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.     

Synthetic studies toward lindenane-type dimers via Diels-Alder reaction

Lindenane-type dimers are natural sesquiterpenoids, mainly from Chloranthaceae, possessing an unique heptacyclic skeleton and diverse bioactivities. Inspection of their structures suggested the possibility of deploying an endo Diels-Alder reaction to construct the key cyclohexene ring. In this article we investigated the feasibility of Diels-Alder reactions between various dienes and dienophiles, leading ultimately to their applications to the realization of [4 + 2] lindenane dimers.     

Ring-opened metathesis polymers from cyclohexenylnorbornene and related Diels-Alder adducts of cyclopentadiene with vinyl and divinyl cyclohydrocarbons

5-(3-Cyclohexen-1-yl)-2-norbornene [CHNB] has been shown to form a very lightly crosslinked polymer (T_g = 127°C) with good elongation via ring-opening metathesis polymerization (ROMP). Based on swelling behavior with added norbornadiene dimer, the low crosslink density is ascribed to ≪0.5% participation by the cyclohexenyl ring. Compared to dicyclopentadiene (DCPD), CHNB polymerizations were less exothermic, required less catalyst, and exhibited greater molding latitude, which are advantageous for Reaction Injection Molding (RIM). Styrene-isoprene and styrene-ethylene/butylene block copolymers were effective impact modifiers for polyCHNB, forming large particle morphologies. Small rubber particles formed from styrene-butadiene block copolymers were not effective for impact enhancement of polyCHNB, in contrast to polyDCPD. Rubber-modified polyCHNB retained impact resistance four to six times longer than polyDCPD samples when aged in air at 50–70°C. Related RIM-ROMP of liquid monomer mixtures prepared by cyclopentadiene cycloadditions with 4-vinyl-1-cyclohexene, cis-1,3-divinylcyclopentane, 3,5-divinylcyclopentene or cis-2,4-divinylbicyclo[3.3.0]oct-6-ene formed highly crosslinked, less ductile copolymers with T_gs as high as 206°C. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3049–3063, 1997     

Highly efficient synthesis of extended triptycenes via Diels-Alder cycloaddition in water under microwave radiation

Microwave-irradiated Diels-Alder reactions of anthracene and endoxides (6a-g) in water afforded the cycloadducts (8a-g) with high efficiencies. The extended triptycenes (2a-g) were readily obtained by dehydration of 8a-g in a mixture of AcOH and Ac₂O with good overall yields.     

Easy access to N-aryl, N-heteroarylbenzoxazolinones and 4-aza analogues via Diels-Alder cycloaddition reactions

An improved method for the synthesis of N-aryl, N-heteroarylbenzoxazolinones and their 4-aza analogues is described. The process involves the Diels-Alder cycloaddition of benzoxazinic or pyridoxazinic dienic systems with dienophiles as a key step.     

A novel and facile stereocontrolled synthetic method for polyhydro-quinolines and pyridopyridazines via a diene-transmissive Diels-Alder reaction involving inverse electron-demand hetero Diels-Alder cycloaddition of cross-conjugated azatrienes

Cross-conjugated azatrienes bearing an electron-withdrawing sulfonyl or benzoyl group on the nitrogen atom underwent, on heating or in the presence of a Lewis acid (TMSOTf), an initial inverse electron-demand hetero Diels-Alder reaction with electron-rich dienophiles (vinyl ether, vinyl thioether, and allenyl ether) to produce 1:1 cycloadducts with high endo selectivity. The initial cycloadducts thus obtained underwent a second Diels-Alder reaction stereoselectively on the newly formed diene unit with electron-deficient dienophiles to give the crossed bis-cycloadducts, octahydroquinolines, with high diastereo-π-facial selectivity. The N-sulfonylazatrienes tethering an ortho-cinnamyloxyphenyl dienophile at the triene terminal underwent an initial intramolecular hetero Diels-Alder reaction of the inverse electron-demand type. The subsequent second Diels-Alder reaction of the formed mono-cycloadducts completed the diene-transmissive hetero Diels-Alder protocol to give benzopyrano[3,4-c]quinolines in a highly stereoselective manner.     

Microwave assisted Diels-Alder cycloaddition of 2-fluoro-3-methoxy-1,3-butadiene

The title fluorodiene (2) reacts with several dienophiles in moderate yields (20-65%, 0.5 h to 3d) when thermal activation is used. When 100 W microwave radiation is used the reaction yields (70-90%, 5-25 min) are greatly improved and the reaction times are much shorter. A microwave procedure is also used for the hydrolysis of vinyl ether cycloadducts to alpha-fluoroketones.     

Facile synthesis of phthalan derivatives via a Pd-catalyzed tandem hydroalkynylation,isomerization, Diels-Alder cycloaddition and aromatization reaction

A novel Pd-catalyzed tandem reaction involving hydroalkynylation,isomerization.Diels-Alder cycloaddition and aromatization reaction to produce phthalan derivatives in moderate yields is reported.The reaction is atom economical and occurs in a highly ordered fashion.The reaction mechanism is discussed.     

