Phenyl Nitrene Research Articles

Photochemistry of N-aryl and N-alkyl dibenzothiophene sulfoximines.

N-phenyl dibenzothiophene sulfoximine has been demonstrated to produce phenyl nitrene and dibenzothiophene S-oxide upon irradiation with UV-A light, and dibenzothiophene S-oxide upon further irradiation releases triplet atomic oxygen. Thus, N-phenyl dibenzothiophene sulfoximine exhibits a rare dual-release capability in its photochemistry. In this work, N-substituted dibenzothiophene sulfoximine derivatives are irradiated with UV-A light to compare their photochemistry and quantum yield of dibenzothiophene S-oxide production with that of N-phenyl dibenzothiophene sulfoximine. Both N-aryl and N-alkyl derivatives of dibenzothiophene sulfoximine are examined to observe their effects on the quantum yield of the photolysis reaction. Adding electron withdrawing N-aryl substituents is shown to increase the quantum yield of dibenzothiophene S-oxide production, while adding electron donating N-aryl substituents is shown to decrease the quantum yield. The quantum yield was slightly lowered or not increased by most N-alkyl substituents. Furthermore, the quantum yield was not augmented by branching and steric hindrance effects associated with the N-alkyl substituents. These results suggest that electronic modulation of the sulfoximine bonds affects the observed photolysis reaction.

Simulating Non-Adiabatic Dynamics of Photoexcited Phenyl Azide: Investigating Electronic and Structural Relaxation en Route to the Formation of Phenyl Nitrene.

Excited state molecular dynamics simulations of the photoexcited phenyl azide have been performed. The semi-classical surface hopping approximation has enabled an unconstrained analysis of the electronic and nuclear degrees of freedom which contribute to the molecular dissociation of phenyl azide into phenyl nitrene and molecular nitrogen. The significance of the second singlet excited state in leading the photodissociation has been established through electronic structure calculations, based on multi-configurational schemes, and state population dynamics. The investigations on the structural dynamics have revealed the N-N bond separation to be accompanied by synchronous changes in the azide N-N-N bond angle. The 100 fs simulation results in a nitrene fragment that is electronically excited in the singlet manifold.

Investigation of phenyl azide photochemistry by conventional and time-resolved spectroscopy. Elucidation of intermediates and reaction mechanisms

Early studies of the photochemistry of parent phenyl azide concerned the products obtained under specific experimental conditions. Depending on the products, various mechanisms and intermediates were proposed. Later work involving conventional spectroscopic studies in low temperature matrices allowed the direct detection and identification of several intermediates. Electron Spin Resonance spectroscopy allowed the determination of a ground triplet phenyl nitrene and IR spectroscopy allowed the direct observation of ketenimine and bicyclic azirine. However, low temperature spectroscopy studies gave contradictory results with the formation of secondary photochemical intermediates and products. Recent studies using-time resolved spectroscopy allowed the direct observation of singlet phenyl nitrene. Furthermore, modern computational tools helped to corroborate experimental spectra of intermediates and to determine electronic structure of other possible intermediates, relative energies and mechanistic pathways. A unified picture of the photochemistry of phenyl azide has emerged from experimental, spectroscopic and theoretical studies. In this review, the results presented in each methodology are discussed.

IR absorption spectra of aniline cation, anilino radical, and phenylnitrene isolated in solid argon

Electron bombardment of aniline (PhNH2) in an Ar matrix mainly generated the aniline cation (PhNH2+), anilino (PhNH) and phenyl (Ph) radicals, and phenylnitrene (PhN). Further irradiation of the electron-bombarded matrix sample at 365 nm depleted PhNH2+ and PhN, and resulted in the formation of PhNH2, PhNH, and Ph. In separate experiments, irradiation of the PhNH2/Ar matrix samples at 265 or 160 nm mainly generated PhNH and Ph radicals, but without the formation of PhNH2+ and PhN. According to the observed photochemical behaviors, quantum-chemically predicted harmonic vibrational wavenumbers of each species, and the information reported in previous photodissociation studies, we unambiguously characterized the IR features of the aromatic species. The information of the vibrational fundamentals of PhNH is new and the formation mechanism is discussed.

Two-step Two-intermediate Photorelease Bolm-McCulla Reaction: Dual Release of Nitrene and Atomic Oxygen Reactive Intermediates.

