Photocyclization Reaction Research Articles

Enantioselective photocyclisation reactions of 2-aryloxycyclohex-2-enones mediated by a chiral copper-bisoxazoline complex

The photocyclisation of the title compounds leads upon direct irradiation at λ = 366 nm in dichloromethane solution to racemic cis-2,3,4a,9b-tetrahydro-1H-dibenzofuran-4-ones (nine examples, 37–74% yield). Since it was found that the substrates show a significant bathochromic absorption shift upon treatment with EtAlCl2, it was attempted to perform the reactions enantioselectively in the presence of a chiral Lewis acid. A complex of Cu(ClO4)2·6H2O and a bisoxazoline ligand gave the best enantioselectivities (up to 60% ee). Two procedures are reported for the enantioselective photocyclisation. The first protocol is based on a direct irradiation at λ = 368 nm (LED) with a catalyst loading of 50 mol% and it delivered the products in 26–76% yield with 22–40% ee. The second protocol is applicable to electron rich 2-aryloxycyclohex-2-enones (31–62% yield, 29–46% ee) and relies on sensitization by thioxanthone (50 mol%) at λ = 419 nm.

A novel strategy for estimation of selective photochromism by the fluorescence change in a multiswitchable dithienylethene system

Dithienylethenes 1 and 2, consisting of three photochromic units linked by imidazole bridges, together with its N-methylated derivatives 3 and 4, have been synthesized. Their photochromic properties and fluorescence behaviors have also been investigated in solution by irradiation with various wavelengths of UV light. It was found that the dithienylethene 1 showed selective photochromism, indicated by the fluorescence change upon irradiation with 365 nm and 302 nm light. However, dithienylethenes 2, 3 and 4 only underwent a photocyclization reaction in the middle switchable unit under the same conditions. Moreover, for dithienylethenes 2 and 4, introduction of fluorine atoms on the cyclopentene evidently enhanced the photochromic properties compared with perhydro-substituted dithienylethenes 1 and 3. And dithienylethene 3 could act as one potential fluorescent molecular switch.

Synthesis, Structure and Photophysical Properties of Pyrene–based [5]Helicenes: an Experimental and Theoretical Study

AbstractPyrene‐cored [5]helicenes were prepared by a facile, efficient Wittig reaction and an intramolecular photocyclization reaction utilising 2,7‐di‐tert‐butylpyrene‐4‐carbaldehyde and naphthalene/pyrene‐based phosphorus ylides. Optical properties based on UV−vis absorption and fluorescence spectra were investigated. X‐ray crystallography revealed that the pyrene‐based [5]helicenes exhibited strong face‐to‐face π−π interactions and edge‐to‐face π−π interactions. HOMO and LUMO energies and molecular orbitals were also studied by density functional theory (DFT) calculations. This study has revealed that the torsion angle of the helical structure plays a role in determining the π−π interactions and the frontier molecular orbital energy levels. Thus, pyrene‐based helicenes need to be considered when one designs new highly efficient organic light‐emitting diodes and organic semiconductor materials.

Structural and Spectral Properties of Photochromic Diarylethenes: Size Effect of the Ethene Bridge.

The effect of the size of the ethene bridge on the structural and spectral properties of photochromic diarylethenes, which remains a poorly understood phenomenon, was studied as applied to diarylethenes containing unsymmetrical (cyclohexenone and cyclopentenone) and symmetrical (cyclohexene and cyclopentene) ethene bridges. Thiophene, oxazole, and imidazole derivatives were used as aryl moieties. An increase in the size of the ethene bridge in the cycloalkenone series was found to be accompanied by a hypsochromic shift of the absorption maximum of the photoinduced form, whereas no difference was found for cycloalkenes. A detailed analysis of the NMR spectra (including 2D experiments) revealed previously unknown effects associated with the existence of an intramolecular hydrogen bond (CH···N) between the six-membered ethene bridge and the azole substituents. The NMR experimental data obtained were confirmed by DFT quantum chemical calculations and X-ray analysis. It was found that an intramolecular hydrogen bond favors an increase of the quantum yield of the photocyclization reaction.

