This paper reports that the split ring resonators and complementary split ring resonators are compounded to construct a novel compact composite metamaterial.The composite metamaterial exhibits a unique property of blocking electromagnetic wave propagating in two directions near the resonant frequency.An example of two-element microstrip antenna array demonstrates that the developed metamaterial enables array performance that is an improvement in comparison with the traditional one,including mutual coupling suppression of 9.07 dB,remarkable side lobe suppression and gain improvement of 2.14 dB.The mechanism of performance enhancement is analysed based on the electric field and Poynting vector distributions in array.The present work not only is a meaningful exploration of new type composite metamaterial design,but also opens up possibilities for extensive metamaterial applications to antenna engineer. 相似文献
Due to the importance of chroman frameworks in medicinal chemistry, the development of novel synthetic methods for these structures is gaining increasing interest of chemists. Reported here is a new (4 + 2) radical annulation approach for the construction of these functional six-membered frameworks via photocatalysis. Featuring mild reaction conditions, the protocol allows readily available N-hydroxyphthalimide esters and electron-deficient olefins to be converted into a wide range of valuable chromans in a highly selective manner. Moreover, the present strategy can be used in the late-stage functionalization of natural product derivatives and biologically active compounds, which demonstrated the potential application. This method is complementary to the traditional Diels–Alder [4 + 2] cycloaddition reaction of ortho-quinone methides and electron-rich dienophiles, since electron-deficient dienophiles were smoothly transformed into the desired chromans.We have developed a (4 + 2) radical annulation approach for the synthesis of diverse chromans. This method is complementary to the traditional Diels–Alder [4 + 2] annulation of ortho-quinone methides and electron-rich dienophiles.Chroman moieties frequently exist as the key subunit in a wide array of natural products, pharmaceuticals, and bioactive molecules.1 For example, vitamin E,2 centchroman,2 cromakalim3 and rubioncolin B4 are well-known active pharmaceutical ingredients in various therapeutic areas (Scheme 1a). Due to their significant importance in medicinal chemistry, developing new methods towards the synthesis of chromans and the installation of a variety of the functional groups in chroman frameworks are gaining increasing attention of the chemical community.5Open in a separate windowScheme 1Selected bioactive molecules and the synthetic methods of chromans.In the past few decades, a great deal of methods have been developed for the assembly of substituted chromans, and among them, the Diels–Alder [4 + 2] cycloaddition reaction provides a highly efficient synthetic platform in the construction of these functional six-membered frameworks.6 Extensive work has been done with this strategy, resulting in a lot of significant progress. The ortho-quinone methides (o-QMs) are generally essential dienes for the Diels–Alder reaction towards the synthesis of chromans, as they are highly reactive for rapid rearomatization via Michael addition of nucleophiles, cycloaddition with a dienophile of 2π partners or 6π-electrocyclization (Scheme 1b).7 Herein, although various valuable chromans have been successfully synthesized with the Diels–Alder [4 + 2] cycloaddition reaction, the use of o-QMs may lead to several potential limitations in some cases. One of the potential limitations is that o-QMs are used mainly as Michael acceptor and electron-deficient dienes to react only with nucleophiles and electron-rich dienophiles. In these considerations, the evolution of synthetic methods for chromans is very important and highly desirable. In particular, novel (4 + 2) cycloaddition strategies capable of synthesizing chromans with the use of easily available materials and electron-deficient dienophiles are of utmost interest.On the basis of retrosynthetic analysis of chroman shown in Scheme 1c, (4 + 2) radical annulation of the corresponding carbon-centered radical R with olefin would be an alternative route, which is able to overcome the above-mentioned potential limitations. Considering that radical species R is normally nucleophilic, thus, it could react with electron-deficient olefins affording chroman products that generally can''t be synthesized by the traditional Diels–Alder [4 + 2] cycloaddition reaction involving o-QMs. Herein, we reported a