Photoinduced Cyclization Research Articles

Photoinduced Intramolecular C−N Coupling For The Synthesis Of Azolo[1,2,4]Benzothiadiazines Derivatives

AbstractThis paper explores an original synthesis of novel heterocyclic compounds derived from photoinduced intramolecular coupling reactions, involving the formation of C−N and C−C bonds. Specifically, two classes of compounds were targeted: 4H‐benzo[e]pyrazolo[1,5‐b][1,2,4]thiadiazines and 5‐amino‐6‐thia‐4,5a‐diazaacephenanthrylenes. The synthetic strategy involved nucleophilic substitution reactions to prepare key sulfonamide starting substrates, followed by photoinduced cyclization (via the Substitution Radical Nucleophilic Unimolecular, SRN1) under mild conditions using visible light. The scope of the reaction was explored with different substituent groups on the starting materials, which demonstrated moderate to very good product yields, with 13 examples and yields of up to 78 %. Moreover, this strategy represents the first synthetic approach to obtain 5‐amino‐6‐thia‐4,5a‐diazaphenanthrylenes.

Photocatalytic Chemoselective Cyclic Oxysulfonylation of 2,3-Allenoic Acids.

Photoinduced cyclization of 2,3-allenoic acids with sulfonyl chloride providing an efficient synthesis of 4-sulfonylated furan-2(5H)-ones under mild reaction conditions has been achieved. The reaction enjoys a high chemoselectivity and tolerates a wide range of functional groups. The catalytic cycle has been validated through control experiments, cyclic voltammetry studies, and Stern-Volmer quenching studies.

Enhancing tetraphenylethene cyclization as photoswitch

AbstractTetraphenylethene (TPE), a star building block with promising aggregation‐induced emission, has received much interest. Given that its intramolecular Woodward‐Hoffmann cyclic intermediate instantaneously converts back to the original state within several picoseconds, the essentially photochromic characteristic of TPE is little investigated. Achieving a visible photocyclization of TPE is still an unsolved issue and considered as the bottleneck in the further advancement of applications. We report a strategy of attaching carbonate ester onto the TPE skeleton (TPE‐4C) to enhance TPE photocyclization stability. As demonstrated, the incorporated cholesteryloxycarbonyloxy substituents in TPE‐4C can increase the energy barrier for cycloreversion, thereby exhibiting extremely thermal stability of photocyclic intermediate upon UV irradiation, prolonging its lifetime from 63 picoseconds to 46 s by 7.2 × 1011‐fold. The photoinduced cyclization of TPE‐4C could be monitored with naked eyes, and the photocyclization/cycloreversion is achieved by turning on/off UV light along with a relative fatigue resistance. Encapsulation of TPE‐4C into the liquid crystal can induce a striking phase transformation (achiral↔chiral), which can be applicable to encode optical information. Employing carbonate ester into the TPE unit plays a vital role in enhancing the unprecedented TPE photocyclization stability, providing a toolbox to allow TPE‐based photocyclization to be visually monitored.

Photoreactive Polymorphic Cocrystals Utilizing a Molecular Template with Dual Halogen and Hydrogen Bonding Capabilities

The ability of 1,2,4,5-tetrachloro-3-iodobenzene to act as a template to form a pair of polymorphic cocrystals with trans-1,2-bis(4-pyridyl)ethylene is reported. After exposure to ultraviolet light, a nearly quantitative yield for the [2 + 2] cycloaddition reaction was observed for both concomitant cocrystals. These crystalline solids engage in both I···N halogen bonds along with nontraditional C–H···N hydrogen bond which results in a one-dimensional chain. In addition, the components homogeneous π–π stack which positions the carbon–carbon double bond on the reactant molecule in a suitable orientation to undergo a photoinduced cyclization reaction.

