Molecular dynamics and metadynamics simulations of [2 + 2] photocycloaddition

Triplet state mechanism of [2 + 2] photocycloaddition forming a cyclobutane ring from two ethylenes is investigated in the context of photocatalysis. High‐level ab initio calculations are combined with ab initio adiabatic molecular dynamics and ab initio metadynamics for rare events modeling. In a photocatalytic scheme, a reactant reaches the triplet state either via intersystem crossing (ISC) or triplet sensitization. The model system adopts a biradical structure, which represents energy intersection with the ground state. The system either completes cyclization or undergoes fragmentation into two olefinic units. The potential and free energy surfaces of the cyclobutane/ethylenes system are mapped with multireference approaches describing possible reaction pathways. To obtain a full picture of a double bond photoreactivity, ab initio adiabatic dynamical calculations were used to estimate reaction yields and to model the effects of excess energy. The potential use of density functional theory based approaches for [2 + 2] photocycloaddition was investigated for future simulations and design of realistic photocatalytic systems. Dynamical aspects of [2 + 2] photocycloaddition via a triplet state manifold are investigated by combining ab initio multireference methods and ab initio molecular dynamics and metadynamics. The reaction pathways are studied for a model system of two ethylenes forming a cyclobutane ring to provide a basis for further studies on design of photocatalytic systems.     

Theoretical studies of spin state‐specific [2 + 2] and [5 + 2] photocycloaddition reactions of n‐(1‐penten‐5‐yl)maleimide

N‐alkenyl maleimides are found to exhibit spin state‐specific chemoselectivities for [2 + 2] and [5 + 2] photocycloadditions; but, reaction mechanism is still unclear. In this work, we have used high‐level electronic structure methods (DFT, CASSCF, and CASPT2) to explore [2 + 2] and [5 + 2] photocycloaddition reaction paths of an N‐alkenyl maleimide in the S₁ and T₁ states as well as relevant photophysical processes. It is found that in the S₁ state [5 + 2] photocycloaddition reaction is barrierless and thus overwhelmingly dominant; [2 + 2] photocycloaddition reaction is unimportant because of its large barrier. On the contrary, in the T₁ state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo‐addition reaction. Mechanistically, both S₁ [5 + 2] and T₁ [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S₁ [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S₁ intermediate, which then decays to the S₀ state as a result of an S₁ → S₀ internal conversion. In the T₁ [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T₁ → S₀ intersystem crossing process to the S₀ state. In the S₀ state, the second C C bond is formed. Our present computational results not only rationalize available experiments but also provide new mechanistic insights. © 2017 Wiley Periodicals, Inc.     

RELATIVE REACTIVITIES OF THE EXCITED STATES OF FUROCOUMARINS FOR [2+2] PHOTOCYCLOADDITION REACTION WITH TETRAMETHYLETHYLENE

Abstract— The Stern-Volmer constants for fluorescence quenching by tetramethylethylene decrease in the order DMC ≫ DHP > F-2 > 8-MOP. The same order was observed for the quantum yields of [2+2] cycloaddition reaction with tetramethylethylene on direct irradiation. In [2+2] photocycloaddition of F-2 with tetramethylethylene in ethanol, the ratio of quantum yields deduced from singlet and triplet states of F-2; φ₃⁰/φ₁⁰, is about 5. The excited triplet state is the reactive state for the [2+2] photocycloaddition of F-2 with tetramethylethylene in solution but the excited singlet state of F-2 becomes very important in biological conditions.     

Photoaddition reactions of acetylene and butadiyne derivatives to benzodithiophene

The photochemical [2pi +2pi] cycloaddition of dimethyl acetylenedicarboxylate to benzo[1,2-b:4,5-b']dithiophene has been used to synthesize substituted cyclobuta[b]thieno[2,3-f][1]benzothiophene. The first [2pi + 2pi] photocycloaddition reaction of a series of butadiynes to benzodithiophene is reported to yield regioselective and acetylene-substituted cyclobutene derivatives containing an aromatic thiophene moiety.     

