Oxygen-saturated Acetonitrile Research Articles

ChemInform Abstract: Organic Synthetic Transformations Using Organic Dyes as Photoredox Catalysts

AbstractReview: 117 refs.

Comparison of the Photochemistry of 3-Methyl-2-phenyl-2H-azirine and 2-Methyl-3-phenyl-2H-azirine

Photolysis of 3-methyl-2-phenyl-2H-azirine (1a) in argon-saturated acetonitrile does not yield any new products, whereas photolysis in oxygen-saturated acetonitrile yields benzaldehyde (2) by interception of vinylnitrene 5 with oxygen. Similarly, photolysis of 1a in the presence of bromoform allows the trapping of vinylnitrene 5, leading to the formation of 1-bromo-1-phenylpropan-2-one (4). Laser flash photolysis of 1a in argon-saturated acetonitrile (λ = 308 nm) results in a transient absorption with λ(max) at ~440 nm due to the formation of triplet vinylnitrene 5. Likewise, irradiation of 1a in cryogenic argon matrixes through a Pyrex filter results in the formation of ketene imine 11, presumably through vinylnitrene 5. In contrast, photolysis of 2-methyl-3-phenyl-2H-azirine (1b) in acetonitrile yields heterocycles 6 and 7. Laser flash photolysis of 1b in acetonitrile shows a transient absorption with a maximum at 320 nm due to the formation of ylide 8, which has a lifetime on the order of several milliseconds. Similarly, photolysis of 1b in cryogenic argon matrixes results in ylide 8. Density functional theory calculations were performed to support the proposed mechanism for the photoreactivity of 1a and 1b and to aid in the characterization of the intermediates formed upon irradiation.

Vinylnitrene Formation from 3,5-Diphenyl-isoxazole and 3-Benzoyl-2-phenylazirine

Photolysis of 1 in argon-saturated acetonitrile yields 2, whereas in oxygen-saturated acetonitrile small amounts of benzoic acid and benzamide are formed in addition to 2. Similarly, photolysis of 2 in argon-saturated acetonitrile results in 1 and a trace amount of 3, whereas in oxygen-saturated acetonitrile the major product is 1 in addition to the formation of small amounts of benzoic acid and benzamide. Laser flash photolysis of 1 results in an absorption due to triplet vinylnitrene 4 (broad absorption with λ(max) at 360 nm, τ = 1.8 μs, acetonitrile) that is formed with a rate constant of 1.2 × 10(7) s(-1) and decays with a rate constant of 5.6 × 10(5) s(-1). Laser flash photolysis of 2 in argon-saturated acetonitrile likewise results in the formation of triplet vinylnitrene 4 but also ylide 5 (λ(max) at 440 nm, τ = 13 μs). The rate constant for forming 4 in argon-saturated acetonitrile is 1.6 × 10(7) s(-1). In oxygen-saturated acetonitrile, vinylnitrene 4 reacts to form the peroxide radical 6 (λ(max) 360 nm, ~0.7 μs, acetonitrile) at a rate of 2 × 10(9) M(-1) s(-1). Density functional theory calculations were performed to aid in the characterization of vinylnitrene 4 and peroxide 6 and to support the proposed mechanism for the formation of these intermediates.

Photocatalytic Monofluorination of Benzene by Fluoride via Photoinduced Electron Transfer with 3-Cyano-1-methylquinolinium