This article is a highlight of the paper by Isor etal. in this issue of Photochemistry and Photobiology. It describes the photolysis of a dibenzothiophene sulfoximine (bearing N-phenyl imino and S-oxide groups) to produce two reactive intermediates in tandem. The sulfoximine undergoes a S-N and S-O photocleavage to release phenyl nitrene and atomic oxygen [O(3 P)]. The phenyl nitrene dimerizes to azobenzene or is trapped by diethylamine to reach an azepine. From there, atomic oxygen arises in a secondary photolysis of dibenzothiophene sulfoxide. A computational analysis also reveals that the S-N bond is labile for initial nitrene release, with the secondary release of atomic oxygen by S-O cleavage. Whether future sulfoximine scaffolds can produce the reverse order release of O(3 P) then nitrene, or release both simultaneously, is yet to be established. Nonetheless, molecules with dual-intermediate release, such as coupled photoaffinity labeling and cellular oxidation, are worth pursuing.

Photochemistry of N-Phenyl Dibenzothiophene Sulfoximine.

Sulfoximines are popular scaffolds in drug discovery due to their hydrogen bonding properties and chemical stability. In recent years, the role of reactive intermediates such as nitrenes has been studied in the synthesis and degradation of sulfoximines. In this work, the photochemistry of N-phenyl dibenzothiophene sulfoximine [5-(phenylimino)-5H-5λ4 -dibenzo[b,d]thiophene S-oxide] was analyzed. The structure resembles a combination of N-phenyl iminodibenzothiophene and dibenzothiophene S-oxide, which generate nitrene and O(3 P) upon UV-A irradiation, respectively. The photochemistry of N-phenyl dibenzothiophene sulfoximine was explored by monitoring the formation of azobenzene, a photoproduct of triplet nitrene, using direct irradiation and sensitized experiments. The reactivity profile was further studied through direct irradiation experiments in the presence of diethylamine (DEA) as a nucleophile. The studies demonstrated that N-phenyl dibenzothiophene sulfoximine underwent S-N photocleavage to release singlet phenyl nitrene which formed a mixture of azepines in the presence of DEA and generated moderate amounts of azobenzene in the absence of DEA to indicate formation of triplet phenyl nitrene.

Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet Tribromophenyl Nitrenes.

Previously unknown the steric heavy atom effect on magnetic anisotropy parameters of triplet phenyl nitrenes is reported. The heavy bromine atom effect is revealed by W-band EPR and theoretical investigations of triplet 2,4,6-tribromophenyl nitrenes bearing different substituents in positions 3 and 5 of the phenyl ring (1a, H/H; 1b, CN/CN; 1c, N3/F; 1d, N3/N3; 1e, Cl/Cl; 1f, Br/Br). The zero-field splitting parameters of nitrenes 1a ( D = 0.9930 cm-1, E = 0.0261 cm-1), 1c ( D = 1.244 cm-1, E = 0.030 cm-1), and 1d ( D = 1.369 cm-1, E = 0.093 cm-1), generated by the photolysis of the corresponding azides in frozen methylcyclohexane solution at 5 K, were determined from the W-band EPR spectra. To clarify the origin of considerable differences in the experimental D values of nitrenes 1a, 1c, and 1d, extensive DFT and CASSCF calculations of these nitrenes as well as of model nitrenes 1b, 1e, and 1f were performed. The calculations show that all nitrenes have nearly the same magnitudes of the spin-spin interactions ( DSS ∼ 1 cm-1), but drastically differ in the spin-orbit coupling parameter (from DSOC = 0.087 cm-1 for 1a to DSOC = 0.765 cm-1 for 1f). Comprehensive analysis of various computational data showed that the magnitude of DSOC of nitrenes 1a-f is the function of the N···Br distance between the nitrene nitrogen and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a-1f, the smaller the N--Br distance and the larger DSOC. These features indicate that the heavy atom effect on magnetic anisotropy of triplet phenyl nitrenes originates from the through-space rather than through-bond electronic interactions between the bromine atoms and the nitrene unit.

Unexpected effect of substituents on the zero-field splitting of triplet phenyl nitrenes

The EPR spectrum of triplet 2,4,6-tribromophenyl nitrene was obtained in glassy methylcyclohexane at 15K. Surprisingly, the zero-field splitting parameter D=0.989cm−1 derived from this spectrum is much lower than that reported previously for triplet 3,5-diazido-2,4,6-tribromophenyl nitrene and has the same value as in phenyl nitrenes composed with light atoms. DFT calculations of the zero-field splitting parameters support this unexpected experimental observation. Experimental and theoretical data provide evidence that the enhanced by bromine atoms spin-orbit contribution to the parameter D (the so called “heavy-atom effect”) is strongly modulated by other substituents attached to the aromatic cycle.