highly selective (4 + 2) radical–annulation reaction to construct the chroman framework with the use of easily available NHPI ester as the radical precursor and olefin as the radical acceptor under mild conditions.Compared with other alkyl radical precursors, the redox-active N-(acyloxy)phthalimides (NHPI esters) come to the fore, since they are cheap, stable, readily available, and non-toxic.8 Bearing above hypothesis in mind, we commenced to investigate the (4 + 2) annulation reaction by utilizing readily available N-hydroxyphthalimide ester A′ and commercially available ethyl acrylate as model substrates. After a great deal of screening on the reaction parameters, only a trace amount of the target product was detected by GC-MS. In contrast, the main product is anisole, which may result from a rapid hydrogen abstraction reaction of the unstable primary alkyl radical intermediate. To restrain the formation of this by-product, we designed N-hydroxyphthalimide esters A and A′′, which could produce more stable tertiary radicals, for the target (4 + 2) annulation reaction instead of A′ (9 74% yield of ethyl-2,2-dimethylchromane-4-carboxylate upon 1 was selectively obtained after irradiation of the reaction system under blue LEDs at room temperature for 12 h, despite a little by-product (
Open in a separate windowaStandard conditions: A (0.2 mmol), ethyl acrylate (0.5 mmol), Eosin Y (2 mol%), DMAc (2.0 mL), blue light, N2, rt, 12 h, isolated yield; n.d. = not detected.In order to explore the substrate scope of the (4 + 2) annulation reaction, we commenced to scrutinize the generality and selectivity with respect to N-hydroxyphthalimide esters. The functional group applicability of N-hydroxyphthalimide esters was investigated by the examination of various electron donating/withdrawing substituents at the varying positions, as illustrated in Scheme 2. Gratifyingly, we found that substances bearing electron-donating substituents (Me, OMe, tBu, SMe, OPh, OBn, and Ph) at the para-position could smoothly be transformed into the corresponding chromans with satisfactory yields (2–8). N-Hydroxyphthalimide esters with halogen substituents, such as fluoride, chloride, bromide and iodide are suitable to produce the corresponding chromans in satisfactory yields, which enable potential application in further functionalization (9–12). Surprisingly, electron-withdrawing substituents, such as MeCO, OCF3, and CF3, were also tolerated under standard conditions (13–15). This reaction could proceed effectively with N-hydroxyphthalimide esters containing one group or multiple groups in different positions, which delivered a variety of chroman compounds in moderate to good yields (16–19, 21–23). The annulation reaction is not limited to the construction of benzene compounds, as ethyl-3,3-dimethyl-2,3-dihydro-1H-benzo[f]chromene-1-carboxylate was also obtained in 68% yield (20). After the simple esterification, drug molecules, such as clofibric acid, fenofibric acid and ciprofibrate, could be transformed into the corresponding N-hydroxyphthalimide esters, further engaging with ethyl acrylate (10 and 24–25), which highlighted the synthetic applicability of this protocol.Open in a separate windowScheme 2Reactions of NHPI esters with ethyl acrylate. Standard conditions: NHPI ester (0.2 mmol), ethyl acrylate (0.5 mmol), Eosin Y (2 mol%), DMAc (2.0 mL), blue light, N2, rt, 12 h, isolated yield.Next, we shifted attention to the scope with respect to a wide range of acrylates, as shown in Scheme 3. Methyl acrylate and butyl acrylate were well amenable with N-hydroxyphthalimide esters (26–27). Other acrylates, such as cyclohexyl, tert-butyl and phenyl, were also competent reaction partners with a satisfactory efficiency (28–30). Ethyl (E)-but-2-enoate was tolerant to afford the desired chroman product, albeit in 29% yield (31). It is particularly noteworthy that dimethyl maleate was demonstrated to be a suitable substrate, leading to the formation of sterically hindered product (32). The sensitive benzylic C–H bond and the fragile furan and thiophene moieties could be retained in the radical cascade reaction, providing a series of functionalized chromans (33–35). Alkoxy and aligned alkoxy on substances did not reduce the reaction efficacy (36–37). Chromans possessing various subtle trimethylsilyl, hydroxyl, primary/secondary bromoalkene, cyano and thiomethylene were accessed in reasonable yields, which provided the basis for late-stage derivatization of products (38–41, 43). Owing to the superiority of lipophilicity, permeability and metabolism, we tried to introduce trifluoromethyl into chroman skeletons. To our delight, 