The photocatalytic fluorination of benzene occurs under photoirradiation of an oxygen-saturated acetonitrile (MeCN) of the 3-cyano-1-methylquinolinium ion (QuCN(+)) containing benzene and tetraethylammonium fluoride tetrahydrofluoride (TEAF·4HF) with a xenon lamp (500 W) attached to a colored-glass filter (λ < 290 nm) to yield fluorobenzene and hydrogen peroxide. The quantum yield of formation of fluorobenzene was 6%. Nanosecond laser flash photolysis measurements were performed to elucidate the mechanistic details for photocatalytic fluorination. Transient absorption spectra taken after the nanosecond laser excitation at 355 nm of a degassed MeCN solution of QuCN(+) and benzene exhibited absorption bands due to QuCN(•) (λmax = 500 nm) and the benzene dimer radical cation (λmax = 900 nm), which were generated by photoinduced electron transfer from benzene to the singlet excited state of QuCN(+). The decay rate of the transient absorption band due to the benzene dimer radical cation was accelerated by the addition of TEAF·4HF. The observed rate constant increased with increasing concentration of TEAF·4HF. The rate constant of the electrophilic addition of fluoride to the benzene radical cation was determined to be 9.4 × 10(9) M(-1) s(-1). Thus, the photocatalytic reaction is initiated by intermolecular photoinduced electron transfer from benzene to the single excited state of QuCN(+). The benzene radical cation formed by photoinduced electron transfer reacts with the fluoride anion to yield the F-adducted radical. However, QuCN(•) can reduce O2 to O2(•-), and this is followed by the protonation of O2(•-) to afford HO2(•). The hydrogen abstraction of HO2(•) from the F-adduct radical affords fluorobenzene and H2O2 as the final products.

Visible-Light-Induced Oxygenation of Benzene by the Triplet Excited State of 2,3-Dichloro-5,6-dicyano-p-benzoquinone

Photocatalytic oxygenation of benzene to phenol occurs under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in an oxygen-saturated acetonitrile solution of benzene and tert-butyl nitrite. The photocatalytic reaction is initiated by photoinduced electron transfer from benzene to the triplet excited state of DDQ.

Oxygenation and chlorination of aromatic hydrocarbons with hydrochloric acid photosensitized by 9-mesityl-10-methylacridinium under visible light irradiation

Efficient photocatalytic oxygenation of toluene occurs under visible light irradiation of 9-mesityl-10-methylacridinium (Acr+–Mes) in oxygen-saturated acetonitrile containing toluene and aqueous hydrochloric acid with a xenon lamp for 15 h. The oxygenated products, benzoic acid (70 %) and benzaldehyde (30 %), were formed after the photoirradiation. The photocatalytic reaction is initiated by intramolecular photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the Acr+ moiety of Acr+–Mes, which affords the electron-transfer state, Acr•–Mes•+. The Mes•+ moiety can oxidize chloride ion (Cl−) by electron transfer to produce chlorine radical (Cl•), whereas the Acr• moiety can reduce O2 to O 2 •− . The Cl• radical produced abstracts a hydrogen from toluene to afford benzyl radical in competition with the bimolecular radical coupling of Cl•. The benzyl radical reacts with O2 rapidly to afford the peroxyl radical, leading to the oxygenated product, benzaldehyde. Benzaldehyde is readily further photooxygenated to yield benzoic acid with Acr•–Mes•+. In the case of an aromatic compound with electron-donating substituents, 1,3,5-trimethoxybenzene, photocatalytic chlorination occurred efficiently under the same photoirradiation conditions to yield a monochloro-substituted compound, 2,4,6-trimethoxychlorobenzene.

Photocatalytic alkoxylation of benzene with 3-cyano-1-methylquinolinium ion

One-pot alkoxylation of benzene with alcohols occurs under photoirradiation of 3-cyano-1-methylquinolinium ion in an oxygen-saturated acetonitrile solution containing benzene and alcohols. The photocatalytic reaction mechanism was clarified by nanosecond laser flash photolysis. The photocataytic alkoxylation of benzene is initiated by photoinduced electron transfer from benzene to the singlet excited state of 3-cyano-1-methylquinolinium ion, followed by the nucleophilic addition of alcohols to benzene radical cation. In the case of photoethoxylation, the optimal product and quantum yields of ethoxybenzene were 20% and 10%, respectively.

