet (CSS) and triplet states at the B3LYP/6-311++G(3df,3pd) level. bObserved band
positions of the most intense matrix sites in Ar- and N2-matrices. The relative band intensities: s = strong, m =
medium strong, w = weak, vw = very weak. cTentative assignment based on the calculated vibrational
displacement vectors for the syn-conformer in the triplet state.
In line with the calculated frequencies of 1358 and 1349 cm–1 for the C–N stretching
vibrations in the two conformers of triplet nitrene 15, two weak bands at 1354.8 and 1345.1 cm–1
can be clearly identified. And, they are quite close to the ν(C–N) modes in the parent triplet
3-thienylnitrene (11) (1350.2 cm–1, Ar-matrix)26 and phenylnitrene (1312.3 cm–1, Ne-matrix).43,44
The generation of nitrene 15 in the 266 nm laser photolysis of azide 13 is reproducible when using
N2 as the matrix host (Figure S3). The observed band positions of triplet nitrene 15 in N2-matrix
shift slightly (< 12 cm–1, Table 1) due to different weak interactions with the surrounding matrix
materials. Attempts to stimulate the conformational conversion between the two conformers of
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nitrene 15 failed by annealing the matrix to 25.0 K or irradiating with visible-light (830 and 532
nm), and mainly the transformation from nitrene 15 to isoxazole 17 happens under the irradiation
conditions.
9
Unlike the spontaneous tunneling transformation from triplet 2-formyl phenylnitrene (2) to
singlet imino ketene 3 in Ar-matrix at 10.0 K (Scheme 1),15 the two conformers of nitrene 15 are
stable in both Ar- and N2-matrices, no noticeable change of their IR bands could be observed
when keeping the corresponding matrix in the dark up to 25.0 K. In fact, the photo-induced
conversion from nitrene 15 to imino ketenes 19/19' via 1,4-H shift is much less efficient when
comparing with the intramolecular cyclization to 3,2-thienoisoxazole (17, bands pointing upward
in Figure 1B). In sharp contrast, no analogous cyclization of nitrene 2 to form an isoxazole 4 was
observed, which is probably due to the sole formation of the unfavourable anti-conformer nitrene
2 in the photolysis of the azide precursor 1, since the carbonyl oxygen atom in this configuration
points away from the electron-deficient nitrene center. Unexpectedly, the previous attempt to
generate the syn-conformer nitrene 2', in which the carbonyl oxygen atom points towards the
electron-deficient nitrene center, through photo-induced ring-opening of the isoxazole 4 (Scheme
1) only yields the anti-conformer nitrene 2.15
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13
14
15
16
17
18
19
20
21
22
23
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29
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31
32
33
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35
36
37
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
As can be seen in Figure 1A, only traces of imino ketene 19 are produced in the photolysis of
the azide 13. In fact, the calculated moderate activation barrier (< 20 kcal mol−1) for the nitrene 2
→ imino ketene 3 transformation15 suggest that it may occur easily once the nitrene could be
generated in the gas phase. As expected, the high-vacuum flash pyrolysis (HVFP) of the azide 13
at ca. 1000 K mainly yields imino ketene 19 together with traces of the cyclization isomer
isoxazole 17 (Figure 2A). Similar to the photo-induced syn ↔ anti isomerization in other imines
in matrices,45 subsequent irradiation of the matrix containing the Ar-matrix isolated HVFP
products with UV-light (365 nm) results in selective anti → syn conversion in imino ketene 19
(Figure 2B), whereas, further 266 nm laser irradiation leads to the reverse conversion (Figure 2C).
