RESUMEN
The oxyboration of arynes was achieved for the first time. A series of 2-aryl-1,3,2-dioxaborolane derivatives were reacted with aryne precursors in the presence of CsF to give the corresponding ring-expanded seven-membered borinic acid esters via selective boron-oxygen bond activation. Preliminary experimental mechanistic studies and density functional theory (DFT) calculations suggest that this unprecedented aryne oxyboration proceeds through the formation of boron ate complexes of arylboronates with CsF, followed by aryne insertion into the boron-oxygen bond.
RESUMEN
Radical initiators such as azo compounds and organic peroxides have been widely used to facilitate numerous transformations of free radicals, which enable the efficient synthesis of structurally complex molecules, natural products, polymers, and functional materials. However, these high-energy reagents are potentially explosive and thus often require special precautions or delicate operating conditions. We postulated that a more convenient and safer alternative for radical chain initiation could be developed by mechanical activation of thermodynamically stable covalent bonds. Here, we show that commodity plastics such as polyethylene and poly(vinyl acetate) are capable of acting as efficient initiators for radical chain reactions under solvent-free mechanochemical conditions. In this approach, polymeric mechanoradicals, which are generated by homolytic cleavage of the polymer chains in response to the applied mechanical energy provided by ball milling, react with tris(trimethylsilyl)silane to initiate radical chain dehalogenation of organic halides. Preliminary calculations support our proposed force-induced radical chain mechanism.
RESUMEN
The ability to correlate the structure of a molecule with its properties is the key to the rational and accelerated design of new functional compounds and materials. Taking photoswitches as an example, the thermal stability of the metastable state is a crucial property that dictates their application in molecular systems. Indigos have recently emerged as an attractive motif for designing photoswitchable molecules due to their red-light addressability, which can be advantageous in biomedical and material applications. The lack of synthetic techniques to derivatize the abundant parent dye and a thorough understanding of the impact of structural factors on the photochemical and thermal properties hinder broad applications of this emerging photoswitch class. Herein, we report an efficient copper-catalyzed indigo N-arylation that enables the installation of a wide variety of aryl moieties carrying useful functional groups. The exclusive selectivity for monoarylation likely originates from a bimetallic cooperative mechanism through a binuclear copper-indigo intermediate. Functional N-aryl-N'-alkylindigos were prepared and shown to photoisomerize efficiently under red light. Moreover, this design allows for the modulation of thermal half-lives through N-aryl substituents, while the N'-alkyl groups enable the independent attachment of functional moieties without affecting the photochromic properties. A strong correlation between the structure of the N-aryl moiety and the thermal stability of the photogenerated Z-isomers was achieved by multivariate linear regression models obtained through a data-science workflow. This work thus builds an avenue leading to versatile red-light photoswitches and a general method for structure-property correlation that is expected to be broadly applicable to the design of photoresponsive molecules.
RESUMEN
In recent decades, more than 100 different mechanophores with a broad range of activation forces have been developed. For various applications of mechanophores in polymer materials, it is crucial to selectively activate the mechanophores with high efficiency, avoiding nonspecific bond scission of the material. In this study, we embedded cyclobutane-based mechanophore cross-linkers (I and II) with varied activation forces (fa) in the first network of the double network hydrogels and quantitively investigated the activation selectivity and efficiency of these mechanophores. Our findings revealed that cross-linker I, with a lower activation force relative to the bonds in the polymer main chain (fa-I/fa-chain = 0.8 nN/3.4 nN), achieved efficient activation with 100% selectivity. Conversely, an increase of the activation force of mechanophore II (fa-II/fa-chain = 2.5 nN/3.4 nN) led to a significant decrease of its activation efficiency, accompanied by a substantial number of nonspecific bond scission events. Furthermore, with the coexistence of two cross-linkers, significantly different activation forces resulted in the almost complete suppression of the higher-force one (i.e., I and III, fa-I/fa-III = 0.8 nN/3.4 nN), while similar activation forces led to simultaneous activations with moderate efficiencies (i.e., I and IV, fa-I/fa-IV = 0.8 nN/1.6 nN). These findings provide insights into the prevention of nonspecific bond rupture during mechanophore activation and enhance our understanding of the damage mechanism within polymer networks when using mechanophores as detectors. Besides, it establishes a principle for combining different mechanophores to design multiple mechanoresponsive functional materials.
