Reaction mechanisms and structural characterization of the reactive intermediates observed after the photolysis of 3-(hydroxymethyl)benzophenone in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions

Nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy was employed to investigate the photoinduced reactions of 3-(hydroxymethyl)benzophenone (1) in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions. Density functional theory calculations were utilized to help the interpretation of the experimental spectra. In acetonitrile, the neutral triplet state 1 [denoted here as (m-BPOH)(3)] was observed on the nanosecond to microsecond time scale. In 2-propanol this triplet state appeared to abstract a hydrogen atom from the solvent molecules to produce the aryphenyl ketyl radical of 1 (denoted here as ArPK of 1), and then this species underwent a cross-coupling reaction with the dimethylketyl radical (also formed from the hydrogen abstraction reaction) to form a long-lived light absorbing transient species that was tentatively identified to be mainly 2-(4-(hydroxy(3-(hydroxymethyl)phenyl)methylene)cyclohexa-2,5-dienyl)propan-2-ol. In 1:1 H(2)O:CH(3)CN aqueous solution at neutral pH, (m-BPOH)(3) reacted with water to produce the ArPK of 1 and then underwent further reaction to produce a long-lived light absorbing transient species. Three photochemical reactions appeared to take place after 266 nm photolysis of 1 in acidic aqueous solutions, a photoreduction reaction, an overall photohydration reaction, and a novel photoredox reaction. TR(3) experiments in 1:1 H(2)O:CH(3)CN aqueous solution at pH 2 detected a new triplet biradical species, which is associated with an unusual photoredox reaction. This reaction is observed to be the predominant reaction at pH 2 and seems to face competition from the overall photohydration reaction at pH 0.     

Photopolymerization of Acrylonitrile: Benzophenone-Isopropanol System as Initiator

Abstract

Irradiation of benzophenone in the presence of a suitable hydrogen donor like alcohol leads to benzpinacol and the ketone as sole products. This reaction proceeds through ketyl radical intermediates. This photoredox system may be used for photopolymerization. Photopolymerization of acrylonitrile using isopropanol and benzophenone combination as initiator has been studied. The formation of the polymer is directly proportional to irradiation time, [acrylonitrile] and [isopropanol]. The rate of polymerization increases with an increase in concentration of benzophenone to an optimum and a further increase in benzophenone concentration decreases the polymerization rate. A suitable mechanism is suggested.     

How and when does an unusual and efficient photoredox reaction of 2-(1-hydroxyethyl) 9,10-anthraquinone occur? A combined time-resolved spectroscopic and DFT study

The photophysics and photochemical reactions of 2-(1-hydroxyethyl) 9,10-anthroquinone (2-HEAQ) were studied using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy techniques and density functional theory (DFT) calculations. In acetonitrile, 2-HEAQ underwent efficient intersystem crossing to the triplet excited state ((2-HEAQ)(3)). A typical photoreduction reaction for aromatic ketones took place via production of a ketyl radical intermediate for 2-HEAQ in isopropanol. In water-containing solutions with pH values between 2 and 10, an unusual photoredox reaction reported by Wan and co-workers was detected and characterized. Observation of the protonated species in neutral and acidic aqueous solutions by fs-TA spectra indicated the carbonyl oxygen of (2-HEAQ)(3) was protonated initially and acted as a precursor of the photoredox reaction. The preference of the photoredox reaction to occur under moderate acidic conditions compared to neutral condition observed using ns-TR(3) spectroscopy was consistent with results from DFT calculations, which suggested protonation of the carbonyl group was the rate-determining step. Under stronger acidic conditions (pH 0), although the protonated (2-HEAQ)(3) was formed, the predominant reaction was the photohydration reaction instead of the photoredox reaction. In stronger basic solutions (pH 12), (2-HEAQ)(3) decayed with no obvious photochemical reactions detected by time-resolved spectroscopic experiments. Reaction mechanisms and key reactive intermediates for the unusual photoredox reaction were elucidated from time-resolved spectroscopy and DFT results. A brief discussion is given of when photoredox reactions may likely take place in the photochemistry of aromatic carbonyl-containing compounds and possible implications for using BP and AQ scaffolds for phototrigger compounds.     

