Kinetic features of photolysis of phenols in polymer matrices

The low quantum yields of photoconversion of phenols and the low probability of formation of phenoxy radicals in solid polymers indicate significant cage effects. An increase in the degree of shielding of the hydroxyl group results in an increase, while incorporation of electron-acceptor substituants in the p position results in a decrease in the reactivity of the phenols in the polymers. The increase in the quantum yields of photoconversion and the decrease in the quantum yields of the fluorescence of phenols with a decrease in the wavelength are due to the occurrence of a reaction and induced radiationless deactivation with the participation of higher excited states.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 18–23, January, 1990.We would like to thank G. A. Nikiforov for providing phenols (I)–(V).     

Direct transition-metal-free intramolecular arylation of phenols

Direct transition-metal-free, base-mediated intramolecular arylation of phenols with aryl halides has been developed. In the presence of 2.5 equiv of t-BuOK in dioxane at 140 degrees C, the intramolecular cyclization of 3-(2-halobenzyloxy)phenols affords 6H-benzo[c]chromenes in high yields. This reaction proceeds by an initial formation of a benzyne intermediate followed by an aromatic sp(2) C-H functionalization (a formal C-H activation) to form the carbon-carbon bond.     

Direct synthesis of 3-fluoroalkyl-2,6-diethoxycarbonyl phenols

Cyclocondensation of α-fluoroalkyl ketones or α-fluoroalkyl aldehyde (1) and diethyl 1,3-acetonedicarboxylate (2) affords 3-fluoroalkyl-2,6-diethoxycarbonyl-5-alkyl-phenols (3) in good yields. This provides a novel method for the preparation of meta-fluoroalkyl polysubstituted phenols.     

Direct photolysis of acetyl benzoyl peroxide and benzoyl peroxide

Conclusion The quantum yields were found for the photodissociation of the O-O bond for acyl peroxides (0.12–0.14). In the framework of our assumptions, the increase in with increasing peroxide concentration may be related to conversion of the triplet excimer.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2815–2817, December, 1986.     

Direct perfluoroalkylation of non-activated aromatic C-H bonds of phenols

A simple procedure for the perfluoroalkylation of the aromatic ring of phenols under mildly basic conditions is described. Treatment of a variety of phenols with perfluoroalkyl iodide in the presence of the radical initiator V-70L and Cs₂CO₃ provided the corresponding perfluoroalkylated products in moderate to good yields. Generally, the reaction proceeded smoothly at room temperature to yield regioselectively perfluoroalkylated products.     

Photodegradation of polyacetaldehyde in solution. I. Direct photolysis

The quantum yields measured for the random scission of main-chain bonds of polyacetaldehyde in benzene solution by 254 nm irradiation at 298°K are on the order of 10^?3–10^?4 scissions per quantum absorbed by the polymer. The quantum yields are unaffected by oxygen but are dependent upon initial polymer concentration and upon the inverse square root of absorbed light intensity. The direct photochemistry involves excitation of carbonyl chromophores in their ¹nπ^* bands, and a Norrish type I process is believed to be responsible for subsequent free-radical random-chain scissions. It has been established that direct photoexcitation processes do not lead to sequential depolymerization of polyacetaldehyde to monomer.     

Direct asymmetric aldol reaction catalyzed by simple prolinamide phenols

Simple prolinamides 1a–f were synthesized, and their catalytic effects on the direct asymmetric aldol reactions in organic solvents and in water were evaluated. Prolinamide phenols 1a–d were found to be effective catalysts for the reaction of aromatic aldehydes with cyclohexanone in neat ketone and in water. The anti-aldol products were obtained with up to 98/2 anti/syn ratio and 96% ee in neat ketone, 98/2 anti/syn ratio and 99% ee in water, respectively.     

Near-UV photolysis of substituted phenols, I: 4-fluoro-, 4-chloro- and 4-bromophenol

