Contrasting Photolytic and Thermal Decomposition of Phenyl

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Article Cite This: J. Phys. Chem. A 2017, 121, 8604-8613

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Contrasting Photolytic and Thermal Decomposition of Phenyl Azidoformate: The Curtius Rearrangement Versus Intramolecular C− H Amination Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”. Huabin Wan,† Jian Xu,† Qian Liu,† Hongmin Li,† Yan Lu,† Manabu Abe,*,‡ and Xiaoqing Zeng*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123 Suzhou, P. R. China Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

‡

S Supporting Information *

ABSTRACT: 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.

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INTRODUCTION

azidoformates are limited, although their applications as nitrene transfer reagents have been frequently reported.18−20 According to the early studies in solutions,21,22 the parent phenyl azidoformate PhO−C(O)N3 splits off molecular N2 upon heating (250 °C) and yields aziridines as the formal intermolecular nitrene trapping products. More importantly, the intramolecular cyclization product 3H-benzooxazol-2-one (Scheme 1) was also obtained, and it becomes the dominant 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 nm) of 4acetylphenoxycarbonyl azide in C6F6 solution;23 both chemical trapping products and electron paramagnetic resonance (EPR) results confirm a triplet multiplicity for the aryloxycarbonylnitrene. The facile metal-free intramolecular C−H bond amination in the thermal decomposition of PhO−C(O)N3 resembles the intensively explored metal-catalyzed sp2 C−H bond insertions of arylnitrenes.24−27

Azidoformates RO−C(O)N3 are versatile reagents in synthetic organic chemistry that have been broadly used in metalcatalyzed C−H amination and aziridination via the intermediacy of highly reactive oxycarbonylnitrenes RO−C(O)N.1−4 Therefore, the decomposition of azidoformates and the reactivity of oxycarbonylnitrenes have been the fundamentally important research topics in nitrene chemistry.5−7 For instance, the photolytic and thermal decomposition of the simplest alkyl azidoformate MeO−C(O)N3 into the Curtius rearrangement product MeONCO and N2 has attracted extensive experimental and theoretical interest in the past few decades.8−13 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 converted into MeONCO upon subsequent visible light irradiation.14 Both triplet ethoxycarbonylnitrene and t-butyloxycarbonylnitrene, generated from laser photolysis of nonazide sulfilimine precursors in solution, have also been directly detected by time-resolved IR spectroscopy.15 Comparing to the extensively explored mechanisms for the decomposition of alkylcarbonyl azides5−7,16 and alkoxycarbonyl azides,8−14,17 the corresponding studies of aryl-substituted © 2017 American Chemical Society

Received: August 10, 2017 Revised: October 23, 2017 Published: October 25, 2017 8604

DOI: 10.1021/acs.jpca.7b07969 J. Phys. Chem. A 2017, 121, 8604−8613

Article

The Journal of Physical Chemistry A Scheme 1. Decomposition of Ph−N3,28 Ph−C(O)N3,29−32 and PhO−C(O)N321,22

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 by a tantalum wire (o.d. 0.4 mm, resistance 0.4 Ω), and deposited (2 mmol h−1) at a high vacuum onto the Rh-plated Cu block matrix support (2.8 K) in a high vacuum (∼1 × 10−6 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 codeposition with noble gas at 70 °C. Photolysis was performed using an ArF excimer laser (Gamlaser EX5/250, 193 nm, 3 mJ, 3 Hz), a Nd3+:YAG laser (266 nm, 532 nm, MPL-F-266, 10 mW), and a high-power flashlight (Boyu T648, 440 ± 20 nm, 20 W). 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 under the organic glassy matrix conditions, the sample solution (20 mg mL−1) in a quartz tube (4.0 mm) was degassed by the freeze−pump−thaw cycle three times. Quantum Chemical Calculation Methods. Structures and IR frequencies of stationary points were calculated using the density functional theory (DFT) B3LYP method45 with the 6-311++G(d,p) basis set. Accurate relative energies of the species were further calculated using the complete basis sets (CBS-QB3)46 and CCSD(T)47 methods. The B3LYP/6-311+ +G(d,p) optimized structures were used for the latter singlepoint energy calculations. Local minima were confirmed by vibrational frequency analysis, and transition states were further confirmed by intrinsic reaction coordinate (IRC) calculation.48,49 Time-dependent (TD)50,51 B3LYP/6-311++G(d,p) calculations were performed for the prediction of UV−vis transitions. These calculations were performed using the Gaussian 09 software package.52 The CASPT2 calculations were performed based on the CASSCF(12,11)/6-311G** optimized structure with an active space of 12 electrons and 11 active orbitals by using the MOLPRO program.53,54

