Visible‐Light‐Induced Organic Photochemical Reactions through Energy‐Transfer Pathways

Visible‐light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible‐light‐induced photochemical reactions occur through single‐electron transfer (SET) pathways. Recently, visible‐light‐induced energy‐transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible‐light‐induced EnT reactions.     

Difunctionalization of Alkenes and Alkynes via Intermolecular Radical and Nucleophilic Additions

Popular and readily available alkenes and alkynes are good substrates for the preparation of functionalized molecules through radical and/or ionic addition reactions. Difunctionalization is a topic of current interest due to its high efficiency, substrate versatility, and operational simplicity. Presented in this article are radical addition followed by oxidation and nucleophilic addition reactions for difunctionalization of alkenes or alkynes. The difunctionalization could be accomplished through 1,2-addition (vicinal) and 1,n-addition (distal or remote) if H-atom or group-transfer is involved in the reaction process. A wide range of moieties, such as alkyl (R), perfluoroalkyl (R_f), aryl (Ar), hydroxy (OH), alkoxy (OR), acetatic (O₂CR), halogenic (X), amino (NR₂), azido (N₃), cyano (CN), as well as sulfur- and phosphorous-containing groups can be incorporated through the difunctionalization reactions. Radicals generated from peroxides or single electron transfer (SET) agents, under photoredox or electrochemical reactions are employed for the reactions.     

Regioselectivity of enzymatic and photochemical single electron transfer promoted carbon-carbon bond fragmentation reactions of tetrameric lignin model compounds

New types of tetrameric lignin model compounds, which contain the common β-O-4 and β-1 structural subunits found in natural lignins, have been prepared and carbon-carbon bond fragmentation reactions of their cation radicals, formed by photochemical (9,10-dicyanoanthracene) and enzymatic (lignin peroxidase) SET-promoted methods, have been explored. The results show that cation radical intermediates generated from the tetrameric model compounds undergo highly regioselective C-C bond cleavage in their β-1 subunits. The outcomes of these processes suggest that, independent of positive charge and odd-electron distributions, cation radicals of lignins formed by SET to excited states of sensitizers or heme-iron centers in enzymes degrade selectively through bond cleavage reactions in β-1 vs β-O-4 moieties. In addition, the findings made in the enzymatic studies demonstrate that the sterically large tetrameric lignin model compounds undergo lignin peroxidase-catalyzed cleavage via a mechanism involving preliminary formation of an enzyme-substrate complex.     

Synthesis of Three‐Membered and Four‐Membered Heterocycles with the Assistance of Photochemical Reactions

Some transformations are not possible with ground‐state reactions even in the presence of a catalyst; hence, they are performed under photochemical conditions. Electron transfer occurred even with the photochemical excitement of one molecule where redox reaction is not possible at the ground state. The side products were obtained from ground‐state reactions. For C─C bond formation during photochemical reactions, there was no requirement of any chemical activation of the substrates. Therefore, these reactions are presented here for the synthesis of three‐membered and four‐membered heterocycles in the context of sustainable processes.     

Photoinduced reactions of chloranil with 1,1-diarylethenes and product photochemistry-intramolecular

