Thermal Cycloaddition Reactions Research Articles

Functionalization of Single-Walled Carbon Nanotubes with End-Capped Polystyrene via a Single-Step Diels-Alder Cycloaddition.

A facile, single-step, [4+2] Diels–Alder cycloaddition reaction for the surface functionalization of single-walled carbon nanotubes (SWNTs) with end-capped polystyrene chains is presented. The thermal cycloaddition reaction took place at high temperature (~230 °C) without any catalyst between the sp2 network of carbon nanotubes, which acted as dienophile, and the diphenylethylene cyclobutene (DPE-CB) terminal group of the polystyrene chain. Anionic polymerization was employed for the synthesis of the polystyrene macromolecule, and successful and quantitative end-capping reaction with the DPE-CB molecule was confirmed by matrix-assisted laser desorption/ionization time of flight mass spectroscopy. Thermogravimetric analysis revealed the wt % of the grafted macromolecule on the CNT surface as well as the grafting density of the polymer chains on the SWNTs (0.027 chains nm−2). Direct evidence for the surface functionalization and the presence of thin polystyrene film was obtained by transmission electron microscopy (TEM) and by atomic force microscopy (AFM).

Preparation of fluorosilicone rubber containing perfluorocyclobutyl aryl ether

A new curing system for high temperature vulcanized fluorosilicone rubber was studied by thermal cycloaddition reaction of trifluoroethylene aryl ether (TFVE). Fluorosilicone rubber containing perfluorocyclobutyl aryl ether (Si‐PFCB) was prepared by using the polysiloxane containing trifluorovinyl aryl ether (Si‐TFVE) as base gum. The basic formula of vulcanized rubber was determined as: 100 parts Si‐TFVE (0.10%TFVE, mol), 50 parts fumed silica. The curing process is as follows: the primary curing process is 180°C, 25 minutes; the post‐curing process is 190°C, 3 hours. It is found that the mechanical properties (the tensile strength is 6.9 MPa, the tearing strength is 35.3 KN/m and the elongation at break is 1685%), thermal property (Tg: −120 ± 5°C) and heat resistance (T5: 450 ± 5°C) of Si‐PFCB are comparable to ordinary high‐temperature vulcanized silicone rubber, while the new curing system has the advantage of no vulcanizing agents and no by‐products.

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions.

The interest on nitrogen-containing heterocycles has expanded rapidly in the synthetic community since they are important motifs for new drugs. Traditionally, they were synthesized through thermal cycloaddition reactions, whereas today, photocatalysis is preferred due to the mild and efficient conditions. With this focus, a new photocatalytic method for the synthesis of nitrogen-containing heterocycles is highly desired. Here, we report a protocol for the biosynthesis of cercosporin, which could function as a metal-free photocatalyst. We then illustrate cercosporin-photocatalyzed protocols for the synthesis of nitrogen-containing heterocycles 1,2,3-thiadiazoles through annulation of azoalkenes with KSCN, and synthesis of 1,4,5,6-tetrahydropyridazines [4+2] through cyclodimerization of azoalkenes under mild conditions, respectively. As a result, there is a new bridge between the microbial fermentation method and organic synthesis in a mild, cost-effective, environmentally friendly and sustainable manner.

Structure-property relationships of d-mannitol-based cationic poly(amide triazoles) and their self-assembling complexes with DNA

A set of poly(amide triazoles) (PT) was synthesized by azide-alkyne cycloaddition reactions from N,N-Bis[2-(propargylamido)ethyl]-N-[2-(tert-butoxycarbonylamino)ethyl] amine and 1,6-diazido-1,6-dideoxy-d-mannitol derivatives, having the secondary hydroxyl groups protected by removable acetal and/or acetyl groups. Either copper-catalyzed (CuAAC) as well as the classical thermal cycloaddition reactions were used to afford in good yields and medium molecular weight-linear PT featuring either 1,4- or 1,4/1,5-disubstituted triazole rings along the polymer chains, respectively. Full structural characterization of all PT was accomplished and comparatively studied. Protecting group removal allowed for modulating the flexibility and polarity of cationic polytriazoles (CP), entitling for structure-activity relationship insight of their self-assembling capabilities in the presence of nucleic acids. Hence, CP and DNA were formulated at different nitrogen-to-phosphate (N/P) ratios and the resulting polyplexes were characterized by Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). All CP were able to condense DNA into positively-charged nanometric-sized and narrow-polydispersed particles that could mediated transfection into human embryonic stem cells (hESC) with negligible toxicity.

