Visible Light-fueled Mechanical Motions with Dynamic Phosphorescence Induced by Topochemical [2+2] Reactions in Organoboron Crystals

In the quest for essential energy solutions towards an ecological friendly future, the transformation of visible light/solar energy into mechanical motions in metal-free luminescent crystals offers a sustainable choice of smart materials for lightweight actuating, and all-organic electronic devices. Such green energy-triggered photodynamic motions with room temperature phosphorescence (RTP) emission in molecular crystals have not been reported yet. Here, we demonstrate three new stoichiometrically different Lewis acid-base molecular organoboron crystals (PS1, PS2, and PS3), which exhibit rapid photosalient effects (ballistic splitting, moving, and jumping) under both ultraviolet (UV) and visible light associated with quantitative single-crystal-to-single-crystal (SCSC) [2+2] cycloaddition of preorganized olefins. Furthermore, these systems respond to sunlight and mobile (white) flashlight with a complete SCSC transformation in a relatively slow fashion. Remarkably, all PS1, PS2, and PS3 crystals display visible light-promoted dynamic green RTP as their emission peaks promptly blue-shift, due to instantaneous photomechanical effects. Time-dependent structural mapping of intermediate photoproducts during fast SCSC [2+2] photoreaction, by X-ray photodiffraction, reveals a rationale for the origin of these photodynamic motions associated with rapid topochemical transformations. The reported light-driven behavior (mechanical motions, dynamic phosphorescence, and topochemical reactivity), is considered advantageous for the strategic design of stimuli-responsive multi-functional crystalline materials.     

Mechanical Effects of Elastic Crystals Driven by Natural Sunlight and Force

Flexible crystals that can capture solar energy and convert it into mechanical energy are promising for a wide range of applications such as information storage and actuators, but obtaining them remains a challenge. Herein, an elastic crystal of a barbiturate derivative was found to be an excellent candidate, demonstrating plastic bending behavior under natural sunlight irradiation. ¹H NMR and high-resolution mass spectrum data of microcrystals before and after light irradiation demonstrated that light-induced [2+2] cycloaddition was the driving force for the photomechanical effects. Interestingly, the crystals retained elastic bending even after light irradiation. This is the first report of flexible crystals that can be driven by natural sunlight and that have both photomechanical properties and elasticity. Furthermore, regulation of the passive light output direction of the crystals and transport of objects by applying mechanical forces and light was demonstrated.     

Formation of a Syndiotactic Organic Polymer Inside a MOF by a [2+2] Photo‐Polymerization Reaction

Getting suitable crystals for single‐crystal X‐ray crystallographic analysis still remains an art. Obtaining single crystals of metal–organic frameworks (MOFs) containing organic polymers poses even greater challenges. Here we demonstrate the formation of a syndiotactic organic polymer ligand inside a MOF by quantitative [2+2] photopolymerization reaction in a single‐crystal‐to‐single‐crystal manner. The spacer ligands with trans,trans,trans‐conformation in the pillared‐layer MOF with guest water molecules in the channels, undergo pedal motion to trans,cis,trans‐conformation prior to [2+2] photo‐cycloaddition reaction and yield single crystals of MOF containing two‐dimensional coordination polymers fused with the organic polymer ligands. We also show that the organic polymer in the single crystals can be depolymerized reversibly by cleaving the cyclobutane rings upon heating. These MOFs also show interesting photoluminescent properties and sensing of small organic molecules.     

Bending,Curling, Rolling,and Salient Behavior of Molecular Crystals Driven by [2+2] Cycloaddition of a Styrylbenzoxazole Derivative

We report interesting photomechanical behaviors of the dynamic molecular crystals of (E )‐2‐(2,4‐dichlorostyryl)benzo[d]oxazole ( BOACl24 ). The photosalient effect of the rod‐like crystal based on a metal‐free olefin driven by photodimerization is observed. Moreover, the needle‐like crystals of BOACl24 exhibit a reversible bending away from a UV light source. The nanofibers curl easily under UV irradiation in an organogel, in which the photo‐induced rolling of a small slice occurs. This suggests that the rapid release of the accumulated strain during photodimerization may lead to a photosalient effect, and the bending or curling happens when the strain is released slowly. Notably, [2+2] cycloaddition takes place between two different conformational isomers of BOACl24 on account of the rotation of the benzoxazole ring around the C−C bond in an excited state before photodimerization. Such topo‐photochemical reaction has not been reported elsewhere.     