It is noteworthy that the transformation from isoxazole 17 to imino ketene 19 also occurs under
the laser irradiation conditions, which is probably due to the formation of nitrene (isoxazole 17 →
nitrene 15) as followed by concomitant 1,4-H migration (nitrene 15 → imino ketene 19). The
dominant formation of imino ketene 19 in the thermal decomposition of
3-azido-2-formylthiophene (13) is in sharp contrast to the pyrolysis of the closely related
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2-acetyl-3-azidothiophene, in which the major product is 3-methylthieno[3,2-c]isoxazole.46
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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27
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29
30
31
32
33
34
35
36
37
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39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. (A) Ar-matrix IR spectrum of the high-vacuum flash pyroysis (ca. 1000 K) products of
3-azido-2-formylthiophene (13). (B) IR difference spectrum showing rearrangement of imino ketenes 19' (anti) to
19 (syn) upon UV-light irradiation (365 nm, 25 min). (C) IR difference spectrum showing rearrangement of imino
ketenes 19 (syn) to 19' (anti) upon 266 nm laser irradiation (2 min). Bands of isoxazole 17, H2O (&), and unknown
species (*) are also marked.
The reversible photo-interconversion between the two conformers of imino ketene 19 allows
unambiguously identification of all the IR bands in the range of 4000–600 cm–1 (Table 2 and
Table S3). The observed difference of 11.3 cm–1 between the two νasym(C=C=O) modes in the anti
and syn conformers agrees with the calculation (12 cm–1) at the B3LYP/6-311++G(3df,3pd) level.
The weaker C=N stretching modes for the two conformers locate at 1590.1 and 1594.7 cm–1,
coinciding with the calculated frequencies of 1601 and 1603 cm–1, respectively. According to the
calculated and observed IR intensities for the two well-resolved bands at 1384.1 (anti) and 1361.2
cm–1 (syn), a ratio of 1: 0.8 for the two conformers among the HVFP products of the azide 13 can
be estimated. This is in consistent with the calculated lower energy of the anti-conformer than the
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syn-conformer by 1.0 kcal mol–1. The assignment of the bands (1596.8, 1493.1, 1471.1, 1381.8,
1102.3, 1081.2, 1063.4, 928.9, 865.1, 821.0, 784.7, 754.8, 726.4, and 683.8 cm–1) to the
cyclization product isoxazole 17 (Figure 1B) is also supported by the agreement with the
calculated IR fundamental modes (Table S4).
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. Calculated and experimentally observed IR frequencies for the syn and anti conformers
of imino ketene 19.
calculateda
observedb
approximate
assignmentc
ν(NH)
syn
anti
syn
anti
sym
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A''
A''
A'
A''
A'
A''
A'
A'
A''
A'
A''
A'
A''
A'
A''
3413 (9)
3171 (<1)
3154 (2)
3422 (7)
3165 (<1)
3140 (<1)
3292.5 vw 3310.2 vw
ν(CH)
ν(CH)
2162 (1049) 2174 (1062)
2129.1 s
1594.7 m
1578.1 w
1361.2 m
2140.4 s
1590.1 m
1580.8 w
1384.1 w
νasym(C=C=O)
ν(C=N)
1603 (232)
1556 (36)
1352 (104)
1305 (5)
1240 (47)
1100 (40)
1038 (35)
933 (14)
916 (<1)
813 (11)
782 (23)
758 (82)
670 (22)
638 (21)
626 (9)
1601 (275)
1544 (17)
1378 (52)
1301 (8)
1249 (101)
1100 (4)
1045 (58)
967 (1)
ν(C=Cring
)
νsym(C=C=O)
ρ(CH)
1307.8 vw 1303.5 vw
1241.1 w
1100.4 w
1058.5 w
953.4 vw
1249.2 w
1066.2 w
ρ(CH) + ρ(NH)
δ(CH) + ρ(NH)
ρ(CH) + ρ(NH)
ip(C=C=O)
τ(CH)
907 (1)
819 (98)
784 (11)
732 (<1)
672 (31)
646 (7)
814.6 vw
789.3 w
753.0 w
675.3 vw
631.0 vw
816.6 m
τ(NH)
792.8 vw
ring breathing
ω(CH) + ω(NH)
ring breathing
ω(CH)
676.6 vw
621 (<1)
552 (5)
ring breathing
ν(CS)
554 (2)
519 (7)
524 (18)
463 (11)
388 (<1)
366 (2)
oop(C=C=O)
458 (1)
ring breathing
ring distortion
ρ(CH)
384 (3)
375 (10)
215 (<1)
123 (2)
209 (<1)
130 (<1)
81 (4)
ring distortion
ρ(C=C=O)
ω(C=C=O)
72 (<1)
aCalculated harmonic IR frequencies (cm–1, scaled by a factor of 0.98)42 and intensities (km mol–1, in parentheses)
b
at the B3LYP/6-311++G(3df,3pd) level. Observed band positions of the most intense matrix sites in Ar-matrix.