The experimental techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance-enhanced multiphoton ionisation time-of-flight spectroscopy have been used to investigate the dynamics of H atom loss processes from gas phase 4-fluorophenol (4-FPhOH), 4-chlorophenol (4-ClPhOH) and 4-bromophenol (4-BrPhOH) molecules, following excitation at many wavelengths, lambda(phot), in the range between their respective S(1)-S(0) origins (284.768 nm, 287.265 nm and 287.409 nm) and 216 nm. Many of the Total Kinetic Energy Release (TKER) spectra obtained from photolysis of 4-FPhOH show structure, the analysis of which reveals striking parallels with that reported previously for photolysis of bare phenol (M. G. D. Nix, A. L. Devine, B. Cronin, R. N. Dixon and M. N. R. Ashfold, J. Chem. Phys., 2006, 125, 133318). The data demonstrates the importance of O-H bond fission, and that the resulting 4-FPhO co-fragments are formed in a select fraction of their available vibrational state density. All spectra recorded at lambda(phot)> or = 238 nm show a feature centred at TKER approximately 5500 cm(-1). These H atom fragments show no recoil anisotropy, and are rationalised in terms of initial S(1)<-- S(0) (pi* <--pi) excitation and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S(0)) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer to (and round) the Conical Intersection (CI) between the S(0) and S(2)((1)pi sigma*) Potential Energy Surfaces (PESs) at larger R(O-H), en route to H atoms and ground state 4-FPhO products. The vibrational energy disposal in the 4-FPhO products indicates that parent mode nu(16a) promotes non-adiabatic coupling at the S(0)/S(2) CI. Spectra recorded at lambda(phot)< or = 238 nm reveal a faster (but still isotropic) distribution of recoiling H atoms, centred at TKER approximately 12 000 cm(-1), attributable to H + 4-FPhO products formed when the optically excited (1)pi pi* molecules couple directly with the (1)pi sigma* PES. Parent mode nu(16b) is identified as the dominant coupling mode at the S(1)((1)pi pi*)/S(2)((1)pi sigma*) CI, and the resulting 4-FPhO radical co-fragments display progressions in nu(18b) (the C-O in-plane wagging mode) and nu(7a) (an in-plane ring breathing mode involving significant C-O stretching motion). Analysis of all structured TKER spectra yields a C-F bond dissociation energy: D(0)(H-OC(6)H(4)F) = 29 370 +/- 50 cm(-1). The photodissociation of 4-ClPhOH shows many similarities, though the 4-ClPhO products formed together with faster H atoms at shorter wavelengths (lambda(phot)< or = 238 nm, by coupling through the S(1)/S(2) CI) show activity in an alternative ring breathing mode (nu(19a) rather than nu(7a)). Spectral analysis yields D(0)(H-OC(6)H(4)Cl) = 29 520 +/- 50 cm(-1). H atom formation via O-H bond fission is (at best) a very minor channel in the photolysis of 4-BrPhOH at all wavelengths investigated. Time-dependent density functional theory calculations suggest that this low H atom yield is because of competition from the alternative C-Br bond fission channel, and that the analogous C-Cl bond fission may be responsible for the weakness of the one photon-induced H atom signals observed when photolysing 4-ClPhOH at longer wavelengths.     

Direct photolysis and electron transfer photooxygenation of enol acetates of 3-phenylpropiophenones

Summary Direct photolysis of enol acetates of 3-phenylpropiophenones1a–c gives rise to the parent propiophenones2a–c and the 1,3-acyl shift products3a–c. By contrast, 2,4,6-triphenylpyrylium tetrafluoroborate sensitized photolysis of substrates1a–c affords the -acetyloxypropiophenones7a–c as the most general products. These results have been rationalized according to the generation of radical pairs in the direct photolysis and radical cations in the photoinduced electron transfer processes.

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Direkte Photolyse und Flektron-Transfer-Photooxygenierung von Enolacetaten von 3-Phenylpropiophenonen

Zusammenfassung Die direkte Photolyse von Enolacetaten der 3-Phenylpropiophenone1a–c ergab die zugrundeliegenden Propiophenone2a–c und die 1,3-acyl-verschobenen Produkte3a–c. Im Gegensatz dazu ergab die 2,4,6-Triphenylpyrylium-tetrafluorborat-sensitivierte Photolyse der Substrate1a–c die -Acetyloxypropiophenone7a–c als generelle Produkte. Diese Ergebnisse sind mit der Erzeugung von Radikalpaaren bei der direkten Photolyse und der Bildung von Radikalkationen beim photoinduzierten Elektronen-Transfer-Prozess zu erklären.

     

Direct photolysis of chlorophenols in aqueous solutions by UV radiation from excilamps

Direct photolysis of 2- and 4-chlorophenol and 2,4-dichlorophenol in aqueous solutions by UV radiation from XeBr (282 nm) and KrCl (222 nm) excilamps at various pH values was subjected to a comparative study.     

Near-UV photolysis of substituted phenols. Part II. 4-, 3- and 2-methylphenol

The photodissociation of jet-cooled 4-, 3- and 2-methylphenol molecules has been investigated using the experimental techniques of resonance enhanced multiphoton ionisation and H (Rydberg) atom photofragment translational spectroscopy. O-H bond fission is found to occur, via a repulsive (1)pisigma state, in a manner analogous to that occurring in phenol and 4-fluorophenol. Excitation to the (1)pipi manifold results in H-atom loss either directly (via a (1)pipi/(1)pisigma conical intersection) or indirectly, following internal conversion to the ground state and subsequent coupling to the (1)pisigma state via a second conical intersection at extended O-H bond lengths. The resulting methylphenoxyl radicals are created with specific vibrational excitation, reflecting the nuclear distortions required to access the (1)pisigma potential energy surface and the geometry changes induced by subsequent H atom loss. The position of the methyl group on the benzene ring is observed to influence the product vibrational energy disposal-not least through its influence on the mode(s) that are activated as a result of coupling to the repulsive (1)pisigma state. O-H bond strengths are reported for 4-, 3- and 2-methylphenol. These are in good agreement with values derived from recent combustion calorimetry studies and serve to highlight the relative destabilisation of the radical caused by methyl substitution at the 3-position.     