Bending,Curling, Rolling,and Salient Behavior of Molecular Crystals Driven by [2+2] Cycloaddition of a Styrylbenzoxazole Derivative

We report interesting photomechanical behaviors of the dynamic molecular crystals of (E )‐2‐(2,4‐dichlorostyryl)benzo[d]oxazole ( BOACl24 ). The photosalient effect of the rod‐like crystal based on a metal‐free olefin driven by photodimerization is observed. Moreover, the needle‐like crystals of BOACl24 exhibit a reversible bending away from a UV light source. The nanofibers curl easily under UV irradiation in an organogel, in which the photo‐induced rolling of a small slice occurs. This suggests that the rapid release of the accumulated strain during photodimerization may lead to a photosalient effect, and the bending or curling happens when the strain is released slowly. Notably, [2+2] cycloaddition takes place between two different conformational isomers of BOACl24 on account of the rotation of the benzoxazole ring around the C−C bond in an excited state before photodimerization. Such topo‐photochemical reaction has not been reported elsewhere.     

Thermo-Induced Single-Crystal-to-Single-Crystal Transformations and Photo-Induced [2+2] Cycloaddition Reactions in Polymorphs of Chalcone-Based Molecular Crystals: Multi-Stimuli Responsive Actuators

The polymorphs of 2ClChMe-4 in Form I (ribbon-like crystal) and Form II (block-like crystal) were prepared, and they exhibited curling/flipping and expansion upon heating on account of single-crystal-to-single-crystal transformations. The irreversible phase transformations occurred separately at 53.2 °C and 57.8 °C for the crystals in Form I and Form II, during which the molecular conformation of 2ClChMe-4 changed and the molecules slipped along the (100) plane. Movement at the molecular level resulted in changes of cell parameters, which in turn led to macroscopic motions of the crystals upon heating. Additionally, the ribbon-like crystals of 2ClChMe-4 showed photo-induced bending driven by [2+2] cycloaddition. Accordingly, an actuator showing reversible bending behavior was fabricated triggered by light and heat successively. Like biomimetic self-actuators, such multi-stimuli mechanical responsive molecular crystals might have potential applications in soft robots, artificial muscles and microfluidic systems.     

Dihydrobiphenylenes through Ruthenium‐Catalyzed [2+2+2] Cycloadditions of ortho‐Alkenylarylacetylenes with Alkynes

A new synthetic route to dihydrobiphenylenes has been developed. The process involves a mild Ru^II‐catalyzed [2+2+2] dimerization of ortho‐alkenylarylacetylenes or its more versatile variant, the Ru‐catalyzed [2+2+2] cycloaddition of ortho‐ethynylstyrenes with alkynes. Mechanistic aspects of this [2+2+2] cycloaddition are discussed.     

二芳基乙烯类酸致变色材料的光机械效应

本文合成了氟代萘乙烯基苯并唑BO1N4F和氟代萘乙烯基苯并噻唑BT1N4F,二者在紫外光照下可发生拓扑[2+2]环加成反应,生成的主产物为β-型二聚体.由于前者的光化学反应效率高于后者,因此,BO1N4F的光二聚反应导致晶体内部产生较大的张力.当累积在棒状晶体中的张力快速释放时,晶体发生爆裂,产生的碎片跳离原位置;当累积在针状晶体中的张力缓慢释放时,晶体发生显著的背光弯曲.此外,光二聚反应将平面形二芳基乙烯分子转化为蝴蝶形环丁烷衍生物,共轭分子的去平面化导致基于二聚体D-BO1N4F的薄膜对三氟乙酸蒸气的检测限低于薄膜BO1N4F.可见,光诱导[2+2]环加成反应不仅能改善二芳基乙烯衍生物在薄膜中的荧光传感性能,而且是分子晶体产生光机械效应的驱动力.     

A density functional theory study of the stereoselectivity of Cu(OTf)2-catalyzed [3+2] cycloaddition of trifluoromethylated N-acylhydrazones and isoprene: A concerted asynchronous mechanism

Here we employ density functional theory calculations to systematically investigate the underlying mechanism of Cu(OTf)₂-catalyzed [3+2] cycloaddition reactions in the synthesis of CF₃-substituted pyrazolidines. About eight possible initial configurations of the [3+2] reaction is considered, and all relevant reactants, transition states, and products are optimized. Based on these structures, internal reaction coordinate paths, and wavefunction analysis results, we conclude that the Cu(OTf)₂-catalyzed [3+2] cycloaddition follows a concerted asynchronous mechanism. The C N bond forms immediately after the formation of the C C bond. Among the eight reaction paths, the energy barrier for the [3+2] reaction that leads to the CF₃-substituted syn-pyrazolidine is the lowest, ∼5.4 kcal/mol, which might result in the diastereoselectivity that is observed in the experiment. This work not only gives the detailed mechanism of the Cu(OTf)₂-catalyzed [3+2] cycloaddition but can also be helpful for the future designation of Cu(OTf)₂-based cycloaddition processes.     

