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.     

New processable polyaromatic amides curable by intramolecular cyclization. XVI

New Processable Polyaromatic amides were prepared from 2,2′-diiododiphenyl-4,4′-dicarbonyl dichloride (I) and several aromatic diamines. Phenylethynyl groups were introduced in these polymides by replacing the iodine groups with copper phenyl acetylide. On thermal curing, 2,2′-di(phenylethynyl)biphenyl group undergoes intramolecular cyclization to form 9-phenyl dibenzanthracene derivative. The cured polymers showed increased heat and chemical stabilities. No melting points were observed for all the polymers below 500°C. The viscosity of the polymers was decreased on substitution of the iodine by phenylethynyl groups.     

New processable polyaromatic esters curable by intramolecular cyclization. XVIII

New processable polyaromatic esters were prepared from 2,2′-diiododiphenyl-4,4′-dicarbonyl dichloride (I), isophthaloyl chloride (II), and/or terephthaloyl chloride (III) with 4,4′-isopropylidene diphenol (IV), 4,4′-sulfonyldiphenol (V), or resorcinol by interfacial condensation. In these polymers phenylacetylenyl groups were introduced by replacing the iodine. This process led to soluble and curable polymers. Polymer films can be prepared. After curing the polymers were insoluble and showed excellent thermal and chemical resistance. The curing process increased the polymers' softening temperature about 20°C.     

New processable polyaromatic ether-keto-sulfones curable by intramolecular cyclization. XV

New processable polyaromatic ether-keto-sulfones were prepared from 2,2′-diiododiphenyl-4,4′-dicarbonyl dichloride (I), bis(p-phenoxybenzene)sulfone (II), isophthaloyl chloride (III), and diphenyl ether (IV) in Friedel–Crafts-type polymerizations. In the most promising of the iodine-containing polymers phenylacetylenyl groups were introduced in place of iodine. This polymer, with an initial monomer ratio of I:II:III:IV = 1:5:7:3, was further investigated. It is soluble in DMF, DMA, pyridine, and sulfuric acid. After curing it was insoluble in all solvents used and lost only 1.1% of its weight at 300°C when heated in air for three days. Hence in this cured state it has excellent chemical and thermal resistance. It can be cast into a film from solution in DMAc and a glass fiber laminate is readily prepared. The film is tough, transparent, and brown in color. The cured film is tougher than the uncured. The glass fiber laminate is also tough and fairly flexible. A distinct advantage of this type of polymer is its ready availability in relatively cheap raw materials. The phenylacetylenyl-group-containing polymer showed a transition temperature at 175°C and two exotherm peaks at 243 and 361°C which disappeared after curing in a DSC thermogram. Before and after curing this polymer displayed softening temperatures at 149 and 171°C, respectively, measured by a Vicat apparatus at a heating rate of 1°C/min. No melting temperatures up to 500°C were observed for any of the polymers in this study.     

New processable polyaromatic keto-sulfones with internal acetylene units

Polyaromatic ketone-sulfones of high molecular weight have been obtained from bis-(3-carboxyphenyl) acetylene dichloride, isophthaloyl chloride, terephthaloyl chloride, and 4,4-diphenoxydiphenyl sulfone in a Friedel-Craft-type polycondensation. The polymer products are white powders and form transparent, strong films. These polymers could be reacted by Diels–Alder cycloaddition with 1,4-diphenyl-1,3-butadiene. These polymers showed only a very slight increase in viscosity over those that were not reacted with diphenylbutadiene.     

New processable polyaromatic ether-keto-sulfones as colorless,clear film-forming materials

New processable polyaromatic ether-keto-sulfones were prepared from 4,4′-diphenoxydiphenyl sulfone, terephthaloyl chloride, isophthaloyl chloride, and bis-p-carboxyphenylacetylene dichloride (IV) in a Friedel-Crafts-type polymerization. The white polymers obtained form colorless, transparent, strong films and can be cast into a glass-fiber laminate. One polymer was prepared which incorporated biphenylene-2,6-dicarboxylic acid dichloride as crosslinking sites; this heat-cured polymer was completely insoluble in sulfuric acid.     

New monomers and polymers via Diels-Alder cycloaddition

The series of new bis(naphthalic anhydrides) was prepared through Diels-Alder cycloaddition. The Diels-Alder cycloaddition was used as a synthetic route to new phenylated monomers as well as to polymers. All polymers synthesized revealed to be soluble in a wide range of organic solvents such as toluene, THF, chloroform, and displayed high thermostability. Therefore, they can be processed easily and are promising candidates for advanced coating systems as well as for electrooptical applications.     

Phenylethynyl-pendant polyphenylquinoxalines curable by an intramolecular cycloaddition reaction

Polyphenylquinoxalines containing 2,2′-bis(phenylethynyl)diphenylene moieties along the polymer backbone have been synthesized. As anticipated, these polymers were found to undergo a novel curing reaction consisting of an intramolecular cycloaddition (IMC) of pendant groups to a dibenzoanthracene backbone structure. The IMC reduces chain mobility, and the fused ring structure increases the glass transition temperature of the polymer. The potential of this approach to curing high-temperature polymers was demonstrated in the processing of one such polymer having an initial T_g of 215°C. Curing at 245°C with no evolution of volatiles produced a T_g of 365°C. This very significant increase in potential use temperature via a volatiles-free IMC cure provides promise for a tough phenyiquinoxaline resin system which can be used to fabricate reinforced composites that have use temperatures far exceeding processing temperatures.     

Segment block dendrimers via Diels-Alder cycloaddition

Segment block dendrimers consisting of polyester and polyaryl ether dendrons were synthesized using reagent free Diels-Alder cycloaddition reactions. Three generations of furan functionalized polyaryl ether dendrons were reacted with maleimide functionalized polyester dendrons of the same generation to obtain segment block dendrimers in good yields. The thermoreversible nature of these macromolecules was investigated by subjecting them to elevated temperatures in the presence of anthracene as a scavenger diene.     

