Controlling the Conformational Changes in Donor–Acceptor [4]‐Dendralenes through Intramolecular Charge‐Transfer Processes

The synthesis of two [4]‐dendralene compounds incorporating thiophene‐(p‐nitrophenyl) donor–acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron‐donating components of the dendralenes are represented by bromothienyl (in 13 ) and ethylenedioxythiophene(EDOT)‐thienyl (in 15 ) end‐groups. The most facile transformation involves the isomerisation of donor–acceptor conjugated systems ( a conformers) into structures in which only the thiophenes are conjugated ( b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single‐crystal X‐ray diffraction studies and the structural changes in both compounds have been monitored by ¹H NMR spectroscopy and absorption studies. The transformations were found to be first‐order processes with rate constants of k=0.0027 s^?1 and k=0.00022 s^?1 for 13 and 15 , respectively. Density functional theory calculations at the B3LYP/6‐31G* level give credence to the proposed mechanism for the a → b conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative ( 15 ) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.     

Intramolecular Benzyne–Phenolate [4+2] Cycloadditions

An intramolecular benzyne–phenolate [4+2] cycloaddition is reported. Benzyne precursors, having vicinal halogen‐sulfonate functionalities, linked with a phenol(ate) by various tether groups undergo efficient intramolecular [4+2] cycloaddition by treatment with either Ph₃MgLi or nBuLi for halogen–metal exchange to form various benzobarrelenes.     

Proton‐Coupled Reduction of the Catalytic [4Fe‐4S] Cluster in [FeFe]‐Hydrogenases

In nature, [FeFe]‐hydrogenases catalyze the uptake and release of molecular hydrogen (H₂) at a unique iron‐sulfur cofactor. The absence of an electrochemical overpotential in the H₂ release reaction makes [FeFe]‐hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one‐electron reduction of [FeFe]‐hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton‐coupled electron transport during the formation of the reduced state Hred′. Charge compensation stabilizes the excess electron at the [4Fe‐4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred′ in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes.     

Enantioselective Intramolecular [2+2] Photocycloaddition Reactions of 4‐Substituted Coumarins Catalyzed by a Chiral Lewis Acid

Eight coumarins, which carry a terminal alkene tethered by a CH₂XCH₂ group to their 4‐position (X=CH₂, CMe₂, O, S, NBoc, NZ, NTs, NBn), were synthesized in overall yields of 51–80 %. Starting materials for the syntheses were either commercially available 4‐hydroxycoumarin or 4‐formylcoumarin. The intramolecular [2+2] photocycloaddition of these coumarins gave diastereoselectively products with a tetracyclic 3,3a,4,4a‐tetrahydro‐1H‐cyclopenta[2,3]cyclobuta[1,2‐c]chromen‐5(2H)‐one skeleton. Direct irradiation at λ=300 nm in dichloromethane (c=10 mM ) led to product formation in good yields for most substrates, presumably via a singlet excited state intermediate. Due to the low coumarin absorption at λ >350 nm the photocycloaddition was slow upon irradiation at λ=366 nm. Addition of a chiral oxazaborolidine‐based Lewis acid (50 mol %) increased the reaction rate at λ=366 nm and induced a significant enantioselectivity in the [2+2] photocycloaddition. Six out of eight coumarin substrates (X=CH₂, CMe₂, O, NBoc, NZ, NTs) gave the respective products in yields of 72–96 % and with 74–90 % enantiomeric excess (ee) upon irradiation in dichloromethane (c=20 mM ) at ?75 °C. The Lewis acid presumably acts by coordination to the coumarin carbonyl oxygen atom, which leads to a bathochromic shift (redshift) of the UV absorption and which increases the singlet state lifetime. A second electrostatic interaction of the hydrogen atom at C3 with the oxygen atom of the oxazaborolidine is likely.     

EPR Study on the Complex Formed by Charge‐Transfer Process between Ground‐state Acceptor 2, 3‐Dicyano‐5, 6‐dichloro‐1, 4‐benzoquinone and Some Donors and on Cation Radical of Perylene (or Pyrene)

EPR study showed that the semi‐quinone radical anion of 2,3‐dicyano‐5,6‐dichloro‐1,4‐benzoquinone (DDQ) was formed in a charge transfer process between ground‐state DDQ as acceptor and each one of following ground state donors, i.e., 4‐methyl‐4′‐tridecyl‐2, 2′‐bipyridyl; 4‐methyl‐4′‐nonyl‐2, 2′‐bipyridyl; bis (2,2′‐bipyridyl) (4‐methyl‐4′‐heptadecyl‐2, 2′‐bipyridyl)ruthenium(2+) perchlorate and perylene. EPR study also showed that there are perylene cation radical and pyrene cation radical in the following experimental conditions: (a) in 98% sulfuric add. (b) 10^?3 mol/L perylene (or pyrene) was dissolved in trifluoroacetic acid‐nitrobenzene (1: 1 V/V).     