Perylenequinonoid-Catalyzed [4 + 1] and [4 + 2] Annulations of Azoalkenes: Photocatalytic Access to 1,2,3-Thiadiazole/1,4,5,6-Tetrahydropyridazine Derivatives

Nitrogen-containing heterocycles are especially considered "privileged" structural scaffolds for the development of new drugs. However, traditional methods of organic synthesis are mainly based on thermal cycloaddition reaction; thus, the exploration of new strategies for the rapid assembly of N-heterocycles under mild conditions is highly desirable. Here, we developed a new method that visible light along with 1 mol % cercosporin, which is one of the perylenequinonoid pigments with excellent properties of photosensitization and can be easily produced by a new isolated endophytic fungus Cercospora sp. JNU001 strain with high yield through microbial fermentation, catalyzes the synthesis of 1,2,3-thiadiazoles and 1,4,5,6-tetrahydropyridazines by a photocatalytic process with good regioselectivity and broad functional-group compatibility under mild conditions. Thus, a bridge between microbial fermentation and organic photocatalysis for the construction of nitrogen-containing heterocycles was set up in a sustainable, environmentally friendly manner.

Benzo[a]heptalenes from Heptaleno[1,2‐c]furans. Part 2

AbstractIt is shown in this ‘Part 2’ that heptaleno[1,2‐c]furans 1 react thermally in a Diels–Alder‐type [4+2] cycloaddition at the furan ring with vinylene carbonate (VC), phenylsulfonylallene (PSA), α‐(acetyloxy)acrylonitrile (AAN), and (1Z)‐1,2‐bis(phenylsulfonyl)ethene (ZSE) to yield the corresponding 1,4‐epoxybenzo[d]heptalenes (cf. Schemes 1, 5, 6, and 8). The thermal reaction of 1a and 1b with VC at 130° and 150°, respectively, leads mainly to the 2,3‐endo‐cyclocarbonates 2,3‐endo‐2a and ‐2b and in minor amounts to the 2,3‐exo‐cyclocarbonates 2,3‐exo‐2a and ‐2b. In some cases, the (P*)‐ and (M*)‐configured epimers were isolated and characterized (Scheme 1). Base‐catalyzed cleavage of 2,3‐endo‐2 gave the corresponding 2,3‐diols 3, which were further transformed via reductive cleavage of their dimesylates 4 into the benzo[a]heptalenes 5a and 5b, respectively (Scheme 2). In another reaction sequence, the 2,3‐diols 3 were converted into their cyclic carbonothioates 6, which on treatment with (EtO)3P gave the deoxygenated 1,4‐dihydro‐1,4‐epoxybenzo[d]heptalenes 7. These were rearranged by acid catalysis into the benzo[a]heptalen‐4‐ols 8a and 8b, respectively (Scheme 2). Cyclocarbonate 2,3‐endo‐2b reacted with lithium diisopropylamide (LDA) at −70° under regioselective ring opening to the 3‐hydroxy‐substituted benzo[d]heptalen‐2‐yl carbamate 2,3‐endo‐9b (Scheme 3). The latter was O‐methylated to 2,3‐endo‐(P*)‐10b. The further way, to get finally the benzo[a]heptalene 13b with MeO groups in 1,2,3‐position, could not be realized due to the fact that we found no way to cleave the carbamate group of 2,3‐endo‐(P*)‐10b without touching its 1,4‐epoxy bridge (Scheme 3).The reaction of 1a with PSA in toluene at 120° was successful, in a way that we found regioisomeric as well as epimeric cycloadducts (Scheme 5). Unfortunately, the attempts to rearrange the products under strong‐base catalysis as it had been shown successfully with other furan–PSA adducts were unsuccessful (Scheme 4).The thermal cycloaddition reaction of 1a and 1b with AAN yielded again regioisomeric and epimeric adducts, which could easily be transformed into the corresponding 2‐ and 3‐oxo products (Scheme 6). Only the latter ones could be rearranged with Ac2O/H2SO4 into the corresponding benzo[a]heptalene‐3,4‐diol diacetates 20a and 20b, respectively, or with trimethylsilyl trifluoromethanesulfonate (TfOSiMe3/Et3N), followed by treatment with NH4Cl/H2O, into the corresponding benzo[a]heptalen‐3,4‐diols 21a and 21b (Scheme 7).The thermal cycloaddition reaction of 1 with ZSE in toluene gave the cycloadducts 2,3‐exo‐22a and ‐22b as well as 2‐exo,3‐endo‐22c in high yields (Scheme 8). All three adducts eliminated, by treatment with base, benzenesulfinic acid and yielded the corresponding 3‐(phenylsulfonyl)‐1,4‐epoxybenzo[d]heptalenes 25. The latter turned out to be excellent Michael acceptors for H2O2 in basic media (Scheme 9). The Michael adducts lost H2O on treatment with Ac2O in pyridine and gave the 3‐(phenylsulfonyl)benzo[d]heptalen‐2‐ones 28a and 3‐exo‐28b, respectively. Rearrangement of these compounds in the presence of Ac2O/AcONa lead to the formation of the corresponding 3‐(phenylsulfonyl)benzo[a]heptalene‐1,2‐diol diacetates 30a and 30b, which on treatment with MeONa/MeI gave the corresponding MeO‐substituted compounds 31a and 31b. The reductive elimination of the PhSO2 group led finally to the 1,2‐dimethoxybenzo[a]heptalenes 32a and 32b. Deprotonation experiments of 32a with t‐BuLi/N,N,N′,N′‐tetramethylethane‐1,2‐diamine (tmeda) and quenching with D2O showed that the most acid CH bond is HC(3) (Scheme 9).Some of the new structures were established by X‐ray crystal‐diffraction analyses (cf. Figs. 1, 3, 4, and 5). Moreover, nine of the new benzo[a]heptalenes were resolved on an anal. Chiralcel OD‐H column, and their CD spectra were measured (cf. Figs. 8 and 9). As a result, the 1,2‐dimethoxybenzo[a]heptalenes 32a and 32b showed unexpectedly new Cotton‐effect bands just below 300 nm, which were assigned to chiral exciton coupling between the heptalene and benzo part of the structurally highly twisted compounds. The PhSO2‐substituted benzo[a]heptalenes 30b and 31b showed, in addition, a further pair of Cotton‐effect bands in the range of 275–245 nm, due to chiral exciton coupling of the benzo[a]heptalene chromophore and the phenylsulfonyl chromophore (cf. Fig. 10).