Experimental and theoretical studies of Diels-Alder dimerization of 1,2,3,4,5-pentachlorocyclopentadiene and of Diels-Alder cycloaddition of polychlorinated cyclopentadienes to norbornadiene

Diels-Alder cyclodimerization of 1,2,3,4,5-pentachlorocyclopentadiene (1) affords 2a as the exclusive reaction product. Diels-Alder cycloaddition of 1 to norbornadiene also proceeds stereoselectively to produce a single [4+2] cycloadduct, 4c. The structures of 2a and 4c were established unequivocally via application of single crystal X-ray crystallographic techniques. The origins of the observed diastereofacial selectivity in each of these cycloaddition processes have been investigated by application of semiempirical (AM1 Hamiltonian) and ab initio (Hartree-Fock 3-21G*) calculations. The computational results thereby obtained, which are based upon consideration of the kinetically favored transition state for each of the two cycloaddition reactions studied, are consistent with experiment. These semiempirical and ab initio methods also have been used to investigate the mechanisms of the Diels-Alder reactions that have been used to prepare aldrin and isodrin (7 and 8, respectively). The results thereby obtained suggest that isodrin formation via Diels-Alder cycloaddition of cyclopentadiene to 1,2,3,4,7,7-hexachloronorbornadiene proceeds with kinetic control of product stereochemistry.     

Synthetic studies of GKK1032s: the asymmetric synthesis of the decahydrofluorene skeleton via a novel cyclization of silyl enol ether and sequential retro Diels-Alder and intramolecular Diels-Alder reactions

GKK1032s, which were isolated from the culture broth of Penicillium sp. GKK1032, exhibit antitumor activity. We constructed the fully elaborated decahydrofluorene skeleton of GKK1032s. The C-ring was constructed by intramolecular cyclization reaction between a chiral epoxide and silyl enol ether, and the dienophile moiety was introduced concurrently. In addition, the AB-rings were stereoselectively constructed using novel sequential retro Diels-Alder (DA) and intramolecular Diels-Alder (IMDA) reactions. Several further modifications of the IMDA adduct were carried out, leading to the asymmetric synthesis of the desired hydroxyester including nine stereo centers.     

Normal electron demand Diels-Alder cycloaddition of indoles to 2,3-dimethyl-1,3-butadiene

The influence of experimental conditions and of a wide range of electron-withdrawing indole substituents has been studied in the thermally activated and Lewis acid activated intermolecular Diels-Alder cycloaddition of electron-poor indoles to 2,3-dimethyl-1,3-butadiene.     

Exploring polymer solubility with thermally-responsive Diels-Alder monomers: Revisiting the monkey's fist

Thermo-responsive monomers were designed to contain a Diels-Alder (DA) adduct such that cyclo-reversion would yield either the maleimide or the furan unit attached to the polymer chain. These thermally responsive monomers were then copolymerized with N-isopropylacrylamide (NIPAM) via reversible addition-fragmentation chain-transfer (RAFT) polymerization to yield linear gradient-copolymer structures as a comparison to existing nanogel/starlike systems to understand how polymer topology and composition influence solution-state properties. Using UV–Vis spectroscopy, it was determined that solution-state properties were thermally dependent and influenced by a number of variables such as comonomer feed ratio, polymer chain end functionality, and polymer backbone length and composition. Manipulation of the feed ratio allowed for control over the cloud point, including the breadth and location of phase separation. Thermal treatment of these copolymers revealed tunable and predictable variations in previously observed transitions, directly correlated to cleavage of the DA adducts and change in polymer backbone composition. Finally, on cooling cycles, a double sigmoid was sometimes observed, indicating a complex globule to random coil transition correlated to polymer chain end composition. These studies help understand how to untie the “monkey's fist.”     

New developments in hyperbranched polymers

In the last 12 years the field of hyperbranched polymers has been well established with a large variety of synthetic approaches and fundamental studies on structure and properties of these unique materials. However, new developments involving hyperbranched materials appeared recently, for example, different synthetic strategies, new reaction mechanisms, formation of more complex architectures, a deeper understanding of the branched structure and their kinetic development, and intensive studies on the material properties and possible applications. This demonstrates the high versatility and the possibilities that are still involved in hyperbranched polymers and render it one of the most active fields in polymer science with a very promising future. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2505–2525, 2000     

