Diphenyl Ketyl Research Articles

Sensitized photoreduction of selected benzophenones. Mass spectrometry studies of radical cross-coupling reactions

The hydrogen atom transfer reaction (HAT) between selected benzophenones (benzophenone BP, 3-carboxybenzophenone 3CB, and 4-carboxybenzophenone 4CB) and 2-propanol was reinvestigated focusing on stable product analysis. As expected, the primary species of these HAT's are the respective diphenyl and dimethyl ketyl radicals that eventually undergo several radical coupling reactions leading to stable photoproducts. However, the mechanisms of these free radical reactions remain unclear and open to question. In this report, we focus on the detailed analysis of the stable photoproducts of these reactions using liquid chromatography coupled with high-resolution mass spectrometry (LC-ESI-QTOF-MS/MS). Products of photopinacolization (benzpinacol and two diastereoisomers of 4CB and 3CB dimers) and isomeric radical cross-coupling adducts of respective diphenyl and dimethyl ketyl radicals were separated chromatographically, and their structures were determined by high-resolution MS/MS, and the mechanisms of the reactions are discussed.

ChemInform Abstract: The Electron Transfer Reactions of Cyano Substituted Pyridines and Quinolines with Thermally Generated Diphenyl Ketyl.

Electron transfer reactions of cyano substituted pyridines and quinolines with thermally generated diphenyl ketyl are reported. When E1/2(-) for the heterocycle is less negative than -2V, electron transfer occurs from the ketyl to the heterocyclic base. In some cases the products obtained from the thermal reaction are the same as those obtained from the photochemical reaction with benzophenone and alcohols. In other cases different products are formed. Two bases, 2-pyridinecarbonitrile and 2,4-pyridinedicarbonitrile, undergo regiospecific reactions in which the course of the reaction is determined by the acidity of the medium. A mechanism in which the heterocyclic nitrogen is involved in the substitution process at the 2 position is proposed.

Time-Resolved Resonance Raman and Density Functional Theory Investigation of the Photoreactions of Benzophenone in Aqueous Solution

A time-resolved resonance Raman study of the photoreactions of benzophenone (BP) in neutral, alkaline, and acidic aqueous solutions is reported. Under neutral and alkaline conditions, a hydrogen abstraction reaction is observed to form the corresponding diphenyl ketyl (DPK) radical with different reaction rates while under acidic condition the protonation of triplet benzophenone occurs simultaneously, which is followed by a faster hydration reaction to produce the corresponding photohydration short-lived intermediates that were observed during experiments and that were tentatively assigned to the m-(3)BP x H(2)O and the o-(1)BP x H(2)O species. The dynamics, structures, and properties of these species are briefly discussed.

Time-Resolved Resonance Raman Identification and Structural Characterization of a Light Absorbing Transient Intermediate in the Photoinduced Reaction of Benzophenone in 2-Propanol

A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic study of the triplet state benzophenone reaction with the 2-propanol hydrogen-donor solvent and subsequent reactions is presented. The TR3 spectra show that the benzophenone triplet state (npi*) hydrogen-abstraction reaction with 2-propanol is very fast (about 10 to 20 ns) and forms a diphenylketyl radical and an associated 2-propanol radical partner. The temporal evolution of the TR3 spectra also indicates that recombination of these two radical species occurs with a time constant of about 1170 ns to produce a LAT (light absorbing transient) intermediate that is identified as the 2-[4-(hydroxylphenylmethylene)cyclohexa-2,5-dienyl]propan-2-ol (p-LAT) species. Comparison of the TR3 spectra with results obtained from density functional theory calculations for the species of interest was used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the TR3 spectra. The structures and properties of the reaction intermediates observed (triplet benzophenone, diphenyl ketyl radical, and p-LAT) are briefly discussed.