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.     

Visible‐Light‐Catalyzed Direct Benzylic C(sp3)–H Amination Reaction by Cross‐Dehydrogenative Coupling

A conceptually new and synthetically valuable cross‐dehydrogenative benzylic C(sp³)–H amination reaction is reported by visible‐light photoredox catalysis. This protocol employs DCA (9,10‐dicyanoanthracene) as a visible‐light‐absorbing photoredox catalyst and an amide as the nitrogen source without the need of either a transition metal or an external oxidant.     

Photoinduced C(sp3)−N Bond Cleavage Leading to the Stereoselective Syntheses of Alkenes

Herein we report a versatile Mizoroki–Heck-type photoinduced C(sp³)−N bond cleavage reaction. Under visible-light irradiation (455 nm, blue LEDs) at room temperature, alkyl Katritzky salts react smoothly with alkenes in a 1:1 molar ratio in the presence of 1.0 mol % of commercially available photoredox catalyst without the need for any base, affording the corresponding alkyl-substituted alkenes in good yields with broad functional-group compatibility. Notably, the E/Z-selectivity of the alkene products can be controlled by an appropriate choice of photoredox catalyst.     

Hydrotrifluoromethylthiolation of Unactivated Alkenes and Alkynes with Trifluoromethanesulfonic Anhydride through Deoxygenative Reduction and Photoredox Radical Processes

An ongoing challenge in trifluoromethylthiolation reactions is the use of less expensive and easily available trifluoromethylthio sources. Herein, we disclose an unprecedented usage of trifluoromethanesulfonic anhydride (Tf₂O) as a radical trifluoromethylthiolating reagent. Hydrotrifluoromethylthiolation of unactivated alkenes and alkynes with Tf₂O in the presence of PMePh₂ and H₂O under visible‐light photoredox catalysis gave the addition products. The trifluoromethylthio radical (^.SCF₃) was first formed from Tf₂O through a photoredox radical processes and deoxygenative reduction of PMePh₂, and H₂O serves as the H‐atom donor for the hydrotrifluoromethylthiolation reaction. This reaction provides a new strategy for radical trifluoromethylthiolation.     

Chemistry of heterocyclic N-oxides and related compounds. 11. Reaction of pyridine N-oxide with metal ketyls and benzophenone dianions

The reaction of pyridine N-oxide with metal ketyls of benzophenone was studied. It is shown that diphenyl(2-pyridyl)carbinol N-oxide is formed with the lithium and sodium derivatives of benzophenone, whereas diphenyl(2-pyridyl)carbinol is formed with the potassium derivative of benzophenone. Diphenyl(2-pyridyl)carbinol N-oxide is obtained in lower yields in the reaction of pyridine N-oxide with benzophenone dianions. Pyridine and 4,4- and 2,2-dipyridyls are simultaneously formed in all of the reactions.See [1] for Communication 10.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 655–658, May, 1979.     

Visible‐Light Photoredox‐Catalyzed and Copper‐Promoted Trifluoromethoxylation of Arenediazonium Tetrafluoroborates

We report the development of photoredox‐catalyzed and copper‐promoted trifluoromethoxylation of arenediazonium tetrafluoroborates, with trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation reagent. This new method takes advantage of visible‐light photoredox catalysis to generate the aryl radical under mild conditions, combined with copper‐promoted selective trifluoromethoxylation. The reaction is scalable, tolerates a wide range of functional groups, and proceeds regioselectively under mild reaction conditions. Furthermore, mechanistic studies suggested that a Cs[Cu(OCF₃)₂] intermediate might be generated during the reaction.     

Excitation of chemiluminescence in the reaction of benzophenone-O-oxide with diphenyldiazomethane. Quantum-induced decomposition of diphenyldiazomethane

The excitation yield (⌽⁺) of triplet benzophenone in the chemiluminescence reaction of thermal decomposition of diphenyldiazomethane (Ph₂CN₂) was measured. Triplet benzophenone was produced by the reaction of benzophenone-O-oxide with Ph₂CN₂ in a MeCN solution. The ⌽⁺ value is equal to 0.05 and temperature-independent. Analysis of the dependences of the observed rate constant of chemiluminescence decay on the concentration of Ph₂CN₂ both in the presence and absence of a luminescence activator suggests the quantum-induced decomposition of diazomethane in the reaction with triplet benzophenone. The rate constant of the reaction of triplet benzophenone with Ph₂CN₂ was estimated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1097–1101, June, 1999.     