Direct detection of a transient oxenium ion in water generated by laser flash photolysis

Laser flash photolysis of the quinol ester 2b in O2-saturated aqueous phosphate buffer at pH 7.1 with excitation at 266 nm generates a transient intermediate with lambda(max) 460 nm that decays in a first-order manner with an aqueous solution lifetime of (170 +/- 10) ns at 22 degrees C. This intermediate is not affected by O2, but reacts rapidly with N3- with an apparently diffusion-limited rate constant of (6.6 +/- 0.2) x 10(9) M-1 s-1. Steady state photolysis of 2b yields the quinol 3b as a major reaction product with a yield of ca. 30-35% after correction for photolytic decomposition of 3b. This is the same product that is quantitatively produced by hydrolysis of 2b in the dark. Photolysis of 2b in the presence of 40 mM N3- completely suppresses the yield of 3b The photolytic intermediate is identified as the aryloxenium ion 1b, that was previously indirectly detected by N3--trapping during the hydrolysis of 2b, based on the chemical behavior of the transient and the quantitative agreement of the N3-/solvent selectivity ratio, kaz/ks, measured directly during the flash photolysis experiment, and indirectly by the azide clock procedure during the hydrolysis reaction. Other, as of yet unidentified, transients are produced during the photolysis reaction. A strong transient absorbance band observed at 360 nm decays in a biphasic manner with two first-order rate constants, neither of which are affected by O2 or N3-. The lifetimes of the two intermediates of ca. 12 and 75 mus are considerably longer than that of 1b. Another very short-lived species can be detected at early reaction times (相似文献   

Direct synthesis of phenols by iron-catalyzed biphasic oxidation of aromatic hydrocarbons with hydrogen peroxide

The hydroxylation of a series of aromatic hydrocarbons with hydrogen peroxide, catalyzed by iron complexes with pyrazine-3-carboxylic acid N-oxide, was investigated, operating in a biphasic reaction medium.

The new catalyst showed a high selectivity to the corresponding phenols, minimizing the over-oxidation reactions to polyoxygenated derivatives and tars which, along with dimers formation, are the major limitations of the classical Fenton’s reagent for a practical synthetic application.

In the case of alkylbenzenes, the competitive side chain oxidation at the benzylic positions also occurred. Electron rich substrates, such as anisole, were oxidized with very poor selectivity.

The reactions were carried out in a biphasic system that allows a convenient recovery and recycling of the catalyst by phase separation techniques. The catalyst showed a complete retention of activity after six consecutive reaction cycles.

The new catalyst appears as a promising tool for the direct synthesis of phenols, in alternative to the conventional multi-step methods.     

Direct synthesis of unprotected phenols using palladium-catalysed cross coupling reactions of functionalised organozinc reagents

Palladium-catalysed reaction of unprotected 2-, 3-, and 4-iodophenols with a range of amino acid derived organozinc reagents (not used in excess) gives the expected products in good to excellent yield, demonstrating that carbon-zinc bonds are not protonated by acidic phenols under the conditions of palladium-catalysed coupling reactions.     

Direct synthesis of enaminone functionalized biaryl ethers by CuI-catalyzed O-arylation of enaminone functionalized phenols

The O-arylation of o-enaminone functionalized phenols, namely, (E)-3-(dimethylamino)-1-(2-hydroxyphenyl)prop-2-en-1-ones, has been achieved via a self-promoted process in the presence of CuI, which provided a class of new biaryl ethers bearing a reactive enaminone fragment. The reactions were performed under mild conditions and the functionalized biaryl ether products have been found as useful building blocks for the assembly of heterocyclic compounds.     

Direct observation of spin-forbidden formation of O(D) in the near-UV photolysis of ozone

Time-of-flight measurements have been made of the O(¹D) fragment following the photolysis of ozone in the near-UV. At 321.9 nm fragments are seen with kinetic energies the values of which are those expected from spin-forbidden dissociation with ground state O₂(X³Σ_g⁻) molecules as the co-product. Spin-allowed dissociation of internally excited ozone molecules is also seen to produce translationally cold O(¹D) and O₂(a¹Δ_g) products. The implications for the dissociation of tropospheric ozone are briefly discussed.     

Direct conversion of aryl halides to phenols using high-temperature or near-critical water and microwave heating

The direct conversion of aryl halides to the corresponding phenols has been achieved using microwave heating. High-temperature or near-critical water is used as the solvent in conjunction with a copper catalyst and a mineral base.     

Direct and sensitized photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase

The direct photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase at 280–300 nm produces mainly 1,3,5-cyclooctatriene and benzene plus ethylene. The yield of the former product is enhanced by added gases, and it is proposed that it is formed in a vibrationally excited state which can revert to bicyclooctadiene unless the excess energy is removed in collisions. Computer modelling of the direct photolysis yielded quantitative agreement with the experimental results, but only when large, arbitrary adjustments were made to the calculated rate constants for the interconversion of cyclooctratriene and bicyclooctadiene. The Hg(6³P₁) sensitized reaction of bicyclooctadiene produces mainly benzene plus ethylene, a process which is also enhanced by added gases.