Enhancement of Chain Amplification in Photoreactions of N-Methoxypyridinium Salts with Alcohols†

Recently we reported a chain-amplified photochemical reaction, initiated by electron transfer from an excited sensitizer to N-methoxypyridinium salts, which leads to N-O bond cleavage (26). Hydrogen atom abstraction by the methoxy radical from an alcohol yields an alpha-hydroxy radical, which reduces another N-methoxypyridinium molecule and propagates the chain. We now report that the chain amplification can be significantly enhanced in the presence of water. Detailed kinetic studies of the reaction of 4-cyano-N-methoxypyridinium salt (CMP) with benzhydrol (BH) showed that the rate constant for reduction of CMP by the diphenyl ketyl radical (1.1 x 10(6) M(-1) s(-1)) increases by more than an order of magnitude in the presence of water. This increase in the rate constant is the result of coupling of the electron transfer to a proton transfer from the ketyl radical to water, which decreases the endothermicity of the reaction. Unfortunately, this increase in the rate constant for one of the two propagation steps is accompanied by a larger increase in the rate constant(s) of the competing termination reaction(s) of the ketyl radical. The observed enhancement in chain amplification is the result of a significant increase in the ratio of propagation to termination rate constants of the reactions of the methoxy radical. The main chain-terminating reactions of the methoxy radical are deuterium abstraction from the solvent, CD(3)CN, and reaction with the sensitizer, thioxanthone. The effect of increase in the ratios of the propagation rate constant of the methoxy radical (hydrogen abstraction from BH) to those of both termination reactions is larger than the unfavorable effect of water on the reactions of the ketyl radical. The increase in chain amplification depends on the concentration of the reactants; at 0.037 M of both reactants, the quantum yield increases form approximately 16 to approximately 45 in the presence of <1% water. The reaction of 4-phenyl-N-methoxypyridinium (PMP) with 4-methoxybenzyl alcohol does not proceed via chain amplification because of large endothermicity for electron transfer from the alpha-hydroxy radical to the pyridinium salt. However, chain amplification could be induced, simply by addition of water, where at approximately 10% water content, a quantum yield of approximately 5 was obtained. Water-induced, proton-coupled electron transfer increases the rate constant for reduction of PMP from a negligible level to becoming the dominant path.

Monitoring the Formation and Decay of Transient Photosensitized Intermediates Using Pump-Probe UV Resonance Raman Spectroscopy. II: Kinetic Modeling and Multidimensional Least-Squares Analysis

Analysis of transient excited-state Raman spectra is a challenging spectroscopic measurement since transient spectral features are often overlapped with dominant ground-state and solvent bands. In the previous manuscript, resolution of component Raman spectra from the time-resolved amine quenching of excited-triplet benzophenone was accomplished using self-modeling curve resolution, a model-free factor analysis technique that relies on correlation in the data along a changing composition dimension. The results are consistent with the production of diphenylketyl radicals by H-atom abstraction from the amine and subsequent free-radical decay by recombination reactions. A kinetic model for this chemistry is developed in the present work, based on the observed Raman scattering data and the structures of product species confirmed by mass spectral analysis. The model is applied to the analysis of the time-dependent Raman scattering data using multidimensional least-squares methods, and it yielded well-resolved spectra of benzophenone excited-triplet states, diphenyl ketyl radical, and the solvent and ground-state precursors. The best-fit kinetic parameters agree well with the time-dependent triplet-state and ketyl-radical concentration profiles.

Photograft polymerization of acrylate monomers and macromonomers on photochemically reduced PTFE films