Substituent-Controlled Regioselective Photoinduced Cyclization of N-Allylbenzamides with N-Sulfonylaminopyridinium Salts

The annulation reactions of N-allylbenzamides with N-sulfonylaminopyridinium salts were developed under metal-free photoinduced mild conditions. Substituent-controlled sulfonaminoarylation and sulfonaminooxylation of benzamides were realized: N-allylbenzamides lead to benzosultams, while N-(2-phenylallyl)benzamides give sulfonamidylated oxazoline derivatives. Control experiments indicated that those reactions undergo a radical pathway with arylsulfonamidyl radicals as the intermediates. The aryl C-H bond functionalization in arylsulfonamidyl was involved for the first time to give benzosultams.

O-Nitrobenzyl Oxime Ethers Enable Photoinduced Cyclization Reaction to Provide Phenanthridines under Aqueous Conditions

In this paper, we describe a novel N-O photolysis of o-nitrobenzyl oxime ethers that enables the synthesis of phenanthridines via intramolecular cyclization reactions. Without the use of additional photocatalysts or photosensitizers, the process proceeds with an efficiency of ≤96% upon exposure of the sample to near-visible light (405 nm) under aqueous conditions. Through the photoinduced production of a fluorescent phenanthridine derivative in HeLa cells, the progress of the reaction under biological conditions was demonstrated. This photoinduced cyclization reaction could be used as a different photochemical instrument to control biological processes by inducing the production of bioactive molecules.

Photo-induced cyclization of olefinic amides towards sulfonamidylated iminoisobenzofurans and benzoxazines

A visible-light induced radical cyclization of olefinic amides with easily available N-sulfonylaminopyridinium salts towards iminoisobenzofurans and benzoxazines has been developed.

Cascade Synthesis of Phenanthrenes under Photoirradiation

We report a photoinduced phenanthrene synthesis from aryl iodides and styrenes through an arylation/cyclization cascade. Compared to prior methods, this approach obviates the need for hazardous reagents and provides access to unsymmetrical phenanthrenes with good functional group tolerance. Mechanistic studies revealed that photoexcitation of aryl iodides leads to homolytic C-I bond cleavage. Arylation of styrenes with the formed aryl radical species furnishes stilbene derivatives, which undergo photoinduced cyclization promoted by iodine generated in situ to yield phenanthrene products.

Photoinduced cyclization of aryl ynones with 4-alkyl-DHPs for the divergent synthesis of indenones, thioflavones and spiro[5.5]trienones

A direct and efficient visible-light-induced cascade cyclization reaction of aryl ynones with 4-alkyl-DHPs for the divergent synthesis of indenones, thioflavones and spiro[5.5]trienones via a 5-exo-trig or 6-exo-trig radical cyclization. This cascade reaction features operational simplicity, metal- and oxidant-free, and excellent functional group tolerance.

Photoinduced Intramolecular Cyclization of Acylsilanes Bearing a Boronate Moiety: Construction of a Highly Strained trans-Fused Bicyclo[3.3.0]octane Skeleton.

A reliable strategy for the construction of trans-fused bicyclo[n.3.0] skeletons was explored by utilizing photoinduced cyclization of acylsilanes bearing a boronate. The substrates having an acylsilane and a boronate in a 1,2-trans relationship were prepared via hydroboration of cycloalkene derivatives. The desired cyclization was efficiently promoted by photoirradiation to give the trans-fused bicyclo[n.3.0] derivatives as a single diastereomer. It is noteworthy that this methodology enables the efficient construction of a highly strained trans-5-5 ring system.

An unexpected photoinduced cyclization to synthesize fully substituted γ-spirolactones via intramolecular hydrogen abstraction with allyl acrylates

A photoinduced intramolecular cyclization of allyl acrylates to synthesize fully substituted spiro-γ-butyrolactone compounds is described. The reaction is believed to proceed through an intramolecular hydrogen abstraction strategy.