c
The relative band intensities: s = strong, m = medium strong, w = weak, vw = very weak. Tentative assignment
based on the calculated vibrational displacement vectors for the syn-conformer.
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Photochemistry of 3-azido-2-formylfuran (14)
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Similar to the photochemistry of 3-azido-2-formylthiophene (13), the 266 nm laser photolysis
of 3-azido-2-formylfuran (14) in solid N2-matrix at 15.0 K causes N2-elimination and furnishes
3-nitrene-2-formylfuran (16 and 16') in syn and anti conformations (Figure 3A). Additionally, the
presence of the 1,4-H migration product imino ketene 20 in syn and anti conformations can also be
inferred by the appearance of two weak IR bands at 2137.0 and 2145.5 cm–1 for the characteristic
νasym(C=C=O) modes in the spectrum. The two conformers of nitrene 16 are stable when keeping
the matrix in the dark up to 25.0 K. Consistent with the calculated absorption at around 470 nm for
the two conformers of the nitrene 16 (Table S1), subsequent visible-light irradiation (532 nm)
promotes cyclization of the nitrene to form isoxazole 18 (Table S4). Additionally, the syn → anti
conformational conversion in imino ketene 20 occurs simultaneously (Figure 3B).
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10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. (A) IR difference spectrum showing the decomposition of 3-azido-2-formylfuran (14) in N2-matrix (15.0
K) upon 266 nm laser irradiation (55 s). (B) IR difference spectrum (3 times expanded along the ΔA axis) showing
the rearrangement of nitrenes 16 and 16' to isoxazole 18 upon subsequent visible-light irradiation (532 nm, 55
min). (C, D) B3LYP/6-311++G(3df,3pd) calculated IR spectra (scaled by a factor of 0.98) for 16 (C: syn) and 16'
(D: anti) in the triplet state. Bands of imino ketenes 20/20', H2O (&), and unknown species (*) are also marked.
The identification for the two conformers of nitrene 16 in the triplet ground state is supported
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by the agreement with the calculated spectra (Table 3). Specifically, the characteristic ν(C=O)
modes in the syn (16) and anti (16') conformers appear at 1649.6 (cal. 1673 cm–1) and 1662.2 cm–1
(cal. 1678 cm–1, Table S5), respectively. They are quite similar to the ν(C=O) vibrational modes in
triplet nitrene 15 (syn: 1649.4 cm−1; anti: 1663.9 cm−1). The ν(CN) mode in the syn-conformer of
nitrene 16 could not be identified due to low IR intensity of 4 km mol–1. In contrast, it appears at
1331.9 cm−1 in the anti-conformer (16') with a calculated IR intensity of 25 km mol–1. The
photochemistry of the azide 14 in N2-matrix is reproducible in Ar-matrix (Figure S4), in which the
bands for nitrene 16 slightly shift by about 10 cm–1 (Table 3). In sharp contrast to the thermal
decomposition of azide 13 (→ isoxazole 17 + imino ketene 19 + N2), the pyrolysis of azide 14 at
ca. 1000 K mainly leads to complete fragmentation products HCN, C2H2, CO, CO2, H2O, and N2
(IR inactive) together with traces of the 1,4-H shift product imino ketene 20 (Figure S5).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
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Table 3. Calculated and observed IR frequencies of syn and anti conformations of nitrene 16.