Microwave-assisted sidewall functionalization of single-wall carbon nanotubes by Diels-Alder cycloaddition

The first Diels-Alder cycloaddition of o-quinodimethane to SWNT has been performed under microwave irradiation.     

beta-Acyloxysulfonyl tethers for intramolecular Diels-Alder cycloaddition reactions

[reaction: see text]. beta-Hydroxy sulfone-based tethers were employed for the first time to achieve thermally mediated intramolecular Diels-Alder cycloaddition. The reactions proceeded with complete regioselectivity and high (10/1) to complete endo/exo-selectivity and resulted in the preferential formation of one of the two possible endo-cycloadducts. The yields and stereoselectivities were proportional to the bulk of the R(1) substituent on the beta-acyloxysulfonyl tether.     

o-Fluoranil chemistry: Diels-Alder versus hetero-Diels-Alder cycloaddition

The title quinone undergoes [4 + 2] cycloadditions in two ways, Diels-Alder on the ring and hetero-Diels-Alder by attack at the oxygens. The latter mode of reaction is strongly favored thermodynamically, but there is a kinetic bias favoring the normal Diels-Alder addition that often prevails, especially with cycloaddends that are not electron-rich.     

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.     

Diels-Alder cycloaddition reactions of 1,3-diazabutadienes with sulfene

Diels-Alder cycloaddition reactions of 1-aryl-4-dimethylamino-2-phenyl(thiomethyl)-1,3-diazabutadienes with sulfene resulting in good yields of 1,2,4-thiadiazine-1,1-dioxide derivatives are reported.     

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.     

Anion radical chain cycloaddition of tethered enones: intramolecular cyclobutanation and Diels-Alder cycloaddition

[reaction: see text] The anion radicals of certain bis(enones), generated by cathodic reduction, are observed to participate in intramolecular cyclobutanation, yielding bicyclo[3.2.0]heptane derivatives through an anion radical chain mechanism. Evidence for stepwise cycloaddition involving distonic anion radical intermediates is presented. In addition to the novel anion radical cyclobutanations, an unprecedented intramolecular anion radical Diels-Alder product is observed. Parallel trends in substrate scope vis-à-vis the Co-catalyzed bis(enone) cyclobutanation are discussed.     

Oligonucleotide conjugation to a cell-penetrating (TAT) peptide by Diels-Alder cycloaddition

Modifed oligonucleotides are routinely employed as analytical probes for use in diagnostics, e.g. in the examination of specific RNA sequences for infectious diseases, however, a major limiting factor in oligonucleotide-based diagnostics is poor cellular uptake of naked oligonucleotides. This problem can be overcome by covalent attachment of a so-called 'cell-penetrating peptide' to form an oligonucleotide peptide conjugate. Stepwise solid phase synthesis of such a conjugate is difficult and expensive due to the conflicting chemistries of oligonucleotides and peptides. A simple approach to overcome this is post-synthetic conjugation. Diels-Alder cycloaddition is an attractive methodology for oligonucleotide peptide conjugation; the reaction is fast, chemoselective and the reaction rate is greatly enhanced in aqueous media - ideal conditions for biological moieties. An oligodeoxyribonucleotide sequence has been derivatised with a series of dienes at the 5'-terminus, using a series of unique dienyl-modified phosphoramidites, and investigation into the effect of diene type on the efficiency of conjugation, using Diels-Alder cycloaddition with a maleimido-derivatised cell-penetrating (TAT) peptide, has been performed. This led to the observation that the optimal diene for conjugation was cyclohexadiene, allowing conjugation of oligodeoxyribonucleotides to a cell-penetrating peptide by Diels-Alder cycloaddition for the first time.     

New processable poly(ether-keto-sulfone)s,poly(arylene sulfone)s,and polyesters curable by intramolecular cyclization. XVII

New processable polyaromatic ether-keto-sulfones were prepared from 2,2′-diiododiphenyl-4,4′-dicarbonyl dichloride (I), bis(p-phenoxybenzene)sulfone (V), isophthaloyl chloride (VI), terephthaloyl chloride (VII), and diphenylether (IX) in Friedel-Crafts-type polymerizations. By varying (VI):(VII) ratio and (V):(IX) ratio and by reducing the polymerization time, soluble, processable polymers were obtained. In these polymers, phenylacetylenyl groups were introduced by replacing the iodine. This process led to soluble and curable polymers. Transparent, tough films and fairly flexible glass fiber laminates can readily be prepared. After curing, the polymers were insoluble and showed excellent chemical and thermal resistance. The curing process increased the polymers' softening temperature by ca. 20°C and produced intersting new useful materials for laminates. Processable poly(arylene sulfone)s were prepared from I, V, and diphenylether-4,4′-disulfonylchloride (X) in a Friedel-Crafts-type polymerization. Different monomer ratios and polymerization times were used. Only low-molecular-weight polymers were obtained. The same result was shown by curable polyester formation from I, VI, VII, and 4,4′-sulfonyldiphenol (XI) in an interfacial polycondensation.     

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.     

A method for bioconjugation of carbohydrates using Diels-Alder cycloaddition

Diels-Alder-type cycloaddition of an electronically matched pair of saccharide-linked conjugated dienes and a dienophile-equipped protein gives neoglycoproteins at ambient temperature in pure water with a reaction half-life of approximately 2 h. Uncoupled saccharides can be recovered by diafiltration with complete conservation of the diene moiety, thus allowing their repeated use. The procedure described is the first for creating a carbon-carbon covalent bond in the bioconjugation step between a saccharide and a protein.