Maximizing Step‐Normalized Increases in Molecular Complexity: Formal [4+2+2+2] Photoinduced Cyclization Cascade to Access Polyheterocycles Possessing Privileged Substructures

AbstractA new complexity building photoinduced cascade which amounts to an unprecedented formal [4+2+2+2] cycloaddition topology is developed to access complex nitrogen polyheterocycles. This photocascade is initiated by the excited state intramolecular proton transfer (ESIPT) in aromatic amino ketones with tethered dual unsaturated pendants, i.e. pyrrole and alkenic moieties, resulting in the formation of four σ‐bonds and setting six new stereogenic centers in a single experimentally simple photochemical step with up to 220 mcbit complexity increases.

Maximizing Step-Normalized Increases in Molecular Complexity: Formal [4+2+2+2] Photoinduced Cyclization Cascade to Access Polyheterocycles Possessing Privileged Substructures.

A new complexity building photoinduced cascade which amounts to an unprecedented formal [4+2+2+2] cycloaddition topology is developed to access complex nitrogen polyheterocycles. This photocascade is initiated by the excited state intramolecular proton transfer (ESIPT) in aromatic amino ketones with tethered dual unsaturated pendants, i.e. pyrrole and alkenic moieties, resulting in the formation of four σ-bonds and setting six new stereogenic centers in a single experimentally simple photochemical step with up to 220 mcbit complexity increases.

Controlled Fabrication and Characterization of Coronene Diimide-Based Insoluble Thin Films Produced by Photoinduced Cyclization.

An insoluble thin film of a coronene diimide (CDI) derivative was fabricated from a soluble precursor of perylene diimide (PDI) by photoirradiation. We prepared a 1,7-diarylated PDI (TP-PDI) that can be converted into a coronene diimide (TP-CDI) derivative via a Scholl-type photocyclization reaction. This reaction was accompanied by structural changes from a twisted structure to a π-extended planar molecule. It was found that this photoconversion reaction occurs for both solution-based and thin-film-based reactants investigated by the changes of UV-vis absorption spectra and 1 H NMR spectra. The photocyclization reactions were found to proceed smoothly in polar solvents. In the thin-film state, the solvent vapor annealing method is a key process for achieving photoconversion reaction. Additionally, the fabrication of multi-layered thin films was achieved without undesirable dissolution of the underlying layers because of different solubilities of TP-PDI and TP-CDI.

Long chain fatty acid affects excited state branching in bilirubin-human serum protein Complex

After binding to human serum albumin, bilirubin could undergo photoisomerization and photo-induced cyclization process. The latter process would result the formation of a product, named as lumirubin. These photo induced behaviors are the fundamental of clinical therapy for neonatal jaundice. Previous studies have reported that the addition of long chain fatty acids is beneficial to the generation of lumirubin, yet no kinetic study has revealed the mechanism behind. In this study, how palmitic acid affects the photochemical reaction process of bilirubin in Human serum albumin (HSA) is studied by using femtosecond transient absorption and fluorescence up-conversion techniques. With the addition of palmitic acid, the excited population of bilirubin prefers to return to its hot ground state (S0) through a 4 ps decay channel rather than the intrinsic ultrafast decay pathways (<1 ps). This effect prompts the Z-Z to E-Z isomerization at the S0 state and then further increases the production yield of lumirubin. This is the first time to characterize the promoting effect of long chain fatty acid in the process of phototherapy with femtosecond time resolution spectroscopy and the results can provide useful information to benefit the relevant clinical study.

Photoinduced Cyclizations of o-Diisocyanoarenes with Organic Diselenides and Thiols that Afford Chalcogenated Quinoxalines.

This study describes the syntheses of 2,3-bis(selanyl)quinoxalines via the photoinduced cyclizations of o-diisocyanoarenes with diaryl or dialkyl diselenides, in addition to providing a detailed discussion of the corresponding mechanism and revealing that the developed procedure can also be applied to prepare 2-thiolated quinoxaline derivatives from o-diisocyanoarenes and thiols. The developed technique does not need the use of additives or metal catalysts and features the advantages of a high conversion, a broad substrate scope, and mild reaction conditions, thereby rendering it a valuable addition to the quinoxaline synthesis toolbox.

