Formation Of Nitrene Research Articles

Oxidative cyclization of 1-(pyridin-2-yl)guanidine derivatives: a synthesis of [1,2,4]triazolo[1,5-a]pyridin-2-amines and an unexpected synthesis of [1,2,4]triazolo[4,3-a]pyridin-3-amines

Oxidative cyclization of 1-(pyridin-2-yl)guanidine derivatives using N-chlorosuccinimide and aqueous potassium carbonate has been investigated. Chlorination of 1-(5-nitropyridin-2-yl)guanidine by N-chlorosuccinimide in methanol followed by addition of aqueous potassium carbonate gave rise to cyclization and afforded 6-nitro-[1,2,4]triazolo[1,5-a]pyridin-2-amine in one-pot. In the course of studying the scope and limitation of the reaction, it was found that some of the examined 1-(pyridin-2-yl)guanidine derivatives gave not only the desired [1,2,4]triazolo[1,5-a]pyridin-2-amines but also unexpected [1,2,4]triazolo[4,3-a]pyridin-3-amine products. As plausible reaction mechanisms of this oxidative cyclization, diazirine formation and nitrene formation are presented.

ChemInform Abstract: Visible‐Light Sensitization of Vinyl Azides by Transition‐Metal Photocatalysis.

Irradiation of vinyl and aryl azides with visible light in the presence of Ru photocatalysts results in the formation of reactive nitrenes, which can undergo a variety of C-N bond-forming reactions. The ability to use low-energy visible light instead of UV in the photochemical activation of azides avoids competitive photodecomposition processes that have long been a significant limitation on the synthetic use of these reactions.

Computational study on the vinyl azide decomposition.

The decomposition mechanism of vinyl azide (CH2CHN3) has been studied by calculations of the electronic structure. In addition, a study based on the topology of the electron charge density distribution and its Laplacian function, within the Quantum Theory of Atoms in Molecules (QTAIM), has been carried out with the aim of comprehending the electron redistribution mechanisms that take place in the formation of vinyl nitrenes. The electronic structure calculations reveal that the decomposition of the s-cis conformer of vinyl azide leads to the formation of ketenimine through a single-step conversion, s-cis-CH2CHN3 → CH2CNH + N2, while the conversion of the s-trans conformer to acetonitrile occurs in two steps, s-trans-CH2CHN3 → cyc-CH2NCH + N2 → CH3CN + N2. The topological analysis of the L(r) function reveals that triplet vinyl nitrene has one lone pair on the valence shell charge concentration (VSCC) of nitrogen and thus could act as a monodentate Lewis base, while singlet vinyl nitrene has two lone pairs on the VSCC of nitrogen and thus could act as a bidentate Lewis base.

Visible‐Light Sensitization of Vinyl Azides by Transition‐Metal Photocatalysis

Irradiation of vinyl and aryl azides with visible light in the presence of Ru photocatalysts results in the formation of reactive nitrenes, which can undergo a variety of C-N bond-forming reactions. The ability to use low-energy visible light instead of UV in the photochemical activation of azides avoids competitive photodecomposition processes that have long been a significant limitation on the synthetic use of these reactions.

Chemoselective deoxygenation of 4,5-dinitroacenaphthenequinone with hexaethyltriamidophosphite