Visible‐Light‐Mediated Decarboxylation/Oxidative Amidation of α‐Keto Acids with Amines under Mild Reaction Conditions Using O2

Photochemistry has ushered in a new era in the development of chemistry, and photoredox catalysis has become a hot topic, especially over the last five years, with the combination of visible‐light photoredox catalysis and radical reactions. A novel, simple, and efficient radical oxidative decarboxylative coupling with the assistant of the photocatalyst [Ru(phen)₃]Cl₂ is described. Various functional groups are well‐tolerated in this reaction and thus provides a new approach to developing advanced methods for aerobic oxidative decarboxylation. The preliminary mechanistic studies revealed that: 1) an SET process between [Ru(phen)₃]²⁺* and aniline play an important role; 2) O₂ activation might be the rate‐determining step; and 3) the decarboxylation step is an irreversible and fast process.     

Photoredox Catalysis Using Heterogenized Iridium Complexes**

Heterogenized photoredox catalysts provide a path for sustainable chemical synthesis using highly tunable, reusable constructs. Here, heterogenized iridium complexes as photoredox catalysts were assembled via covalent attachment to metal oxide surfaces (ITO, ZrO₂, Al₂O₃) in thin film or nanopowder constructs. The goal was to understand which materials provided the most promising constructs for catalysis. To do this, reductive dehalogenation of bromoacetophenone to acetophenone was studied as a test reaction for system optimization. All catalyst constructs produced acetophenone with high conversions and yields with the fastest reactions complete in fifteen minutes using Al₂O₃ supports. The nanopowder catalysts resulted in faster and more efficient catalysis, while the thin film catalysts were more robust and easily reused. Importantly, the thin film constructs show promise for future photoelectrochemical and electrochemical photoredox setups. Finally, all catalysts were reusable 2–3 times, performing at least 1000 turnovers (Al₂O₃), demonstrating that heterogenized catalysts are a sustainable catalyst alternative.     

Visible light photoredox-catalyzed phosphorylation of quinoxalin-2(1H)-ones

A visible light photoredox-catalyzed C-3 phosphorylation of quinoxalin-2(1H)-ones with diphenylphosphine oxide has been developed. The reaction was effectively accelerated using an inexpensive eosin B as a photoredox catalyst under ambient air at room temperature without any other metal, oxidant, or additive. This approach offers a facile way to prepare 3-diphenylphosphorylated quinoxalin-2(1H)-one derivatives.     

Alkali and alkaline-earth metal ketyl complexes: isolation, structural diversity, and hydrogenation/protonation reactions