2,2,2-trifluoroethyl acrylate gave rise to the corresponding chromans with 52% yield (42). The unactivated alkynyl moiety and alkenyl moiety survived in the photoredox catalysis (44–46).Open in a separate windowScheme 3Reactions of A with various olefins. Standard conditions: A (0.2 mmol), olefin (0.5 mmol), Eosin Y (2 mol%), DMAc (2.0 mL), blue light, N2, rt, 12 h, isolated yield.It is well-known notorious that compounds possessing nitrogen atoms are a very important class of biologically active and functional molecules. Thus, we turned our attention from acrylates to acrylamide derivatives. We were delighted to find that N,N-dimethylacrylamide was a suitable radical receptor to give the target molecule in moderate yield (47). Similarly, a series of chroman products were obtained with cyclic and acyclic acrylamides (48–51). Subsequently, we continued to investigate the reaction of different secondary acrylamides with N-hydroxyphthalimide ester A. These secondary acrylamides bearing NH-isopropyl, -cylopropyl, -benzyl, -phenylethyl and -aryl functionalities, could smoothly be transformed into the desired (4 + 2) annulation products under standard conditions (52–57). Besides acrylates and acrylamides, this method was successfully applied to other Michael acceptors resulting in the synthesis of various functionalized chromans (58–61). In order to demonstrate the potential applicability of this methodology, a variety of natural products, their derivatives and functional molecules, such as isoborneol (62), cedrol (63), citronellol (64), cholesterol (65), and dehydroabietylamine (66), were examined, and all these structures could be embedded into target products in 56–70% yields.The (4 + 2) annulation protocol is not limited to the synthesis of chromans. Under standard conditions, the thiochromane derivative could be formed, although less efficiently (Scheme 4a). With curiosity, we tried to use the commercially available pinacol vinylboronate instead of acrylates for this transformation because of the widespread use of organoboron compounds in organic synthesis. The target compound 2-(2,2-dimethylchroman-4-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, which is a versatile building block in functionalization of chromans, was obtained in 48% yield under the slightly revised conditions (Scheme 4b). It is noting that the reaction could be conducted smoothly to afford 60% yield under sunlight irradiation, showing the potential of industrial application (Scheme 4c). Furthermore, the versatility of chroman 1 was also explored. The oxidative dehydrogenation process of 1 led to the formation of value-added ethyl 2,2-dimethyl-2H-chromene-4-carboxylate 69 by using DDQ as the oxidant (Scheme 4d). 1 could also be reduced to (2,2-dimethylchroman-4-yl)methanol 70 with lithium aluminum hydride in ethyl ether (Scheme 4d).Open in a separate windowScheme 4The synthetic applications. (a) The synthesis of thiochromane. (b) Pinacol vinylboronate as a substrate. (c) Sunlight condition. (d) The derivatization of products.To further gain mechanistic insights into this process, a series of experiments were conducted. When the model reaction was performed under standard conditions but in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a radical scavenger, the target product was not detected (Scheme 5a). Notably, when butylated hydroxytoluene (BHT) was added to this reaction system, the annulation reaction was significantly suppressed, meanwhile, a coupling product was detected by GC-MS and HRMS (Scheme 5b). These results indicated a radical-involved pathway for this transformation. Subsequently, the carbon radical was captured by an intramolecular aromatic ring, giving the cyclization product 69 in excellent yield (Scheme 5c). Moreover, the intermolecular kinetic isotope effect (KIE) experiment was carried out by using A and A-d5 as competitive substrates. Under standard conditions, a KIE value of 1.05 was observed, indicating that the cleavage of the aromatic C–H bond might not be the rate-determining step in the transformation (Scheme 5d).Open in a separate windowScheme 5The control experiments. (a) The addition of TEMPO. (b) The addition of BHT. (c) Intramolecular reaction. (d) KIE experiment.On the basis of the above experimental results, we proposed a possible mechanism cycle for the reaction, as shown in Scheme 6. Initially, the photocatalyst Eosin Y (EY) was transformed into the excited species EY* (E1/2[EY˙+/EY*] = −1.1 V vs. SCE) under the irradiation with visible light. As a redox-active species, EY* was able to reduce N-hydroxyphthalimide ester (E1/2[A/I] = −0.8 V, see the CV in the ESI†) via