Effect of donor-π-acceptor structure on photochromism of dithienylethene-based dyes

Dithienylethene-based donor-π-acceptor photochromic dyes were synthesized to investigate the effect of the structure on photo-switching properties. Specifically, 3,5-dimethoxyphenyl and 4,4′-tert-butyltriphenylamino groups (donors) and benzoic and cyanoacetic acid groups (acceptors) were attached to an oxazole-based dithienylethene core in a horizontal or vertical fashion. The photochromic properties of these dyes were probed by UV–visible absorption spectroscopy upon exposure to UV (365 nm) and visible (>400 nm) light. The dye bearing 3,5-dimethoxyphenyl and carboxylic acid groups showed the best photochromic behavior in terms of quantum yield, and response time, which confirmed that strong electron-withdrawing substituents can hinder photo-induced cyclization by decreasing electron density at thiophene ring carbons 2 and 2’. Moreover, the introduction of an electron-withdrawing group onto the cycloethene moiety of vertical-type dyes led to a lower photochromic quantum yield than the introduction of an electron-withdrawing group at thiophene ring positions 5 and 5’.

Photoinduced cyclization of alkynoates to coumarins with N-Iodosuccinimide as a free-radical initiator under ambient and metal-free conditions

An efficient photoinduced strategy for the preparation of coumarins was developed. In the presence of N-iodosuccinimide (NIS) as a free-radical initiator and under LED (380–385 nm) irradiation and metal-free conditions, the reaction of alkynoates underwent smoothly to afford the corresponding coumarins in high yields at room temperature with broad substrate scope via free radical intramolecular cyclization and ester rearrangement.

Single-strand and ladder-type polymeric acceptors based on regioisomerically-pure perylene diimides towards all-polymer solar cells

Since dibromo perylene diimides (DBPDIs) contain two isomers that are difficult to separate, isomeric effect on properties and applications of PDI polymers has been rarely studied. Few ladder-type polymers have been used in organic solar cells. Herein, single-strand polymers (PrPDIT) were synthesized by the polymerization of regioisomerically-pure 1,7-dibromo-PDIs (1,7-DBPDIs) and 2,5-bis(trimethylstannyl)thiophene. Furthermore, ladder-type polymers (FPrPDIT) were obtained by the highly efficient photo-induced cyclization of polymers PrPDIT. The effects of isomers and molecular weight on the absorption, electrochemistry, thermal stability, and photovoltaic application of polymers were studied. Pure 1,7-DBPDIs lead to polymers with molecular weight distribution as low as 1.08. These polymers with strong absorption in 300–700 nm, low-lying lowest unoccupied molecular orbital (LUMO) energy levels around −4.0 eV and high thermal stability could be excellent electron acceptors. Regioisomerically-pure polymers demonstrate better photovoltaic performance than that of mixed ones. The highest power conversion efficiency (PCE) of all-polymer solar cells based on PrPDIT and FPrPDIT is up to 5.95% and 4.52% respectively.

Dihetarylethene photocyclization as a synthetic route to fluorescent compounds

It was shown that UV irradiation of imidazoline-bridged dihetarylethenes affords fused products exhibiting fluorescence. The starting compounds were prepared by three-component condensation of N,N’-dimethylurea with arylglyoxals and enols. A method was proposed for the synthesis of polycyclic imidazole derivatives by photoinduced cyclization followed by elimination of a water molecule. The advantages of the method include readily accessible starting compounds, process simplicity, and the ease of isolation of the target products. The crystal structure of the cyclization products was determined. It was shown that unlike the starting imidazol-2-ones, all photochemically prepared polycyclic products exhibit intense fluorescence.