Post-polymerization modification of aromatic polyimides via Diels-Alder cycloaddition

We report a facile post-polymerization modification route to functionalized aromatic polyimides via Diels-Alder cycloaddition. Aromatic polyimides are important, versatile high-performance polymers; however, their structural diversity is restricted by the requirements of the step-growth polymerization. We prepared polyimides with alkynes in their main-chain as macromolecular dienophiles and quantitatively grafted tetraphenylcyclopentadienone based dienes. The resulting solution-processable, wholly aromatic polyimides show a considerable increase in surface area due to the induced conformational changes and bulky, rigid, and contorted molecular structures. The orthogonality of the reaction is exploited to insert functional groups, namely bromine and sulfonates, along the polymer backbone. In a further extension, the phenylene segments undergo cyclodehydrogenation to form nanographene segments within the polymer chains. The Diels-Alder cycloaddition onto polyimides is therefore demonstrated to be an effective, widely applicable route to tunable high-performance polymers with value-added functionality and thus considerable potential in a wide range of advanced materials.     

New processable polyaromatic ether–keto-sulfones curable by Diels-Alder cycloaddition

New Processable polyaromatic ether-keto-sulfones were prepared from the acid chloride of bis-m-carboxyphenyl acetylene (XII), isophthaloyl chloride (XX), diphenyl ether (XVIII), and 4,4′-diphenoxydiphenyl sulfone (XIX) in a Friedel-Crafts-type polymerization. These polymers were cured by Diels-Alder cycloaddition with 1,4-diphenyl-1,3-butadiene. The cured polymers showed an increase in T_g and in thermal and heat stabilities. The polymers form colorless, transparent, brittle films and can be cast into a glass fiber laminate. Both meta-and para-substituted acid chlorides of biscarboxyphenyl-1,3-butadiene yielded insoluble polymers under the same conditions but form processable polymers where combined with acetylene units in the polymer chain. Polymers that contained both acetylene and butadiene units were prepared but could not be cured by an intramolecular Diels-Alder cycloaddition reaction.     

碳纳米管上分子印迹微反应器的构建及催化Diels-Alder反应研究

分子印迹聚合物（MIP）在合成与催化方面的应用受到越来越多的关注.传统的本体聚合生成的聚合物往往具有活性中心的利用率低、活性中心不均一、底物难以到达等问题.由于纳米材料具有很高的比表面积,将MIP制备成纳米级别的材料将有可能解决这些问题.本研究的目的是在纳米材料上构建分子印迹微反应器(MIM).通过多壁碳纳米管(MWNTs)上羟基的酯化反应在碳纳米管表面枝接双键,以蒽和马来酸的加成反应产物为模板分子,甲基丙烯酸(MAA)为功能单体,乙二醇二甲基丙烯酸酯(EDMA)为交联剂,通过微波辐射聚合在碳纳米管表面枝接一层厚度约为30 nm 的分子印迹催化剂. 该催化剂对蒽和马来酸的加成反应具有明显的催化作用,反应开始后的180min内,催化反应速率是不加催化剂反应速率的1.77倍. 催化动力学遵守Michaelis-Menten方程,催化反应最大速率（vmax）为0.713 umolL-1S-1,米氏常数（KM）为17735.24 umolL-1.     

Computational investigation of the suitability of heterocyclic aromatic compounds with two heteroatoms in the 1 and 3 ring positions as dienes for the diels alder reaction

Semiempirical and density functional theory computational studies were carried out with the target determining the reactivity of five membered heterocycles with heteroatoms in the 1 and 3 positions as dienes for Diels-Alder reactions. The relative reactivity was evaluated in their reaction with acetylene, ethylene, and cyclopropene as dienophiles for cycloaddition. Qualitative criteria such as uniformity of heterocycle bond orders, change of bond orders and frontier molecular orbital energies in transformation of reactants into transition state structures were used to determine the relative reactivity in comparison with furan. These results are compared with the computed activation barriers as well as with experimental findings, where available. If cycloaddition is feasible with these heterocycles, a new synthetic transformation of simple organic compounds to valuable prostaglandin derivatives can be accomplished.     

Total synthesis of (+/-)-crinine via the regioselective Stille coupling and Diels-Alder reaction of 3,5-dibromo-2-pyrone

The regioselective synthesis and Diels-Alder cycloaddition of 3-(3,4-methylenedioxyphenyl)-5-bromo-2-pyrone provided a new synthetic route to crinine. The vinyl bromide group can be used as a handle for further derivatization.     