The objective of this study was to explore and optimize the photograft polymerization of acrylate monomers and macromonomers upon premodified poly(tetrafluoroethylene) (PTFE) substrates, with a view toward applying these grafted materials as biomaterials. PTFE films were photochemically reduced by exposure to short wavelength ultraviolet (UV) light in the presence of the reducing agents, i.e., diphenyl ketyl radical anion, generated from a solution of benzophenone and sodium hydride in dimethylformamide (Noh et al., J. Polym. Sci. A, Polym. Chem., 35, 1499 (1997)). The photochemically reduced PTFE films were subsequently modified by photograft polymerization of acrylic acid, acrylamide, and poly(ethylene glycol) monoacrylate (PEG-Ac) by employing long wavelength UV irradiation with benzil dimethyl ketal as an initiator. The photochemically reduced PTFE films were good substrates for photograft polymerization, leading to surfaces with chemical compositions close to those of the respective homopolymers as determined by electron spectroscopy for chemical analysis. Photograft polymerization of PEG-Ac on the photochemically reduced surface was also explored with different concentrations (1, 2, 4, 6, and 8%; v/v) and molecular weights (200, 400, 1000, and 2000 g/mol) of PEG-Ac, different durations (5, 10, and 20 min) of long wavelength UV irradiation, and different concentrations (0.1, 0.2, 0.4, and 0.8%; v/v) of benzil dimethyl ketal. Higher concentrations and lower molecular weights of PEG-Ac generally led to more extensive photograft polymerization, whereas the concentration of benzil dimethyl ketal and duration of UV irradiation did not substantially influence the extent of photograft polymerization. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3467–3482, 1997

Surface modification of poly(tetrafluoroethylene)with benzophenone and sodium hydride by ultraviolet irradiation

Poly(tetrafluoroethylene) (PTFE) films were surface modified in a solution of benzophenone and sodium hydride in dry dimethylformamide by ultraviolet (UV) light irradiation. The extent of surface modification was characterized after durations of UV light irradiation from 5–20 min at temperatures from 19–60°C. The modified films were analyzed by electron spectroscopy for chemical analysis, diffuse reflectance ultraviolet-visible light spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, dynamic contact angle measurement, and low-voltage scanning electron microscopy. PTFE surfaces produced by this modification demonstrated extensive defluorination, oxygen incorporation, surface unsaturation, and reduction in both advancing and receding dynamic water contact angles in a manner that was more extensive at long durations of irradiation and at high temperatures. Morphological damage depended upon treatment conditions, but extensive surface modification could be obtained without substantial morphological damage to PTFE films. Control experiments indicated that the surface modification proceeded by photoexcitation of either diphenyl ketyl radical anion or benzhydrol anion, the products of reaction of benzophenone with sodium hydride. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1499–1514, 1997

Surface modification of poly(tetrafluoroethylene)with benzophenone and sodium hydride by ultraviolet irradiation

Poly(tetrafluoroethylene) (PTFE) films were surface modified in a solution of benzophenone and sodium hydride in dry dimethylformamide by ultraviolet (UV) light irradiation. The extent of surface modification was characterized after durations of UV light irradiation from 5-20 min at temperatures from 19-60°C. The modified films were analyzed by electron spectroscopy for chemical analysis, diffuse reflectance ultraviolet-visible light spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, dynamic contact angle measurement, and low-voltage scanning electron microscopy. PTFE surfaces produced by this modification demonstrated extensive defluorination, oxygen incorporation, surface unsaturation, and reduction in both advancing and receding dynamic water contact angles in a manner that was more extensive at long durations of irradiation and at high temperatures. Morphological damage depended upon treatment conditions, but extensive surface modification could be obtained without substantial morphological damage to PTFE films. Control experiments indicated that the surface modification proceeded by photoexcitation of either diphenyl ketyl radical anion or benzhydrol anion, the products of reaction of benzophenone with sodium hydride.

Photoreduction of organic dyes in ketone amine systems

Nucleophilic radicals, such as α-amino and ketyl radicals, produced by reaction of benzophenone triplets with amines and alcohols respectively, react efficiently by electron transfer with cationic organic dyes, such as phenosafranine, thiopyronine, methylene blue, and crystal violet. Time-resolved laser flash photolysis studies reveal a high reactivity of α-amino radicals toward these cationic dyes; in contrast, ketyl radicals are less reactive. The electron transfer rate constants for the reaction between the α-amino radical of triethanol amine and these dyes range from 2.7 × 10 9 M −1 s −1 (phenosafranine) to 7.5 × 10 9 M −1 s −1 (methylene blue). In contrast, rate constants ranging from 1.1 × 10 8 M −1 s −1 (phenosafranine) to 1.2 × 10 9 M −1 s −1 (methylene blue) were measured for the reaction of the dyes with the diphenyl ketyl radical. In dye mixtures used in conjunction with ketone-amine systems, stepwise electron transfer from an initially formed semireduced dye to a molecule of another dye occurs. For example, the primarily formed crystal violet radical reacts very rapidly ( k = 2 × 10 9 M −1 s −1) with phenosafranine, thus forming the semireduced phenosafranine and regenerating crystal violet.