RESUMEN
Visualization of mechanochemical damages, especially for those in the molecular-scale (e.g., bond scission in polymeric materials), is of great industrial and academic significance. Herein, we report a novel strategy for in situ and real-time visualization of mechanochemical damages in hydrogels by utilizing prefluorescent probes via oxygen-relayed free-radical trapping. Double-network (DN) hydrogels that generate numerous mechanoradicals by homolytic bond scission of the brittle first network at large deformation are used as model materials. Theoretical calculation suggests that mechanoradicals generated by the damage of the first network undergo an oxygen-relayed radical-transfer process which can be detected by the prefluorescent probe through the radical-radical coupling reaction. Such an oxygen-relayed radical-trapping process of the prefluorescent probe exhibits a dramatically enhanced emission, which enables the real-time sensing and visualization of mechanochemical damages in DN hydrogels made from brittle networks of varied chemical structures. To the best of authors' knowledge, this work is the first report utilizing oxygen as a radical-relaying molecule for visualizing mechanoradical damages in polymer materials. Moreover, this new method based on the probe post-loading is simple and does not introduce any chemical structural changes in the materials, outperforming most previous methods that require chemical incorporation of mechanophores into polymer networks.
RESUMEN
Double-network (DN) hydrogels have recently been demonstrated to generate numerous radicals by the homolytic bond scission of the brittle first network under the influence of an external force. The mechanoradicals thus generated can be utilized to trigger polymerization inside the gels, resulting in significant mechanical and functional improvements to the material. Although the concentration of mechanoradicals in DN gels is much higher than that in single-network hydrogels, a further increase in the mechanoradical concentration in DN gels will widen their application. In the present work, we incorporate an azoalkane crosslinker into the first network of DN gels. Compared with the traditional crosslinker N,N'-methylenebis(acrylamide), the azoalkane crosslinker causes a decrease in the yield stress but significantly increases the mechanoradical concentration of DN gels after stretching. In the azoalkane-crosslinked DN gels, the concentration of mechanoradicals can reach a maximum of â¼220 µM, which is 5 times that of the traditional crosslinker. In addition, DN gels with the azoalkane crosslinker show a much higher energy efficiency for mechanoradical generation. Interestingly, DN gels crosslinked by a mixture of azoalkane crosslinker and traditional crosslinker also exhibit excellent radical generation performance. The increase in the mechanoradical concentration accelerates polymerization and can broaden the application range of force-responsive DN gels to biomedical devices and soft robots.
RESUMEN
In sharp contrast to the use of conventional magnesium-based Grignard reagents (R-MgX), the application of calcium-based heavy Grignard reagents (R-CaX) in organic synthesis remains poorly explored. This is mainly due to the lack of experimentally simple ways to access such organocalcium nucleophiles from readily available starting materials under mild conditions. Here, we show that a mechanochemical technique using ball milling allows the generation of calcium-based heavy Grignard reagents from aryl halides and commercially available calcium metal without complicated pre-activation processes. Notably, all experimental operations can be carried out in air. Our operationally simple protocol enables the rapid development of novel cross-electrophile-coupling reactions mediated by arylcalcium nucleophiles, which are rather difficult using conventional Grignard reagents. This method will allow synthetic chemists to readily access the novel and unique reactivity of organocalcium nucleophiles.
RESUMEN
Herein, we report a novel strategy for introducing a luminophore into generic polymers facilitated by mechanical stimulation. In this study, polymeric mechanoradicals were formed in situ under ball-milling conditions to undergo radical-radical coupling with a prefluorescent nitroxide-based reagent in order to incorporate a luminophore into the polymer main chains via a covalent bond. This method allowed the direct and conceptually simple preparation of luminescent polymeric materials from a wide range of generic polymers such as polystyrene, polymethyl methacrylate, and polyethylene. These results indicate that the present mechanoradical coupling strategy may help to transform existing commodity polymers into more valuable functional materials.
RESUMEN
The phosphorylation of amide represents an unprecedented environmentally friendly and easily achievable method to constitute C-P bonds in organic synthesis. In this study, the mechanisms for the nickel-catalyzed direct decarbonylative phosphorylation of amides recently reported by Szostak's team were systematically studied with density functional theory calculations. The reaction mainly undergoes four steps: oxidative addition (rate-determining step), phosphorylation, decarbonylation, and reductive elimination. The structures of the substrate and Na2CO3 were found to be critical for the reaction efficiency. Substrates bearing electron-withdrawing groups like carbonyl groups near the amide bond facilitate the reaction by weakening the C-N bond, and Na2CO3 can not only neutralize the H atom in the phosphate ligand as an alkali but also activate the Ni-N bond through the coordination bond with the adjacent carbonyl of the amide group.
