Cross‐linked poly(ε‐caprolactone) (PCL)‐based polyesterurethane (PUR) systems have been synthesized through Diels–Alder reactions by reactive extrusion. The Diels–Alder and retro‐Diels–Alder reactions proved to be useful for enhancing the molecular motion of PCL‐based systems, and therefore their crystallization ability, in the design of cross‐linked semicrystalline polymers with one‐way and two‐way shape‐memory properties. Successive reactions between α,ω‐diol PCL (PCL2), furfuryl alcohol, and methylene diphenyl 4,4′‐diisocyanate straightforwardly afforded the α,ω‐furfuryl PCL‐based PUR systems, and subsequent Diels–Alder reactions with N,N‐phenylenedimaleimide afforded the thermoreversible cycloadducts. The cross‐linking density could be modulated by partially replacing PCL‐diol with PCL‐tetraol. Interestingly, the resulting PUR systems proved to be semicrystalline cross‐linked polymers, the melting temperature of which (close to 45 °C) represented the switching temperature for their shape‐memory properties. Qualitative and quantitative measurements demonstrated that these PUR systems exhibited one‐way and two‐way shape‐memory properties depending on their cross‐linking density. 相似文献
A novel synthetic strategy gives reversible cross‐linked polymeric materials with tunable fluorescence properties. Dimaleimide‐substituted tetraphenylethene (TPE‐2MI), which is non‐emissive owing to the photo‐induced electron transfer (PET) between maleimide (MI) and tetraphenylethene (TPE) groups, was used to cross‐link random copolymers of methyl (MM), decyl (DM) or lauryl (LM) methacrylate with furfuryl methacrylate (FM). The mixture of copolymer and TPE‐2MI in DMF showed reversible fluorescence with “on/off” behavior depending on the Diels–Alder (DA)/retro‐DA process, which is easily adjusted by temperature. At high temperatures, the retro‐DA reaction is dominant, and the fluorescence is quenched by the photo‐induced electron transfer (PET) mechanism. In contrast, at low temperatures, the emission recovers as the DA reaction takes over. A transparent PMFM/TPE‐2MI polymer film was prepared which shows an accurate response to the external temperature and exhibited tunable fluorescent “turn on/off” behavior. These results suggest the possible application in areas including information security and transmission. An example of invisible/visible writing is given. 相似文献
The evolution of a ternary molecular system (imine, diene and nitrile) is analyzed to disclose the pathways leading to a divergent synthetic outcome. The Lewis acid catalyzed reaction between cyclohexadiene, 2‐phenyl‐indol‐3‐one and acetonitrile yields the imino‐Diels–Alder adduct as the major product together with minor amounts of the Mannich–Ritter‐amidine product. The experimental and computational data show that the relative orientation of the initial reactants dictates the synthetic outcome. The exo approach between imine and diene leads to the Diels–Alder adduct in a concerted process, whereas the endo mode leads to a polarized intermediate, which is trapped by acetonitrile to yield the multicomponent adduct. 相似文献
The 10‐halo (Cl or Br) anthracene‐9‐nitrile oxides (1a,b) were obtained directly from the treatment of 9‐anthracenylaldoxime with N‐halosuccinimide (NCS or NBS) in DMF. The 3‐(10′‐halo‐9′‐anthracenyl)‐5‐isoxazolecarboxylic esters ( 5a,b and 6a,b ) were prepared via 1 ,3‐dipolar cycloaddition between the obtained nitrile oxides 1a (or 1b ) and two different dipolarophiles: ethyl β‐pyrrolidinocrotonate (an enamine of ethyl acetoacetate) or dimethyl acetylenedicarbox‐ylate (DMAD) respectively. The 10 (or 10′)‐ position of the anthracene in either anthracene‐9‐nitrile oxide or 3‐(9′‐anthracenyl) isoxazole molecules (3,4) is readily halogenated by N‐halosuc‐cinimide in DMF. X‐ray studies showed that 5a possesses two aromatic ring systems that lie at 74.4° from coplanarity. The bond linking the two ring systems is 1.4893(18) Å, indicating only partial conjugation between the two ring systems. The crystal lattice showed unique head‐to‐tail intermolecular stacking of anthracene rings. 相似文献
An anthracene‐containing meso‐fused carbaporphyrin, which has extended π‐conjugation pathways as compared to the corresponding naphthalene‐containing carbaporphyrin, has been synthesized. The weak global aromaticity of the anthriporphyrin also allowed its use as the diene for a Diels–Alder reaction with dimethyl acetylenedicarboxylate (DMAD). The resulting phlorin contains an interesting bicyclic structure. Moreover, to the best of our knowledge, this phlorin is the first Diels–Alder adduct of a diene forming part of the global π‐conjugation pathway of an aromatic porphyrinoid. 相似文献