Radical Cyclization of Trichloroacetamides: Synthesis of Lactams

Trichloroacetamides can act as radical precursors to synthesize nitrogen-containing heterocycles in a variety of processes, mainly involving atom transfer radical cyclizations (ATRC), mediated by Cu(I) or Ru(II) catalysts, and the hydride reductive method, employing either Bu₃SnH or (Me₃Si)₃SiH, or recently NaBH₃CN. Additionally, amine-mediated single-electron transfer cyclizations, as well as radical processes promoted by Ni, Fe, Mn, Ti, and Ag, have been developed. 1 Introduction 2 Atom Transfer Radical Cyclizations (ATRC) 2.1 Copper-Mediated Reactions 2.2 Ruthenium(II)-Mediated Reactions 2.3 ATRC Promoted by Other Metals 3 Photocyclization Reactions 4 Reductive Cyclization via Single-Electron Transfer Processes 4.1 Ni/AcOH-Promoted Cyclizations 4.2 Titanocene(III)-Catalyzed Reactions 4.3 Silver Acetate Mediated Reactions 4.4 Amine-Mediated Single-Electron Transfer 5 Cyclization Involving M–H Hydrogen Atom Donors (M = Sn, Si, B) 5.1 Bu₃SnH-Mediated Reactions 5.2 (Me₃Si)₃SiH (TTMSS)-Mediated Reactions 5.3 Borohydride-Mediated Radical Cyclizations 6 Concluding Remarks

Diastereoselective photocyclization of a photochromic diarylethene having a benzo[b]phosphole P-oxide group

A photochromic diarylethene derivative 1 having a chiral benzo[b]phosphole P-oxide at the aryl moiety underwent a diastereoselective photocyclization reaction in acetonitrile upon irradiation with ultraviolet (UV) light. It was found that the diastereomeric excess (d.e.) value changes depending on the conversion ratio from the open- to closed-ring isomer as well as the irradiation wavelength. This is attributed to the difference in the photocycloreversion quantum yields of diastereomers of the closed-ring isomer. The diarylethene showed a high photocyclization quantum yield owing to a high population of photoreactive conformers of the open-ring isomer by the steric effect of the benzo[b]phosphole P-oxide group.

Substituent effect in the photochromism of two isomeric asymmetric diarylethenes having pyrrole and thiophene units

Two new asymmetric isomeric diarylethenes having pyrrole and thiophene units have been synthesized by one-pot reaction and characterized by single crystal X-ray diffraction analysis. The two prepared diarylethenes had disparate crystal structures, and they exhibited distinctly different photochromic behavior, both in solution and in the solid state. Their photochromism, fatigue resistance, and fluorescence were investigated systematically. The methyl group at the reactive carbon atom could significantly enhance the quantum yield of cyclization step and decrease the quantum yield of cycloreversion step, whereas a cyano group at the same position could notably suppress the photocyclization reaction and promote the photocycloreversion reaction. The results indicated that the substituent at the reactive carbon atom could readily modulate the optoelectronic and physical properties for these diarylethenes.

A new dithienylethene dimer with terminal tertiary amine redox centers: Electrochemical, UV–vis–NIR spectral and electronic transfer charges induced by a stepwise photochromic process

Photochromic diarylethene dimers (R-DTE-CCCC-DTE-R, R = benzene, 1oo; R = triphenylamine, 2oo; DTE = dithienylethene) with remarkable stepwise photochromic process were synthesized successfully. Dimer 2oo, whose two terminal redox-active triphenylamine centers were separated by two photochromic DTE-acetylide moieties, could accomplish photoswitchable charge delocalization between the N⋯N electro-active centers. Upon UV-light irradiation, due to the progressive increase of π-conjugated systems as the order of 2oo → 2co → 2cc, the electronic interaction of two N-centers in 2oo was increasingly facilitated following the stepwise photocyclization. Furthermore, for monocationic species [2oo]+/[2co]+/[2cc]+, their absorption strength was gradually increased during the conversion of [2oo]+→[2co]+→[2cc]+, which demonstrated that the extent of charge delocalization along the molecular backbones was progressively enhanced during the stepwise photocyclization. The lower electronic density of reactive carbon atoms during the oxidation of N0 to N+ could result more difficult photocyclization reaction to produce oxidized species [2oo]2+/[2co]2+. Additionally, compound 2 exhibited eight switchable states through stepwise photochromic and reversible redox processes.