calculateda
CSS
observedb
Ar-matrix
syn
sym approximate
assignmentc
triplet
N2-matrix
syn
anti
syn
anti
anti
syn
anti
3224 (<1)
3198 (3)
2965 (10)
1735 (169)
1549 (15)
1478 (29)
1392 (110)
1379 (55)
1287 (21)
1237 (108)
1096 (18)
1078 (2)
997 (10)
914 (19)
910 (<1)
831 (3)
3221 (1)
3201 (2)
2864 (55)
1676 (208)
1591 (8)
1551 (4)
1391 (42)
1315 (4)
1242 (172)
1168 (135)
1108 (4)
1011 (7)
951 (15)
925 (4)
3217 (<1)
3199 (3)
2895 (38)
1673 (265)
1542 (19)
1415 (21)
1390 (35)
1330 (4)
1280 (34)
1230 (57)
1132 (44)
1047 (22)
1025 (6)
971 (<1)
882 (<1)
848 (7)
3217 (<1)
3198 (3)
2877 (75)
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A'
A''
A''
A'
A'
A''
A''
A'
A''
A'
ν(CH)
ν(CH)
2864.1 vw
2860.3 vw
1651.1 m
1541.5 m
1417.6 m
1391.1 m
ν(OC–H)
ν(C=O)
ν(C=C)
ρ(OC–H)
ρ(CH)
1678 (171) 1649.6 m
1662.2 s
1540.2 m
1417.1 m
1367.1 m
1331.9 m
1306.3 m
1223.5 m
1120.8 w
1660.9 s
1542 (16)
1413 (40)
1372 (23)
1328 (25)
1300 (74)
1202 (67)
1119 (14)
1062 (5)
1038 (21)
969 (<1)
879 (<1)
845 (7)
1540.2 m
1417.1 m
1387.9 m
1541.5 m
1417.6 m
1369.3 m
1333.3 m
1307.6 m
1226.7 m
1121.2 vw
1072.0 vw
1033.0 w
ν(CN) + ρ(CH)
ν(C–Cring) + ρ(CH)
ν(C=C)
1282.7 w
1249.7 m
1133.9 m
1042.2 m
1286.9 w
1250.3 m
1132.6 m
ρ(CH)
1073.1 vw 1046.0 w
1035.0 m
ν(COring) + ρ(CH)
ν(C–Cring) + ρ(CH)
ω(OC–H)
880 (4)
τ(CH)
823 (5)
848.8 w
805.3 s
766.7 m
848.8 w
766.7 m
748.4 m
849.4 vw
805.5 s
849.4 vw
774.8 m
748.3 m
ring breathing
ring breathing
ω(CH)
790 (51)
782 (31)
716 (21)
694 (3)
771 (57)
719 (49)
688 (49)
643 (4)
803 (69)
771 (48)
654 (<1)
646 (9)
770 (50)
741 (50)
698 (4)
774.8 m
ring distorting
ring breathing
ring distorting
ring distorting
658 (<1)
559 (8)
649.3 vw
651.3 vw
655 (20)
539 (4)
525 (5)
569 (6)
468 (4)
529 (4)
470 (2)
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340 (2)
416 (10)
266 (8)
191 (9)
135 (3)
100 (2)
328 (<1)
314 (3)
388 (19)
A'
A''
A''
A'
ρ(CN)
299 (4)
276 (<1)
233 (13)
162 (1)
ω(CN) + ω(OC–H)
ω(CN) + ω(OC–H)
ρ(OC–C)
268 (11)
178 (14)
106 (8)
217 (16)
175 (10)
129 (1)
128 (<1)
A''
ω(C=O)
aCalculated harmonic IR frequencies (cm–1, scaled by a factor of 0.98)42 and intensities (km mol–1, in parentheses)
for the closed-shell singlet (CSS) and triplet states at the B3LYP/6-311++G(3df,3pd) level. bObserved band
positions of the most intense matrix sites in Ar- and N2-matrices. The relative band intensities: s = strong, m =
medium strong, w = weak, vw = very weak. cTentative assignment based on the calculated vibrational
displacement vectors for the syn-conformer in the triplet state.
