High-temperature polymer cycloaddition

Since there are 30 double bonds to react in fullerene Cgo, the [2-1-2] cycloaddition of Cgo molecules at these bonds results in the formation of so-called fullerene polymers [19]. Although it seemed important to clarify the structure and properties of the most basic unit of these polymers such as dimers and trimers, the method for preparation of these polymers such as high-pres-sure/high-temperature treatment or photoirradiation was not suitable to stop the [2-1-2] reaction at the stage of dimerization or trimerization. 

This result proves that well-defined structures with low degree of heterogeneity of the multiarm star-shaped polymers can be synthesized. Moreover, the method reported herein can also provide a synthetic pathway for the introduction of block copolymers synthesized via different polymerization routes (RAFT, ROP, etc.) onto the anthracene-end-functionalized multiarm star-shaped polymers. Although the Diels-Alder cycloaddition between anthracene and maleimide derivatives has proven to provide good results in the formation of complex architectures, the major drawback of this method remains the requirement of high temperature and relatively long reaction times. 

Furyl derivatives 76, with an allylether or allylamine-type linkage to a methylenecyclopropane framework, readily undergo high pressure-promoted intramolecular cycloaddition" to give spirocyclopropane tricyclic products 77. No cycloaddition reaction occurred at ambient pressure and the products were mostly tar and polymers. Lewis acid catalysis was only marginally successful (Scheme 7.18). At 1.0 GPa and a slightly elevated temperature (60-70 °C) the intramolecular Diels-Alder reaction occurs readily and is exo-diastereo-selective. To quantify the pressure effect on the kinetics the volumes of activation were determined. 

While metallocenes are ubiquitous in organometallic and polymer chemistry, few such complexes have been reported to catalyze the Diels-Alder process in high enantioselectivity [127,128,129]. Thebis(tetrahydroindenyl)zirconium tri-flate 60 and the corresponding titanocene are electrophilic to the extent that they catalyze the low-temperature cycloadditions of acrylate and crotonate imides with cyclopentadiene with good diastereoselectivity and excellent enantioselec-tion (Scheme 48). The reactivity of 60 is noteworthy since the corresponding reaction using the crotonyl imide with highly reactive catalysts 31a or 44 requires temperatures of -15 and 25 °C, respectively. 

Inter molecular Additions to Cyclohexenones and Related Systems. Calculations have been reported that deal with a transition state analysis of the regioselectivity encountered in triplet-state (2 + 2)-cycloaddition reactions of cyclohex-2-enone. Diastereoselective (2 + 2)-cycloaddition of ethene to cyclohexenone carboxylates in the presence of chiral auxiliaries has been described. Yields of bicyclo[4.2.0]octanone derivatives can be obtained with de as high as 81%. The enone (16) can be tethered to a polymer substrate via the tether group R. Irradiation of this material in toluene with ethene at —78°C gives a 68% yield of the adduct (17). The de of the product is reasonable at 72%. Addition of 1,1-diethoxyethene to the cyclohexenone (18, R = Ph) results in the formation of the two stereoisomeric head-to-tail regioisomers (19) and (20). The outcome of the reaction is dependent on the rate of formation of 1,4-biradical intermediates. This can be seen in the dependence of the cisjtrans ratio on the solvent and on the temperature at which the reaction is carried out. Thus with enone (18, R = Ph) in acetone at 3°C, a cjt ratio of 4.4 is obtained, and this changes to 2.1 at —40°C. In acetonitrile the cjt ratio is only 1.9 at the same temperature. With the other derivative of (18), the cjt ratio is 1.3 in acetone and 0.8 in acetonitrile. Photoaddition of ethene to the enone carboxylate [21, R = (-)-8-(2-naphthyl)menthyl] results in the formation of the diastereoisomers (22) and (23) with a de of 56% at —78°C. The diastereo-selectivity can be enhanced by the addition of naphthalene derivatives to the solution. Thus with naphthalene, a de of 71% is obtained, and this can be increased to 83% with 1-phenylnaphthalene. ... 

Reactions between a diene and a dienophile yielding cyclohexene derivatives through carbon-carbon or carbon-heteroatom bond formation are referred to as Diels-Alder (DA) reactions. The concerted [4 + 2] cycloaddition was first described in 1928 by Diels and Alder, ° and can be adapted for the efficient conjugation of peptides and proteins to synthetic polymers. Despite being recognized as highly selective and additive-free, DA reactions require elevated temperatures to obtain the desired product. Furthermore, the reversibility of the reaction limits its application in materials science. ... 

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