Air-Driven Potassium Iodide-Mediated Oxidative Photocyclization of Stilbene Derivatives.

A new method has been developed for the potassium iodide-mediated oxidative photocyclization of stilbene derivatives. Compared with conventional iodine-mediated oxidative photocyclization reactions, this new method requires shorter reaction times and affords cyclized products in yields of 45-97%. This reaction proceeds with a catalytic amount of potassium iodide and works in an air-driven manner without the addition of an external scavenger. The radical-mediated oxidative photocyclization of stilbene derivatives using TEMPO was also investigated.

Regioselective Photocyclizations of Di(quinolinyl)arylamines and Tri(quinolinyl)amine with Emission Color Changes and Photoreaction-Induced Self-Assemblies.

Bis[2,4-di(trifluoromethyl)quinoline-7-yl]amine (1), bis[2,4-di(trifluoromethyl)quinoline-7-yl]methylamine (2), bis[2,4-di(trifluoromethyl)quinoline-7-yl]phenylamine derivatives, Q2 NPhX; X=NO2 (3 a), I (3 b), H (3 c), OMe (3 d), and NH2 (3 e), tris[2,4-di(trifluoromethyl)quinoline-7-yl]amine (4), and bis[2,4-di(pentafluoroethyl)quinoline-7-yl]-4-nitrophenylamine (5) were prepared as functional fluorophores. On irradiating the solution samples, 1 showed no noticeable alteration, whereas 2, 3 a-d, and 4 showed emission color changes from yellowish green to blue, indicating that a photoreaction took place. Analyses of the photoproduct based on absorption and emission spectra, (1) H NMR spectra, and X-ray crystallography indicated that photocyclization reactions occurred regioselectively and quantitatively to form bent-bent dipyridocarbazoles. In 3 a-d, the reaction rates depended on the solvent polarity and the substituent on the benzene ring. The photoreactions were accelerated with decreasing solvent polarity and with increasing electron-withdrawing character of the substituents. The photocyclization of triquinolineamine 4 was faster than that of 3 a in all solvents. The results of semiempirical quantum-chemical PM6 calculations suggested that the observed regioselective photocyclization could be explained by stabilization of the excited triplet transition state for the bent-bent form because of the molecular geometry with the CH-NQ hydrogen bonds. The solution of 5 in MeOH displayed photoreaction-induced self-assembly behavior to form twisted tape-like fibers of width 200 nm, as determined by TEM imaging.

A computational study of the mechanism of the photocyclization reaction of α-methylamino ketone

The mechanism for the photochemical cyclization reaction is studied theoretically using a model system of α-N-methylamidoacetophenone with the M06-L/6-311G(d,p) method.

Multistate and Multicolor Photochromism through Selective Cycloreversion in Asymmetric Platinum(II) Complexes with Two Different Dithienylethene-Acetylides.