Synthetic Chemistry of Quadricyclane

This articale is the first attempt to review chemical transformation of one most interesting synthon among strained, saturated hydrocarbons - tetracyclo[3.2.0.02,704,6]heptane (quadricyclane, Q), from the point of view of synthetic organic chemistry. Thermal [2+2+2] cycloaddition reactions of Q with olefins, acetylenes, carbonyl, nitrogen, sulfur containing compounds, carbenes and silenes along with cycloaddition reactions involving transition metals are reviewed in a first part. Second part of the review covers photochemical transformations, ionic reactions (electrophilic and nucleophilic), oxidation and reduction of Q. Keywords: Quadricyclane, thermal cycloaddition reactions, photochemical cycloaddition reactions, electrophilic reactions, lithiation of C-H bond, oxidation, reduction

Thermal cycloaddition reactions of azulene-1,5-quinones and azulene-1,7-quinones with cyclic dienes and cycloheptatriene

[2+4]-[6+4] Cycloadducts were obtained from the thermal cycloaddition reactions of azulenequinones with cyclic 4π systems such as 1,3-diphenylisobenzofuran and isobenzofuran while the reaction of cycloheptatriene and azulenequinones gave ene products as well as cage molecules. The first addition site of 1,3-diphenylisobenzofuran was different for 3-bromoazulene-1,5-quinone and azulene-1,5-quinone; the former reacted on the cyclopentenone part and the latter on the C-6–C-7 bond to avoid formation of an unstable cyclopentadienone structure.

Stereoselectivity in the thermal cycloaddition reactions of tetrafluoroethylene to derivatives of α-(4-ethoxyphenyl)acrylic acid

Esters of chiral auxiliaries with α-(4-ethoxyphenyl)acrylic acid undergo diastereoselective addition reactions with tetrafluoroethylene at 130°–170° to afford tetrafluorocyclobutanecarboxylic esters. The oxazoline derived from (+)-(1 S,2 R)-norephedrine and α-(4-ethoxyphenyl)acrylic acid shows negligible stereoselectivity.

