rDA Reaction Research Articles

ChemInform Abstract: A Novel Approach to Conformationally Strained 2,2′‐Bipyridine Thiacrown Ethers and Their Chiral Sulfoxides by Sequential Homo‐Coupling/DA—rDA Reaction of 5,5′‐Bi‐1,2,4‐triazine‐Containing Thiamacrocycles.

AbstractHomocoupling of bis(triazinylsulfide) (IV) in the presence of KCN followed by Diels—Alder/retro‐Diels—Alder reaction with cyclic olefins provides access to title strained bipyridine thiacrown ethers (VII) and (X).

Synthesis and in situ core reorganization of smart polymers

A smart poly(ethylene oxide) polymer containing two thermally reversible groups located at the chain center (F2-PEO) was synthesized by a maleimide–furan (MF) [4 + 2] Diels–Alder (DA) reaction between one bis[(furan-2-yl)methyl] terephthalate (F2) and two maleimide-terminated poly(ethylene glycol) methyl ethers (MI-PEOs). MF adducts underwent retro-DA (rDA) addition at a higher temperature, and the maleimide–anthracene (MA) addition reaction could occur simultaneously. Therefore, when F2-PEO and bis((anthracen-10-yl)methyl) succinate (AN2) were refluxed in toluene for 48 h, the cores of F2-PEOs were replaced with the MA adducts in situ to give new polymers AN2-PEOs. Moreover, when F2-PEOs were mixed with the molecules containing four anthracene units (AN4), the rDA reaction of the MF adducts and the MA addition reactions occurred simultaneously, generating four-arm star PEOs. The core reorganization of polymers has been confirmed by SEC, UV, 1H NMR, and MALDI-TOF mass spectra.

A novel approach to conformationally strained 2,2′-bipyridine thiacrown ethers and their chiral sulfoxides by sequential homo-coupling/DA–rDA reaction of 5,5′-bi-1,2,4-triazine-containing thiamacrocycles

The synthesis of conformationally strained 2,2′-bipyridine thiamacrocycles 5, 6, 9, 10 and their chiral sulfoxides 11– 14 is elaborated using, (1) homo-coupling of 1,2,4-triazine sulfide 3 with potassium cyanide and (2) Diels–Alder/retro Diels–Alder (DA–rDA) with 2,5-norbornadiene or 1-pyrrolidino-1-cyclopentene as the key steps. The crystal structure determinations of 4– 6 and the theoretical calculations at DFT/B3LYP/6-311G ∗∗ level were conducted thus establishing conformational preferences of the target thiamacrocycles

Tuning the Kinetics and Energetics of Diels−Alder Cycloaddition Reactions to Improve Poling Efficiency and Thermal Stability of High-Temperature Cross-Linked Electro-Optic Polymers

A comprehensive theoretical and experimental study of the bromo effect on Diels−Alder (DA) and retro-Diels−Alder (rDA) reactions between the anthracene-containing dienes and maleimide dienophile has been conducted to improve thermal stability and poling efficiency of electro-optic (EO) polymers. Calculations with density functional theory (DFT) reveal that the bromo substitution on anthracene-based dienes leads to significantly increased cycloaddition exothermicities (9−12 kcal/mol) but has a minor effect on their activation barriers (0.6−3 kcal/mol) when reacted with N-phenylmaleimide dienophile. These calculated values correlate very well with the experimental results obtained from a series of model compounds. The DA/rDA cross-linking protocols based on these compounds can be applied to several nonlinear optical (NLO) polymers and result in large EO coefficients (r33) (as high as 69 pm/V) and greatly improved thermal stability (up to 250 °C). This demonstrates that controlled DA/rDA reactions can be use...

Thermally reversible crosslinked polyethylene using Diels–Alder reaction in molten state

Thermally reversible crosslinked polyethylene was prepared by Diels–Alder (DA) and retro Diels–Alder (rDA) reaction. Maleimide/furan adduct was used as crosslinking agent. Dienophile named 11-maleimido-undecanoic acid was first synthesized and between this dienophile and commercial 3-(2-furyl) propanoic acid, the DA reaction was studied to determine DA and rDA reactions temperatures in the solid state. Then, an original modification method was employed to graft the two molecules onto the Lotader® poly(ethylene-co-glycidyl methacrylate) in one step procedure. The DA and rDA reactions between diene and dienophile grafted moieties are followed by FT-IR analysis on a thin film. Readily polymer network is synthesized and the cycle of DA and retro-DA reactions is repeatable with no significant polymer degradation.