Gold‐Catalyzed Intramolecular [3+2] Cycloadditions of 1‐Aryl‐1‐allene‐6‐enes

Treatment of 1‐aryl‐1‐allen‐6‐enes with [PPh₃AuCl]/AgSbF₆ (5 mol %) in CH₂Cl₂ at 25 °C led to intramolecular [3+2] cycloadditions, giving cis‐fused dihydrobenzo[a]fluorene products efficiently and selectively. The reactions proceeded with initial formation of trans/cis mixtures of 2‐alkyl‐1‐isopropyl‐2‐phenyl‐1,2‐dihydronaphthalene cations B, which were convertible into the desired cis‐fused cycloadducts through the combined action of a gold catalyst and a Brønsted acid. Theoretic calculation supports the participation of the trans‐B cation as reaction intermediate. Although HOTf showed similar activity towards several 1‐aryl‐1‐allen‐6‐enes, it lacks generality for this cycloaddition reaction.     

Efficient Intramolecular Charge Transfer in Oligoyne‐Linked Donor–π–Acceptor Molecules

Studies are reported on a series of triphenylamine–(C?C)_n–2,5‐diphenyl‐1,3,4‐oxadiazole dyad molecules (n=1–4, 1 , 2 , 3 and 4 , respectively) and the related triphenylamine‐C₆H₄–(C?C)₃–oxadiazole dyad 5 . The oligoyne‐linked D–π–A (D=electron donor, A=electron acceptor) dyad systems have been synthesised by palladium‐catalysed cross‐coupling of terminal alkynyl and butadiynyl synthons with the corresponding bromoalkynyl moieties. Cyclic voltammetric studies reveal a reduction in the HOMO–LUMO gap in the series of compounds 1 – 4 as the oligoyne chain length increases, which is consistent with extended conjugation through the elongated bridges. Photophysical studies provide new insights into conjugative effects in oligoyne molecular wires. In non‐polar solvents the emission from these dyad systems has two different origins: a locally excited (LE) state, which is responsible for a π*→π fluorescence, and an intramolecular charge transfer (ICT) state, which produces charge‐transfer emission. In polar solvents the LE state emission vanishes and only ICT emission is observed. This emission displays strong solvatochromism and analysis according to the Lippert–Mataga–Oshika formalism shows significant ICT for all the luminescent compounds with high efficiency even for the longer more conjugated systems. The excited‐state properties of the dyads in non‐polar solvents vary with the extent of conjugation. For more conjugated systems a fast non‐radiative route dominates the excited‐state decay and follows the Engelman–Jortner energy gap law. The data suggest that the non‐radiative decay is driven by the weak coupling limit.     

β‐Substituted Terthiophene [2]Rotaxanes

Towards polythiophene polyrotaxanes : The β‐substituted terthiophene [2]rotaxanes have been synthesized (see figure). Basic optical and electrochemical properties of the synthesized [2]rotaxanes are also reported.

     

Synthesis of redox-active, intramolecular charge-transfer chromophores by the [2+2] cycloaddition of ethynylated 2H-cyclohepta[b]furan-2-ones with tetracyanoethylene

Ethynylated 2H-cyclohepta[b]furan-2-ones 5-15 have been prepared by Pd-catalyzed alkynylation of 3-iodo-5-isopropyl-2H-cyclohepta[b]furan-2-one (2) with the corresponding ethynylarenes or the reaction of 2-iodothiophene with 3-ethynyl-5-isopropyl-2H-cyclohepta[b]furan-2-one (4) under Sonogashira-Hagihara conditions. Compounds 5-15 reacted with tetracyanoethylene in a formal [2+2] cycloaddition reaction, followed by ring opening of the initially formed [2+2] cycloadducts, cyclobutenes, to afford the corresponding 1,1,4,4-tetracyanobutadienyl (TCBD) chromophores 16-26 in excellent yields. The intramolecular charge-transfer interactions between the 2H-cyclohepta[b]furan-2-one ring and TCBD acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of the novel TCBD derivatives 16-26 was examined by cyclic voltammetry and differential pulse voltammetry, which revealed multistep electrochemical reduction properties, depending on the number of TCBD units in the molecule. Moreover, a significant color change was observed by UV/Vis spectroscopy under electrochemical reduction conditions.     