Enhancing the scope of the Diels-Alder reaction through isonitrile chemistry: emergence of a new class of acyl-activated dienophiles

α,β-Unsaturated imides, formylated at the nitrogen atom, comprise a new and valuable family of dienophiles for servicing Diels-Alder reactions. These systems are assembled through extension of recently discovered isonitrile chemistry to the domain of α,β-unsaturated acids. Cycloadditions are facilitated by Et(2)AlCl, presumably via chelation between the two carbonyl groups of the N-formyl amide. Applications of the isonitrile/Diels-Alder logic to the IMDA reaction, as well as methodologies to modify the N-formyl amide of the resultant cycloaddition product, are described. It is expected that this easily executed chemistry will provide a significant enhancement for application of Diels-Alder reactions to many synthetic targets.     

Click chemistry by microcontact printing on self-assembled monolayers: a structure-reactivity study by fluorescence microscopy

Microcontact printing (μCP) has developed into a powerful tool to functionalize surfaces with patterned molecular monolayers. μCP can also be used to induce a chemical reaction between a molecular ink and a self-assembled monolayer (SAM) in the nanoscale confinement between stamp and substrate. In this paper, we investigate the Huisgen 1,3-dipolar cycloaddition, the Diels-Alder cycloaddition and the thiol-ene/yne reaction induced by μCP. A range of fluorescent alkyne inks were printed on azide SAMs and fluorescence microscopy was used to monitor the extent of the 1,3-dipolar cycloaddition on a glass substrate. The rate of cycloaddition depends on the reactivity of the alkyne and on the presence of Cu(I). The cycloaddition is accelerated by Cu(I) but it also proceeds readily in the absence of Cu(I). In addition, a range of fluorescent diene inks were printed on alkene SAMs on glass. In this case, fluorescence microscopy was used to monitor the rate of the Diels-Alder cycloaddition as well as its retro-reaction. Finally, fluorescent thiol inks were printed on alkene SAMs on glass, and fluorescent alkenes and alkynes were printed on thiol SAMs. It is shown that reactions by μCP follow structure-reactivity relationships similar to solution reactions. Under optimized conditions all reactions lead to dense microarrays of addition products within minutes of printing time.     

Lewis acid-promoted hetero Diels-Alder cycloaddition of alpha-acetoxynitroso dienophiles

[reaction: see text] alpha-Acetoxynitroso compound 3 has been prepared as a new stable, isolable, and reactive dienophile in nitroso Diels-Alder reactions. The yield of the [4 + 2] cycloaddition of alpha-acetoxynitroso dienophile with 1,3-dienes could be enhanced in the presence of 20 mol % Lewis acid. An unexpected retro hetero-Michael reaction from 26 was observed, leading to the cleavage of the N-O bond of the cycloadduct. This tandem nitroso Diels-Alder/retro hetero-Michael sequence has been used with cyclic and acyclic 1,3-dienes.     

Highly selective Diels-Alder reactions of directly connected enyne dienophiles

The paper describes the course of cycloadditions of Diels-Alder dienophiles containing linked enyne sites, each substituted with activating groups. Consistently, it was found that in the enyne cases the Diels-Alder reaction occurred specifically at the acetylenic center. Furthermore, it was found that the regiochemical sense of the cycloaddition was apparently determined by the remote activating group bound to the olefinic site. This remote acrylyl group totally dominated the course of the cycloaddition, relative to the activating group bound directly on the acetylene site. Explanations for these findings at the computational level are provided. The computations also rationalize the strong preference for cycloaddition to occur at the acetylene linkage and encompass the otherwise surprising regiochemical dominance by the remote ester on the olefinic site. The high selectivities available through such reactions provide important new opportunities in the synthesis of orsenillate type substructures that are found in a variety of natural products of contemporary interest.     

Diels-Alder cycloaddition of novel buta-1,-3-diene derivatives possessing a (diethoxyphosphinoyl)difluoromethyl unit

A series of new buta-1,3-diene derivatives possessing a (diethoxyphosphinoyl)difluoromethylene unit at the terminal carbon was prepared to examine the reactivity for Diels-Alder cycloaddition with various representative dienophiles.     