Trithia‐ and dithiaselenapentalenes from benzylidene‐1,2‐dithioles and heterocumulenes

Deprotonation of the 5-aryl-3-benzyl-1λ4,2-dithiol-1-ylium iodides (6a–6d) obtained by reaction of the 1-aryl-4-phenylbutan-1,3-diones (5a–5d) with hydrogen sulfide and iodine in ethanol gave the stable 5-aryl-3-benzylidene-3H-1,2-dithioles (3a–3d), respectively. The dithioles (3a–3d) underwent thermal cycloaddition reactions with isoselenocyanates and isothiocyanates to give the 2-(substituted amino)-5-aryl-3-phenyl-6,6aλ4-dithia-1-selenapentalenes (7a–7h) andthe 2-(substituted amino)-5-aryl-3-phenyl-1,6,6aλ4-trithiapentalenes (8a–8l), respectively. The dithioles(3a–3d) reacted with isocyanates to give the N-substi-tuted-2-phenyl-2-(5-aryl-3H-1,2-dithiol-3-ylidene) acetamides (11a–11h). © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 233–244, 1997.

Stereochemistry and mechanisms of thermal cycloaddition reactions of conjugated enynes. The stereospecific formation of six tetrahedral centers in a single reaction

Thermal condensations of penta-1,5-dien-3-ynes with two molecules of olefinic dienophiles proceed stereospecifically with retention of the configurations of the dienophiles, and with both molecules of the dienophile in each reaction adding to the same face of the dienyne.

Dioxopyrrolines. LXI. Cycloaddition Reaction of 4-Benzoyl-5-ethoxycarbonyl-1-phenyl-1H-pyrrole-2,3-dione with 1,3-Dienes: Competitive Occurrence of Normal and Hetero Diels-Alder Reaction and Claisen Rearrangement of the Hetero Diels-Alder Product.

Thermal cycloaddition reaction of 4-benzoyl-5-ethoxycarbonyl-1H-pyrrole-2, 3-dione (dioxopyrroline) 1 with 1, 3-dienes caused two types of Diels-Alder (D-A) reaction in a competitive manner. One is the hetero D-A reaction in whch dioxopyrroline acts as an electron-deficient diene and the 1, 3-diene acts as an electron-rich dienophile. The other is the normal D-A reaction in which diosopyrroline acts as an electron-deficient dienophile. The 1, 3-dienes bearing electron-rich substituents undergo the D-A reaction via the normal pathway, while the 1, 3-dienes which do not bear an electron-donating group predominantly undergo the hetero D-A reaction. When the normal D-A pathway is sterically hindered, the hetero D-A pathway occurs exclusively.

Theoretical studies on the substituent effect of the thermal cycloaddition of ethylene and formaldehyde

Ab initio HF molecular orbital calculations using 3-21G and 6-31G ∗ basis sets and including electron correlation (MP2 to MP4) have been applied to investigate the concerted supra-antara [2 s + 2 a] thermal cycloaddition reaction of some substituted ethylenes and formaldehyde. The stepwise reaction paths have also been studied for one of the reactions by using UHF/3-21G and MP2/3-21G. The calculated results indicate that although the presence of some particular substituents can decrease the activation barriers obviously, the title reaction is difficult to carry out via the supra-antara [2 s + 2 a] thermal cycloaddition pathway under moderate conditions. However, the reactions are probably carried out via stepwise pathways when there are strong electron-withdrawing groups in formaldehyde and/or electron-releasing groups in ethylene.

Thermal and Lewis Acid-Promoted Hetero Diels–Alder Reactions of α,β : γ,δ-Unsaturated Carbodithioic Esters and Carbothioic O-Esters; 5,6-Dihydro-2H-naphtho[1,2-b]thiopyran-3-carbodithioates and -carbothioic O-Esters

Abstract Phenyl esters of 2-substituted 5,6-dihydro-2H-naphtho[1,2-b]thiopyran-3-carbodithioic acids (2) reacted with 2-norbornene, dimethyl acetylenedicarboxylate, diethyl azodicarboxylate, N-phenylmaleimide, dimethyl fumarate, 2-chloroacrylonitrile, methyl acrylate, and styrene in refluxing benzene to give the [4+2]cycloadducts, while the thermal cycloaddition reaction of O-methyl esters of 2-substituted 5,6-dihydro-2H-naphtho[1,2-b]thiopyran-3-carbothioic acids (3) was successful only with the triple-bonded dienophiles such as dimethyl acetylenedicarboxylate and ethyl propiolate. In the presence of a Lewis acid (AlCl3 or EtAlCl2), the reactions of both 2 and 3 were found to proceed efficiently even at room temperature.

