Subsequent Diels Research Articles

Cascade hydroarylation/Diels-Alder cycloaddition of alkynylindoles with electron-deficient alkynes and alkenes.

Herein, a novel cascade gold(I)-catalyzed hydroarylation of alkynylindoles and subsequent Diels-Alder cycloaddition with electron-deficient alkynes and alkenes is described. A variety of azepino-fused hydrocarbazoles and carbazoles were obtained in moderate to excellent yields. Key features of this methodology are low catalyst loadings, high regioselectivity, broad functional group tolerances, access to important heterocycles, and 100% atom economy.

Preparation and performance of low‐dielectric benzocyclobutene resins containing dicyclopentadiene

AbstractLow‐dielectric‐constant polymer materials are widely used in the electronic industry owing to their unique characteristics. Benzocyclobutene (BCB) resin is one such material with excellent dielectric properties and thermal stabilities. In this work, a novel BCB‐functionalized monomer (DCPDNO‐BCB) was synthesized from 2,6‐dimethyl phenol‐DCPD novolac (DCPDNO) and 4‐Bromobenzocyclobutene through Ullmann coupling. The structure of DCPDNO‐BCB was characterized by 1H NMR, 13C NMR, and ESI‐MS, and poly(DCPDNO‐BCB), obtained by ring‐opening and subsequent Diels–Alder reaction at the appropriate temperature, was found to exhibit low dielectric properties (k = 2.53 at 10 MHz), low water absorption, high thermal stability (Td5 = 430°C), and good film flatness.

Role of the Oxethyl Unit in the Structure of Vegetable Oil-Based Plasticizer for PVC: An Efficient Strategy to Enhance Compatibility and Plasticization.

Developing vegetable oil-derived primary plasticizers for poly(vinyl chloride) (PVC) is still a challenge because of their insufficient compatibility. As described in this work, we report the synthesis of plasticizers through the esterification of polyethylene glycol methyl ether and dimer acid, in which dimer acid is renewable material prepared via a two-step reaction (1) the hydrolysis of fatty acids from soybean oil at 70 °C and (2) subsequent Diels–Alder reaction at 250 °C. The resulting plasticizers, dimer acid-derived polyethylene glycol methyl ether esters (DA-2n, 2n = 2, 4, 6 or 8 referring to the number of oxethyl units per molecule), were blended with PVC. It was found that the tensile properties, transparency, and thermal stability of plasticized PVC (PVC-DA-2n) increased significantly with an increase in the number of oxyethyl units. Fourier-transform infrared spectroscopy analysis revealed that its good compatibility can be attributed to the strong interaction between oxyethyl units and PVC. As the number of the oxyethyl units of plasticizer increased, the glass transition temperature (Tg) of the corresponding plasticized PVC samples decreased from 62.3 (PVC-DA-2) to 35.4 °C (PVC-DA-8). Owing to the excellent plasticization of DA-8, the performances of PVC-DA-8 were comparable or better than that of the PVC plasticized using commercial dioctyl terephthalate (DOTP). The simple but efficient method of this study provides a new avenue for the preparation of vegetable oil-based plasticizers for PVC.

A Flow Microreactor Approach to a Highly Efficient Diels–Alder Reaction with an Electrogenerated o-Quinone

We have demonstrated a Diels–Alder reaction of an o-quinone generated in an electrochemical flow microreactor. In the flow microreactor system, 4-tert-butyl-o-benzoquinone was easily electrogenerated from 4-tert-butylpyrocatechol in the absence of chemical oxidants and then rapidly used, without decomposing, in a subsequent Diels–Adler reaction with various fulvenes to give the desired products efficiently.

Toughened and hydrophobically modified polyamide 11 copolymers with dimer acids derived from waste vegetable oil

ABSTRACTA series of dimer acid (DA)‐modified polyamide 11 copolymers is synthesized via melt polycondensation from 11‐aminoundecanoic acid, hexamethylene diamine (HMDA), and DA, prepared by the hydrolysis of waste vegetable oil and a subsequent Diels–Alder reaction. The molecular structures and the polymers' molar masses are characterized by nuclear magnetic resonance, Fourier‐transform infrared spectroscopy, size exclusion chromatography. The thermal and mechanical properties are modified with increasing the composition of DA in PA copolymers by increasing the chain irregularity of the PA, thereby reducing the hydrogen bonding strength of the amide groups. Finally, water contact angle and water absorption experiments show that the hydrophobicity of DA‐modified PA copolymers is increased as compared to neat PA11. As a result, these thermoplastic materials could offer improved sustainability and performances by the addition of renewable DA monomer in PA copolymers. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47174.