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 reactions of the closely related phenyl azide Ph−N328 and phenyl carbonyl azide Ph−C(O)N3,13,29−32 in which the corresponding nitrene intermediates (Ph−N and Ph−C(O)N have been directly detected by various spectroscopic methods, as followed by ringexpansion and Curtius rearrangement (Scheme 1), respectively. Note 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 PhO− C(O)N3 and the absence of the evidence for its Curtius rearrangement to PhONCO in the previous studies requires a reinvestigation of the underlying mechanism from both aspects of experiment and theory. Continuing our interest in the decomposition of α-oxoazides (RC(O)N3,33−36 RS(O)N3,37,38 R2P(O)N3,39 and RS(O)2N340−43), herein we report a comprehensive study of the decomposition of PhO−C(O)N3 by combining laser photolysis, flash vacuum pyrolysis, matrixisolation IR and EPR spectroscopies, and quantum chemical calculations.

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EXPERIMENTAL SECTION Caution! Covalent azides are potentially hazardous and explosive. Although we did not experience any incident during the work with PhOC(O)N3, safety precautions (face shields, leather gloves, and protective leather clothing) are recommended for handing the azide. Sample Preparation. Phenyl azidoformate, PhOC(O)N3, was prepared according to published protocol.44 The purity was checked by NMR spectroscopy (1H NMR (400 MHz, CDCl3, tetramethylsilane (TMS)) δ = 7.18 (d, 2H), 7.26 (t, 1H) 7.39 (d, 2H) ppm, 13C NMR (100 MHz, CDCl3, TMS) δ = 120.98, 126.70, 129.73, 150.72, 156.41 ppm) with a Bruker Avance III HD400 spectrometer at 25 °C. 1-15N sodium azide (98 atom % 15 N, EURISO-TOP GmbH) was used for the preparation of 15 N labeled sample. The commercially available 3H-benzooxazol-2-one (Energy Chemical, 98%) was purified through vacuum sublimation at 90 °C before the matrix isolation experiment. Matrix−Isolation IR Spectroscopy. Matrix IR spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer (Bruker 70 V) in a reflectance mode by using a transfer optic. A KBr beam splitter and mercury cadmium telluride (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. The azide sample was mixed by passing a flow of noble gas (Ne, Ar) though a U-trap (−20 °C)

RESULTS AND DISCUSSION Photolysis of Phenyl Azidoformate with 193 nm Laser. Given a strong absorption at ∼190 nm for PhOC(O)N3 (Figure S1), the photolysis of the azide was performed 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. 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−1 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−1 (b), which are likely to be associated with CO stretching vibrations. Given the theoretically predicted (TD-B3LYP/6311++G(d,p)) visible-light absorptions for the nitrene in the triplet state (syn: 545 nm, anti: 554 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 carrier of the band at 1649.1 cm−1 (b). Additionally, traces of the azide precursor (a) were also depleted by the visible light. As the result, the aforementioned new species bearing IR bands at ∼2300− 2100 cm−1 and 1711.0 (f) and 1695.0 cm−1 (g) were produced. 8605