Deoxygenation under the action of the P(III) derivatives is widely used in organic chemistry [1, 2]. Among the variety of reactions a separate group includes the deoxygenation of nitro compounds with the P(III) derivatives resulting in the nitrogencontaining heterocycles of different structures [3]. The tertiary phosphines and phosphites are most commonly used as the P(III) derivatives. One of the most studied reactions of this type is the cyclization of nitroarenes to form the corresponding nitrogen-containing heterocycles (indoles, pyrrolopyrimidines, carbazoles, quinolines) under the action of the phosphorous acid esters. Presumably, the reaction proceeds through the intermediate formation of nitrenes. If 2-nitrobiaryls were used, the nitrene inserts on the C–H bond adjacent to the aryl moiety, to give the corresponding carbazoles [4, 5] or δ-carbolines [6]. The reaction of 2nitrophenylsulfide with an excess of triethyl phosphite, resulting in phenothiazine, can be attributed to the same type of the reactions [7, 8]. If the aryl contains a double bond ortho-positioned relative to the nitro group (o-nitrostyrene), the nitrene inserts on the β-C– H bond of the vinyl moiety to give the substituted indoles [9–11]. The reactions of o-dinitrobenzene with the P(III) derivatives should be specially noted. For example, under the action of triethyl phosphite, diphenylethylphosphinite, or methyldiethylphosphonite one of the nitro groups is substituted by the phosphorus moiety and ethyl nitrite is eliminated [12]. At the same time, the reduction of 2,2'-dinitrobiphenyl with the gaseous PH3 stops when benzocinnoline-2,2'dioxide forms [13].

Mechanisms and Kinetics for the Thermal Decomposition of 2-Azido-N,N-Dimethylethanamine (DMAZ)

To gain insight into the mechanisms and kinetics of 2-azido-N,N-dimethylethanamine's (DMAZ's) thermal decomposition postulated reaction paths were simulated with ab initio and density functional theory quantum chemistry models. Four reaction types were modeled: (i) spin-allowed and spin-forbidden paths involving N-N(2) bond fission and nitrene formation, (ii) HN(3) elimination with the formation of (dimethylamino)ethylene, (iii) N-N(2) bond fission with the formation of molecules with three- or four-membered heterocyclic rings, and (iv) simple scission of C-H, C-N, and C-C bonds. The geometries of stationary points of the reactions were obtained with a MPWB1K/6-31+G(d,p) model. To locate and model the geometries of minimum energy intersystem crossing points for triplet nitrene formation and isomerization, unrestricted broken spin symmetry calculations were performed. Employed to model an analogous path for methyl azide's decomposition, this approach was found to yield results similar to those obtained with a CASSCF(10,8)/aug-cc-pVDZ model. Of the four reaction types studied, N-N(2) bond fissions with singlet or triplet nitrene formation were found to have the lowest barriers. Barriers for paths to cyclic products were found to be 2-4 kcal/mol higher. Kinetic rate expressions for individual paths were derived from the quantum chemistry results, and spin-allowed nitrene formation was found to be dominant at all temperatures and pressures examined. The expression 2.69 × 10(9) (s(-1))T(1.405) exp(-39.0 (kcal/mol)/RT), which was derived from QCISD(T)/6-31++G(3df,2p)//MPWB1K/6-31+G(d,p) results, was found to be representative of this reaction's gas-phase rate. Adjusted on the basis of results from self-consistent reaction field models to account for solvation by n-dodecane, the expression became 1.11 × 10(9) (s(-1))T(1.480) exp(-37.6 (kcal/mol)/RT). Utilizing this result and others derived in the study, a model of the decomposition of n-dodecane-solvated DMAZ was constructed, and it generated simulations that well-reproduce previously published measured data for the process.

Photochemistry of 2-Naphthoyl Azide. An Ultrafast Time-Resolved UV–Vis and IR Spectroscopic and Computational Study

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S(1) state. Nitrene formation correlates with the decay of the relaxed S(1) state only upon 350 nm excitation (S(0) → S(1) excitation). When S(n) (n ≥ 2) states are populated upon excitation (λ(ex) = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S(1) and higher singlet excited states (S(n)) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S(1) state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.

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.