Thermally Induced Cross-Linking and Degradation Reactions of Benzocyclobutene-Based Polymers

We describe the use of MALDI-TOF mass spectrometry and collision-induced dissociation (CID) fragmentation techniques to examine the thermally induced cross-linking chemistry of dibenzocyclobutene (BCB)2–resorcinol-based materials. The overall goal was to gain a better understanding of benzocyclobutene ring-opening and the subsequent Diels–Alder cycloaddition reactions, which could assist in troubleshooting problems associated with unwanted side-product formation in these materials. Experimental results indicate that relative changes in average molecular mass (e.g., Mn) and signature side-product formation can be used to predict the optimum curing temperature used for cross-linking/polymerization. CID fragmentation studies identified two low kinetic energy degradation pathways for the BCB2–resorcinol-based oligomers examined in this study: (1) ether bond cleavage with an associated 1,3-hydrogen transfer and (2) transannular bond cleavage across the cyclooctadiene linkage. This information was used to devel...

ChemInform Abstract: Inherently Chiral Heterocyclic Resorcinarenes Using a Diels—Alder Reaction.

AbstractThe title compounds are prepared with unusually high diastereoselectivity in one‐pot via thermal generation of o‐quinomethide resorcinarene derivatives and subsequent Diels—Alder reaction with dienophiles, such as α‐methylstyrene.

ChemInform Abstract: Tandem Gold‐Catalyzed Dehydrative Cyclization/Diels—Alder Reactions: Facile Access to Indolocarbazole Alkaloids.

AbstractA gold‐catalyzed synthesis of cyclic 2‐oxodienes from readily prepared propargyl alcohols and the subsequent Diels—Alder reaction are reported.

Tandem Gold-Catalyzed Dehydrative Cyclization/Diels–Alder Reactions: Facile Access to Indolocarbazole Alkaloids

A gold-catalyzed synthesis of cyclic 2-oxodienes from readily prepared propargyl alcohols and the subsequent Diels-Alder reaction are reported. The dehydrative cyclization reactions proceeded smoothly, and the dienes formed in situ were demonstrated to undergo cycloaddition with a variety of dienophiles. This method offers a new strategy for the synthesis of indolocarbazole alkaloids, whereby the convergent synthetic design allows for differentiation between the indole nitrogens.

ChemInform Abstract: (R/S)‐BINOL‐α‐Phosphoryloxy Enecarbamate‐Mediated and (R/S)‐Titanium(IV) BINOLates‐Catalyzed Enantioselective Intramolecular Heck/Aza‐Diels—Alder Cycloaddition (IHADA): An Expedient Methodology.

AbstractThe conversion of o‐vinyl substituted N‐Boc‐N‐acetyl anilines to enecarbamates and subsequent Diels—Alder reaction in the presence of enantiopure BINOL as chiral source is developed.

Preparation of a Porous polymer by a catalyst‐free diels‐alder reaction and its structural modification by post‐reaction

ABSTRACTA microporous polymer is prepared by a catalyst‐free Diels–Alder reaction. A cyclopentadiene with both a diene and a dienophile functionality and a dienophilic maleimide are used for the Diels–Alder reaction. 1,3,5‐Tris(bromomethyl)‐2,4,6‐trimethylbenzene is reacted with sodium cyclopentadienide to produce the multicyclopentadiene‐functionalized monomer. A crosslinked polymer (CDAP) is obtained by the reaction of the cyclopentadiene monomer with N,N′‐1,4‐phenylenedimaleimide. The thermal dissociation of the cyclopentadiene dimeric unit and the subsequent Diels–Alder reaction with the maleimide group are investigated by the model reaction. We are able to restructure the crosslinked polymer network by taking advantage of the thermal reversibility of the Diels–Alder linkage. After the post thermal treatment, the BET surface area of the polymer (CDAP‐T) is greatly increased from 317 to 1038 m2 g−1. CDAP‐T is functionalized with pyrene by bromination with N‐bromosuccinimide and the subsequent substitution reaction with aminopyrene. The adsorption property of the pyrene‐functionalized polymer for an aromatic dye is investigated using malachite green. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3646–3653

ChemInform Abstract: Mild Boronic Acid Catalyzed Nazarov Cyclization of Divinyl Alcohols in Tandem with Diels—Alder Cycloaddition.

AbstractA convenient one‐pot method is presented for the stereoselective synthesis of highly functionalized bicyclic compounds (III) via boronic acid catalyzed Nazarov cyclization of divinyl alcohols and subsequent Diels—Alder reaction with maleic anhydride or phenylmaleimide.

Synthesis of azahomosteroid ring system through intramolecular [4+2] cycloaddition of in situ generated azaisobenzofuran intermediates

Azahomosteroid ring systems were synthesized through two-component coupling of γ,δ-unsaturated Fischer carbene complexes with o-alkynylheteroaryl carbonyl derivatives. The reaction occurs through the formation of azaisobenzofuran as transient intermediate; the latter undergoes a subsequent Diels–Alder cycloaddition reaction with in-built dienophile with high regio- and stereoselectivity.