“Click Chemistry” in Tailor-Made Polymethacrylates Bearing Reactive Furfuryl Functionality: A New Class of Self-Healing Polymeric Material

This investigation reports the effective use of the Diels-Alder (DA) reaction, a "click reaction" in the preparation of thermally amendable and self-healing polymeric materials having reactive furfuryl functionality. In this case, the DA and retro-DA (rDA) reactions were carried out between the tailor-made homo- and copolymer of furfuryl methacrylate prepared by atom-transfer radical polymerization and a bismaleimide (BM). The kinetic studies of DA and rDA reactions were carried out using Fourier transform infrared spectroscopy. The DA polymers were insoluble in toluene at room temperature. When the DA polymers were heated at 100 degrees C in toluene, it was soluble. This is because of the cleavage between furfuryl functionality and BM. The chemical cross-link density was determined by the Flory-Rehner equation. The cross-linked polymer showed much greater adhesive strength at room temperature, but the adhesive strength was quite low at higher temperature. The self-healing capability was studied by using scanning electron microscopy analysis. The thermal and dynamic mechanical properties of the thermally amendable cross-linked materials were investigated by thermogravimetric analysis and dynamic mechanical analysis.

Healing behavior of a matrix crack on a carbon fiber/mendomer composite

The cure kinetics of a thermally mendable polymer based on Diels-Alder (DA) and retro-Diels-Alder (rDA) reactions, mendomer 401, was investigated using dynamic differential scanning calorimetry (DSC). The resulting behavior was modeled using a conventional cure kinetic model for thermosetting polymers. A composite panel with two layers of carbon fabric and mendomer 401 was fabricated following the cure cycle suggested by the cure kinetics model. Micro-indentation tests were performed to investigate mechanical properties and dynamic mechanical thermal analysis (DMTA) was conducted to study thermal behavior and to find the trigger temperature for healing. Self-healing behavior of the carbon fiber/mendomer composite was demonstrated using electrical resistive heating over the glass transition temperature.

Effect of the polymer matrix on the thermal behaviour of a furan-maleimide type adduct in the molten state

The effect of three different polymer matrices on the stability of the exo-adduct formed from N-Phenylmaleimide (PM) and furfuryl alcohol (FAl) toward retro Diels-Alder reaction was studied. First, a stereospecific synthesis of the exo-adduct was carried out at the condensed state and for a best understanding, its thermal properties was reported. The temperature of the rDA reaction of the exo-adduct in polyolefins (a low density polyethylene and a random copolymer of ethylene and glycidyl methacrylate) was found similar to the pure exo-adduct rDA reaction temperature. In a poly(vinylacetate)-g-polycaprolactone matrix this temperature was distinctly lower. In this last matrix, the reversibility of the rDA reaction was proved by the formation of the endo-adduct FAlPM during cooling.

Letter: Electron Impact Mass Spectrometry Study of a Series of Substituted 5-Aminoalkylmethyl-Cytosines and Their 1-N-(o-, m- and p-)Bromobenzyl-Substituted Derivatives

The electron-impact mass spectra of 5-aminoalkylmethyl-substituted cytosine and of their 1-N- o-(m- or p-)bromobenzyl-substituted derivatives are discussed. The influence of 5-aminoalkyl and 1-N-bromobenzyl substituents on the mode of mass fragmentation occurring upon electron impact ionization was demonstrated. The fragmentation pathways of all compounds and the characteristic ions in these spectra originating from the McLafferty rearrangement, RDA reactions and simple cleavages are presented.

Structures of oxygenated cyclohexa-1,3-diene–maleic anyhydride cycloadducts. Structural evidence suggests a stepwise retro-Diels–Alder reaction

The structures of a series of cyclohexadiene-maleic anhydride cycloadducts show structural parameters consistent with the early stages of the retro-Diels-Alder (rDA) reaction in the ground state structure. The symmetrical adduct shows effects consistent with a synchronous rDA reaction. An asynchronous rDA reaction is suggested in the structure of 8, and the first evidence suggesting a two-step retro-Diels-Alder mechanism is provided by the deoxygenated adduct 9.

Synthesis and properties of readily recyclable polymers from bisfuranic terminated poly(ethylene adipate) and multi-maleimide linkers

Polymers that can repeatedly polymerize and depolymerize at moderate conditions was designed and synthesized. Furyl-telechelic poly(ethylene adipate) [PEA2F] was prepared and copolymerized with bis- and tris-maleimides by Diels–Alder [DA] reaction at 60°C. Though the reaction with bismaleimide proceeded partially to give a polymer whose molecular weight was only twice as large as PEA2F, the reaction with trismaleimide gave a product with a 3D-network structure. The latter product shows elasticity at room temperature. By keeping at 145°C, these DA products were easily disconnected by retro-Diels–Alder [rDA] reaction to reproduce the starting macromonomer, PEA2F. The repetition of the cycle of the DA and rDA reactions induce the reduction of neither the mechanical properties of the DA products nor the molecular weight of the rDA products. Therefore, we have successfully obtained readily recyclable polymers.