Quantum mechanical inspection of the Diels-Alder approach to biaryls mechanism

The Diels-Alder reaction of substituted cyclohexadienes with substituted phenylacetylenes offers an attractive alternative for the synthesis of biaryl compounds via a two-step cycloaddition/cycloelimination pathway. Quantum mechanical calculations using B3LYP and M06-2X density functional methods for the reaction of 2-chloro-6-nitrophenylacetylene with 1-carbomethoxy-cyclohexadiene show the reaction proceeds by a stepwise diradical [4+2] cycloaddition followed by concerted [2+4] cycloelimination of ethylene. [2+2] cycloadducts are also the result of stepwise addition. [2+2] cycloadducts isomerize to [4+2] cycloadducts via diradical pathways, which involve the same diradical intermediate in cycloaddition. There is also a competitive conrotatory ring opening followed by trans-cis double bond isomerization pathway of the [4.2.0] bicycle (the [2+2] cycloadduct) to give the cis,cis,cis-1,3,5-cyclooctatriene.     

Phase Transition-Promoted Rapid Photomechanical Motions of Single Crystals of a Triene Coordination Polymer

Molecular crystals with the ability to transform light energy into macroscopic mechanical motions are a promising class of materials with potential applications in actuating and photonic devices. In regard to such materials, coordination polymers that exhibit dynamic photomechanical motion, associated with a phase transition, are unknown. Herein, we report an intriguing photoactive, one-dimensional Zn^II coordination polymer, 1 , derived from 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene and 3,5-difluorobenzoate. Single crystals of 1 under UV light irradiation exhibit rapid shrinking and bending, violent bursting-jumping, splitting, and cracking behavior. Single-crystal X-ray diffraction analysis and ¹H NMR spectroscopy reveal an unusual photoinduced phase transition involving a single-crystal-to-single-crystal [2+2] cycloaddition reaction that results in photomechanical responses. Interestingly, crystals of 1 , which are triclinic with space group , are transformed into a higher symmetry, monoclinic cell with space group C2/c. This process represents a rare example of symmetry enhancement upon photoirradiation. The photomechanical activity is likely due to the sudden release of stress associated with strained molecular geometries and significant solid-state molecular movement arising from cleavage and formation of chemical bonds. A composite membrane fabricated from 1 and polyvinyl alcohol (PVA) also displays interesting photomechanical behavior under UV light illumination, indicating the material's potential as a photoactuator.     

Mechanically facilitated retro [4+2] cycloadditions

Poly(methyl acrylate)s (PMAs) of varying molecular weights were grown from a [4+2] cycloaddition adduct of maleimide with furan containing two polymerization initiators. Subjecting the corresponding PMA (>30 kDa) chains to ultrasound at 0 °C resulted in a retro [4+2] cycloaddition reaction, as observed by gel permeation chromatography (GPC) and UV-vis spectroscopy, as well as labeling of the liberated maleimide and furan moieties with appropriate chromophores featuring complementary functional groups. Similar results were obtained by sonicating analogous polymers that were grown from a thermally robust [4+2] cycloaddition adduct of maleimide with anthracene. The generation of anthracenyl species from these latter adducts allowed for the rate of the corresponding mechanically activated retro [4+2] cycloaddition reaction to be measured. No reduction in the number average molecular weight (M(n)) or liberation of the maleimide, furan, or anthracene moieties was observed (i) for polymers containing the cycloaddition adducts with M(n) < 20 kDa, (ii) for high molecular weight PMAs (M(n) > 60 kDa) featuring terminal cycloaddition adducts, or (iii) when the cycloaddition adducts were not covalently linked to a high molecular weight PMA. Collectively, these results support the notion that the aforementioned retro [4+2] cycloaddition processes were derived from a vectorially opposed mechanical force applied to adducts embedded within the polymer chains.     

Biocatalytic Strategies towards [4+2] Cycloadditions

Long sought after [4+2] cyclases have sprouted up in numerous biosynthetic pathways in recent years, raising hopes for biocatalytic solutions to cycloaddition catalysis, an important problem in chemical synthesis. In a few cases, detailed pictures of the inner workings of these catalysts have emerged, but intense efforts to gain deeper understanding are underway by means of crystallography and computational modelling. This Minireview aims to shed light on the catalytic strategies that this highly diverse family of enzymes employs to accelerate and direct the course of [4+2] cycloadditions with reference to small-molecule catalysts and designer enzymes. These catalytic strategies include oxidative or reductive triggers and lid-like movements of enzyme domains. A precise understanding of natural cycloaddition catalysts will be instrumental for customizing them for various synthetic applications.     

Thermally-Induced Intramolecular [4+2] Cycloaddition of Allylamino- or Allyloxy-Tethered Alkylidenecyclopropanes

A thermally-induced intramolecular [4+2] cycloaddition reaction of allylamino- or allyloxy-tethered alkylidenecyclopropanes has been reported in this paper, giving a new protocol for the rapid construction of polycyclic skeleton molecules in moderate to excellent yields with a broad substrate scope. On the basis of control experiments and DFT calculations, we disclosed that the reaction proceeded through a [4+2] cycloaddition and trace of water assisted 1,3-H shift process to give the target product.     