Competition Between Azido Cleavage and Triplet Nitrene Formation in Azidomethylacetophenones

Photolysis of p- and m-azidomethylacetophenone (1a, 1b) in argon-saturated solutions yields predominantly imine 2a, 2b, whereas irradiation of 1a, 1b in oxygen-saturated solutions results in heterocycles 3a, 3b, aldehydes 4a, 4b and nitriles 5a, 5b. Density functional theory calculations place the energy of the first and second excited state of the triplet ketones (T1K and T2K) in 1a, 1b in close proximity to each other. The triplet transition state for cleaving the C–N bond in 1a, 1b to form azido and benzyl radicals 1aB, 1bB is located only 3 kcal mol–1 (1 kcal = 4.184 kJ) above T1K, indicating that azido cleavage is feasible. The calculations place the energy of the triplet azido group (TA) in 1a, 1b ∼25 kcal mol–1 below T1K; thus, this process is also easily accessible via energy transfer. Further, the transition state barrier for TA to expel N2 and form triplet nitrenes is less than 1 kcal mol–1 above TA in 1a, 1b. Laser flash photolysis of 1a, 1b reveals the formation of the triplet excited ketones of 1a, 1b, which decay to form benzyl radicals 1aB, 1bB and triplet alkylnitrenes. The triplet ketones and the benzyl radicals are quenched with molecular oxygen at rates close to diffusion, whereas the triplet nitrenes react more slowly with oxygen (∼5 × 105 M–1 s–1). We conclude that the triplet alkylnitrenes intercept the benzyl radicals to form 2 in argon-saturated solution, whereas the benzyl radicals are trapped to form 4 in oxygen-saturated solution; thus, the triplet nitrenes react with oxygen to form 3.

Photoderivatized Polymer Thin Films at Quartz Crystal Microbalance Surfaces: Sensors for Carbohydrate−Protein Interactions

Photoderivatized polymer-coated gold surfaces have been developed following a perfluorophenylazide-based double ligation strategy. Gold-plated quartz crystal microbalance (QCM) crystals were initially covalently functionalized with a monolayer of poly(ethylene glycol) (PEG), using photo- or thermolytic nitrene formation and insertion. The polymer surfaces were subsequently used as substrates for photoinsertion of carbohydrate-derivatized photoprobes, yielding different recognition motifs for selective protein binding. The resulting robust and biocompatible sensor surfaces were applied to a flow-through QCM instrument for monitoring lectin-carbohydrate interactions in real time. The results clearly show the predicted lectin selectivity, demonstrating the applicability of the approach.

General Photo‐Patterning of Polyelectrolyte Thin Films via Efficient Ionic Bis(Fluorinated Phenyl Azide) Photo‐Crosslinkers and their Post‐Deposition Modification

AbstractWe have extended the well known bisfluorinated(phenyl azide) (bisFPA) methodology to develop an ionic bisFPA process suitable for photo‐crosslinking a wide variety of polyelectrolyte thin films. The crosslinking efficiencies (0.1–1.0 crosslink per photo‐reaction) are sufficiently high for the gel fraction to exceed 80 % for crosslinker concentrations of only a few weight %. This method is based on the photo‐induced formation of singlet nitrenes from FPAs and their insertion into unactivated C–H or other bonds, which thus general and not dependent on the presence of specific chemical functional groups. By derivatizing with ionic charge groups, we obtained ionic bisFPAs that can be properly dispersed into polyelectrolyte thin films. The sorbed moisture always present in these films however severely limits the photo‐crosslinking efficiency, apparently through nitrene protonation and intersystem crossing. This can be avoided by dehydration of the films, in some cases, to 130 °C for 10 min in nitrogen before photo‐exposure. We found that efficient photo‐crosslinking can then be achieved for polyelectrolytes even when they have nucleophilic groups. These include poly(styrenesulfonic acid) and their salts, poly(acrylic acid) and their salts, poly(dimethyldiallylammonium salts), as well as the electrically‐conducting poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonic acid) complex (PEDT:PSSH). We further demonstrate using this ionic bisFPA methodology both photo‐patterning and post‐deposition chemical modifications of polyelectrolyte thin films. This opens broad new possibilities in membrane, sensor and actuator technologies, as well as for organic semiconductor plastic electronics (such as field‐effect transistors) and polyelectrolyte‐based devices.