Bio-based thermosetting resins composed of cardanol novolac and bismaleimide

Cardanol novolac (CDN) was synthesized by the reaction of cardanol (CD) and paraformaldehyde in the presence of oxalic acid. The prepolymerized compounds of CD/4,4′-bismaleimidediphenylmethane (BMI) and CDN/BMI with CD/maleimide unit ratios 1/2, 1/4 and 1/6 at 200 °C were finally compression-molded at 250 °C for 5 h to produce cured CD/BMI (cCD/BMI) and cured CDN/BMI (cCDN/BMI) resins. Although the proton nuclear magnetic resonance (1H-NMR) and Fourier transform infrared spectroscopy (FTIR) analyses of the model reaction product of CD and N-phenylmaleimide (PMI) at 200 °C for 8 h suggested the occurrence of the ene reaction and subsequent Diels–Alder reaction, the FTIR analysis of cCD/BMI and cCDN/BMI suggested the occurrence of the ene reaction and addition copolymerization. The cCD/BMI and cCDN/BMI with CD/maleimide ratio lower than 1/2 did not show glass transition until 300 °C and had a 5% weight loss temperature higher than 450 °C. The cCD/BMI and cCDN/BMI with CD/maleimide ratio 1/4 showed the most balanced flexural properties (flexural strength 60–80 MPa, flexural modulus 2.0–2.5 GPa). 4,4′-Bismaleimidediphenylmethane (BMI) was cured finally at 250 °C with cardanol (CD) or CD novolac (CDN) prepared by the reaction of CD and paraformaldehyde with CD/maleimide unit ratios 1/2, 1/4 and 1/6. The obtained cured resins with CD/maleimide ratio lower than 1/2 did not show glass transition until 300°C and had 5% weight loss temperature higher than 450°C. The curing mechanism was elucidated by means of the proton nuclear magnetic resonance (1H-NMR) and Fourier transform infrared spectroscopy (FTIR) analyses of the model reaction products of CD and N-phenylmaleimide (PMI) and the cured resins.

Olefin‐Metathesis‐Based Synthesis of Furans by an RCM/Deprotonation/Phosphorylation Sequence and Their Diels–Alder Reactions

AbstractButenolides, obtained by ring‐closing metathesis (RCM) of acrylates, undergo quantitative deprotonation with amide bases. Trapping of the resulting anions with electrophiles, for example, chlorophosphates, give furans. Subsequent Diels–Alder reaction and acid‐catalysed rearrangement of the resulting oxabicyclonorbornadienes give substituted benzenes.

Stereoselective synthesis of optically active dihydrofurans and dihydropyrans via a ring closing metathesis reaction

A ring closing metathesis reaction of dienes and a ring closing enyne metathesis reaction derived from allyl, homoallyl and homopropargyl alcohol backbones are described. 2-Heteroaryl substituted allyl, homoallyl and homopropargyl alcohols have been easily and efficiently resolved through enzymatic resolution with high ee (93–99%) and known stereochemistry. Enantiomerically enriched dienes derived from allyl and homoallyl alcohols afforded the corresponding enantiomerically enriched dihydrofuran and dihydropyran derivatives, respectively, with chemical yields which varied between 72% and 88%. On the other hand, enantiomerically enriched enynes derived from homoallyl and homopropargyl alcohols gave the corresponding optically active dihydropyrans with conjugated diene units with chemical yields between 70% and 80%. A subsequent Diels–Alder reaction of the dihydropyran derivatives with a diene unit with tetracyanoethylene resulted in the formation of a diastereomeric dihydroisochromene ring system as the sole product.

A biomimetic total synthesis of allomicrophyllone: protective Diels–Alder reaction as a stratagem

A biomimetic total synthesis of bioactive tetracyclic natural product allomicrophyllone has been achieved in which a protective Diels–Alder reaction employing a disposable sacrificial 1,3-diene directs the regioselectivity of the subsequent Diels–Alder reaction.

Synthesis of 2-Fluoro-3-phenylthio-1,3-butadiene and 2-Trifluoromethyl-3-phenylthio-1,3-butadiene and Their Reactivities toward Various Dienophiles

A general approach for the synthesis of novel 2-fluoro-3-phenylthio-1,3-butadiene (8) and 2-trifluoromethyl-3-phenylthio-1,3-butadiene (9) from monobromoalkene and dibromoalkenes has been developed. Subsequent Diels-Alder reactions of these dienes with symmetrical and unsymmetrical dienophiles in the presence of Lewis acids gave a variety of fluorinated six-membered carbocycles in moderate to high yields.

Synthesis of isoquinolines through the coupling of Fischer carbene complexes with o-alkynylpyridine carbonyl derivatives

The reaction of Fischer carbene complex with o-alkynylpyridine carbonyl derivatives has been investigated. This involves the generation of furo[3,4- c]pyridine as transient intermediates through the coupling of o-alkynylpyridine carbonyl derivatives with carbene complex and subsequent Diels–Alder trapping with suitable dienophiles resulted in the formation of isoquinoline derivatives and the entire sequence can be run in one pot. When an olefinic tether was present, intramolecular Diels–Alder cycloaddition occurred followed by ring opening to yield tricyclic alcohols.

Expedient synthesis of highly substituted α-pyrones from Baylis–Hillman adducts and their conversion to poly-substituted aromatics

An efficient synthetic protocol of fully substituted α-pyrones has been developed starting from the Baylis–Hillman adducts. Subsequent Diels–Alder reaction of the α-pyrones and DMAD produced poly-substituted aromatic compounds in high yields.