RESUMEN
An azaborine-based phosphine-Pd catalyst was introduced by the Liu group to promote trans hydroboration of the C≡C triple bond of internal 1,3-enyne substrates. Despite the excellent yield and selectivity observed experimentally, the mechanism and the origin of this special trans selectivity remained unknown. Herein, a comprehensive theoretical investigation was performed to clarify these issues. Accordingly, two main mechanisms (inner- and outer-sphere) were proposed and examined. Different from the conventional inner-sphere mechanism, in which the transition metal is involved in H-B bond cleavage, this reaction follows an outer-sphere mechanism, in which Pd does not directly participate in H-B bond cleavage. More specifically, the favorable pathway followed a Tsuji-Trost type reaction, in which the H-B bond was weakened by the formation of a four-coordinate boron intermediate (i.e., the boron is attached to the terminal carbon of the alkyne group). It then underwent a hydride-transfer process with the assistance of a second borane molecule, and finally reductive elimination generated the trans hydroboration product. Further analysis ascribed the origin of the special trans selectivity to the unique steric effect and electronic effect introduced by the special κ1 -P-η2 -BC coordination pattern.
RESUMEN
The mechanism of the nickel-catalyzed silylation of aryl methyl ethers has been systematically investigated by using DFT methods. This theoretical study supports a catalytic cycle that involves the formation of a nickel-silyl complex, C-O bond cleavage, C-Si reductive elimination, the addition of methoxide to boron, and finally regeneration of the catalyst. Notably, it was found that activation of the C-O bond proceeded through an oxidative addition pathway with a three-centered transition state. The silyl anion generated in situ works as a ligand to the nickel center and promotes this process. Meanwhile, the role of the base added (KOtBu) is also elucidated. The potassium cation helps to stabilize the oxidative addition transition state through noncovalent interactions, while the resting state is destabilized due to steric repulsion introduced by the tert-butoxide anion. This is further confirmed by a comparison made computationally between the reaction with KOtBu and that with KOMe or NaOtBu as the base.
RESUMEN
The C(sp(3) )H bond activation of 8-methylquinoline followed by alkyne insertion catalyzed by a Rh(III) complex has been studied by using density functional theory (DFT) calculations. Contrary to common belief, the CH bond activation of methylquinoline does not occur by the traditional intramolecular concerted metalation/deprotonation (CMD) mechanism but by an external base CMD mechanism. The use of free acetate or copper(II) acetate as base permits the CH activation step, as observed experimentally. However, the following insertion is possible only if copper(II) acetate is used. The insertion followed by metathesis occurs via a cationic Rh(III) complex and is irreversible, which ensures the efficiency of the entire process. Therefore the use of copper is crucial for completing the catalytic cycle. The present work should help to rationalize the origins of the experimental results described in the literature.
RESUMEN
The mechanism of the C-H activation of aldehydes and the succeeding acylation of an alkene using a hypervalent iodine reagent is investigated by theoretical calculations. In contrast to the initial proposed mechanism, the present calculations show that the hypervalent iodine is the initiator of the radical reaction. The formation of acyl radical is rate-determining, and the resulting radical acts as the chain carrier. The kinetic isotope effect (KIE) of deuterated aldehyde, as well as other experimental observations, can now be rationalized from the newly proposed mechanism.
RESUMEN
Significant rate enhancements in the Diels-Alder reaction of alkynes and 2-pyrones bearing a Lewis basic group are observed when a combination of alkynyltrifluoroborates and BF3·OEt2 is used. This process generates functionalized aromatic compounds with complete regiocontrol. The observed rate enhancement was studied by density functional theory methods and appears to originate from coordination of the diene substrate to a mixture of alkynylborane intermediates, followed by a Lewis acid-mediated product equilibration step. Evidence for this mechanism is presented, as is the enhanced promotion of the cycloaddition via the use of alternative Lewis acid promoters.
Asunto(s)
Alquinos/química , Boranos/química , Bases de Lewis/química , Pironas/química , Teoría Cuántica , Ciclización , Estructura MolecularRESUMEN
The designation of E/Z-geometrical isomers in 3-acylidene 2-oxindoles by NMR spectroscopy can lead to erroneous assignment of alkene stereochemistry because of the narrow chemical shift range observed over a large series of analogues. In contrast, UV-Vis spectroscopy offers a convenient and more reliable method for alkene stereochemical assignment. A combination of X-ray crystallography and theoretical studies shows that the observed differences in UV-Vis spectroscopic behaviour relate to the twisted conformation of the Z-isomers that provides reduced conjugation and weaker hypsochromic (blue-shifted) absorbances relative to those of the E-isomers.