Synthesis and Chemical and Electrochemical Characterization of Fe-S Carbonyl Clusters. X-ray Crystal Structures of [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)] and [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)

A reinvestigation of the redox behavior of the [Fe(3)(&mgr;(3)-S)(CO)(9)](2)(-) dianion led to the isolation and characterization of the new [Fe(5)S(2)(CO)(14)](2)(-), as well as the known [Fe(6)S(6)(CO)(12)](2)(-) dianion. As a corollary, new syntheses of the [Fe(3)S(CO)(9)](2)(-) dianion are also reported. The [Fe(5)S(2)(CO)(14)](2)(-) dianion has been obtained by oxidative condensation of [Fe(3)S(CO)(9)](2)(-) induced by tropylium and Ag(I) salts or SCl(2), or more straightforwardly through the reaction of [Fe(4)(CO)(13)](2)(-) with SCl(2). The [Fe(6)S(6)(CO)(12)](2)(-) dianion has been isolated as a byproduct of the synthesis of [Fe(3)S(CO)(9)](2)(-) and [Fe(5)S(2)(CO)(14)](2)(-) or by reaction of [Fe(4)(CO)(13)](2)(-) with elemental sulfur. The structures of [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)] and [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)] were determined by single-crystal X-ray diffraction analyses. Crystal data: for [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)], monoclinic, space group P2(1)/c (No. 14), a = 24.060(5), b = 14.355(6), c = 23.898(13) Å, beta = 90.42(3) degrees, Z = 4; for [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)], monoclinic, space group C2/c (No. 15), a = 34.424(4), b = 14.081(2), c = 19.674(2) Å, beta = 115.72(1) degrees, Z = 4. The new [Fe(5)S(2)(CO)(14)](2)(-) dianion shows a "bow tie" arrangement of the five metal atoms. The two Fe(3) triangles sharing the central Fe atom are not coplanar and show a dihedral angle of 55.08(3) degrees. Each Fe(3) moiety is capped by a triply bridging sulfide ligand. The 14 carbonyl groups are all terminal; two are bonded to the unique central atom and three to each peripheral iron atom. Protonation of the [Fe(5)S(2)(CO)(14)](2)(-) dianion gives reversibly rise to the corresponding [HFe(5)S(2)(CO)(14)](-) monohydride derivative, which shows an (1)H-NMR signal at delta -21.7 ppm. Its further protonation results in decomposition to mixtures of Fe(2)S(2)(CO)(6) and Fe(3)S(2)(CO)(9), rather than formation of the expected H(2)Fe(5)S(2)(CO)(14) dihydride. Exhaustive reduction of [Fe(5)S(2)(CO)(14)](2)(-) with sodium diphenyl ketyl progressively leads to fragmentation into [Fe(3)S(CO)(9)](2)(-) and [Fe(CO)(4)](2)(-), whereas electrochemical, as well as chemical oxidation with silver or tropylium tetrafluoroborate, in dichloromethane, generates the corresponding [Fe(5)S(2)(CO)(14)](-) radical anion which exhibits an ESR signal at g = 2.067 at 200 K. The electrochemical studies also indicated the existence of a subsequent one-electron anodic oxidation which possesses features of chemical reversibility in dichloromethane but not in acetonitrile solution. A reexamination of the electrochemical behavior of the [Fe(3)S(CO)(9)](2)(-) dianion coupled with ESR monitoring enabled the spectroscopic characterization of the [Fe(3)S(CO)(9)](-) radical monoanion and demonstrated its direct involvement in the generation of the [Fe(5)S(2)(CO)(14)](n)()(-) (n = 0, 1, 2) system.