DOI: 10.1021/acs.jpca.7b07969 J. Phys. Chem. A 2017, 121, 8604−8613

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The Journal of Physical Chemistry A

at 1195.6 cm−1. 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 ∼1618 cm−1 with a large IR intensity of 188 km mol−1. The absence of this band in the 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)N3 by using a 266 nm laser was performed. The IR difference spectra reflecting the changes of the respective matrices are shown in Figure 2. Comparing to the 193 nm laser irradiation (Figure 1A), less product formed after the 266 nm laser photolysis of PhOC(O)N3 (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 associated IR band at 2200.6 cm−1, 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 909.3 cm−1 (νcalcd: 928 cm−1), which associates with the characteristic O−N stretching vibration with an observed 15N isotopic shift of 7.1 cm−1 (Figure S2). Similarly, the band at 2200.6 cm−1 exhibits a 15N 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 193 and 266 nm laser photolysis of the azide. As shown in Figure 1B, traces of radicals ·NCO (d, 1920.1 cm−1, Δν(14/15N) = 7.3 cm−1)56,57 and PhO· (e, 1552.4, 1486.5, 787.4, and 637.6 cm−1)58 were simultaneously consumed upon the visible light irradiation. Given the known photochemistry of phenol (→ PhO· + H·, 275 nm)59 and anisole (→PhO· + ·CH3, 276 nm)60 in solid Ar matrices, the formation of 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. The identification of the two lowest-energy ortho-NCOcyclohexadienone (f, 2240.3, 1711.2, 1259.1, 705.9, and 687.3 cm−1) and para-NCO-cyclohexadienone (g, 2256.0, 1694.6, 1282.4, 894.8 cm−1) isomers is mainly based on the distinct photobehaviors toward the aforementioned irradiations (532 and 266 nm) and also the calculated spectra (Table S4). For example, both the ortho (f) and para (g) isomers 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

Figure 1. (A) Ne-matrix IR difference spectrum showing the decomposition of PhOC(O)N3 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 of PhOC(O)N3 (a), PhOC(O)N (b), PhONCO (c), ·NCO (d), Ph−O· (e), 6-isocyanate-2,4-cyclohexadienone (f), 4-isocyanate-2,5-cyclohexadienone (g), 1-isocyanate-butadienyl-4-ketene (h), and H2O (*) are labeled. Spectrum B is expanded in intensity by five times for clarity. Bands for the depleted species point downward, and those of the formed species point upward.

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 recently disclosed conformational properties of MeOC(O)N,14 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 (Figure 1D) states in the lower-energy syn conformation are compared with the observed IR bands (Figure 1B, pointing downward). 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 MeOC(O)N (syn: 1639.9 cm−1; anti: 1597.7 cm−1, Ar matrix)14 and H2NC(O)N (1638.0 cm−1, Ar matrix).55 Its assignment to the CO stretching vibration is also supported by the 15N-labeling experiment, and consistent with the theoretical calculation no noticeable 15N isotopic shift was observed for this band when a 1:1 mixture of PhO−C(O)15NαNβNγ and PhO−C(O)NαNβ15Nγ was used as the nitrene precursor (Figure S2). The calculated strongest IR bands for PhOC(O)N in the singlet state (syn: 1777 cm−1; anti: 1764 cm−1) mainly belong to the C−N stretching vibration with large 15N isotopic shifts (syn: 14 cm−1; anti: 12 cm−1). In contrast, the C−N stretching vibration in the triplet nitrene is symmetrically coupled with the C−O stretching and appears at 953.5 cm−1 with a well-resolved 15N isotopic shift of 5.6 cm−1. The asymmetrically coupled stretching mode locates 8606

DOI: 10.1021/acs.jpca.7b07969 J. Phys. Chem. A 2017, 121, 8604−8613

Article

The Journal of Physical Chemistry A Table 1. Observed and Calculated Vibrational Frequencies (>600 cm−1) of PhOC(O)N calculateda

observedb

syn triplet

syn singlet

anti triplet

anti singlet

3254 (4) 3203 (4) 3193 (13) 3182 (9) 3173 (