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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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The Journal of Physical Chemistry

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†*

†

College of Chemistry, Chemical Engineering and Materials Science, Soochow

University, 215123, Suzhou, P. R. China. E-mail: [email protected] ‡

Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1

Kagamiyama,

Higashi-Hiroshima

Hiroshima

739-8526,

[email protected]

1

ACS Paragon Plus Environment

Japan.

E-mail:


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ABSTRACT The decomposition of phenyl azidoformate, PhOC(O)N3, has been 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 has been identified by matrix-isolation IR spectroscopy (with

15

N

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 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)N3 are versatile reagents in synthetic organic chemistry that have been broadly used in metal-catalyzed 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 non-azide 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 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 3



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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 nm) of 4-acetylphenoxycarbonyl azide in C6F6 solution,23 both chemical trapping products and 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

Scheme 1. Decomposition of Ph-N3,28 Ph-C(O)N3,29-32 and PhO-C(O)N3.21-22

The

experimentally

Curtius-rearrangement

observed in

both

intramolecular thermal

and

cyclization photolytic

rather

decomposition

than 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 ring-expansion and Curtius-rearrangement (Scheme 1), respectively. It should be 4


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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 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, matrix-isolation IR and EPR spectroscopy, and quantum chemical calculations.

EXPERIMENTAL SECTION Caution! Covalent azides are potentially hazardous and explosive. Although we have not experienced 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, 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 5



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spectrometer at 25 °C. 1-15N sodium azide (98 atom % was used for the preparation of

15

15

N, EURISO-TOP GmbH)

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 FT-IR spectrometer (Bruker 70V) in a reflectance mode by using a transfer optic. A KBr beam splitter and 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 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 by a tantalum wire (o.d. 0.4mm, 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 (~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 co-deposition with noble gas at 70 °C. Photolysis was performed using an ArF excimer laser (Gamlaser EX5/250, 193 nm, 3 mJ, 3Hz), a Nd3+:YAG laser (266 nm, 532 nm, MPL-F-266, 10mW), and a high-power flashlight (Boyu T648, 440±20 nm, 20W).

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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 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 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 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 single-point 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 carried out 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

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RESULTS AND DISCUSSION Photolysis of Phenyl Azidoformate with 193 nm Laser Given a strong absorption at about 190 nm for PhOC(O)N3 (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 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 8


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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 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‒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/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 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 around 2300–2100 cm‒1 and 1711.0 (f) and 1695.0 cm‒1 (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 9



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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 states (Figure 1D) in the lower-energy syn-conformation are compared with the observed IR bands (Figure 1B, pointing downward).

Table 1. Observed and calculated vibrational frequencies (> 600 cm‒1) of PhOC(O)N. Calculateda syn-triplet 3254 (4) 3203 (4) 3193 (13) 3182 (9) 3173 (

Contrasting Photolytic and Thermal Decomposition ... - ACS Publications

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