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14
15
16
17
18
19
20
21
22
23
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Quantum Chemical Calculations
To understand the mechanism for the observed rearrangement reactions of the two
structurally related heteroarylnitrenes 15 and 16, their potential energy profiles in the closed-shell
singlet
state
(CSS)
have
been
computationally
explored
at
the
CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level, and the results for nitrene 16 are
depicted in Figure 4 (Figure S6 for nitrene 15).
Figure 4. Calculated relative Gibbs free energy profile (kcal mol–1, including zero-point vibrational energy
corrections) for the rearrangement of closed-shell singlet state 3-nitrene-2-formylfuran (16) at the
CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level.
For 3-nitrene-2-formylfuran (16) in the CSS state, the anti-conformer (16') is lower in energy
than the syn-conformer (16) by 8.6 kcal mol–1. In contrast, the syn-conformer becomes more stable
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in the triplet ground state by 0.5 kcal mol–1. The calculated activation barrier between the two
conformers in the triplet state is more than 10 kcal mol–1, rendering the thermally initiated
conformational conversion in cryogenic matrices (< 25.0 K) unlikely. Furthermore, the large
energy gaps (ΔEST) between the CSS and triplet states for nitrene 16 (anti: 18.6 kcal mol–1; syn:
27.7 kcal mol–1) suggest that the initially generated singlet nitrene may immediately relax to the
triplet state through rapid intersystem crossing (ISC) at low temperatures. By analogy, the two
conformers for 3-nitrene-2-formylthiophene (15) in the triplet ground state differ by 2.7 kcal mol–1,
and the barrier associated with the conformational conversion from the anti-conformer (15') to
lower-energy syn-conformer (15) is 6.9 kcal mol–1 (Figure S6). Similar to the electronic properties
of phenylnitrene,47 the calculations with the second-order multiconfigurational perturbation theory
(CASPT2) on the two heteroarylnitrenes 15/15' and 16/16' demonstrate that the open-shell singlet
(OSS) state is higher in energy than the triplet ground state by about 15 kcal mol–1 at the
CASPT2(8,8)/cc-pVTZ//CASSCF(8,8)/cc-pVTZ level. Furthermore, the CSS state is above the
OSS state by about 20 kcal mol–1 (Table 4).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 4. Calculated energy gaps ΔEST (kcal mol–1) between the closed-shell singlet state and
the triplet state for nitrenes 15 and 16.
species
syn-15
anti-15'
syn-16
anti-16'
B3LYPa
29.7
CBS-QB3
28.8
CCSD(T)b
26.6
mPW1PW9a
33.5
CASPT2c
33.6 (15.3)
36.0 (14.9)
33.6 (15.7)
36.1 (15.2)
35.5
34.9
32.6
39.4
30.2
28.9
27.7
34.5
27.2
18.0
18.6
31.5
b
aAt the 6-311++G(3df,3pd) basis set. At the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level. Positive
c
values refer to the triplet being more stable. At the CASPT2(8,8)/cc-pVTZ//CASSCF(8,8)/cc-pVTZ level. The
energy gaps between the open-shell singlet state and the triplet state are given in parentheses.