A self-consistent and cross-checked scale of spin-delocalization substituent constants, the .sigma.JJ.bul. scale

On the basis of the 19 F data of substituted α,β,β-trifluorostyrenes and the rate constants of their thermal cycloaddition reactions, a self-consistent and cross-checked scale of spin-delocalization substituent constants σ . JJ is proposed for 21 para-substituents and for 10 met-substituents. By means of adopting different π mb values, three methods for the calculation of the σ . JJ , values at five different temperatures and their averaged values are presented

Conformational Preferences of Vinyl Piiosphine Oxides in Solution Alud in the Solid State

Abstract The insight into s-cis vs. s-trans conformational equilibria in simple activated olefins is of prime importance for transition state modeling and for understanding of stereo-chemical results of the reactions involving such olefins. Some acrylates and vinyl sulfoxides have been found recently to choose the corresponding s-cis conformation as the reactive conformation in their thermal cycloaddition and conjugate addition reactions. In the course of our ste-reochemical studies involving chiral phosphine oxides we were prompted to address analogous conformational issue in regard to the conjugated vinyl-phosphoryl moiety. The two P-epimeric vinyl phosphine oxides and served us as the model compounds and their detailed analysis in solution and in the solid state will be presented. The presentation will include 2D 1H NMR as well as the solid state and solution 13C NMR data, the X-ray and IR analysis and also, comparison with some reference compounds. The significance of the depicted s-cis conformations as well as of the syn-coplanar array of the P=O and C=O dipoles in the studied compounds is implied.

Anodic Oxidation for the Syntheses ofo- andp-Tropoquinone Mono- and Bisacetals

The preparation of o- and p-tropoquinone acetals by means of anodic oxidation is described, together with metal salt oxidations. The reactivities of acetals to Grignard reagents and photochemical and thermal reactions were investigated as well as the electrochemical reactions. Conformational changes of 4,4,5,5-tetramethoxy-2,6-cycloheptadien-1-ones and 6,6,7,7-tetramethoxy-2,4-cycloheptadien-1-one have been studied. 1. Introduction 2. Tropoquinone Acetals 2.1. Preparation 2.2. Mechanism of the Anodic Oxidation 3. Reactions of Tropoquinone Acetals 3.1. Alkylation 3.2. Thiele-Type Addition Reaction 3.3. Photochemistry 3.4. Thermal Cycloaddition Reaction 4. Properties of Tropoquinone Acetals 4.1. Cyclic Voltammetry 4.2. Conformational Changes of Bisacetals 5. Conclusion

ChemInform Abstract: Superjacent and Subjacent Orbitals Participations for Kinetically Controlled Cycloaddition Reactions of 2H‐Cyclohepta(b)furan‐2‐ones and 8,8‐Dicyanoheptafulvene to 2,3‐Bis(methoxycarbonyl)‐7‐oxabicyclo(2.2.1)heptadiene.

AbstractThermal cycloaddition of the cycloheptatrienes (I) or (VI) with the oxabicyclo(2.2.1)heptadiene (II) gives mixtures of the (4 + 2) cycloadducts (III) and (V) or (VII) and (IX) and the cycloreversion products (IV) or (VIII).

Fulvene cycloaddition reactions in water: influence on rate and selectivity

The thermal cycloaddition reaction between dimethylpentafulvene 1 and 1,4-benzoquinone 2 or maleic anhydride was investigated under several solvent and concentration conditions. In aqueous media the reaction rates were highly accelerated and the endo/exo - ratio for 3/5 could be controlled from 9:1 to 1:9 depending on the formal concentration in water.

ChemInform Abstract: A Stereoselective Synthesis of D‐Normorphinan Derivatives by Means of an Intramolecular Diels‐Alder Reaction.

AbstractIntramolecular thermal cycloaddition reaction of the benzocyclobutenes (I) yields the hydrophenanthrenes (II) ‐ (VI) which are valuable synthons for the preparation of D‐normorphinan derivatives.