Early Events in the Photochemistry of Aryl Azides from Femtosecond UV/Vis Spectroscopy and Quantum Chemical Calculations

The photochemistry of para- and ortho-biphenylyl azides and 1-naphthyl azide was studied by ultrafast spectroscopy. In every case, the singlet azide second excited states were observed by transient absorption spectroscopy and were found to have lifetimes of hundreds of femtoseconds. The decay of the S(2) states of the azides was accompanied by the growth of transient absorption of the corresponding singlet nitrenes. The intermediate S(1) state of the azides could not be observed due to its low instantaneous concentration resulting from fast fragmentation and nitrene formation. Quantum chemical calculations predict that the S(2) state of the azide is bound and that there is a much lower barrier toward arylnitrene formation from the S(1) state of the azide. Vibrational cooling of para-biphenylnitrene (11 ps) was experimentally observed. The lifetime of singlet ortho-biphenylnitrene was 16 ps in acetonitrile and was not affected by perdeuteration of the aryl ring. The lifetime of singlet 1-naphthylnitrene is 12 ps in acetonitrile at ambient temperature.

Catalytic activation of nitrogen derivatives with transition-metal complexes

Abstract Studies regarding the transition-metal-catalyzed decomposition of nitrogen derivatives toward Curtius rearrangement and the formation of nitrenes which undergo C-H insertion and aziridination reactions are presented. These processes lead to the formation of C-N bond with a high level of selectivity and efficiency.

Photolysis of alpha-azidoacetophenones: trapping of triplet alkyl nitrenes in solution.

[reaction: see text] Selective excitation of the ketone chromophore in alpha-azidoacetophenones, 1, leads to intramolecular triplet energy transfer to the azido group, which forms the corresponding triplet alkyl nitrene, 2. Azides 1 also undergo alpha-cleavage to form benzoyl and methyl azido radicals in competition with nitrene formation. Thus the major photoproduct, 2-benzoylamino-1-phenylethanone, 3, comes from trapping of 2 with a benzoyl radical. This appears to be the first observation of bimolecular trapping of triplet alkyl nitrenes in solution.

Conformational changes in regioregular polythiophenes due to crosslinking

K. A. Murray, A. B. Holmes, S. C. Moratti and G. Rumbles, J. Mater. Chem., 1999, 9, 2109 DOI: 10.1039/A902683E

1H, 13C, 14N and 15N NMR Study of copolycondensates obtained by 1, 3 dipolar cycloaddition of α, ω‐diazidoalkanes with 1, 1'‐(methylene di‐4, 1‐phenylene) bismaleimide, with N, N'‐1, 4‐phenylene dimaleimide and with isopropylidene‐1, 4‐phenylene dimethacrylate

AbstractCopolycondensates of α, ω‐diazidoalkanes with 1, 1'‐(methylenedi‐4, 1‐phenylene) bismaleimide, with N, N'‐1, 4‐phenylene dimaleimide and with isopropylidene‐1, 4‐phenylene dimethacrylate were synthetized using 1, 3 dipolar cycloaddition reaction (DCA). Their structures were determined by 1H, 13C, 14N, 15N NMR. The results show that at high temperature (150°C), the copolycondensates present a mixture of triazoline/succinimide, of aziridine/succinimide, of enamine/maleimide and of anil/succinimide junctions. At low temperature (20°C), only the triazoline structure is formed. A reaction scheme is proposed via two reactions, DCA and nitrene formation, occuring simultaneously and independently.

Controlled pyrolysis of the new energetic binder azide polyester PAP‐G

AbstractThe degradation mechanism of the azide polyester PAP‐G (poly‐2,2‐bis(azidomethyl)‐1,3‐propandiol‐glutarate) on controlled pyrolysis differs from that of the polyether GAP (glycidyl azide polymer). The decomposition is more intensive, involves more of the carbon backbone and occurs at lower temperatures. As an energetic component in a propellant it tends to disintegrate at an early stage giving off cabonaceous material of the polymer structural backbone.Mixed metal oxides in the doped binder samples affect neither the reaction enthalpy nor the reaction products. However, they accelerate the nitrene formation by shifting the reaction interval to lower temperatures. Shifts up to 20 K were observed. In the case of PAP‐G the effect is most expressed by the additive MOVO whereas for GAP, in opposite, copper chromite is most effective.