Stoichiometry and mechanism of protonation of alkali metal salts of benzophenone radical anions by weak proton donors and its relevance to the base-catalyzed decomposition of benzopinacol

The Stoichiometry of the protonation of lithium and potassium salts of benzophenone radical anions and of the lithium salt of the fluorenone radical anion by methanol has been measured and found to be [(Ar2C=O)−]/[MeOH] =2∶1. This result, which was obtained by the method of magnetic titration, implies that paramagnetism decays by the reaction between a ketyl anion and a ketyl radical (i.e., a protonated ketyl anion). The reactivities of alkali metal salts of fluorenone radical anions in relation to methanol exhibit a pronounced dependence on the nature of the counterion. No kinetic deuterium isotope effect has been found for the protonation of the lithium salt of the benzophenone radical anion in tetrahydrofuran (THF) bytert-pentyl alcohol. The lithium salt of the benzophenone radical anion inN,N,N′,N′-tetramethylethylenediamine (TMEDA) behaves markedly differently. Namely, its protonation by methanol exhibits 1 ∶ 1 Stoichiometry and it reacts considerably more slowly withsec-butyl alkohol,K(THF)/K(TMEDA) = 2.5. Benzopinacol undergoes decomposition by an alkoxide base to diphenyl ketyl, which decays into an equimolar mixture of benzophenone and benzhydrol. The reaction follows second-order kinetics and the specific rate constants exhibit an inverse relationship with respect to the initial concentration of the alkoxide. With a very strong base benzopinacol decomposes into two diphenyl ketyl anions. On the basis of this information as well as on studies of products, relevant mechanisms are proposed for the protonation of ketyl anions and for the decomposition of aromatic pinacols in basic media.

Reduction of triphenyl methane dyes by the diphenyl ketyl radical and hydrated electrons: a time-resolved study

The reduction of four triphenyl methane (TPM) dyes (brilliant green (BG+), malachite green (MG+), crystal violet (CV+) and methyl green (MEG2+)) by diphenyl ketyl radicals (BPH.) and hydrated electrons (eaq−) was studied by pulse radiolytic kinetic spectrophotometry. The rate constants for the reaction of BPH. with the cationic dyes are (8.0±1.0) × 108, (1.5±0.2) × 109, (4.0±0.5) × 108 and (1.0±0.15) × 1010 dm3mol−1s−1 respectively and for the reaction with eaq− are (9.5±1.0) × 109, (9.3±1.0) × 109, (4.0±0.7) × 1010 and (3.7±0.4) × 1010 dm3 mol−1s−1 respectively. The transient spectra resulting form the reduction of the dyes were characterized. The time-resolved spectra and pH variation indicate that the dye radical generated from the eaq- reaction is subsequently protonated. The dye radicals decay by a second-order process without giving rise to bleach recovery.

The electron transfer reactions of cyano substituted pyridines and quinolines with thermally generated diphenyl ketyl

AbstractElectron transfer reactions of cyano substituted pyridines and quinolines with thermally generated diphenyl ketyl are reported. When E1/2(‐) for the heterocycle is less negative than ‐2V, electron transfer occurs from the ketyl to the heterocyclic base. In some cases the products obtained from the thermal reaction are the same as those obtained from the photochemical reaction with benzophenone and alcohols. In other cases different products are formed. Two bases, 2‐pyridinecarbonitrile and 2,4‐pyridinedicarbonitrile, undergo regiospecific reactions in which the course of the reaction is determined by the acidity of the medium. A mechanism in which the heterocyclic nitrogen is involved in the substitution process at the 2 position is proposed.