Polymer‐based colorimetric and “turn off” fluorescence sensor incorporating benzo[2,1,3]thiadiazole moiety for Hg2+ Detection

A conjugated polymer was synthesized by the polymerization of 4,7‐dibromobenzo[2,1,3]thiadiazole ( M‐1 ) with tri{1,4‐diethynyl‐2,5‐bis(2‐(2‐methoxyethoxy)‐ethoxy)}‐benzene ( M‐2 ) via Pd‐catalyzed Sonogashira reaction. The polymer shows strong orange fluorescence. The responsive optical properties of the polymer on various metal ions were investigated through photoluminescence and UV–vis absorption measurements. The polymer displays highly sensitive and selective on‐off Hg²⁺ fluorescence quenching property in tetrahydrofuran solution in comparison with the other cations including Mg²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Cd²⁺, and Pb²⁺. More importantly, the fluorescent color of the polymer sensor disappears after addition of Hg²⁺, which could be easily detected by naked eyes. The results indicate that this kind of polymer sensor incorporating benzo[2,1,3]thiadiazole moiety as a ligand can be used as a novel colorimetric and fluorometric sensor for Hg²⁺ detection. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Reaction-phase-selective inter- and intramolecular photochemical reaction of 2-pyridone derivatives

Phase-selective photochemical reaction of 2-pyridone derivatives was examined. Irradiation of 1 in benzene mainly gave rearrangement products 2. However, intermolecular [4 + 4] photocycloaddition proceeded quantitatively in the solid state, affording photodimers 3. An effective pi-pi stacking and dipole-dipole interaction between two pyridone moieties might play important roles in an effective arrangement of 1 for photodimerization in their crystal structures. [reaction: see text]     

Poly(ProDOT‐Et2): A High‐Contrast,High‐Coloration Efficiency Electrochromic Polymer

The synthesis and electrochemical polymerization of 3,3‐diethyl‐3,4‐dihydro‐2H‐thieno‐[3,4‐b][1,4]dioxepine (ProDOT‐Et₂) was performed resulting in a stable electrochromic polymer capable of switching between an absorbing blue neutral state and a highly transmissive sky‐blue oxidized state in sub‐second time frames. High optical switching contrast ratios (up to 75% at λ_max) and high composite coloration efficiencies (505 cm²/C) were measured.     

Diastereoselective [2+2] photocycloaddition of polymer-supported cyclic chiral enone with ethylene

The phenylmenthyl derivative, previously shown to be very effective chiral auxiliaries in the diastereoselective [2+2] photocycloaddition of cyclic enones with the simplest olefin, ethylene, was attached to poly(ethylene glycol)-grafted Wang resin. We then investigated the diastereoselective [2+2] photocycloaddition on solid support in several solvents. As the result, we accomplished good selectivity in toluene as well as the recycle of the polymer-supported chiral auxiliary. This is the first example that bicyclo[4.2.0]octane derivative has been obtained photochemically on the solid support.     

Gadolinium Photocatalysis: Dearomative [2+2] Cycloaddition/Ring-Expansion Sequence with Indoles

Lanthanide photocatalysts are much less investigated in synthetic chemistry than rare and expensive late transition metals. We herein introduce Gd^III photocatalysis of a highly regioselective, intermolecular [2+2] photocycloaddition/ring-expansion sequence with indoles, which could provide divergent access to cyclopenta[b]indoles and indolines. A simple and commercially available Gd(OTf)₃ salt is sufficient for this visible-violet-light-induced transformation. The reaction proceeds either through a transient or start-to-end dearomatization cascade and shows excellent regioselectivity (usually >95:5 r.r.), broad scope (59 examples), good functional group tolerance and facile scale-up under mild, direct visible-light-excitation conditions. Mechanistic investigations reveal that direct excitation of the Gd(OTf)₃/indole mixture gives an excited state intermediate, which undergoes the subsequent [2+2] cycloaddition and cyclobutane-expansion cascade.     

Density functional theory study of the styrylbenzoquinoline dyad and the related dibenzoquinolylcyclobutane formed in the [2 + 2] photocycloaddition reaction

The structure and electronic properties of the biphotochromic dyad with two styrylbenzo[f]quinoline photochromes, as well as the corresponding cyclobutane with two benzo[f]quinoline (BQ) substituents, are studied by DFT at the M06-2X/6-31G* level, the cyclobutane being a product of the [2 + 2] photocycloaddition (PCA) reaction of the dyad. According to calculations, the dyad forms π-stacked folded conformers, which, when excited, can form excimers that are precursors of cyclobutane. TD DFT calculations and natural transition orbital (NTO) analysis indicated that the lowest singlet excited S₁ state in the dyad is localized on the SBQ photochrome, including the ethylene group that undergoes PCA. Thus, the conditions for concerted electrocyclic reactions are satisfied, and the direct PCA follows the Woodward–Hoffmann rules. In contrast, in cyclobutane, the S₁ state is localized on the BQ substituent rather than on the cyclobutane core. Therefore, the reverse ring-opening (retro-PCA) reaction cannot follow the Woodward-Hoffmann rules and inevitably involves a step of excitation energy transfer from BQ to cyclobutane, which means the predissociation mechanism.     