A study on the reactions of NADH models with electron-deficient alkenes. A probe for the extreme of concerted electron-hydrogen atom transfer mechanism

The reactions of 9-fluorenylidenemalononitrile (FDCN) and 1,1-diphenyl-2,2-dicyanoethylene (DPCN) with Hantzsch ester (HEH), N-methyl Hantzsch ester (Me-HEH), and 1-benzyl-1,4-dihydronicotinamide (BNAH) in oxygen-saturated acetonitrile have been studied. The aerobic reactions with HEH give solely reduction products. However, reactions with Me-HEH and BNAH not only result in reduction products, but also give varying amounts of oxidation products. The amount of oxidation product appears to be related to the electronic character and bulkiness of reactants. We propose that all these reactions follow a general mechanism of concerted electron-hydrogen atom transfer mechanism. If the electron-transfer complex is very tight, only ‘concerted hydride transfer reaction’ occurs. However, if the electron-transfer complex is not so tight, oxygen can capture the radicaloid intermediate to result in oxidation products.

Mechanistic investigation on the reaction of 1,1-di-p-substituted phenyl-2,2-dinitroethylene with 1-benzyl-1,4-dihydronicotin-amide in oxygen saturated acetonitrile—clear evidence for intermediate mechanism

The reaction of 1-benzyl-1,4-dihydronicotinamide with a series of 1,1-di-p-substituted-phenyl-2,2-dinitroethylenes in oxygen-saturated acetonitrile produced various amounts of the corresponding ethanes and diaryl ketones according to the electronic structure of the substituent groups indicating a spectrum of intermediate mechanism between polar mechanism and SET mechanism.

9,10-Dicyanoanthracene photosensitized oxidation of aryl alkanols: evidence for an electron transfer mechanism

9,10-Dicyanoanthracene (DCA) photosensitizes the oxidation of a series of para substituted aryl alkanols in oxygen-saturated acetonitrile. Product analysis and Hammett correlations support an electron transfer mechanism for the title reaction.

Oxygenation of α-Methylstyrene with Molecular Oxygen, Catalyzed by 10-Methylacridinium Ion via Photoinduced Electron Transfer

Photooxygenation of α-methylstyrene with oxygen occurs efficiently in the presence of 10-methylacridinium perchlorate (AcrH+ClO4-) under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield acetophenone as the main oxygenated product. No photoinduced oxygenation of α-methylstyrene occurs in the absence of AcrH+ under otherwise the same experimental conditions. Little photodegradation of AcrH+ occurs in the present photocatalytic system, which provides a clean method of photoinduced oxygenation reaction with molecular oxygen, alternate to the ene reaction of singlet oxygen. The photocatalytic oxygenation of α-methylstyrene with oxygen is shown to proceed via photoinduced electron transfer from α-methylstyrene to the singlet excited state of AcrH+ (1AcrH+*) on the basis of the fluorescence quenching of 1AcrH+* by α-methylstyrene, the quantum yield determination, and the detection of radical intermediates by laser flash photolysis and ESR measurements.

Dehydrogenation vs Oxygenation in Photosensitized Oxidation of 9-Substituted 10-Methyl-9,10-dihydroacridine in the Presence of Scandium Ion

Photooxidation of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) with oxygen occurs efficiently in the presence of 9,10-dicyanoanthracene (DCA) and scandium triflate [Sc(OTf)3] under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield the 9-substituted 10-methylacridinium ion (AcrR+) and H2O2 or the 10-methylacridinium ion (AcrH+) and the oxygenated products of R such as ROOH, depending on the type of substitutent R. No DCA-photosensitized oxidation of AcrHR occurs in the absence of Sc3+ under otherwise the same experimental conditions. The observed selectivities for the C(9)−H vs C(9)−C bond cleavage of AcrHR in the DCA-photosensitized oxidation of AcrHR in the presence of Sc(OTf)3 agree with those for the cleavage of radical cations of AcrHR (AcrHR•+) depending on the type of substituent R. Such product selectivities, being consistent with the electron-transfer oxidation of AcrHR, combined with quantum yield determination, the 1O2 phosphorescence decay dynamics, and the dete...