Four asymmetric bis(dithienylethene-acetylide) platinum(II) complexes trans-Pt(PEt3)2(L1o)(L5o) (1oo), trans-Pt(PEt3)2(L2o)(L5o) (2oo), trans-Pt(PEt3)2(L3o)(L5o) (3oo), and trans-Pt(PEt3)2(L4o)(L5o) (4oo) with two different dithienylethene-acetylides (L1o-L5o) were designed to modulate stepwise, multistate, and multicolor photochromism by modifying ring-closure absorption wavelengths. Upon irradiation under UV light, 1oo converts only to 1oc without the observation of 1co and dually ring-closed species 1cc. In contrast, both mixed ring-open/closed species oc and co as well as dually ring-closed species cc are observed upon UV light irradiation of 2oo-4oo, implying that a substantial stepwise photochromic process occurs following 2oo-4oo → 2oc-4oc/2co-4co → 2cc-4cc. The conversion percentage of dually ring-closed species at the photostationary state (PSS) is progressively increased following 1cc (0%) → 2cc (40%) → 3cc (86%) → 4cc (>95%), coinciding with the progressive red-shift of ring-closure absorption bands in free L1c (441 nm) → L2c (510 nm) → L3c (556 nm) → L4c (591 nm). Particularly, compound 2 affords four states (2oo, 2co, 2oc, and 2cc) with different colors (colorless, purple, blue, and dark blue, respectively) through a selective photochemical cycloreversion process upon irradiation with appropriate wavelengths of light. Although stepwise photocyclization reactions 3oo → 3co/3oc → 3cc and 4oo → 4co/4oc → 4cc are observed, multicolor photochromism of 3oo and 4oo could not be achieved because ring-closure absorption bands between L3c/L4c and L5c are significantly overlapped. The stepwise photochemical processes are well demonstrated by NMR, UV-vis, and infrared (IR) spectroscopy and time-dependent density functional theory (TD-DFT) computational studies.

Diastereoselective Strategies towards Thia[n]helicenes.

In the present study, we have investigated different strategies for diastereoselective synthesis of thia[n]helicenes. We describe the introduction of different chiral auxiliaries at various positions and investigated their effect in the photocyclization reaction. Different chiral groups were placed at the sterically hindered position of the helical core and their interactions with various solvents and metals like copper were investigated. The use of Cu(I) salts has led to high diastereoselectivity in the photocyclization process and we were successful in obtaining the thia[5]helicene in enantiomerically pure form in good yield. The single diastereomer obtained was characterized by X-ray crystallography. From the study of the barrier of racemization of these thia[5]helicenes, the stability was found to be comparable to unsubstituted tetrathia[7]helicenes and substituted diazadithia[7]helicenes. This approach provides an easy access to enantiopure helicenes.

Photocyclization Reactions of Diarylethenes via the Excited Triplet State.

Cyclization reactions of three diarylethene derivatives, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), 1,2-bis(2-hexyl-3-benzothienyl)perfluorocyclopentene (BTHex), and 1,2-bis(2-isopropyl-3-benzothienyl)perfluorocyclopentene (BTiPr), via their excited triplet states were studied by means of steady-state and nanosecond transient absorption spectroscopy. The excited triplet states of BT, BTHex, and BTiPr were generated by energy transfer from the photoexcited triplet states of sensitizers such as xanthone, phenanthrene, and pyrene. The single-step quantum yields of the cyclization reactions from the excited triplet states of BT, BTHex, and BTiPr were determined to be 0.34, 0.53, and 0.65, respectively. The triplet energies of these three BTs were estimated to be 190-200 kJ mol(-1).

ChemInform Abstract: Tetrabutylammonium‐Assisted Diastereoselective [6π]‐Photocyclization of Acrylanilides.

AbstractTetrabutylammonium bromide is remarkably effective for improving the diastereoselectivity for [6π]‐photocyclization reactions of acrylanilides to form predominantly trans‐dihydroquinolines.

Photon-Quantitative 6π-Electrocyclization of a Diarylbenzo[b]thiophene in Polar Medium.

The high reactivity of 6π-electrocyclization in polar solvents has remained one of the important challenges for diarylethenes because of the emergence of a twisted intramolecular charge transfer (TICT) state at the excited state in such polar media, which usually quenches the photocyclization reaction. Herein we report on the preparation and highly efficient photocyclization of 2,3-diarylbenzo[b]thiophenes with nonsymmetric side-aryl units in a polar solvent. While the dithiazolylbenzo[b]thiophene showed a suppressed quantum yield of 6π-electrocyclization of 54 % in methanol, the replacement of a thiazole unit with a thiophene ring led to a photon-quantitative 6π-cyclization reaction. The nonsymmetrical modification into the side-aryl units was considered to enhance the CH/π interactions between side-aryl units to support a photoreactive conformation in methanol. The stabilization of the photochromic reactive conformation is expected to suppress the formation of the TICT state at the excited state, leading to highly efficient photoreactivity.