In addition to the ISC to the triplet ground state, CSS singlet nitrene anti-16' may undergo
cyclization to form either isoxazole 18 or azirine 22 through the attack the electron-deficient
nitrene center to the carbonyl oxygen or carbon atoms by overcoming comparable activation
barriers of 5.5 (TS1) and 4.5 kcal mol–1 (TS2), respectively. The former pathway is
thermodynamically more favorable due to large energy release of 33.2 kcal mol–1. This is fully
consistent with the experimental observation of the isomerization of nitrene 16 to isoxazole 18 in
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matrix (Figure 3). The simultaneous formation of traces of imino ketene 20 from nitrene 16 can
also be explained by even larger energy release of 45.2 kcal mol–1, while the activation barrier is
higher by 8.4 kcal mol–1, which makes this pathway less competitive than forming isoxazole 18
(Figure 4). Alternatively, the formation of imino ketene 20 might be caused by the photo-induced
1,4-H migration in isoxazole 18 with concerted N‒O bond fragmentation. In line with the
observation of imino ketene 19 among the pyrolysis products of the azide precursor 13, the 1,4-H
shift in nitrene 15 to imino ketene 19 is more exothermic than the cyclization to 3,2-thieno
isoxazole (17), and the latter has been obtained during the laser photolysis of the azide 13 under
matrix-isolation conditions.
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Recently, the spontaneous conversion of triplet 2-formyl phenylnitrene (32) to singlet
6-imino-2,4-cyclohexadien-1-ketene (13) has been observed in solid Ar-matrix at 10.0 K, for
which a mechanism of hydrogen-atom tunneling (HAT) has been proposed.15 The tunneling
mechanism has analyzed by estimating the barrier height 17.2 kcal mol–1 (Figure S7) and the
3
3
barrier width 2.50 Bohr for the 1,4-H migration from nitrene 2 to imino ketene 3 according to
intrinsic reaction coordinate (IRC) scans at the B3LYP/6-311++G(3df,3pd) level (Figure 5). As a
further step, the putative triplet imino ketene 33 immediately relaxes to the singlet ground state 13
in the cryogenic matrix.
Figure 5. B3LYP/6-311++G(3df,3pd) calculated intrinsic reaction coordinate (IRC) profiles for rearrangement of
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triplet nitrenes (32, syn-315, and syn-316) to triplet ketenes (33, syn-319, and syn-320) via intramolecular 1,4-H shift.
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The estimated widths of the barriers (in Bohr) are also indicated.
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According to the Jeffreys-Wentzel-Kramers-Brillouin (JWKB) approximation (Eq 1),48 the
tunneling probability P(E) through a parabolic barrier depends on the mass (m) of the tunneling
atom, the width of barrier (w), and the height of the barrier (V0–E). The calculated tunneling
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probability P(E) for nitrene 2 to form ketene 3 was estimated to be 7.14 × 10−18 at the
B3LYP/6-311++G(3df,3pd) level. Since the tunneling rate (k) is a product of the transmission
coefficient and the frequency of attempts, a rate of 6.12 × 10−4 s−1 can be derived based on the
experimentally observed frequency of about 2855 cm−1 for the C–H stretching vibration in the
H-formyl group.15
2m(V0 E)
2
P(E)  e 
(Eq 1)
h
By analogy, the tunneling probabilities for the conversion from the two nitrenes syn-315 and
syn-316 to ketenes syn-319 and syn-320 have also been estimated by performing similar potential
energy scans at the same theoretical level (Figure 5). The results suggest that both the barrier
heights (V0–E) of 20.8 (Figure S8) and 24.5 kcal mol–1 (Figure S9) and widths (w) of 3.10 and
3.48 Bohr in these two rearrangement reactions are larger than those in the nitrene 32 → ketene 33
conversion (Figure 5). As a result, the corresponding tunneling probabilities (P(E)) 4.16 × 10−24
and 3.27 × 10−29 are significantly smaller. Considering the experimentally observed C–H
stretching frequencies of 2858.9 (syn-315) and 2864.1 cm−1 (syn-316), the rate constants are 3.57 ×
10−10 and 2.81 × 10−15 s−1, which are more than five orders of magnitude slower than that for the
tunneling transformation of nitrene 32. Therefore, hydrogen-atom tunneling will be far too slow to
be observable for nitrenes syn-315 and syn-316.