Mechanism of reduction of 1,1-diphenyl-2,2-dinitroethylene by 1-benzyl-1,4-dihydronicotinamide: transition state with partial diradical and partial covalent bonding character

The mechanism of reduction of 1,1-diphenyl-2,2-dinitroethylene (DPDN) by 1-benzyl-1,4-dihydronicotinamide (BNAH) in acetonitrile has been investigated. Based on product analysis, isotopic tracing and electrochemical analysis, the reaction takes place by a hydride transfer mechanism giving 1,1-diphenyl-2,2-dinitroethane (DPDNH). Single crystal X-ray analysis shows that DPDN conforms to idealized C2 symmetry, but steric repulsions between the bulky substituents result in an appreciable twist about the central bond, such that the phenyl rings make a dihedral angle of 77.7°and the planar C–NO2 fragments make a dihedral angle of 68.5°. A small kinetic H/D isotope effect was obtained, that we propose is due to steric hindrance. Reaction in oxygen-saturated acetonitrile produced DPDNH and benzophenone in the ratio of 59.2∶22.0 as the final products with a total yield of 68.6% by GC. Control experiments were performed, by stirring a solution of DPDN or DPDNH alone in oxygen-saturated acetonitrile, or by stirring a solution of DPDN or DPDNH alone in aqueous acetonitrile containing a small amount of hydrochloric acid or in acetonitrile containing triethylamine. These produced no benzophenone. The results clearly indicate the trapping of a radical species by oxygen in the reaction. A curve-crossing model for the reaction projects that the transition state has partial diradical and partial covalent bonding character. As DPDN has a low-lying π* orbital (LUMO), the radical anion DPDN−· is a stabilized radical. It is known that the reaction of alkyl and benzyl radicals with oxygen is exothermic with a rate close to the diffusion-controlled limit. Thus, with use of More O’Ferrall’s two-dimensional potential energy diagram, the results are rationalized by a mechanistic change induced by steric hindrance so that the transition state collapses in two directions leading to the formation of DPDNH (polar pathway) and benzophenone (ET pathway), respectively.

Photooxidation of Dicyanoaurate(I) Induced by Metal-to-Ligand Charge Transfer Excitation

Abstract The irradiation of [Au(CN)2]- in oxygen-saturated acetonitrile leads to photooxidation of Au(I). In the presence of additional chloride [Au(CN)2CL2]- is formed with φ= 0.5 x 10-4 at λirr = 254 nm. It is assumed that [Au(CN)2]- in its metal-to-ligand charge transfer state undergoes an excited state electron transfer to oxygen in the primary photochemical step.

Sensitized Photochromism: Effect on Degradation Product Distribution

In the presence of a triplet sensitizer such as camphorquinone the photodegradation of spiro-indolinonaphthoxazines in oxygen-saturated acetonitrile solutions can be achieved with visible light. From the resulting degradation product distribution it becomes evident that the formation of 3,3-dimethyloxindole and its derivatives is enhanced when compared to UV-induced photolysis, thus suggesting that these compounds originate from the spiro-indolinonaphthoxazine promoted to the first excited triplet state by energy transfer from the sensitizer.

Electron-transfer-induced photo-oxygenation reactions and the special salt effect: determination of efficiencies of free radical ion formation from solvent-separated radical ion pairs in oxygen-saturated acetonitrile solutions