The reaction between N(4S) and C2H3: Rate constant and primary reaction channels

The rate constant and the product branching ratios have been determined at T=298 K for the reaction between ground state atomic nitrogen (4S) and the vinyl radical (C2H3) at a nominal pressure of 1 Torr He. The kinetic technique employed was discharge-flow coupled to a collision-free sampling mass spectrometer. The rate constant was determined by monitoring the decay of the vinyl radical in the presence of excess [N], yielding a value for k(N+C2H3) of (7.7±2.9)×10−11 cm3 molecule−1 s−1. Three primary reaction channels have been experimentally observed: N+C2H3→C2H2+NH (1a), C2H2N+H (1b), and C2H3N (1c). The lowest energy isomers of the C2H2N radical and the C2H3N adduct molecule are CH2CN and CH3CN, respectively and their identification as products of the reaction is consistent with experimental results. Contributions from the higher energy isomers CH2NC or cyc-C2H2N in channel (1b) and CH3NC or H2C=C=NH in channel (1c) are not consistent with the experimental results and can be ruled out. Contribution from other higher energy isomers such as the radicals HC–CH=N, HC=C=NH and H2C=C=N in channel (1b) and the adduct species vinyl nitrene and 2H-azirine in channel (1c) cannot be ruled out in the absence of knowledge of heats of formation of the radical species and ionization energies for both the radical and adduct species. The following branching ratios were determined at T=298 K: Γ1a=0.16, Γ1c=0.04. No other potential products were detected. It can therefore be inferred that channel (1b) accounts for most if not all of the remaining products, i.e., Γ1b=0.80. The magnitude of the rate constant and the nature of the observed products for N+C2H3 are compared with those for the reactions N+CH3 and N+C2H5. The formation of the adduct molecule is considered in terms of initial formation of vinyl nitrene or 2H-azirine followed by a series of ring openings, ring closings, and an H atom transfer to yield the lowest energy isomer acetonitrile, CH3CN. The possible role of the N+C2H3 reaction in the atmospheric chemistry of Titan, Neptune, and Triton is briefly considered.

Laser photolytic studies on sensitizers for negative photoresists: 4,4′-diazido-3,3′-dimethoxybiphenyl in poly(methyl methacrylate) films

The time evolution of the transient absorption spectra of 4,4′-diazido-3,3′-dimethoxybiphenyl (DADMB)(N3—R—N3), a sensitizers for negative photoresists, under laser irradiation with low and high fluences has been investigated in poly(methyl methacrylate)(PMMA) films and in cyclohexane solution. The spectra were measured in the time region 20 ns–1200 s. The results from a DADMB–cyclohexane solution containing diethylamine indicated that the triplet azido nitrene (N3—R—N:) was predominantly generated even at room temperature, not didehydroazepine derivatives. The formation of the triplet azido nitrene in solution and in PMMA films was complete within the duration of the laser pulse. The triplet azido nitrene underwent dimerization in solution within 1 ms, producing an azo compound (N3—R—NN—R—N3). In contrast, a reaction related to the formation of azide nitrene in PMMA films continued up to 1200s. Laser irradiation with a high fluence gave the dinitrene (:N—R—N:). The dinitrene in solution underwent dimerization and/or further polymerization. The results for PMMA films heavily doped with DADMB suggested that DADMB molecules form aggregates in PMMA films: A photoreaction with a high fluence was complete within 300 s.

S-vinyl thio-oximes from thioketenes and benzylnitrene

Sterically hindered thioketenes when heated with benzyl azide gave S-vinyl thio-oximes in a sequence involving nitrene formation, [2 + 1] cycloaddition, and rearrangement.