ESR - a retinal protein with unusual properties from Exiguobacterium sibiricum.

This review covers the properties of a retinal protein (ESR) from the psychrotrophic bacterium Exiguobacterium sibiricum that functions as a light-driven proton pump. The presence of a lysine residue at the position corresponding to intramolecular proton donor for the Schiff base represents a unique structural feature of ESR. We have shown that Lys96 successfully facilitates delivery of protons from the cytoplasmic surface to the Schiff base, thus acting as a proton donor in ESR. Since proton uptake during the photocycle precedes Schiff base reprotonation, we conclude that this residue is initially in the uncharged state and acquires a proton for a short time after Schiff base deprotonation andM intermediate formation. Involvement of Lys as a proton donor distinguishes ESR from the related retinal proteins- bacteriorhodopsin (BR), proteorhodopsin (PR), and xanthorhodopsin (XR), in which the donor function is performed by residues with a carboxyl side chain. Like other eubacterial proton pumps (PR and XR), ESR contains a histidine residue interacting with the proton acceptor Asp85. In contrast to PR, this interaction leads to shift of the acceptor's pKa to more acidic pH, thus providing its ability to function over a wide pH range. The presence of a strong H-bond between Asp85 and His57, the structure of the proton-conducting pathways from cytoplasmic surface to the Schiff base and to extracellular surface, and other properties of ESR were demonstrated by solving its three-dimensional structure, which revealed several differences from known structures of BR and XR. The structure of ESR, its photocycle, and proton transfer reactions are discussed in comparison with homologous retinal proteins.

Preparation and Structural Properties of Highly Twisted Polycyclic Aromatic Hydrocarbons with [4]Helicene Components

Novel di-tert-butylated polycyclic aromatic hydrocarbons (PAHs) with [4] helicene components were prepared through an oxidative photocyclisation reaction. The structural properties of the PAHs were evaluated with 1H NMR and X-ray crystallography. It was detected that the PAHs have highly twisted π-conjugated systems and a deep fjord region. Although they are not planar, these PAHs can readily form dimers in solutions and undergo staggered one-dimensional array columnar crystal packing by intermolecular π–π interactions.

Highly sensitive and selective fluorescent sensor for zinc ion based on a new diarylethene with a thiocarbamide unit.

A new photochromic diarylethene has been synthesized by using thiocarbamide as a functional group and perfluordiarylethene as photoswitching trigger via a salicylidene Schiff base linkage. The diarylethene could be used as a multicontrollable fluorescence switch when triggered by base/acid, light, and metal ions. The results showed that the absorption and fluorescence characteristics of the diarylethene exhibited sequence-dependent responses through efficient interaction of specific salicylidene Schiff base-linked thiocarbamide unit with tetrabutylammonium hydroxide/trifluoroacetic acid and photoirradiation. Moreover, the diarylethene was highly selective toward Zn(2+) ion with obvious fluorescence change from light blue to bright yellow in acetonitrile. The deprotonated form of the diarylethene had typical photochromism, but it showed an irreversible photocyclization reaction after binding with Zn(2+). Finally, two logic circuits were constructed by using the fluorescence intensity as the output signal with the inputs of the combinational stimuli of light and chemical species.

Tetrabutylammonium-assisted diastereoselective [6π]-photocyclization of acrylanilides

Tetrabutylammonium salts are remarkably effective for increasing diastereoselectivities in [6π]-photocyclization reactions of acrylanilides. This TBA+-assisted photocyclization is applicable to a variety of acrylanilides to afford trans-dihydroquinolones. Using a d5-labeled substrate, it was elucidated that a tetrabutylammonium ion shields a zwitterionic intermediate from an intermolecular H-transfer, which enables preferential occurrence of the [1,5] H-shift to give trans products stereoselectively.