Photoinduced reactions of chloranil (CA) with 1,1-diarylethenes 1 [(p-X-Ph)(2)C=CH(2), X = F, Cl, H, Me] in benzene afforded products 4-14, respectively, with the bicyclo[4.2.0]oct-3-ene-2,5-diones 4, the 6-diarylethenylcyclohexa-2,5-diene-1,4-diones 5, and 2,3,5, 6-tetrachlorohydroquinone 13 as the major primary products. The cyclobutane products 4 are formed via a triplet diradical intermediate without involvement of single electron transfer (SET) between the two reactants, while 5 is derived from a reaction sequence with initial SET interaction between (3)CA and the alkene. The 9-arylphenanthrene-1,4-diones 6 and its 10-hydroxy-derivatives 7 are secondary photochemical products derived from 5. The isomeric cage products 9-11 are formed from 4 via intramolecular benzene-alkene [2 + 2] (ortho-)photocycloadditions induced by the triplet excited enedione moiety. The relative amount of the two groups of products (4 and its secondary products 9-11 via non-SET route vs 5 and its secondary products 6, 7, 8, 12, and 14 via SET route) shows a rather regular change, with the ratio of non-SET route products gradually increasing with the increase in oxidation potential of the alkenes and in the positive free energy change for electron transfer (DeltaG(ET)) between (3)CA and the alkene, at the expense of the ratio of the products from the SET route. The competition between the SET and non-SET routes was also found to be drastically influenced by solvent polarity, with the SET pathways more favored in polar solvent. Photo-CIDNP investigations suggest the intermediacy of exciplexes or contact ion radical pairs in these reactions in benzene, while in acetonitrile, SET process led to the formation of CA(*)(-) and cation radical of the alkene in the form of solvent separated ion radical pairs and free ions.     

Photochemical C–N bond forming reactions for the synthesis of five-membered fused N-heterocycles

Abstract

Few conversions cannot take place with ground-state reactions even with the help of a catalyst, therefore they are made to occur under photochemical conditions. The transfer of electrons took place even with the photochemical excitement of one molecule where redox reaction cannot occur at the ground state. The ground-state reactions resulted in the formation of side products. The substrates did not require any sort of chemical activation for C–N bond construction in the course of photochemical reactions. The source of energy; light has always been the interest of researchers in order to induce chemical reactions ever since the starting of scientific chemistry. The present review encloses the chemistry of photochemical transformations with a focus on their synthetic uses. The organic photochemical reactions prevent the polluting or harmful reagents thus, provides a possibility for sustainable procedures as well as green chemistry. This review article displays the formation of numerous of five-membered fused nitrogen-heterocyclic compounds.     

A synthetic strategy for the preparation of cyclic peptide mimetics based on SET-promoted photocyclization processes

A novel method for the synthesis of cyclic peptide analogues has been developed. The general approach relies on the use of SET-promoted photocyclization reactions of peptides that contain N-terminal phthalimides as light absorbing electron acceptor moieties and C-terminal alpha-amidosilane or alpha-amidocarboxylate centers. Prototypical substrates are prepared by coupling preformed peptides with the acid chloride of N-phthalimidoglycine. Irradiation of these substrates results in the generation of cyclic peptide analogues in modest to good yields. The chemical efficiencies of these processes are not significantly affected by (1) the lengths of the peptide chains separating the phthalimide and alpha-amidosilane or alpha-amidocarboxylate centers and (2) the nature of the penultimate cation radical alpha-heterolytic fragmentation process (i.e., desilylation vs decarboxylation). An evaluation of the effects of N-alkyl substitution on the amide residues in the peptide chain showed that N-alkyl substitution does not have a major impact on the efficiencies of the photocyclization reactions but that it profoundly increases the stability of the cyclic peptide.     

Copper-catalyzed aerobic oxidative C-H functionalizations: trends and mechanistic insights

The selective oxidation of C-H bonds and the use of O(2) as a stoichiometric oxidant represent two prominent challenges in organic chemistry. Copper(II) is a versatile oxidant, capable of promoting a wide range of oxidative coupling reactions initiated by single-electron transfer (SET) from electron-rich organic molecules. Many of these reactions can be rendered catalytic in Cu by employing molecular oxygen as a stoichiometric oxidant to regenerate the active copper(II) catalyst. Meanwhile, numerous other recently reported Cu-catalyzed C-H oxidation reactions feature substrates that are electron-deficient or appear unlikely to undergo single-electron transfer to copper(II). In some of these cases, evidence has been obtained for the involvement of organocopper(III) intermediates in the reaction mechanism. Organometallic C-H oxidation reactions of this type represent important new opportunities for the field of Cu-catalyzed aerobic oxidations.     