The special salt effect exerted by LiClO 4 (LP) on the rate of dicyanoanthracene (DCA)-photo-induced electron transfer oxygenation of tetraarylcyclopropane ( 1) and 1,1-diarylethylenes ( 3a–d) in acetonitrile, was used to determine several important parameters of the solvent-separated radical ion pairs (SSIP): (1) the efficiency α o of dissociation into solvated free radical ions, (2) the ratio of rate constants for the separation of SSIP and the back electron transfer within the SSIP ( k sep/ k back) and (3) the ratio of rate constants for scavenging DCA − from SSIP by LP (or Li +) and the back electron transfer ( k L/ k back). The efficiencies α o turned out to be relatively large, i.e. 0.66 for 1, and, increasing with increasing oxidation potentials, between 0.38 for 3a and 0.8 for 3d. Based on the assumption that k L equals the diffusion-controlled rate constant k diff in MeCN, k sep = 1.7 × 10 7 s −1 and k back = 8 × 10 6 s −1 were estimated for 1. Within the limits of error, k sep = (8±2) × 10 7 s −1 was obtained for substrates 3a–d, whereas k back was found to decrease from 1.64 × 10 8 s −1 for 3a to 2 × 10 7 s −1 for 3d, indicating that back electron transfer, associated with Δ G back values of −2.2 to −2.8 eV, increases with decreasing energy content of the SSIP. This result represents another example for back electron transfer occurring in the Marcus ‘inverted region’. However, by assuming k L ⪕ k diff, the absolute values of k back are about one to two orders of magnitude smaller than those obtained by using the generally accepted value of 5 × 10 8 s −1 for k sep. To explain the lower values of k sep and k back, it is proposed that the encounter distance for electron transfer with the electron donors used in this study may be somewhat larger (about 7–10 Å) than the distance (of about 6.5–7.5 Å) usually assumed for these processes with less bulky electron donors.

Hydroxyanthraquinones as sensitizers of electron-transfer-induced photo-oxygenation reactions

Hydroxy-substituted 9,10-anthraquinones (1-AQ, 2-AQ, 1,2-AQ, 1,4-AQ, 1,8-AQ, 2,6-AQ, 3-Me-1,6,8-AQ and 1,2,5,8-AQ) may serve as photosensitizers of electron-transfer-induced reactions with suitable electron-donor substrates. In oxygen-saturated acetonitrile solution, 1,1-bis(4-methoxyphenyl)-2,2-diphenylcyclopropane ( 1 ) and 1,1-bis(4-methoxyphenyl)- trans -2,3-diphenylcyclopropane ( 3 ) undergo exclusively electron-transfer-induced oxygenations yielding dioxolanes 2 and cis - 4 (60%) plus trans - 4 (40%) respectively. All the hydroxy-AQs examined sensitize the reaction with 1 , but only 2-AQ, 2,6-AQ and 3-Me-1,6,8-AQ are able to sensitize the reaction with 3 . Using trans -2,3-bis(4-methoxyphenyl)-1,1-diphenylcyclopropane ( 5 ) as substrate, hydroxy-AQs photosensitize the electron-transfer-induced rearrangement of 5 to propene 6 rather than the oxygenation of 5 to a dioxolane derivative. Using tetraphenylethylene ( 7 ) as an ethylenic substrate that is unreactive towards O 2 ( 1 Δ g ), only 2- AQ, 2,6-AQ and 3-Me-1,6,8-AQ were able to sensitize the electron-transfer-induced oxygenation of 7 which yields benzophenone ( 8 ) as the main product (85%). With 1,1-bis(4-methoxyphenyl)ethylene ( 11 ) as substrate, which reacts rather sluggishly with O 2 ( 1 † g ) to give 4,4′-dimethoxybenzophenone ( 13 ), the electron-transfer-induced oxygenation reaction proceeds with an appreciable efficiency to give 3,3,6,6-tetra(4-methoxyphenyl)-1,2-dioxane ( 12 ). All of the hydroxy-AQs examined serve as sensitizers, but the product ratio 12:13 depends on the hydroxy- AQ used. With efficient O 2 ( 1 Δ g ) acceptors such as cis - and trans -α,α′-dimethylstilbenes ( 14, 15 ), electron-transfer- induced oxygenation, leading to products 16–18 , has almost no chance to compete with the O 2 ( 1 Δ g ) reaction, when hydroxy-AQs are used as sensitizers. Within the limits of proton nuclear magnetic resonance ( 1 H NMR) detection, only the O 2 ( 1 † g ) product, allylic hydroperoxide 19 , is observed. The mechanisms involved are discussed.