过渡金属催化[2 + 2 + 2]环加成反应合成吡啶衍生物

环加成反应可以一步同时构建多个化学键,是目前国内外研究最为活跃的领域之一。相比于传统方法,过渡金属催化的[2+2+2]环加成反应是合成吡啶衍生物的有效手段。本文从反应机理、非手性吡啶化合物合成和手性吡啶化合物合成三个方面阐述了近年来吡啶衍生物的研究情况,涉及Co、Rh、Ru、Fe、Ni、Ti等金属催化体系。     

Stereoselective preparation of methylenecyclobutane-fused angular tetracyclic spiroindolines via photosensitized intramolecular [2+2] cycloaddition with allene

Irradiation of 3-(hexa-4,5-dienyl)indole derivatives in the presence of 3′,4′-dimethoxyacetophenone by a high-pressure mercury lamp through Pyrex glass gave the corresponding [2+2] cycloaddition products stereoselectively in high yields. The major product was a methylenecyclobutane-fused angular tetracyclic spiroindoline derivative produced by the [2+2] cycloaddition through a parallel orientation. The minor product was a hexahydromethanocarbazole derivative through a crossed orientation. Electron-withdrawing substituents, such as acyl or alkoxycarbonyl, on the indole nitrogen were suitable for this reaction.     

4-氧-3-氮杂-三环[5.3.1.02,5]十一烷-9-甲酸甲酯的合成及晶体结构

以2-金刚烷酮为起始原料,通过烯烃与氯磺酰异氰酸酯的[2+2]环加成等5步反应合成了新型双环β-内酰胺类化合物5.采用1H NMR,IR和MS等手段对所得化合物的结构进行了表征.此外,还通过将化合物5酰化并采用X射线单晶衍射分析方法进一步测定了化合物5的空间立体结构.结果表明,烯烃3与氯磺酰异氰酸酯的[2+2]环加成反...     

Metal–Organic Organopolymeric Hybrid Framework by Reversible [2+2] Cycloaddition Reaction

Organic polymers are usually amorphous or possess very low crystallinity. The metal complexes of organic polymeric ligands are also difficult to crystallize by traditional methods because of their poor solubilities and their 3D structures can not be determined by single‐crystal X‐ray crystallography owing to a lack of single crystals. Herein, we report the crystal structure of a 1D Zn^II coordination polymer fused with an organic polymer ligand made in situ by a [2+2] cycloaddition reaction of a six‐fold interpenetrated metal–organic framework. It is also shown that this organic polymer ligand can be depolymerized in a single‐crystal‐to‐single‐crystal (SCSC) fashion by heating. This strategy could potentially be extended to make a range of monocrystalline metal organopolymeric complexes and metal–organic organopolymeric hybrid materials. Such monocrystalline metal complexes of organic polymers have hitherto been inaccessible for materials researchers.     

Rhodium-Catalyzed Intermolecular Cycloaromatization Route to Cycloparaphenylenes that Exhibit Aggregation-Induced Emission

A high-yielding new route to substituted cycloparaphenylenes has been developed via reductive aromatization of a diyne bearing two cyclohexadiene units giving a cyclophenylene-ethynylene (CPE) followed by the cationic rhodium(I)/dppe complex-catalyzed intermolecular [2+2+2] cycloaddition (cycloaromatization) of the CPE with monoynes. The thus-obtained products, substituted [8]cycloparaphenylene-triphenylenes ([8]CPPTs), exhibited definite aggregation-induced emission (AIE). This molecule is noteworthy as a novel AIE-active cycloarylene that does not have well-known AIE luminogens, such as tetraphenylethene and 1,2,4,5-tetraphenylbenzene skeletons. The single-crystal X-ray diffraction analyses of the AIE-active [8]CPPTs revealed their highly ordered packing structures in which the rotation of the triphenylene moieties is restricted.     

Diels-Alder approach to biaryls: elucidation of competing tandem [2+2] cycloaddition/[1,3] sigmatropic shift pathway

Reaction of 2-halo-6-nitrophenylacetylene with an electron deficient diene gives rise to a [4+2] cycloaddition/cycloreversion biaryl product and a bicyclo[4.2.0]octadiene resulting from a competing [2+2] cycloaddition pathway. The cyclobutene can be opened to give a mixture of cyclooctatriene and biaryl in varying amounts depending on heat and light exposure. The conversion of the cyclobutene into biaryl occurs through a [1,3] sigmatropic carbon shift followed by [4+2] cycloextrusion of ethylene gas.