High‐Field Pulsed EPR Spectroscopy for the Speciation of the Reduced [PV2Mo10O40]6− Polyoxometalate Catalyst Used in Electron‐Transfer Oxidations

An in‐depth spectroscopic EPR investigation of a key intermediate, formally notated as [PV^IVV^VMo₁₀O₄₀]^6? and formed in known electron‐transfer and electron‐transfer/oxygen‐transfer reactions catalyzed by H₅PV₂Mo₁₀O₄₀, has been carried out. Pulsed EPR spectroscopy have been utilized: specifically, W‐band electron–electron double resonance (ELDOR)‐detected NMR and two‐dimensional (2D) hyperfine sub‐level correlation (HYSCORE) measurements, which resolved ⁹⁵Mo and ¹⁷O hyperfine interactions, and electron–nuclear double resonance (ENDOR), which gave the weak ⁵¹V and ³¹P interactions. In this way, two paramagnetic species related to [PV^IVV^VMo₁₀O₄₀]^6? were identified. The first species (30–35 %) has a vanadyl (VO²⁺)‐like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO²⁺ was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PV^VMo₁₀O₃₉]^8?[V^IVO²⁺]. This species originates from the parent H₅PV₂Mo₁₀O₄₀ in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electron‐transfer/oxygen‐transfer reaction oxidation reactions. In the second (70–65 %) species, the V^IV remains embedded within the polyoxometalate framework and originates from reduction of distal H₅PV₂Mo₁₀O₄₀ isomers to yield an intact cluster, [PV^IVV^VMo₁₀O₄₀]^6?.     

Charge Fluctuations and Ethylene‐Group‐Ordering Transition in β′′‐(BEDT‐TTF)4[(H3O)Fe(C2O4)3]⋅Y Molecular Charge‐Transfer Salts

The polarized infrared reflectance and Raman spectra of the three quasi‐two‐dimensional β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?Y bifunctional charge‐transfer salts, where BEDT‐TTF=bis(ethylenedithio)tetrathiafulvalene and Y=C₆H₅Br, (C₆H₅CN)_0.17(C₆H₅Br)_0.83, (C₆H₅CN)_0.4(C₆H₅F)_0.6, have been measured as a function of the temperature. Signatures of charge inhomogenity have been found in both Raman and infrared spectra of the β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?Y superconductors. A 100 K transition to a mixed insulating/metallic state is clearly seen for the first time in the temperature dependence of the electronic spectra of superconducting β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?C₆H₅Br. We suggest that this phase transition is due to subtle changes in the ethylene groups ordering, which are related to a structural phase transition in the anionic layer. The infrared and Raman spectra of quasi‐two‐dimensional metal α‐′pseudo‐κ′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]C₆H₄Br₂ are also investigated.     

Catalytic Asymmetric [4+2]‐Cycloadditions Using Tropolones: Developments,Scope, Transformations,and Bioactivity

An organocatalyzed asymmetric [4+2]‐cycloaddition between tropolones and electron‐deficient dienophiles is presented. Complex and biologically interesting dihydrohomobarrelenone scaffolds are formed through a Diels–Alder reaction utilizing bifunctional Brønsted‐base catalysis, affording the corresponding bridged bicyclic cycloadducts in up to quantitative yields with good enantio‐ (up to 92 % ee) and diastereoselectivity (up to >20:1 d.r.). The synthetic value of the obtained products is explored by downstream transformations, including photoisomerizations, and their biological relevancy by in vivo testing in MCF‐7 cancer cells.     

Enantioselective Formal [4+2] Cycloadditions to 3‐Nitroindoles by Trienamine Catalysis: Synthesis of Chiral Dihydrocarbazoles

The first enantioselective formal [4+2] cycloadditions of 3‐nitroindoles are presented. By using 3‐nitroindoles in combination with an organocatalyst, chiral dihydrocarbazole scaffolds are formed in moderate to good yields (up to 87 %) and enantioselectivities (up to 97 % ee). The reaction was extended to include enantioselective [4+2] cycloadditions of 3‐nitrobenzothiophene. The reaction proceeds through a [4+2] cycloaddition/elimination cascade under mild reaction conditions. Furthermore, a diastereoselective reduction of an enantioenriched cycloadduct is presented. The mechanism of the reaction is discussed based on experimental and computational studies.