Asunto(s)
Indoles/química , Aldehídos/química , Cristalografía por Rayos X , Electrones , Modelos Moleculares , Oxindoles , Espectroscopía de Protones por Resonancia Magnética , Espectrofotometría Ultravioleta , Estereoisomerismo , TermodinámicaRESUMEN
Azobenzene has been widely explored as a photoresponsive element in materials science. Although some studies have investigated the force-induced isomerization of azobenzene, the effect of force on the rupture of azobenzene has not been explored. Here we show that the light-induced structural change of azobenzene can also alter its rupture forces, making it an ideal light-responsive mechanophore. Using single-molecule force spectroscopy and ultrasonication, we found that cis and trans para-azobenzene isomers possess contrasting mechanical properties. Dynamic force spectroscopy experiments and quantum-chemical calculations in which azobenzene regioisomers were pulled from different directions revealed that the distinct rupture forces of the two isomers are due to the pulling direction rather than the energetic difference between the two isomers. These mechanical features of azobenzene can be used to rationally control the macroscopic fracture behaviours of polymer networks by photoillumination. The use of light-induced conformational changes to alter the mechanical response of mechanophores provides an attractive way to engineer polymer networks of light-regulatable mechanical properties.
RESUMEN
Silicon-stereogenic optically active silylboranes could potentially allow the formation of chiral silyl nucleophiles as well as the synthesis of various chiral silicon compounds. However, the synthesis of such silicon-stereogenic silylboranes has not been achieved so far. Here, we report the synthesis of silicon-stereogenic optically active silylboranes via a stereospecific Pt(PPh3)4-catalyzed Si-H borylation of chiral hydrosilanes, which are synthesized by stoichiometric and catalytic asymmetric synthesis, in high yield and very high or perfect enantiospecificity (99% es in one case, and >99% es in the others) with retention of the configuration. Furthermore, we report a practical approach to generate silicon-stereogenic silyl nucleophiles with high enantiopurity and configurational stability using MeLi activation. This protocol is suitable for the stereospecific and general synthesis of silicon-stereogenic trialkyl-, dialkylbenzyl-, dialkylaryl-, diarylalkyl-, and alkylary benzyloxy-substituted silylboranes and their corresponding silyl nucleophiles with excellent enantiospecificity (>99% es except one case of 99% es). Transition-metal-catalyzed C-Si bond-forming cross-coupling reactions and conjugate-addition reactions are also demonstrated. The mechanisms underlying the stability and reactivity of such chiral silyl anion were investigated by combining NMR spectroscopy and DFT calculations.
RESUMEN
Since the discovery of Grignard reagents in 1900, the nucleophilic addition of magnesium-based carbon nucleophiles to various electrophiles has become one of the most powerful, versatile, and well-established methods for the formation of carbon-carbon bonds in organic synthesis. Grignard reagents are typically prepared via reactions between organic halides and magnesium metal in a solvent. However, this method usually requires the use of dry organic solvents, long reaction times, strict control of the reaction temperature, and inert-gas-line techniques. Despite the utility of Grignard reagents, these requirements still represent major drawbacks from both an environmental and an economic perspective, and often cause reproducibility problems. Here, we report the general mechanochemical synthesis of magnesium-based carbon nucleophiles (Grignard reagents in paste form) in air using a ball milling technique. These nucleophiles can be used directly for one-pot nucleophilic addition reactions with various electrophiles and nickel-catalyzed cross-coupling reactions under solvent-free conditions.
RESUMEN
Methods for direct functionalization of C-H bonds mediated by N-oxyl radicals constitute a powerful tool in modern organic synthesis. While several N-oxyl radicals have been developed to date, the lack of structural diversity for these species has hampered further progress in this field. Here we designed a novel class of N-oxyl radicals based on N-hydroxybenzimidazole, and applied them to the direct C-H functionalization reactions. The flexibly modifiable features of these structures enabled facile tuning of their catalytic performance. Moreover, with these organoradicals, we have developed a metal-free approach for the synthesis of acyl fluorides via direct C-H fluorination of aldehydes under mild conditions.
RESUMEN
A remarkably strained cyclopyrenylene trimer CP3 was synthesized and it underwent the first biaryl C-C σ-bond cleavage by direct oxygen insertion without the aid of any metal agents. A priori highly strained CP3 exhibits the longest wavelength emission among all pyrene-based fluorophores due to the intensive electronic interactions between pyrenes. The color of the emission drastically changes from orange to light blue upon oxidation. Theoretical studies revealed that the release of ring strain reasonably drives the reaction between two CP3 molecules and O2. This strain-induced transformation could be also applied for sulfur atom insertion into a biaryl σ-bond.