Dimethyl acetylenedicarboxylate (DMAD) is a very important and useful reagent for the preparation of dimethyl heterocyclic‐o‐dicarboxylates, which are key intermediates in the synthesis of fused pyridazine derivatives. The synthesis of thiopyranes by the Diels‐Alder reaction of dithiocarboxylate derivatives, synthesis of various cyclazines by [2 + 8] cycloaddition reactions, and synthesis of dimethyl pyrazolo[3,4‐b]pyridine‐5,6‐dicarboxylates and polycyclic heterocycles containing the 1,6‐naphthyridine ring system by the reaction of o‐aminonitrile compounds with DMAD are described here. 相似文献
An enantioselective chemical synthesis of arene cis‐dihydrodiols has been realized from 2‐pyrones through sequential ytterbium‐catalyzed asymmetric inverse‐electron‐demand Diels–Alder (IEDDA) reaction of 2‐pyrones and retro‐Diels–Alder extrusion of CO2. By using this strategy, a series of substituted arene cis‐dihydrodiols can be obtained efficiently with high enantioselectivity (>99 % ee in many cases). Based on this strategy, efficient and concise asymmetric total syntheses of (+)‐MK7607 and 1‐epi‐(+)‐MK7607 were accomplished. 相似文献
Crosslinked poly[styrene‐co‐(furfuryl methacrylate)] has been produced by the Diels–Alder (D‐A) reaction between the furan ring (within the copolymer) and the maleimide (MI) group (within the other reactant, bismaleimide). The retro D‐A reaction was followed by the analysis of MI groups produced at different times at five constant temperatures. The process was shown to follow first‐order kinetics, and the rate constants were determined. The findings are believed to be the first to provide quantitative information on the breakdown by a retro D‐A reaction of crosslinking in a polymer system. The D‐A and retro D‐A processes constitute a thermoreversible gelling system with respect to the formation and breakdown of crosslinks.
First‐order rate plot for the retro D‐A breakdown of crosslinked poly(ST‐co‐FM). 相似文献
One of the most important reactions in fullerene chemistry is the Diels–Alder (DA) reaction. In two previous experimental studies, the DA cycloaddition reactions of cyclopentadiene (Cp) and 1,2,3,4,5‐pentamethylcyclopentadiene (Cp*) with La@C2v‐C82 were investigated. The attack of Cp was proposed to occur on bond 19 , whereas that of Cp* was confirmed by X‐ray analysis to be over bond o . Moreover, the stabilities of the Cp and Cp* adducts were found to be significantly different, that is, the decomposition of La@C2v‐C82Cp was one order of magnitude faster than that of La@C2v‐C82Cp*. Herein, we computationally analyze these DA cycloadditions with two main goals: First, to compute the thermodynamics and kinetics of the cycloadditions of Cp and Cp* to different bonds of La@C2v‐C82 to assess and compare the regioselectivity of these two reactions. Second, to understand the origin of the different thermal stabilities of the La@C82Cp and La@C82Cp* adducts. Our results show that the regioselectivity of the two DA cycloadditions is the same, with preferred attack on bond o . This result corrects the previous assumption of the regioselectivity of the Cp attack that was made based only on the shape of the La@C82 singly occupied molecular orbital. In addition, we show that the higher stability of the La@C82Cp* adduct is not due to the electronic effects of the methyl groups on the Cp ring, as previously suggested, but to higher long‐range dispersion interactions in the Cp* case, which enhance the stabilization of the reactant complex, transition state, and products with respect to the separated reactants. This stabilization for the La@C82Cp* case decreases the Gibbs reaction energy, thus allowing competition between the direct and retro reactions and making dissociation more difficult. 相似文献
With the goal of synthesizing new [n]paracyclophanes, the expansion of the scope of a strategy originally disclosed by Winterfeldt et al., was investigated. This approach involves sequential Diels–Alder/retro‐Diels–Alder reactions, the applications of which have been constrained so far to steroid derivatives. An efficient access to new functionalized [9]‐, [10]‐, and [16]paracyclophanes, including original cage architectures, was developed from readily available building blocks using thermal electrocyclization and a cycloaddition/cycloreversion sequence as the key steps. 相似文献