a single-electron-transfer (SET) process, generating the EY˙+ radical cation and the corresponding N-hydroxyphthalimide ester radical anion I. The intermediate I underwent rapid homolytic fragmentation to generate carbon-centered nucleophilic radical II by releasing the phthalimide anion and carbon dioxide. Subsequently, the carbon radical II was captured by ethyl acrylate to form the electrophilic radical III, which underwent rapid intramolecular radical cyclization to afford aryl radical IV. Then, the intermediate IV was converted into cation Vvia a SET oxidation. On the other hand, the EY˙+ radical cation was transformed into Eosin Y to accomplish the photocatalytic cycle. The rapid deprotonation of V leads to the formation of the product 1.Open in a separate windowScheme 6Proposed reaction mechanism. 相似文献
The swimming mode of two interacting squirmers under gravity in a narrow vertical channel is simulated numerically using the lattice Boltzmann method (LBM) in the range of self-propelling strength 0.1 ≤ α ≤ 1.1 and swimming type −5 ≤ β ≤ 5. The results showed that there exist five typical swimming patterns for individual squirmers, i.e., steady upward rising (SUR), oscillation across the channel (OAC), oscillation near the wall (ONW), steady upward rising with small-amplitude oscillation (SURO), and vertical motion along the sidewall (VMS). The parametric space (α, β) illustrated the interactions on each pattern. In particular, the range of oscillation angle for ONW is from 19.8° to 32.4° as α varies from 0.3 to 0.7. Moreover, the swimming modes of two interacting squirmers combine the two squirmers’ independent swimming patterns. On the other hand, the pullers (β < 0) attract with each other at the initial stage, resulting in a low-pressure region between them and making the two pullers gradually move closer and finally make contact, while the result for the pushers (β > 0) is the opposite. After the squirmers’ interaction, the squirmer orientation and pressure distribution determine subsequent squirmer swimming patterns. Two pushers separate quickly, while there will be a more extended interaction period before the two pullers are entirely separated. 相似文献
The focus of this work was on developing a green, low-cost, and efficient biosorbent based on the biological structure and properties of MT and applying it to the remediation of cationic dyes in dye wastewater. The adsorption performance and mechanism of MT on methylene blue (MB) and crystal violet (CV) were investigated by batch adsorption experiments. The results demonstrated that the highest adsorption values of MT for MB (411 mg/g) and CV (553 mg/g) were greatly higher than the reported values of other biosorbents. In addition, the adsorption behaviors of methylene blue (MB) and crystal violet (CV) by MT were spontaneous exothermic reactions and closely followed the pseudo-second-order (PSO) kinetics and Langmuir isotherm. Further, the depleted MT was regenerated using pyrolysis mode to convert depleted MT into MT-biochar (MBC). The maximum adsorption of Cu2+ and Pb2+ by MBC was up to 320 mg/g and 840 mg/g, respectively. In conclusion, this work presented a new option for the adsorption of cationic dyes in wastewater and a new perspective for the treatment of depleted biosorbents. 相似文献
To investigate the changes in chemical composition of flaxseed oil during thermal-induced oxidation and the resultant effect on thermal properties, samples with different oxidation levels were obtained by being heated at 180 °C for two hours and four hours. The oxidation degree was evaluated using peroxide value (PV), extinction coefficient at 232 nm and 268 nm (K232 and K268), and total polar compounds (TPC). Using chromatography, the fatty acid profile and triacylglycerol (TAG) profile were examined. Differential scanning calorimetry (DSC) was used to determine the crystallization and melting profiles. Thermal-induced oxidation of flaxseed oil led to a significant increase (p < 0.05) in PV, K232, K268, and TPC, but the relative content of linolenic acid (Ln) and LnLnLn reduced dramatically (p < 0.05). TPC derived from lipid degradation affected both crystallization and melting profiles. Statistical correlations showed that the onset temperature (Ton) of the crystallization curve was highly correlated with K232, TPC, and the relative content of LnLnLn (p < 0.05), whereas the offset temperature (Toff) of the melting curve was highly correlated with the relative content of most fatty acids (p < 0.05). This finding provides a new way of rapid evaluation of oxidation level and changes of chemical composition for flaxseed oils using DSC. 相似文献