Coverage-driven dissociation of azobenzene on Cu(111): a route towards defined surface functionalization.

We investigate the surface-catalyzed dissociation of the archetypal molecular switch azobenzene on the Cu(111) surface. Based on X-ray photoelectron spectroscopy, normal incidence X-ray standing waves and density functional theory calculations a detailed picture of the coverage-induced formation of phenyl nitrene from azobenzene is presented. Furthermore, a comparison to the azobenzene/Ag(111) interface provides insight into the driving force behind the dissociation on Cu(111). The quantitative decay of azobenzene paves the way for the creation of a defect free, covalently bonded monolayer. Our work suggests a route of surface functionalization via suitable azobenzene derivatives and the on surface synthesis concept, allowing for the creation of complex immobilized molecular systems.

Covalent Functionalization of Graphene with Reactive Intermediates

Graphene, a material made exclusively of sp(2) carbon atoms with its π electrons delocalized over the entire 2D network, is somewhat chemically inert. Covalent functionalization can enhance graphene's properties including opening its band gap, tuning conductivity, and improving solubility and stability. Covalent functionalization of pristine graphene typically requires reactive species that can form covalent adducts with the sp(2) carbon structures in graphene. In this Account, we describe graphene functionalization reactions using reactive intermediates of radicals, nitrenes, carbenes, and arynes. These reactive species covalently modify graphene through free radical addition, CH insertion, or cycloaddition reactions. Free radical additions are among the most common reaction, and these radicals can be generated from diazonium salts and benzoyl peroxide. Electron transfer from graphene to aryl diazonium ion or photoactivation of benzoyl peroxide yields aryl radicals that subsequently add to graphene to form covalent adducts. Nitrenes, electron-deficient species generated by thermal or photochemical activation of organic azides, can functionalize graphene very efficiently. Because perfluorophenyl nitrenes show enhanced bimolecular reactions compared with alkyl or phenyl nitrenes, perfluorophenyl azides are especially effective. Carbenes are used less frequently than nitrenes, but they undergo CH insertion and C═C cycloaddition reactions with graphene. In addition, arynes can serve as a dienophile in a Diels-Alder type reaction with graphene. Further study is needed to understand and exploit the chemistry of graphene. The generation of highly reactive intermediates in these reactions leads to side products that complicate the product composition and analysis. Fundamental questions remain about the reactivity and regioselectivity of graphene. The differences in the basal plane and the undercoordinated edges of graphene and the zigzag versus arm-chair configurations warrant comprehensive studies. The availability of well-defined pristine graphene starting materials in large quantities remains a key obstacle to the advancement of synthetic graphene chemistry.

3,4,5,6‐Tetrafluorophenylnitren‐2‐yl: A Ground‐State Quartet Triradical

The photochemistry of 2-iodo-3,4,5,6-tetrafluorophenyl azide (7 d) has been investigated in argon and neon matrices at 4 K, and the products characterized by IR and EPR spectroscopy. The primary photochemical step is loss of a nitrogen molecule and formation of phenyl nitrene 1 d. Further irradiation with UV or visible light results in mixtures of 1 d with azirine 5 d', ketenimine 6 d', nitreno radical 2 d, and azirinyl radical 9. The relative amounts of these products strongly depend on the matrix and on the irradiation conditions. Nitreno radical 2 d with a quartet ground state was characterized by EPR spectroscopy. Electronic structure calculations in combination with the experimental results allow for a detailed understanding of the properties of this unusual new type of organic high-spin molecules.

Identification and Chemistry of Phenylnitrene in Premixed Pyridine/Oxygen/Argon Flame with Tunable Synchrotron Photoionization

The triplet state phenylnitrene (PhN) species generated from the low-pressure (4.0 kPa) premixed laminar pyridine/oxygen/argon flame was detected and identified using tunable synchrotron vacuum ultraviolet photoionization and molecular-beam mass spectrometry techniques. The ionization energies of PhN were determined experimentally by photoionization efficiency spectra and theoretically by calculations. The results indicate that PhN has a 3A2 ground state and its first and second adiabatic ionization energies are 8.04 and 9.150.05 eV, respectively. Furthermore, the formation and consumption pathways of PhN are proposed according to the species detected in the present work. PhN is the first nitrogen-containing diradical detected in combustion chemistry, and so it should be added to the kinetic model of pyridine flames.