Diels-Alder反应可控制备交联等规聚丙烯

交联改性可以提升聚合物材料的机械性能、热稳定性及化学稳定性。 含有蒽基团的高质均分子量等规聚丙烯(iPP)可由配位共聚合反应制得,随后通过双官能的马来酰亚胺试剂与iPP中悬挂蒽基团间的Diels-Alder反应可以成功制备交联的iPP膜材料。 材料的交联度可以通过凝胶重量分析及膜红外分析得出,并由制备过程中交联剂的加入量有效地调控。 通过示差扫描量热仪(DSC)与广角X射线衍射仪(WAXD)表征发现,随交联度增加,交联改性限制了分子链的运动,iPP的结晶度逐渐降低、结晶能力逐渐下降。 对交联材料的动态热机械分析仪(DMA)测试表明,随交联度增加,iPP的损耗模量逐渐降低,材料韧性变差。 交联iPP的T_g逐渐升高,说明材料的耐热性得到提升。 由于交联形成了聚合物链间的交联网络,形成了更多受力点并且限制了聚合物链的自由移动,交联材料的拉伸强度随交联度的升高而增加,而断裂伸长率逐渐下降。     

Synthesis of aromatic phosphates via cycloaddition of phosphate dienes

Cycloadditions of phosphate dienes and quinones can be used to generate aromatic phosphates. Diethyl 3-methoxy-1-methylene-2-propenyl phosphate shows reactivity, stability, and regioselectivity comparable to the corresponding silyloxy diene. Because the phosphate ester in the product originates in the diene, cycloadditions with quinones will place this ester regiospecifically in the B-ring and allow facile distinction between the B- and A-ring oxygens of the cycloadduct.     

Salt, concentration, and temperature effects on an asparagine-based, aqueous Diels-Alder cycloaddition

Results are reported on the salt, concentration, and temperature effects in an aqueous, auxiliary-mediated asymmetric Diels-Alder cycloaddition. The auxiliaries were prepared under basic aqueous conditions from asparagine and trimethylacetaldehyde, and coupled to acryloyl chloride to generate dienophiles in one pot. Temperature and concentration modestly impacted cycloaddition stereochemistry. Experiments with a range of salts examined the speculation that complexation between the counterion of the auxiliary carboxylate and the dienophile carbonyl promotes the formation of the minor endo product, reducing cycloaddition diastereoselectivity for the endo products. The transformation was poorly selective for the carboxylic acid but gave moderate selectivities for several metallated carboxylates. The magnitude of the diastereoselectivity of the endo products was weakly dependent on carboxylate counterion and more strongly influenced by the basicity of the salt anion. Data presented suggest that Lewis acid catalysis reduces cycloaddition diastereoselectivity for the endo products.     

New carbazole-containing monomers and polymers

Five novel vinyl monomers, all of which contain 3-substituted carbazolyl groups [i.e., N-(9-ethyl-3-carbazolyl)-maleimide, 9-ethyl-3-carbazolyl acrylamide, 9-ethyl-3-carbazolyl methacrylamide, 9-ethyl-3-hydroxymethylcarbazolyl acrylate, and 9-ethyl-3-hydroxymethylcarbazolyl methacrylate], and their corresponding polymers were synthesized. Two methods were used for the synthesis of the last two monomers.     

New approach toward the total synthesis of (+)-aphidicolin by tandem transannular Diels-Alder/aldol strategy

The synthesis of a 15-membered macrocyclic triene containing all the required substituents of ring A of (+)-aphidicolin (1) is reported. This compound underwent a thermal transannular cycloaddition followed by an intramolecular aldol reaction to yield tetracycle 32 containing 8 chiral centers which is considered a key intermediate for the synthesis of (+)-aphidicolin and related analogs.     

Novel carbamoyl phosphonate monomers and polymers from unsaturated isocyanates

Carbamoyl phosphonate derivatives of methacrylic acid and α-methyl styrene were synthesized in good yield by reacting dimethyl and diethylphosphite with isocyanato ethylmethacrylate (IEM) or with meta-isopropenyl-α,α-dimethylbenzylisocyanate (m-TMI). The IEM derivatives yield soluble homopolymers with common radical initiators. The m-TMI carbamoyl phosphonates were copolymerized with styrene and with acrylates yielding soluble materials with better than 50 mol % incorporation of the phosphonate monomer. The side chain carbamoyl phosphonate group drastically lowers the T_g relative to those of the parent polymers. Thermogravimetric studies suggest the NHC(O)? P(O)(OR)₂ bond to be stable to about 225°C. The radiopacifying salt UO₂(NO₃)₂6 H₂O yields transparent films with the phosphonate-containing polymers. The T_g increases with uranyl content. © 1993 John Wiley & Sons, Inc.     

A theoretical study of the mechanism and stereoselectivity of the Diels-Alder cycloaddition between difluoro-2-methylencyclopropane and furan

A theoretical study of the molecular mechanism and stereoselectivity of the Diels-Alder cycloaddition reaction between difluoro-2-methylencyclopropane and furan has been carried out at the B3LYP/6-31G+^∗∗ level of theory. The calculation of activation and reaction energies indicates that the 3-endo cycloadduct is favored both kinetically and thermodynamically, which is in agreement with the experimental data. Analysis of the bond order and charge transfer in the transition states shows that this reaction takes place via a synchronous-concerted mechanism.