The protection of iron against corrosion with polymer films prepared by cathodic polymerization of halogenated xylenes

Polymer films were prepared by cathodic polymerization of halogenated xylenes at an iron electrode in a non-aqueous electrolyte solution. The ability of the films to protect against corrosion was examined in an aerated 0.5 M Na2SO4 solution by polarization measurements of an iron rotating disk electrode uncoated and coated with the polymers. The best protection of the electrode was obtained with a polymer film prepared by cyclic sweeps of potential in an oxygen-saturated acetonitrile solution of α,α′-dibromo-p-xylene. Mechanisms of the corrosion protection were investigated on the basis of the Koutecky-Levich equation. Diffusion of oxygen in the aqueous bulk solution was not a rate-determining step in the corrosion process of the iron electrode coated with the polymer film. Rate constants of anodic and cathodic charge transfer reactions and diffusion coefficients of oxygen and ferrous ion through the film, obtained by computer simulation of the polarization curves, provide evidence for the protective mechanisms.

Mechanisms of photo-oxidation of NADH model compounds by oxygen

When an oxygen-saturated acetonitrile (MeCN) solution containing the NADH model compound, 9,10-dihydro-l0-methylacridine (AcrH2), is irradiated with u.v. light firstly in the presence and secondly in the absence of perchloric acid (HClO4), AcrH2 is oxidized by oxygen to yield the 10-methylacridinium ion (AcrH+) and 10-methyl-9-acridone (AcrO) respectively, and reduction of oxygen yields hydrogen peroxide. The u.v. irradiation of a neutral aqueous solution containing 1-benzyl-1,4-dihydronicotinamide (BNAH) also results in the oxidation of BNAH by oxygen to yield BNA+ and H2O2. Kinetic studies and detection of radical intermediates by e.s.r. spectroscopy have revealed that the photo-oxidation of NADH model compounds by oxygen proceeds via radical-chain reactions, which are initiated by electron transfer from the singlet excited state of NADH model compounds to oxygen. The chain carrier for the photo-oxidation of AcrH2 in the presence of HClO4 in MeCN as well as for the photo-oxidation of BNAH in a neutral aqueous solution is hydroperoxy radical (HO2˙), while acridinylperoxy radical (AcrO2˙) acts as a chain carrier for the photo-oxidation of AcrH2 in the absence of HClO4, in MeCN.Although no photo-oxidation of AcrH2 by oxygen occurs under visible-light irradiation, AcrH2 is efficiently oxidized to AcrH+ in the presence of a flavin analogue used as a sensitizer, in MeCN under otherwise identical conditions. The flavin-sensitized photo-oxidation of AcrH2 by oxygen proceeds via one-electron reduction of a flavin analogue by AcrH2 followed by the efficient oxidation of the reduced flavin by oxygen without the appreciable contribution of radical-chain processes.

Electrochemical oxidation of 2,2,6,6-tetramethylpiperidines in acetonitrile: mechanism of N-cyanomethylation

Electrochemical oxidation of 2,2,6,6-tetramethylpiperidine (1a), its 4-oxo derivative (1b), and 2,2,6,6-tetramethylmorpholine (1c) in deoxygenated acetonitrile gave the corresponding N-cyanomethylated products (2). The process was investigated by cyclic voltammetry, controlled-potential electrolysis, and electrolysis in the cavity of an e.s.r. spectrometer. The sequence of cyanomethylation is proposed to be as follows: one-electron transfer from (1) to generate the radical cation, [graphic omitted]H+˙(3); deprotonation of (3) to give the aminyl radical, [graphic omitted]: (4); hydrogen abstraction by (4) from the solvent to afford the cyanomethylene radical; cross-coupling of (4) with the solvent-derived radical. The generation of the radical (4) was confirmed by the e.s.r. experiments. The decay of the 4-oxopiperidinyl radical (4b) obeyed first-order kinetics and exhibited a large primary deuterium isotope effect in [2H3]acetonitrile, indicating that the hydrogen abstraction is rate-determining. The voltammetric behaviour of (1) in oxygen-saturated acetonitrile suggested that the aminyl radical (4) is oxidized at a potential more positive than is the starting amine (1).