纳米银粒子对[Ru(bpy)_3]~(2+)光谱学性质的影响及电解质效应

研究了[Ru(bpy)3]2+溶液中引入纳米银粒子的光谱学性质变化规律以及[Ru(bpy)3]2+与纳米银粒子所构成的溶液体系([Ru(bpy)3]2+-Ag)的电解质效应.研究结果表明,[Ru(bpy)3]2+吸附在纳米银粒子表面使纳米银粒子相互桥连形成规则的类链状网络聚集体.纳米银粒子造成[Ru(bpy)3]2+溶液荧光猝灭,且大尺寸的纳米银粒子引起的荧光猝灭程度较大.在[Ru(bpy)3]2+-Ag体系中引入电解质造成纳米银粒子不同程度的聚集和生长.电解质对纳米银聚集影响为:CaCl2MgCl2Ca(NO3)2KClKNO3.随着[Ru(bpy)3]2+-Ag体系中引入电解质含量的增加,溶液的荧光强度先降低而后又逐渐增强,直至达到定值,表明一定量的电解质可产生荧光猝灭释放效应.电解质对荧光强度影响顺序为:Ca(NO3)2CaCl2MgCl2KClKNO3.采用透射电子显微镜、紫外-可见吸收分光光度计和荧光分光光度计等手段从分子间相互作用和能量传输等方面初步探讨了纳米银粒子对表面吸附[Ru(bpy)3]2+溶液光谱学性质的影响机制以及电解质效应.     

Intramolecular [2 + 2] photocycloaddition of alkenes incorporated in a carbohydrate template. Synthesis of enantiopure bicyclo[3.2.0]heptanes and -[6.3.0]undecanes

Intramolecular [2 + 2] photocycloaddition of alkenes with a furano sugar placed between them have been investigated under both copper(I)-catalyzed and sensitized conditions. The copper(I)-catalyzed photocycloaddition of the dienes 4a, 4b, and 4c led to unexpected formation of the thermodynamically less stable cis-syn-cis 4-5-5 tricyclic adducts 5a, 5b, and 5c, respectively. The sensitized photocycloaddition of the diene 14 also gave the cis-syn-cis adduct 15 showing that the copper(I) catalyst does not have any influence on the stereochemical course through coordination with the anomeric ring oxygen of the furano sugar. The identical stereochemical course observed under both catalyzed and sensitized photoaddition reactions have been attributed to be of steric origin. Bis(dienes) 25a and 25b, which gave an intractable mixture on copper(I)-catalyzed irradiation, underwent smooth photocycloaddition in the presence of benzophenone, and the resulting 1,2-divinyl cyclobutanes underwent spontaneous [3.3]-rearrangement at room temperature to produce bicyclo[6.3.0]undecanes 30a and 30b, respectively. This investigation provides an approach for the construction of enantiopure bicyclo[3.2.0]heptanes and -[6.3.0]undecanes.     

Programmable photoresponsive materials based on a single molecule via distinct topochemical reactions

Engineering the preorganization of photoactive units remains a big challenge in solid-state photochemistry research. It is of not only theoretical importance in the construction of topochemical reactions but also technological significance in the fabrication of advanced materials. Here, a cyanostilbene derivative, (Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(naphthalen-2-yl) acrylonitrile (BNA), was crystallized into two polymorphs under different conditions. The two crystals, BNA-α and BNA-β, have totally different intra-π-dimer and inter-π-dimer hierarchical architectures on the basis of a very simple monomer, which provides them with distinct reactivities, functions and photoresponsive properties. Firstly, two different types of solid-state [2 + 2] photocycloaddition reaction: (i) a typical olefin–olefin cycloaddition reaction within the symmetric π-dimers of BNA-α and (ii) an unusual olefin-aromatic ring cycloaddition reaction within the offset π-dimers of BNA-β have been observed, respectively. Secondly, the crystal of BNA-α can be bent to 90° without any fracture, exhibiting outstanding flexibility upon UV irradiation, while the reversible photocycloaddition/thermal cleavage process (below 100 °C) accompanied by unique fluorescence changes can be achieved in the crystal of BNA-β. Finally, micro-scale photoactuators and light-writable anti-counterfeiting materials have been successfully fabricated. This work paves a simple way to construct smart materials through a bottom-up way that is realized by manipulating hierarchical architectures in the solid state.

Two crystals with different hierarchical architectures are observed based on a single molecule, achieving different types of [2 + 2] photocycloaddition. Finally, controllable photoactuators and light-writable materials are successfully fabricated.     