CONCLUSIONS
The photochemistry of 3-azido-2-formylthiophene (13) and 3-azido-2-formylfuran (14) has
been studied by combining matrix-isolation and quantum chemical calculations. Two new
heteroaryl nitrenes, 3-nitrene-2-formylthiophene (15) and 3-nitrene-2-formylfuran (16), in two
conformations with triplet ground state have been generated and characterized by using
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matrix-isolation IR spectroscopy. In sharp contrast to the spontaneous hydrogen-atom tunneling of
the structurally related 2-formyl phenylnitrene (8) to an imino ketene 9 in solid Ar-matrix at 10.0
K, the two conformers of nitrenes 15 and 16 are persistent in cryogenic Ar- and N2-matrices up to
25.0 K due to higher activation barriers and the associated barrier heights for the 1,4-H migration.
However, both nitrenes are photosensitive, and green-light irradiation (532 nm) results in
cyclization to form 3,2-thienoisoxazole (17) and 3,2-furoisoxazole (18), respectively. In the gas
phase, the thermal decomposition of the azide 13 mainly yields imino ketene in two conformations
19/19', which can be interconverted in matrices under selective irradiation conditions.
Furthermore, the photoisomerization from isoxazole 17 to imino ketene 19 has also been observed.
Whereas, pyrolysis of gaseous azide 14 results in complete fragmentation with traces of imino
ketene 20.
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ASSOCIATED CONTENT
Supporting Information
Experimental band positions and calculated vibrational frequencies, calculated vertical transitions,
energies and atomic coordinates for all species discussed in the paper. This material is available
free of charge via the internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*X. Q. Zeng: E-mail, xqzeng@suda.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (21673147). We
thank Yan Lu for performing the CASPT2 calculations.
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Figure 1. (A) IR difference spectrum showing the decomposition of 3-azido-2-formylthiophene (13) in Ar-
matrix (10.0 K) upon 266 nm laser irradiation (11 s). (B) IR difference spectrum showing the rearrangement
of nitrenes 15 (syn) and 15' (anti) to isoxazole 17 upon subsequent visible light irradiation (532 nm, 15
min). (C, D) B3LYP/6-311++G(3df,3pd) calculated IR spectra (scaled by a factor of 0.98) for nitrenes 15
(syn) and 15' (anti) in the triplet state. Bands of imino ketenes 19 (syn), 19' (anti), H2O (&), and unknown
species (*) are also marked.
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Figure 2. (A) Ar-matrix IR spectrum of the high-vacuum flash pyroysis (ca. 1000 K) products of 3-azido-2-
formylthiophene (13). (B) IR difference spectrum showing rearrangement of imino ketenes 19' (anti) to 19
(syn) upon UV-light irradiation (365 nm, 25 min). (C) IR difference spectrum showing rearrangement of
imino ketenes 19 (syn) to 19' (anti) upon 266 nm laser irradiation (2 min). Bands of isoxazole 17, H2O (&),
and unknown species (*) are also marked.
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Figure 3. (A) IR difference spectrum showing the decomposition of 3-azido-2-formylfuran (14) in N2-matrix
(15.0 K) upon 266 nm laser irradiation (55 s). (B) IR difference spectrum (3 times expanded along the ΔA
axis) showing the rearrangement of nitrenes 16 and 16' to isoxazole 18 upon subsequent visible-light
irradiation (532 nm, 55 min). (C, D) B3LYP/6-311++G(3df,3pd) calculated IR spectra (scaled by a factor of
0.98) for 16 (C: syn) and 16' (D: anti) in the triplet state. Bands of imino ketenes 20/20', H2O (&), and
unknown species (*) are also marked.
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Figure 4. Calculated relative Gibbs free energy profile (kcal mol–1, including zero-point vibrational energy
corrections) for the rearrangement of closed-shell singlet state 3-nitrene-2-formylfuran (16) at the
CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level.
119x73mm (300 x 300 DPI)
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