Single electron transfer-promoted photocyclization reactions of linked acceptor-polydonor systems: effects of chain length and type on the efficiencies of macrocyclic ring-forming photoreactions of tethered alpha-silyl ether phthalimide substrates

Results of an investigation, aimed at gaining information about the factors governing the efficiencies of single electron transfer (SET)-promoted photocyclization reactions of linked acceptor-polydonor systems, are described. One set of substrates used in this effort includes alpha-trimethylsilyl ether terminated, polymethylene- and polyethylenoxy-tethered phthalimides and 2,3-naphthalimides. Photocyclization reactions of the polyethylenoxy-linked phthalimides and naphthalimides were observed to take place in higher chemical yields and with larger quantum efficiencies than those of analogs containing polymethylene tethers of near equal length. These findings show that the rates of formation of 1,omega-zwitterionic biradicals that serve as key intermediates in the photocyclization processes are enhanced in substances that contain oxygen donor sites in the chain. The findings suggest that these donor sites facilitate both initial SET to acceptor excited states and ensuing intrachain SET, resulting in migration of the cation radical center to the terminal alpha-trimethylsilyl ether position. In addition, an inverse relationship was observed between the quantum yields of photocyclization reactions of the tethered phthalimides and naphthalimides and the length of the polyethylenoxy chain. Finally, the roles played by chain type and length in governing photoreaction efficiencies were investigated by using intramolecular competition in photoreactions of polyethylenoxy and polymethylene bis-tethered phthalimides. Mechanistic interpretations and synthetic consequences of the observations made in this study are discussed.     

Photochemical transformations of cyclic acetal radical cations in Freon matrices at 77 K

The efficiency of photochemical reactions of radical cations of cyclic acetals (1,3-dioxolane, 1,3-dioxane) is measured in different Freon matrices at 77 K and the influence of the latter on the reaction path is discovered. The possible nature of the paramagnetic complexes that form in photochemical reactions of cyclic acetal radical cations in Freon-11 is suggested.     

An Unexpected Michael–Aldol–Smiles Rearrangement Sequence for the Synthesis of Versatile Optically Active Bicyclic Structures by Using Asymmetric Organocatalysis

A facile and simple organocatalytic procedure to generate optically active 6‐alkyl‐ and 6‐aryl‐substituted bicyclo[2.2.2]oct‐5‐en‐2‐ones is presented. The reaction is catalysed by a 9‐amino‐9‐deoxyepiquinine trifluoroacetic acid salt, which activates α,β‐unsaturated cyclic ketones for the 1,4‐addition of β‐keto benzothiazoyl sulfones in a stereoselective fashion. Subsequent intramolecular aldol reaction and Smiles rearrangement gives rise to important optically active bicycles, which are a common motif in natural products, ligands in asymmetric catalysis and substrates for Cope rearrangements, photochemical reactions, radical cyclisations and metathesis. Different bicyclic structures were obtained by utilisation of various cyclic enones or by performing ring‐expanding reactions. Furthermore, two possible mechanistic pathways are outlined and discussed.     

Determination of kinetic parameters for interfacial enzymatic reactions on self-assembled monolayers

This paper reports a method to characterize the kinetic constants for the action of enzymes on immobilized substrates. This example uses cutinase, a serine esterase that hydrolyzes 4-hydroxyphenyl valerate moieties that are immobilized on a self-assembled monolayer of alkanethiolates on gold. The product of the enzyme reaction is a hydroquinone, which is redox active and therefore permits the use of cyclic voltammetry to monitor the extent of reaction in situ. A kinetic model based on the Michaelis-Menten formalism is used to analyze the dependence of initial rates of reaction on both the substrate density and the enzyme concentration. The resulting value of k(cat)/K(M) for the interfacial reaction is comparable to that for a homogeneous phase reaction with a substrate of similar structure. This strategy of using monolayers presenting substrates for the enzyme and cyclic voltammetry to measure reaction rates provides quantitative and real-time information on reaction rates and permits a level of analysis of interfacial enzyme reactions that to date has been difficult to realize.     