The use of hexamethylphosphoric triamide (HMPA) as a stabilizing ligand allowed successful isolation of a series of structurally characterizable alkali metal and calcium ketyl complexes. Reaction of lithium and sodium with one equivalent of fluorenone and reaction of sodium with one equivalent of benzophenone in THF, followed by addition of two equivalents of HMPA, yielded the corresponding ketyl complexes 1, 2, and 11, respectively, as microketyl-bridged dimers. If one equivalent of HMPA was used in the reaction of sodium with fluorenone, a further aggregated complex, the mu3-ketyl-bridged tetramer 3, was isolated, whereas analogous reaction of benzophenone with sodium afforded the trimeric ketyl complex 13, rather than a simple benzophenone analogue of 3. In the reaction of potassium with fluorenone, the use of two equivalents of HMPA gave the tetramer 4, rather than a dimeric complex analogous to 1 or 2. Compared to the tetrameric sodium complex 3, there is an extra HMPA ligand that bridges two of the four K atoms in 4. When 0.5 equiv of HMPA was used in the above reaction, complex 5, a THF-bridged analogue of 4, was isolated. In the absence of HMPA, the reaction of sodium with an excess of fluorenone yielded the tetrameric ketyl complex 6, in which two of the four Na atoms are each terminally coordinated by a fluorenone ligand, and the other two Na atoms are coordinated by a THF ligand. Two bridging THF ligands are also observed in 6. Reaction of 1,2-bis(biphenyl-2,2'-diyl)ethane-1,2-diol (7) with two equivalents of LiN(SiMe3)2 or NaN(SiMe3)2 in the presence of four equivalents of HMPA easily afforded 1 or 2, respectively, via C-C bond cleavage of a 1,2-diolate intermediate. The reaction of calcium with two equivalents of fluorenone or benzophenone in the presence of HMPA gave the corresponding complexes that bear two independent ketyl ligands per metal ion. In the presence of 3 or four equivalents of HMPA, the fluorenone ketyl complex was isolated in a six-coordinate octahedral form (10), while the benzophenone ketyl complex was obtained as a five-coordinate trigonal bipyramid (13). The radical carbon atoms in both benzophenone ketyl and fluorenone ketyl complexes are still in an sp2-hybrid state. However, in contrast with the planar configuration of the whole fluorenone ketyl unit, the radical carbon atom in a benzophenone ketyl species is not coplanar with any of the phenyl groups; this explains why benzophenone ketyl is more reactive than fluorenone ketyl. Hydrolysis of 2 or 11 with 2N HCI yielded the corresponding pinacol-coupling product, while treatment of 2 or 11 with 2-propanol, followed by hydrolysis, gave the pairs fluorenone and fluorenol or benzophenone and benzhydrol, respectively. A possible mechanism for these reactions is proposed.     

New Radical Borylation Pathways for Organoboron Synthesis Enabled by Photoredox Catalysis

Radical borylation using N‐heterocyclic carbene (NHC)‐BH₃ complexes as boryl radical precursors has emerged as an important synthetic tool for organoboron assembly. However, the majority of reported methods are limited to reaction modes involving carbo‐ and/or hydroboration of specific alkenes and alkynes. Moreover, the generation of NHC‐boryl radicals relies principally on hydrogen atom abstraction with the aid of radical initiators. A distinct radical generation method is reported, as well as the reaction pathways of NHC‐boryl radicals enabled by photoredox catalysis. NHC‐boryl radicals are generated via a single‐electron oxidation and subsequently undergo cross‐coupling with the in‐situ‐generated radical anions to yield gem‐difluoroallylboronates. A photoredox‐catalyzed radical arylboration reaction of alkenes was achieved using cyanoarenes as arylating components from which elaborated organoborons were accessed. Mechanistic studies verified the oxidative formation of NHC‐boryl radicals through a single‐electron‐transfer pathway.     

One‐Pot Tandem Photoredox and Cross‐Coupling Catalysis with a Single Palladium Carbodicarbene Complex

The combination of conventional transition‐metal‐catalyzed coupling (2 e^? process) and photoredox catalysis (1 e^? process) has emerged as a powerful approach to catalyze difficult cross‐coupling reactions under mild reaction conditions. Reported is a palladium carbodicarbene (CDC) complex that mediates both a Suzuki–Miyaura coupling and photoredox catalysis for C?N bond formation upon visible‐light irradiation. These two catalytic pathways can be combined to promote both conventional transition‐metal‐catalyzed coupling and photoredox catalysis to mediate C?H arylation under ambient conditions with a single catalyst in an efficient one‐pot process.     

Conjugated polymers as photoredox catalysts: a new catalytic system using visible light to promote aryl aldehyde pinacol couplings

The conjugated polymer poly-(p)-phenylene (PPP) was synthesized and used as a photoredox catalyst to promote pinacol coupling of aryl-aldehydes with visible light. The reaction required the use of a sacrificial electron donor (Et₃N), and was accelerated by the addition of Lewis and Brønsted acids. A distinct advantage of this photocatalytic system is the robust nature of the system, which is not overly sensitive to impurities, oxygen, or temperature, and proceeds cleanly with few side reactions. As a comparison with the PPP system, the reactivity of Ru(bpy)₃Cl₂, a popular photoredox catalyst was compared. The PPP system was superior to the Ru(bpy)₃Cl₂ for the pinacol couplings in both rate and yield.     