First determination of ionization energies of phenylnitrene

The continuous flowing beams of the triplet state phenylnitrene (PhN) species have been generated by respective pyrolysis of PhN 3 at 83(±0.5) °C and phenylisocyanate (PhNCO) at 105(±0.5) °C in the quartz inlet tube loosely filled with a 30 Å molecular sieve powder supported on the quartz wool. PE spectrum of PhN is recorded in situ for the first time. The ionization energies, corresponding to different ionic states of PhN +, are determined by PES experiment and both G2 and DFT calculations. The study indicates that PhN is a diradical with C 2v symmetry and has a 3A 2 ground state.

The interplay of theory and experiment in the study of phenylnitrene.

The intra- and intermolecular chemistry of phenylnitrene (PhN), its singlet-triplet energy separation, and its electronic spectra are interpreted with the aid of ab initio molecular orbital theory. The key to understanding singlet PhN is the recognition that this species has an open-shell electronic structure, in contrast to the related species, phenylcarbene, which has a closed-shell electronic structure. The thermodynamics of nitrenes, benzazirines, dehydroazepines, aminyl radicals, and their hydrocarbon analogues are also discussed.

A Laser Flash Photolysis Study of Some Simple Para-Substituted Derivatives of Singlet Phenyl Nitrene

The photochemistry of several para-substituted aryl azides was studied by laser flash photolysis (LFP) techniques in pentane. Para F, Cl, Br, I, CH3, CF3, COCH3, OCH3, and NO2 substituted aryl azides were studied. LFP of aryl azides generates singlet aryl nitrenes which were detected in absorption with maxima between 300 and 365 nm in pentane. The nitrene decay can be fit to an exponential function and analyzed to yield observed rate constant, kobs, which was measured as a function of temperature. The value of kobs = kR + kISC where kR and kISC are the rate constants of nitrene rearrangement to azirine intermediates and intersystem crossing to the lower energy triplet states, respectively. Values of kobs decrease as the temperature is reduced and reach a limiting value attributed to kISC. Assuming that kISC is temperature independent allows values of kR to be deduced along with the Arrhenius parameters to cyclization for para CH3, CF3, COCH3, F, Cl, and Br singlet phenyl nitrenes. The Arrhenius parameters to rearrangement closely resemble those of parent singlet phenyl nitrene with the exception of the para bromo compound. For para I and OCH3, kISC is large and prevents measurement of kR. The increased rate of intersystem crossing in these two singlet nitrenes is due to heavy-atom and to electron-donation effects, respectively. No transient absorptions were detected upon LFP of p-nitrophenyl azide.

Photochemistry of fluorophenyl azides in diethylamine. Nitrene reaction versus ring expansion

Several fluorophenyl azides were photolyzed with diethyl amine at room temperature. Pentafluoro and 2,6-difluorophenyl azides gave hydrazines as the major products. On the other hand, several mono- and difluorophenyl azides gave azepines under similar conditions. Ortho-fluoro singlet phenyl nitrene produces the azirine by ring closure away from the substituent.

ChemInform Abstract: Photochemistry of Fluorinated Phenyl Nitrenes: Matrix Isolation of Fluorinated Azirines.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

How Do Different Fluorine Substitution Patterns Affect the Electronic State Energies of Phenylnitrene?