Ultrafast energy delocalization and electron transfer dynamics in 2-aminopurine-containing trinucleotides

The fate of electronically excited states in DNA base stacks is of tremendous importance for subsequent photochemical damage reactions in the genome. In this study we present a femtosecond broadband pump-probe study on the adenine isomer 2-aminopurine (Ap) incorporated into trinucleotides. After selective excitation of Ap we can monitor energy delocalization between neighboring Ap moieties as well as excited state electron transfer, depending on the sequence of the trinucleotide. Our results establish the time scale for intrastand excimer formation and reveal the lifetime of the excimer state.     

Copper(I)-catalyzed chlorine atom transfer radical cyclization reactions of unsaturated alpha-chloro beta-keto esters

Copper(I) chloride catalyzed chlorine atom transfer radical cyclization reactions of a series of olefinic alpha-chloro beta-keto esters were investigated. It was found that alpha-dichlorinated beta-keto esters were suitable substrates; the chlorine transfer mono or tandem radical cyclization reactions catalyzed by CuCl complex with bis(oxazoline) or bipyridine proceeded smoothly in dichloroethane at room temperature or 80 degrees C, providing cyclic and bicyclic compounds in moderate to high yield. [reaction: see text]     

The photochemistry of polydonor-substituted phthalimides: Curtin-Hammett-type control of competing reactions of potentially interconverting zwitterionic biradical intermediates

The results of studies designed to obtain information about the factors that control the chemical efficiencies/regioselectivities and quantum yields of single electron transfer (SET)-promoted reactions of acceptor-polydonor systems are reported. Photochemical and photophysical investigations were carried out with bis-donor tethered phthalimides and naphthalimides of general structure N-phthalimido- and N-naphthalimido-CH2CH2-D-CH2CH2-NMsCH2-E (E = SiMe3 or CO2NBu4 and D = NMs, O, S, and NMe). These substrates contain common terminal donor groups (NMsCH2SiMe3 or NMsCH2CO2NBu4) that have known oxidation potentials and cation radical fragmentation rates. Oxidation potentials and fragmentation rates at the other donor site in each of these substrates are varied by incorporating different heteroatoms and/or substituents. Photoproduct distribution, reaction quantum yield, and fluorescence quantum yield measurements were made. The results show that photocyclization reactions of alpha-trimethylsilylmethansulfonamide (E = SiMe3)- and alpha-carboxymethansulfonamide (E = CO2NBu4)-terminated phthalimides and naphthalimides that contain internal sulfonamide, ether, and thioether donor sites (D = NMs, O, or S) are chemically efficient (80-100%) and that they take place exclusively by a pathway involving sequential photoinduced SET (zwitterionic biradical desilylation or decarboxylation) biradical cyclization. In contrast, photoreactions of alpha-trimethylsilylmethansulfonamide- and alpha-carboxymethansulfonamide-terminated phthalimides and naphthalimides that that contain an internal tertiary amine donor site (D = NMe) are chemically inefficient and follow a pathway involving alpha-deprotonation at the tertiary amine radical cation center in intermediate, iminium radical-containing, zwitterionic biradicals. In addition, the quantum efficiencies for photoreactions of alpha-trimethylsilylmethansulfonamide- and alpha-carboxymethansulfonamide-terminated phthalimides are dependent on the nature of the internal donor (eg., phi = 0.12 for D = NMs, E = SiMe3; phi = 0.02 for D = S, E = SiMe3; phi = 0.04 for D = NMe, E = SiMe3). The results of this effort are discussed in terms of how the relative energies of interconverting zwitterionic biradical intermediates and the energy barriers for their alpha-heterolytic fragmentation reactions influence the chemical yields and quantum efficiencies of SET promoted photocyclization reactions of acceptor-polydonor substrates.     

