Diels-Alder anthracene-maleimide

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

The single-electron transfer from one excited component to the other component acceptor, as the critical step prior to cycloaddition of photo-induced Diels Alder reactions, has been demonstrated [43] for the reaction of anthracene with maleic anhydride and various maleimides carried out in chloroform under irradiation by a medium-pressure mercury lamp (500 W). The (singlet) excited anthracene ( AN ), generated by the actinic light, is quenched by dienophile... 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Kochi and co-workers studied photoinduced Diels-Alder cycloadditions via direct photoexcitation of anthracene as a diene with maleic anhydride and various maleimides as dienophiles. Here, fluorescence-quenching experiments, time-resolved absorption measurements, and the effect of solvent polarity provide striking evidence for an ion-radical pair to be the decisive intermediate [83],... 

Figure 1.11 Fujita s self-assembled coordination cage, which is prepared by simple mixing of an exo-tridentate organic ligand and end-capped Pd (I I) ion in a 4 6 ratio. Cache optimized structure of the ternary complex anthracene-maleimide metallocage. Molecular structure ofthesyn-isomerofthe 1,4-Diels-Alder adduct.

We specifically chose the reaction between anthracene and maleimide (Figure 5.3.1) for a number of reasons. Most importantly, we assumed that the completely different overall geometry of reactants and products would facilitate enrichment of catalysts that are capable of multiple turnovers. Anthracene is planar, in contrast with the 120° angles between the different rings in the Diels-Alder product. A ligand that can bind to anthracene should, therefore, not be able to bind to the product except after extensive refolding. The availability of sensitive UV absorbance and fluorescence assays for anthracene was another practical reason to choose this reaction. 

Figure 8.20 Diels-Alder reactions of maleimide (23) and anthracene (30) within cage 2.

Three unique di-diene have been utilized by Meek (9, 10) in the preparation of Diels-Alder copolymers. Reaction of 1,8-diphenyl-octatetraene with ftts-maleimides in refluxing chloroform for several days affords high yields of polymers with softening temperatures well over 300°, but low intrinsic viscosities. The copolymerization of 1,5-di(9-anthryl)-l,4-pentadiene-3-one and anthralazine with fo s-maleimides employs the diene nature of anthracene to obtain polymers with... 

Ribozyme-catalyzed reactions involving C-C bond formations have also been reported. Seelig and Jaschke (233) presented the in vitro selection of ribozyme catalysts for the Diels-Alder reaction between maleimide and anthracene, employing a 2 X lO -member library of 160-mer modified ONs (L28) with 120 randomized positions. The selection strategy used is shown in Fig. 10.40. Library L28 was prepared from the corresponding dsDNA sequences, and transcription initiation was performed in the presence of ternary complexes between guanosine monophosphate (10.57), PEG (10.58), and anthracene (10.59, step a. Fig. 10.40). The library obtained contained a 5 -anthracene-PEG appendage and was incubated with biotin-modified maleimide... 

Boydston and co-workers recently revisited the mechanochemical chain scission of star-shaped polymers in acoustic fields using a fluorogenic tum-on mechanophore (Fig. 30a) [232]. They synthesized PMA-L 61.6 and PMA-S3 28.6/ 85.9 with Mspan = 2Maim = 57.2 kDa (Fig. 30b). In both samples they linked one of the PMA arms to the core by an anthracene-maleimide Diels-Alder adduct. This mechanophore was previously studied by Bielawski and co-workers [57, 58]. Upon cycloreversion by mechanical force, it produces an anthracene moiety which displays strong UV-Vis and photoluminescence signals. 

Fig. 30 (a) Anthracene-maleimide Diels-Alder adduct that can generally fluoresce an anthracene moiety upon mechanical force, (b) Mechanophore-containing star and linear polymers bearing similar (c) Photoluminescence spectra in DMF of PMA-S3 28.6/85.9 upon increasing... 

Scheme 113 Synthesis of block copolymer via anthracene-maleimide-type Diels-Alder reaction.

PEO-PSt-PtBA arms, PEO-fi-PSt, and PEO-b-PMMA was synthesized via Diels-Alder reaction of maleimide and anthracene end-functionalized polymers using ATRP or NMP. Durmaz et elegantly combined 1,3-dipolar cycloaddition with... 

Figure 12.20 Diels-Alder click reactions of maleimide with (a) furan and (b) anthracene.

Problem 12.10 Discuss how the following block copolymers can be prepared via Diels-Alder reaction of maleimide- and anthracene-end functionalized polymers (a) PMMA-fc-PSt, (b) PEG-fc-PSt, (c) PrBA-i>-PSt, and (d) PMMA-ii-PEG block copolymers. 

Figure 19 Diels-Alder reactions of (a) anthracene, (b) triphenylene, and (c) naphthalene with maleimide inside cage 5. (d) The crystal structure of the Diels-Alder adduct of 2,3-diethylnaphthalene within the cage.

Molecular dynamic simulations show that the confined space of SWCNTs may influence both the kinetics and thermodynamics of the Diels-Alder reactions between anthracenes and N-substituted maleimide guests, favoring an unusual regioselectivity. Thus, in the confined space of CNTs, the 1,4-exo adduct of a Diels-Alder cycloaddition may be produced instead of the 9,10-adduct, which is favored in bulk. It is possible that the nanotube chirality could influence the outcome of the reaction, as this could alter the preferred orientation of the reactants inside the SWCNT. However, it was snspected that the diameter of the SWCNT is likely to have by far the largest influence on the reaction outcome, as this will have the biggest influence on how close the reactants can approach each other [195]. 

Tunca et al. [27, 28,38] reported in a series of papers on a multi-click approach for the preparation of brush copolymers. The backbone consisted of homopolymers, statistical polymers, or block copolymers from ONBEs with orthogonal side groups for Diels-Alder click reaction [27,28,38], azide/alkyne click reaction, [27, 28, 38] and nitroxide coupling [27]. In a grafting-to approach, maleimide-or ONBE-functionalized polymers (PEG, P BA, PMMA (56)) were attached by Diels-Alder click reaction with the anthracene groups pendant at the polymer backbone (55) (Scheme 9.8b). PCL chains were attached by an azide/alkyne click reaction, while a combination of Diels-Alder and azide/alkyne click reaction allowed the synthesis of graft copolymers with PS-/ -PEG-, PS-h-PMMA-, or PS-/ -P BA side chains. 

There are many well-studied photochemical reactions that are clean, high yielding, relatively fast, and require no chemical catalysts. A number of these have been examined for SCNP formation, including the photochemically triggered Diels-Alder reaction between 2,5-dimethylbenzophenone and maleimide, the photo-dimerization of coumarin, the photodimerization of anthracene, and the photo-induced nitrile imine mediated tetrazole-ene cycloaddition. ... 

Diels-Alderase ribozymes (DAR), isolated from a combinatorial RNA library, cause a (2 X 10 )-fold acceleration of the Diels-Alder cycloaddition of anthracene covalently tethered to ribozyme and a biotinylated maleimide in aqueous-buffered medium (Scheme 5.15). Jaschke recently reported the action of Diels-Alderase ribozymes as true catalysts, in the sense that they catalyze the cycloaddition of anthracene that is not covalently tethered to RNA and biotin maleimide in aqueous-buffered medium. 

Durmaz, H., Colakoglu, B., Tunca, U., and Hizal, G. (2(X)6a) Preparation of block copolymers via Diels-Alder reaction of maleimide- and anthracene-end functional polymers. Journal cf Polymer Science Part A-Polymer Chemistry, 44,1667. 

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