Cycloaddition Reactions and Dendritic Polymer Architectures – A Perfect Match

Summery: The potential of cycloaddition (CA) reactions for the synthesis of dendritic polymers is pointed out. The [4 + 2] Diels Alder cycloaddition as well as 1,3-dipolar CA reactions including “click chemistry” are addressed, and the advantages of these reactions like high selectivity, thus high tolerance towards additional functionalities, high yields and synthesis under mild reaction conditions are highlighted. New perfectly branched dendrimers as well as hyperbranched polymers have been prepared and modified using the 1,3-dipolar cycloaddition reaction of azines with alkynes. The 1,3-dipolar CA reaction of bisazine with maleimides results in hyperbranched and thus, irregular and broadly distributed polymers though with a degree of branching of 100% due to special intermediate formation. The [4 + 2] Diels Alder cycloaddition was successfully applied for the synthesis of highly branched polyphenylene structures using the AB₂ + AB and the A₂ + B₃ approach. CA reactions are also very suitable for highly efficient polymer analogous reactions and thus, they can also be used to prepare complex polymer architectures like dendronized polymers.     

Intramolekulare diels-alder-reaktionen mit substituierten 1,2,4,5-tetrazinen und 1,2,4-triazinen

1,2,4,5-tetrazines as well as 1,2,4-triazines carrying sidechain dienophiles react in an intramolecular Diels-Alder cycloaddition with inverse electron demand to yield novel bicyclic pyridazines and pyridines.     

Theoretical investigation of cycloaddition reactions with Lewis acid mediation

This review covers theoretical investigations of cycloaddition reactions with Lewis acid (LA) mediation, except for Diels-Alder reactions. The key factors influencing these processes are analyzed, as well as the details of the reaction mechanisms.     

Total synthesis of (±)-cycloclavine and (±)-5-epi-cycloclavine

Novel routes to the naturally occurring indole alkaloid cycloclavine and its unnatural C(5)-epimer are described. Key features include the rapid construction of the heterocyclic core segments by two Diels-Alder reactions. An indole annulation was accomplished by a late-stage intramolecular Diels-Alder furan cycloaddition, and a methylenecyclopropane dienophile was used for a stereoselective intramolecular [4 + 2] cycloaddition to give the cyclopropa[c]indoline building block present in cycloclavine.     

Diels-Alder cycloaddition of o-quinonedimethides and alkylidene-5H-furan-2-ones: new and rapid access to lambertellol cores and arthrinone derivatives

An efficient synthesis of deoxy-lambertellols was reported through a highly chemo- and diastereoselective intermolecular Diels-Alder cycloaddition between trans-1,2-disiloxybenzocyclobutenes and 2-methylprotoanemonine. Such transformation with δ-substituted γ-alkylidenebutenolides, to prepare new analogues of these tricyclic spirolactones, which would be very difficult to prepare by other ways, was also studied.     

呋喃环参与的Diels-Alder反应及其应用

呋喃环在作为Diels-Alder反应的二烯组份中得到了广泛的研究.本文就呋喃环参与的分子间和分子内D-A反应及其应用作一综述.     

Intramolecular Diels-Alder and tandem intramolecular Diels-Alder/1,3-dipolar cycloaddition reactions of 1,3,4-oxadiazoles

The scope of intramolecular Diels-Alder and a novel tandem Diels-Alder/1,3-dipolar cycloaddition cascade of 1,3,4-oxadiazoles is disclosed. In the cases examined, the tandem cycloadditions construct three new rings with formation of four new C-C bonds and set all six stereocenters about a central six-membered ring in a single step including three contiguous and four total quaternary centers without a trace of a second diastereomer.     

Synthesis and Use of 6-, 7-and 8-Membered P-Heterocycles

The synthesis of new families of P-heterocycles by reductive modifications, ring enlargement and Diels-Alder cycloaddition is described.     

Iminium ion catalysis: direct comparison of imidazolidinone and diarylprolinol ether reactivity

Comparison of the relative reactivities of the benchmark catalysts for iminium ion catalysed Diels-Alder cycloaddition under optimised literature conditions showed the imidazolidinone scaffold to possess significantly superior levels of activity in the Diels-Alder cycloaddition when compared to diarylprolinol silyl ethers.     

Cycloaddition across the benzofuran ring as an approach to the morphine alkaloids

The intramolecular Diels-Alder reaction of several amidofurans tethered onto a benzofuran ring was examined as a strategy for the synthesis of morphine. Bromo substitution on the furan ring did not provide sufficient activation to allow the cycloaddition to take place across the aromatic benzofuran. However, the presence of a large o-methylbenzyl group on the amido nitrogen atom causes the reactive s-trans conformation of the amidofuran to be highly populated, thereby facilitating its Diels-Alder cycloaddition across a tethered benzofuran.