Density functional theory and atoms-in-molecule study on the role of two-electron stabilizing interactions in retro Diels–Alder reaction of cycloadducts derived from substituted cyclopentadiene and p-benzoquinone

A systematic investigation on the cycloreversion reaction of the cycloadduct formed between substituted cyclopentadiene and p-benzoquinone (1-19) is reported at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory. The computed activation barrier exhibits a fairly high sensitivity to the nature of substituents at the C7-position. Gibbs free energy of activation for 1 and 19 are found to be 20.3 and 30.1 kcal mol(-1), respectively, compared to 7, which is estimated to be 24.7 kcal mol(-1). Quantitative analysis of the electronic effects operating in both the cycloadduct as well as the corresponding transition state for the retro Diels-Alder (rDA) reaction performed using the natural bond orbital (NBO) and atoms in molecule (AIM) methods have identified important two-electron stabilizing interactions. Among four major delocalizations, sigma(C7-X) to sigma*(C1-C5) [and to sigma*(C2-C6)] is identified as the key contributing factor responsible for ground state C1-C5 bond elongation, which in turn is found to be crucial in promoting the rDA reaction. A good correlation between the population of antibonding orbital [sigma*(C1-C5)] of the ground state cycloadduct and Gibbs free energy of activation is observed. The importance of factors that modulate ground state structural features in controlling the energetics of rDA reaction is described.

Cleavable linkers for porous silicon-based mass spectrometry.

Desorption/ionization on silicon mass spectrometry (DIOSMS) uses porous silicon (pSi) to generate gas-phase ions of small (< 3000 Da) molecules without a matrix by using standard MALDI (matrix-assisted laser desorption/ionization) instrumentation. The unique laser desorption/ionization surface properties of DIOSMS allow for the simultaneous detection of a broad range of small molecules as their molecular ions, and the chemical properties of DIOSMS facilitate the attachment of a variety of organic fragments through surface Si H and Si OH groups. We aim to combine these advantages with the proven ability of covalent immobilization to facilitate combinatorial chemistry and the extraction of molecular information from complex mixtures. Previous reports in this vein include the use of self-assembled monolayers of functionalized alkanethiols to purify mixtures (usually of proteins) by specific molecular interactions or properties such as hydrophobicity or charge complementarity prior to MALDI analysis. DIOS has been similarly employed to detect small molecules bound to pSi-immobilized proteins. We report herein that Diels–Alder adducts undergo retro-[4+2] fragmentation in DIOSMS analysis, thus providing a convenient way to covalently attach and detach probe structures for chip-based mass analysis. The most useful covalent linkers for our purposes should be stable under organic synthetic conditions, yet cleavable during desorption/ionization. While photocleavable groups are commonly used in solid-phase organic synthesis, most (but not all) of these systems require a workup step after irradiation to complete the cleavage, and are therefore poorly suited to detachment in the MALDI laser pulse. The retroDiels–Alder (rDA) reaction was one of the first dissociation pathways to be investigated in mass spectrometry, and has since been characterized with a variety of ionization methods. Some studies showed that the fragmentation is highly stereospecific for cis-annelated systems, which suggests a concerted mechanism. We have found that Diels–Alder adducts undergo [4+2] cycloreversion readily upon desorption/ionization by the laser pulse in DIOS analyses. Thus, the maleimide–furan adduct 1 bearing a 3-quinoline carboxamide moiety showed only a single intense peak in the DIOSMS spectrum, which corresponds to the protonated diene [3·H] (Scheme 1). The

Structural Investigations into the retro-Diels-Alder Reaction. Experimental and Theoretical Studies.

The manifestations of the retro-Diels Alder reaction in the ground-state structures of a range of cyclopentadiene and cyclohexadiene cycloadducts 9-15 have been investigated by a combination of techniques. These include low-temperature X-ray crystallography, density functional calculations (B3LYP/6-31G(d,p)) on both the ground states and transition states, and the measurement of (13)C-(13)C coupling constants. We have found that the carbon-carbon bonds (labeled bonds a and b), which break in the rDA, are longer in the cycloadducts 9-15 than in their corresponding saturated analogues 9s-15s, which cannot undergo the rDA reaction. The degree of carbon-carbon bond lengthening appears to be related to the reactivity of the cycloadduct, thus the more reactive benzoquinone cycloadducts 5b and 13 have longer carbon-carbon bonds. Those cycloadducts 14 and 15 which are predicted to undergo asynchronous reactions show differing degrees of carbon-carbon bond lengthening, reflecting the differing degrees of bond breaking at the calculated transition states for the rDA.