Quenching of ketone triplet states by silanes

Triplet state quenching of the ketones benzophenone, p-methylbenzophenone, p-phenylbenzophenone and benzil by silanes containing an SiH bond was investigated in a 9:1 (v/v) mixture of acetonitrile and acetone. It was found that both the n,π* and n,π*−π,π* triplet states are quenched if their energies are above 280 kJ mol −1. Quenching rate constants k H were determined using steady state phosphorescence quenching and the time-resolved measurement of the triplet—triplet absorption. From an analysis of the behaviour of the triplet—triplet absorption of benzophenone as a function of time at variable silane concentrations, a reaction scheme was derived which includes the combination reactions of diphenyl ketyl, solvent and silyl radicals and the addition of silyl radical to the ground state of benzophenone. k H values obtained in this way agree with the phosphorescence quenching data and are of the order of 3 × 10 6 to 3.4 × 10 8 dm 3 mol −1 s −1. Furthermore, it was found that the log k H values depend on the magnitude of the wavenumber of the SiH vibration of the silanes.

Isotope and temperature effects on the lifetime of excited diphenyl ketyl radicals

The lifetime of excited diphenyl ketyl radicals is lengthened by deuterium substitution. The largest effect is observed by substitution at the hydroxylic position; for example, the lifetimes are 3.9, 4.2, 8.7 and 10.5 ns for (C 6H 5) 2COH, (C 6D 5) 2COH, (C 6H 5) 2COD and (C 6D 5) 2COD, respectively, in toluene or toluene- d 8 at room temperature. Deuterium substitution in the solvent has no effect other than providing a different atom for the hydroxylic position in the ketyl radical.

Photopolymerisation mit keton/aryldiazoniumsalz-initiatorsystemen

Photoinitiator systems for the polymerization of methyl methacrylate (MMA), based on combinations of benzophenone (Ph 2CO, light absorbing species), p-chlorobenzenediazonium tetrafluoroborate (ArN 2 +) and tetrahydrofuran (THF), have been investigated by means of quantum yield measurements of ArN 2 +-decomposition ( Φ N2) and MMA-consumption ( Φ p). In the presence of THF, the main reaction of ArN 2 +-decomposition consists of an electron-transfer reaction from Ph 2CO triplet state forming aryl radicals. MMA quenches the T 1 of Ph 2CO, which decreases the Φ N2-values in the presence of monomer. The Ph 2CO or the Ph 2CO/THF initiated MMA polymerizations possess low Φ p, caused by triplet quenching and termination by diphenyl ketyl radicals. In ArN 2 + containing systems, Φ p are much higher. This behaviour can be interpreted at low ArN 2 + concentrations (< 10 −2 M) by means of electron-transfer from nucleophilic ketyl radicals. At higher concentrations, the efficiency is increased because the electron transfer from Ph 2CO triplet state to ArN 2 + competes with the monomer quenching.

The laser versus the lamp: Reactivity of the diphenyl ketyl radical in the ground and excited states

The diphenyl ketyl radical which is formed upon photolysis of α-phenyl benzoin is produced in its excited state upon intense pulsed laser irradiation. Using the techniques of time-resolved absorption and emission spectroscopy, reaction rate constants for the ground and excited states of this radical were obtained. For the radical quenchers employed, the excited state reactivity is found to be typically several orders of magnitude greater than that of the ground state. It is concluded that the excited state of diphenyl ketyl radical reacts predominantly by electron transfer processes.

Time-resolved EPR studies of the benzophenone—diphenyl kethyl radical system. Possible evidence for quartet—doublet intersystem crossing

The photoreduction of benzophenone by several substrates has been studied by time-resolved EPR. When cyclohexadiene, MTHF, and decane are employed as hydrogen donors to the excited triplet state of benzophenone the diphenyl ketyl radical which is produced exhibits emissive electron spin polarization with rise times on the order of a few microseconds. It is suggested that the experimental observations can be explained by intersystem crossing to a quartet state in the excited ketyl radical.

Fluorescence quenching of the diphenyl ketyl radical

Excitation of α-phenylbenzoin solution in acetonitrile generates emission from the diphenyl ketyl radical with a lifetime of 3.7 ns. This is quenched by methyl methacrylate and p-chlorobenzenediazonium tetrafluoroborate. The rate constant with diazonium salt is about the same for ground and excited radicals, while reaction of the excited radical with methyl methacrylate is 3 × 10 5 times faster than for the ground state.