Effect of mercaptan on photoreduction of acetone. Non-repair hydrogen transfer reactions

Abstract— Ultraviolet (u.v.) irradiation of solutions of benzhydrol in acetone leads to formation of -2-propanol, benzpinacol and some benzophenone, apparently from the free radicals (CH₃)₂COH, II, and (C₆H₅)₂COH, I. 2-Propanol is formed more rapidly and benzophenone is formed to a much larger extent and persists longer when the solution contains mesityl mercap-tan, as radical II is reduced by mercaptan and radical I is oxidized by thiyl radical. The same hydrogen atom transfer reactions, which retard by a repair mechanism the photoreduction of benzophenone by 2-propanol, accelerate and alter the course of photoreduction of acetone by benzhydrol. Irradiation of acetone leads to 2-propanol, and this is formed more rapidly in the presence of mercaptan. Irradiation of benzophenone in acetone leads to no apparent reaction. The courses of reaction of the several systems are discussed.     

Visible‐Light‐Irradiated Graphitic Carbon Nitride Photocatalyzed Diels–Alder Reactions with Dioxygen as Sustainable Mediator for Photoinduced Electrons

Photocatalytic Diels–Alder (D–A) reactions with electron rich olefins are realized by graphitic carbon nitride (g‐C₃N₄) under visible‐light irradiation and aerobic conditions. This heterogeneous photoredox reaction system is highly efficient, and the apparent quantum yield reaches a remarkable value of 47 % for the model reaction. Dioxygen plays a critical role as electron mediator, which is distinct from the previous reports in the homogeneous Ru^II complex photoredox system. Moreover, the reaction intermediate vinylcyclobutane is captured and monitored during the reaction, serving as a direct evidence for the proposed reaction mechanism. The cycloaddition process is thereby determined to be the combination of direct [4+2] cycloaddition and [2+2] cycloaddition followed by photocatalytic rearrangement of the vinylcyclobutane intermediate.     

Preparation and photochemical properties of poly[2-(4-benzoylphenyl)-2-(4-propenoylphenyl)-propane] and its copolymer with styrene

2-(4-Benzoylphenyl)-2-phenyl propane ( 4 ) was prepared by benzoylation of 2,2-diphenylpropane ( 2 ). Acylation of ( 4 ) with 3-chloropropanoic chloride gave 2-(4-benzoylphenyl)-2-(4-propenoylphenyl)propane ( 5 ). A monomer 2-(4-benzoylphenyl)-2-(4-propenoylphenyl)propane ( 6 ) was prepared through dehydrochlorination of ( 5 ). The homopolymer of 6 (P6) and the copolymer with styrene ( P6 / S) were prepared by radical polymerization. Laser flash photolysis was employed to determine the absorption and emission spectra of transients, their lifetimes (τ) and the rate constant (k_q) of triplet quenching in benzene at laboratory temperature for 4 , P6 , and P6 / S. P6 exhibits a transient absorption maximum in a different spectral region than do the model 4 and copolymer P6/S . The products of k_q and τ determined by laser flash photolysis for these transients are higher than th Stern–Volmer constants based on inhibition of degradation. Degradation leading to formation of quenchers is the likely cause of this difference although crosslinking may also contribute. Irradiation of polymers ( P6 and P6/S ) at 366 nm leads to main chain scission with aquantum yield of 0.13 under N₂ for P6 and 0.03 for P6/S . In this bichromophoric structural unit, the benzophenone residue absorbs about 80–90% of the incident energy. Its triplet energy is about 5 kJ mol^?1 lower than that of the 1-(4-alkylphenyl)-2-propene-1-one chromophore. Different possible pathways of degradation are discussed namely the Norrish Type II reaction of the alkyl aryl ketone and direct reaction of triplet benzophenone with the main chain. In the mechanism favored the benzophenone triplet is proposed to be in equilibrium with the upper acetophenone-like chromophore from which the Norrish Type II reaction leading to chain fragmentation takes place.