Retro Diels−Alder Reactions of 5,6-Disubstituted-7-oxabicyclo[2.2.1]hept-2-enes: Experimental and Density Functional Theory Studies

Several 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes (1-4) were synthesized on > or = 0.1 mol scale. The heat-induced retro Diels-Alder (rDA) decomposition of these derivatives was studied by thermal analysis, and the kinetics of the rDA were measured for 4. First-order rate constants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures between 124 and 150 degrees C, were used to calculate Arrhenius activation parameters Ea (34.5 +/- 0.5 kcal/mol) and ln A (1.77 +/- 0.03 x 10(4)). The observed activation energy was significantly larger (by 9.5 kcal/mol) than that previously measured for the maleic anhydride adduct 1, and this was attributed to the difference in LUMO energies for the two dienophiles. Modeling of the activation parameters found for 4 with density functional theory (DFT) calculations for similar compounds 5 and 6 gave close quantitative correlations for deltaH double dagger, deltaG double dagger, deltaS double dagger. The rDA reactions studied were found to be entropy-driven.

Reaction of morphinan-6,8-dienes with azadienophiles

Reaction of various morphinan dienes, i.e. thebaine ( 1a), N-demethyl- N-formylthebaine ( 1b), 6-demethoxythebaine ( 1c), β-dihydrothebaine ( 2a), 4-acetoxy-β-dihydrothebaine ( 2b), 7-chloro-6-demethoxythebaine ( 3a) and 7-bromo-6-demethoxythebaine ( 3b) with 4-phenyl-4 H-1,2,4-triazoline-3,5-dione (PTAD) gave rise to new Diels-Alder (DA) adducts. DA-reaction of 1a, 1b and 1c with PTAD led to the products of the β-face attack of the dienophile at the diene unit. The W coupling ( 4J 5β,18) in the 1H-NMR spectra was indicative of these structures. α-Face addition took place in the case of morphinan dienes with opened ring E, and a by-product was formed due to the S E reaction of PTAD with the adducts. The structure of these derivatives was confirmed by means of NMR spectroscopic methods. The retro Diels-Alder (rDA) reaction of the adducts 4a and 4b readily took place in polar-aprotic solvents in the presence of bases with low nucleophilic character.

Synthese einer Calicen‐Vorstufe für Retro‐Diels‐Alder‐Reaktionen

Synthesis of a Calicene Precursor for Retro‐Diels‐Alder ReactionsIn view of retro‐Diels‐Alder reactions (RDA reactions), the calicene precursor 9 has been synthesized in a comparably simple four‐step synthesis by dibromocarbene addition at dibenzobarrelene (10 → 11, 44%), halogenlithium exchange followed by reaction with cyclopentenone (11→12, 91%) and H2O as well as HBr elimination (12→14→9, 43%) (Scheme 5). First experiments with respect to the thermal behavior of 9 show that, although RDA reaction seems to be relatively easily occurring according to the results of ‘Curie‐Point’ pyrolysis, only anthracene and no calicene 2 has been detected so far.

Synthese und Pyrolyse einer Triafulven‐Vorstufe

Synthesis and Pyrolysis of a Triafulvene PrecursorIn view of retro‐Diels‐Alder reactions (RDA reactions), the triafulvene precursor 3 has been prepared in a simple three‐step synthesis by dibromocarbene addition at dibenzo‐barrelene (11→12; 44%), halogen‐Li exchange followed by methylation (12→14, 100%) and HBr elimination (14→3, 62%) (Scheme 3). Reactivity of the so far unknown bridged 1,1‐dibromocyclopropane 12 has been explored, including reductions, allylic rearrangements, and ‘carbene dimerizations’ (Scheme 4). First experiments with respect to the thermal behavior of 3 show that RDA reaction, although occurring in most cases, is not the predominant pathway. When 3 is heated in a sealed tube without solvent, two dimers 26 and 27 are isolated in a total yield of 55% (Scheme 6). On the other hand, gas‐phase pyrolysis of 3 at 400° mainly produces rearranged 28 (56%; Scheme 7). It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 (Scheme 8).

Triafulven- und Calicen-Vorstufen für retro-Diels-Alder Reaktionen

An attractive concept for the synthesis of triafulvene (1) and calicene (2) by retro-Diels-Alder reaction (RDA reaction) of precursors 3 and 4 is presented. Precursors 3 and 4 are easily available in three steps starting with dibenzobarrelene (5). First thermal-fragmentation experiments are summarized.