On-Demand Activation of Transesterification by Chemical Amplification in Dynamic Thiol-Ene Photopolymers

Chemical amplification is a well-established concept in photoresist technology, wherein one photochemical event leads to a cascade of follow-up reactions that facilitate a controlled change in the solubility of a polymer. Herein, we transfer this concept to dynamic polymer networks to liberate both catalyst and functional groups required for bond exchange reactions under UV irradiation. For this, we exploit a photochemically generated acid to catalyse a deprotection reaction of an acid-labile tert-butoxycarbonyl group, which is employed to mask the hydroxy groups of a vinyl monomer. At the same time, the released acid serves as a catalyst for thermo-activated transesterifications between the deprotected hydroxy and ester moieties. Introduced in an orthogonally cured (450 nm) thiol-click photopolymer, this approach allows for a spatio-temporally controlled activation of bond exchange reactions, which is crucial in light of the creep resistance versus reflow ability trade-off of dynamic polymer networks.     

Photochemical and Thermal Reactions of Azido‐oligopyridines: Diazepinones,a New Class of Metal‐Complex Ligands

Azido‐oligopyridines were prepared, and their photochemical reactions resulted in diazepinones linked to pyridine moieties. The thermal reactions of azido‐oligopyridines with triple bonds yielded 2H‐azirines directly attached to oligopyridines.     

Iodine(V) reagents in organic synthesis. Part 4. o-Iodoxybenzoic acid as a chemospecific tool for single electron transfer-based oxidation processes

o-Iodoxybenzoic acid (IBX), a readily available hypervalent iodine(V) reagent, was found to be highly effective in carrying out oxidations adjacent to carbonyl functionalities (to form alpha,beta-unsaturated carbonyl compounds) and at benzylic and related carbon centers (to form conjugated aromatic carbonyl systems). Mechanistic investigations led to the conclusion that these new reactions are initiated by single electron transfer (SET) from the substrate to IBX to form a radical cation which reacts further to give the final products. Fine-tuning of the reaction conditions allowed remarkably selective transformations within multifunctional substrates, elevating the status of this reagent to that of a highly useful and chemoselective oxidant.     

Some aspects of photochemical systems for direct light-induced hydrogen production

Various possible pathways for photochemical conversion of light energy, including light-induced electron transfer and hydride transfer, are described. Several problems diminishing the photoconversion efficiency as well as side reactions affecting the stability of these systems are discussed. Oxidation of photosensitizers by singlet oxygen as well as attack by OH radicals is supposed to be the main degradation pathway for dyes and for the photoinduced reactions. The stability of viologens (acting as electron transfer agents) is mainly affected by hydrogenation, for which a reaction mechanism is presented. The dependence of rate constants on the free enthalpy of reaction is discussed with respect to quantum yields for light energy conversion. Following this, quantum yields of cyclic water splitting based on diffusion-controlled reactions are very low. Selective catalysis or vectorial processes (with a spatial charge separation) could enhance the quantum yields.     

N-centered odd-electron ions formation from collision-induced dissociation of electrospray ionization generated even-electron ions: single electron transfer via ion/neutral complex in the fragmentation of protonated N,N'-dibenzylpiperazines and protonated N-benzylpiperazines

Single electron transfer (SET) via ion/neutral complex (INC) was proposed and confirmed to be the key step in the formation of N-centered odd-electron ions from fragmentation of protonated even-electron ions in the present study. Upon collisional activation, the model compounds, protonated N,N′-dibenzylpiperazine and protonated N-benzylpiperazines initially dissociated to form intermediate INCs consisting of N-benzylpiperazine (or piperazine) and benzyl cation. In these ion/neutral complexes, SET reaction and direct separation as well as other reactions were observed and characterized experimentally and theoretically. Density functional theory calculations demonstrated that the energy requirement for homolysis of the precursor ion was so large that it could not be achieved, whereas the heterolytic dissociation followed by electron transfer via INC was energetically preferred. The SET process occurred only when the radical products were more stable than the separation products. The energy barrier for SET in the compounds studied was roughly estimated by comparison with other competing reactions. When the INC contained electron donor with lower ionization energy and electron acceptor with higher electron affinity, the SET reaction was more efficient.