Substituted phenylnitrenes, reacting out of their closed-shellsinglet (CSS) states, can serve as photoaffinity labeling reagents if they engage in bond insertion reactions.1-3 Such reactivity is similar to that exhibited by singlet phenylcarbene.4 However, the reactivity of unsubstituted CSS phenylnitrene (PhN) is dominated by unimolecular ring expansion to form didehydroazepine.4,5 Substituents on the aromatic ring are required for bimolecular insertion reactions to compete with ring expansion, e.g., polyfluorinated phenylnitrenes, which were first examined by Banks and Prakash.6,7 Platz has demonstrated that the barrier to ring expansion in 2,6-difluorophenylnitrene ((o,oF2)PhN) is increased by 3-5 kcal/mol compared to unsubstituted PhN.4,8 Platz has also shown that proper location of the fluorines on the aromatic ring is critical to this effect; 3,5difluorophenylnitrene ((m,m-F2)PhN), like PhN, undergoes ring expansion faster than it participates in insertion reactions.2-4 Elucidating the energies for different electronic states of PhN has been the goal of several experimental9,10 and theoretical11-13 studies. Theory and experiment are in good agreement on the relative ordering of the lowest energy states. The ground state is a triplet (A2), the first excited state is an open-shell singlet (A2) lying about 18 kcal/mol higher in energy, and higher still is the first CSS (1A1). No experimental number is available for the relative energy of the latter state, but reasonable quality singleand multireference configuration interaction (CISD+Q) calculations place it 31-39 kcal/mol above the triplet. The electronic configuration of this state is dominated by double occupation of the nitrogen p orbital that lies in the plane of the aromatic ring. The next higher lying CSS (2A1) is derived from double excitation of that pair of electrons out of the inplane p orbital and into the aromatic π system. Kim et al.12 have estimated the energy of this state to be about 52 kcal/mol above the triplet, but the errors associated with this calculation may be substantial (vide infra). The energy of this state bears on the ring expansion reactivity of PhN insofar as it correlates with the CSS ground state of didehydroazepine, i.e., the ring expansion is a Wagner-Meerwein shift of a C-C bond to the electron-deficient nitrogen using the empty in-plane p orbital. On the basis of this analysis, Platz has speculated that different fluorine substitution patterns may preferentially stabilize the 1A1 state and hence lead to higher activation barriers for ring expansion.4 Of course, a more complete explanation includes the recognition that there must be an avoided crossing in the state-symmetry correlation diagrams, but the crux of the argument remains a differential effect on the relative energies of the two CSS states (Figure 1). In order to address this question, we have employed density functional theory14 (DFT) to calculate15,16 the relative energies of the triplet and CSS states of PhN, (o,o-F2)PhN, (m,m-F2)PhN, and 4-fluorophenylnitrene (p-FPhN) and their corresponding didehydroazepines. To assess the quality of this level of theory, we have compared the DFT results for PhN to multireference second-order perturbation theory (CASPT2N)17,18 employing an eight-electron/eight-orbital active space and also to single-reference coupled cluster19 calculations including all single, double, and perturbative triple20 excitations (CCSD(T)). These calculations are summarized in Table 1. The gap between the A2 and 1A1 states for PhN appears to be well predicted by DFT. On the basis of the results for methylene, Roos has suggested that the CASPT2N level overstabilizes triplets relative to singlets by 3-5 kcal/mol.21 Our own calculations for various carbenes and nitrenium ions have shown similar overstabilizations of triplet states by up to 8 kcal/ mol.22 The CCSD(T) calculations also support the DFT results. Because of the size of PhN, calculations at this level were not performed using a triple-u basis. The larger stabilization of the triplet at this level of theory compared to DFT is consistent with lesser flexibility in the basis set and the observed tendency of CCSD(T) to overstabilize triplets relative to CSSs by 1-2 kcal/mol in carbenes and nitrenium ions.22 Similar levels of DFT have previously been shown to accurately predict singlet-

Photochemistry of Fluorinated Phenyl Nitrenes: Matrix Isolation of Fluorinated Azirines.

2,6-Difluorophenylnitrene and pentafluorophenylnitrene were generated in solid argon at 10 K by irradiation of the corresponding phenyl azides and characterized by IR and UV spectroscopy. Selective irradiation with lambda = 444 nm results in the formation of the corresponding azirines, while ketene imines are not produced. On lambda = 366 nm irradiation the azirines rearrange back to the nitrenes. The assignment of the azirines is confirmed by ab-initio calculations.

THE PHOTOCHEMISTRY OF IODO, METHYL AND THIOMETHYL SUBSTITUTED ARYL AZIDES IN TOLUENE SOLUTION AND FROZEN POLYCRYSTALS

The photolysis of para-methyl and para-thiomethylphenylazide at 77 K produces the corresponding triplet nitrenes which can be detected by electron paramagnetic resonance (EPR) spectroscopy. Photolysis of these azides in frozen toluene at 77 K leads to insertion of the nitrene into a benzylic C-H bond of the matrix in modest yields. Photolysis of iodinated aryl azides under these conditions does not produce triplet nitrenes that can be detected by EPR spectroscopy. In contrast to the para-methyl and para-thiomethyl substituted phenyl nitrenes, photo-induced coupling of iodo-substituted phenyl nitrenes to toluene proceeds in very poor yield.