DNA‐Templated [2+2] Photocycloaddition: A Straightforward Entry into the Aplysinopsin Family of Natural Products

Biosynthetic considerations inspired us to harness the templating properties offered by DNA to promote a [2+2] photoinduced cycloaddition. The method was developed based on the dimerization of (E)‐aplysinopsin, which was previously shown to be unproductive in solution. In sharp contrast, exposure of this tryptophan‐derived olefin to light in the presence of salmon testes DNA (st‐DNA) reproducibly afforded the corresponding homo‐dimerized spiro‐fused cyclobutane in excellent yields. DNA provides unique templating interactions enabling a singular mimic of the solid‐state aggregation necessary for the [2+2] photocycloaddition to occur. This method was ultimately used to promote the prerequisite dimerizations leading to both dictazole B and tubastrindole B, thus constituting the first example of a DNA‐mediated transformation to be applied to the total synthesis of a natural product.     

Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy

The first study of pseudo‐bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time‐resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo‐gem‐divinyl[2.2]paracyclophane. From X‐ray crystal diffraction measurements we found that the ground‐state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S₁ lifetimes we assigned a clear relation between the conformation and the excited‐state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited‐state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo‐para‐divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S₁ state.     

Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications

The intramolecular [2+2] photocycloaddition of 3-alkenyl-2-cycloalkenones was performed in an enantioselective fashion (nine representative examples, 54–86 % yield, 76–96 % ee) upon irradiation at λ=366 nm in the presence of an AlBr₃-activated oxazaborolidine as the Lewis acid. An extensive screening of proline-derived oxazaborolidines showed that the enantioface differentiation depends strongly on the nature of the aryl group at the 3-position of the heterocycle. DFT calculations of the Lewis acid–substrate complex indicate that attractive dispersion forces may be responsible for a change of the binding mode. The catalytic [2+2] photocycloaddition was shown to proceed on the triplet hypersurface with a quantum yield of 0.05. The positive effect of Lewis acids on the outcome of a given intramolecular [2+2] photocycloaddition was illustrated by optimizing the key step in a concise total synthesis of the sesquiterpene (±)-italicene.     

Gadolinium Photocatalysis: Dearomative [2+2] Cycloaddition/Ring‐Expansion Sequence with Indoles

Lanthanide photocatalysts are much less investigated in synthetic chemistry than rare and expensive late transition metals. We herein introduce Gd^III photocatalysis of a highly regioselective, intermolecular [2+2] photocycloaddition/ring‐expansion sequence with indoles, which could provide divergent access to cyclopenta[b]indoles and indolines. A simple and commercially available Gd(OTf)₃ salt is sufficient for this visible‐violet‐light‐induced transformation. The reaction proceeds either through a transient or start‐to‐end dearomatization cascade and shows excellent regioselectivity (usually >95:5 r.r.), broad scope (59 examples), good functional group tolerance and facile scale‐up under mild, direct visible‐light‐excitation conditions. Mechanistic investigations reveal that direct excitation of the Gd(OTf)₃/indole mixture gives an excited state intermediate, which undergoes the subsequent [2+2] cycloaddition and cyclobutane‐expansion cascade.     

ESIPT-mediated photocycloadditions of 3-hydroxyquinolinones: development of a fluorescence quenching assay for reaction screening

Irradiation of 1,2-dimethyl-3-hydroxyquinolinone (DMQ) leads to excited state intramolecular proton transfer (ESIPT) generating a 3-oxidoquinolinium species which undergoes [3 + 2] photocycloaddition with dipolarophiles. A parallel, fluorescence quenching assay using a microplate format has been developed to evaluate fluorescence quenching of this species with a range of dipolarophiles.     

Photocatalyzed Intramolecular [2+2] Cycloaddition of N-Alkyl-N-(2-(1-arylvinyl)aryl)cinnamamides

N-Alkyl-N-(2-(1-arylvinyl)aryl)cinnamamides are converted into natural product inspired scaffolds via iridium photocatalyzed intramolecular [2+2] photocycloaddition. The protocol has a broad substrate scope, whilst operating under mild reaction conditions. Tethering four components forming a trisubstituted cyclobutane core builds rapidly high molecular complexity. Our approach allows the design and synthesis of a variety of tetrahydrocyclobuta[c]quinolin-3(1H)-ones, in yields ranging between 20–99 %, and with excellent regio- and diastereoselectivity. Moreover, it was demonstrated that the intramolecular [2+2]-cycloaddition of 1,7-enynes—after fragmentation of the cyclobutane ring—leads to enyne-metathesis-like products.     

Photoswitching of the antiferromagnetic coupling in an oxamato-based dicopper(II) anthracenophane

Thermally reversible photomagnetic (ON/OFF) switching behavior has been observed in a dinuclear oxamatocopper(II) anthracenophane upon UV light irradiation and heating; the two Cu(II) ions (S(Cu) = 1/2) that are antiferromagnetically coupled in the dicopper(II) metallacyclic precursor (ON state) become uncoupled in the corresponding [4+4] photocycloaddition product (OFF state), as substantiated from both experimental and theoretical studies.