Formation Of Diels-Alder Adduct Research Articles

The mechanism of conversion of substituted glycals to chiral acenes via Diels-Alder reaction: a computational study.

Synthesis of linearly fused aromatic systems using a glycal-based diene with an aryne is a long-standing topic of interest in glycal chemistry. We have examined the mechanistic pathways for the transformation of substituted glycals to chiral fused aromatic cores via Diels-Alder (DA) reaction using the SMDACN-M06-2X/6-31G(d) level of theory. The DA reactions of E (1a) and Z (1a') forms of C-2 alkenyl glycal and an aryl glycal (1b) as a diene were examined with a benzyne intermediate generated as a dienophile. The computational results reveal that 1a and 1b can only be transformed into the fused aromatic cores by the base-catalyzed reaction because a [1,5] sigmatropic hydrogen shift is not feasible. The activation free energy barrier for the base-catalyzed proton abstraction process is 4.2 kcal mol-1 and there is almost no barrier for stereoisomeric 1a DA-complexes. The activation free energy barrier values for stereoisomeric 1b DA-complexes for the base-catalyzed proton abstraction process are 10.8 and 12.4 kcal mol-1. The appropriate orientation of glycal-ring-oxygen and hydrogen at the 5th position of Z (1a') forms of C-2 alkenyl glycal facilitates the [1,5] sigmatropic hydrogen shift; however, the base-catalyzed reaction is energetically more favored than the former case. The rate-determining step for 1a and 1a' is the ring-opening step (18.2 and 19.5 kcal mol-1 for the S-stereoisomer), whereas the DA adduct formation step is the rate-determining step for 1b (16.1 kcal mol-1 for the S-stereoisomer). The structural analysis reveals the formation of the preferred S-stereoisomer over the R-stereoisomer with the respective dienes.

Diels–Alder adduct formation at solid interfaces between fullerenes and acenes

Understanding organic-organic interfaces is rather challenging due to their large complexity regarding morphology, molecular orientation at the interface, interdiffusion, and energetics. One additional important but often neglected aspect are chemical reactions occuring at such interfaces. For solid interfaces between pentacene and Buckminster-Fullerene (C60) recently very efficient Diels–Alder (D–A) adduct formation has been reported. Considering the importance of pentacene/C60 as prototypical donor-acceptor combination to study fundamental processes in organic photovoltaics, understanding this effect is essential. In this work, we provide detailed NEXAFS-based investigations with respect to the temperature-dependence and reaction zone depth of this effect. Moreover, we widely vary the interface morphology and observe that the D–A adduct formation is most efficient for bulk heterojunctions of pentacene and C60. By also investigating further material combinations such as PEN/C60-PCBM and interfaces between C60 and functionalized acenes, we observe trends for the occurrence of the D–A adduct formation correlated with the different chemical properties of the involved compounds.

Effects of Molecular Orientation in Acceptor–Donor Interfaces between Pentacene and C60 and Diels–Alder Adduct Formation at the Molecular Interface

Interfaces between pentacene and Buckminster‐fullerene (C60) have attracted interest due to their application as oligomeric model system for organic solar cells. As the actual device characteristics in such implementations are crucially controlled by the interface structure, detailed investigations of this interface on a molecular level are mandatory. In this study, the influence of the orientation of the pentacene molecules in highly ordered crystalline bottom layers on the characteristics of such internal interfaces is analyzed. It has been shown that the interface structure is driven by temperature‐controlled diffusion of C60 molecules to the pentacene step edges in the case of uprightly oriented pentacene. For lying pentacene in the bottom layer, no step‐edge decoration is observed while the wetting of the pentacene layer is enhanced. Furthermore, the stability of the interface against intercalation and reorientation has been analyzed by means of NEXAFS spectroscopy, showing that the orientation of the pentacene molecules at the interface remains unchanged. Instead, strong indication for chemical modification of the molecular entities by the formation of Diels–Alder adducts between C60 and pentacene is observed. Finally, it is shown that C60 forms crystalline islands in thicker films only on top of uprightly oriented pentacene while rather amorphous films are formed on lying pentacene.

Diffusion and Reaction Phenomena in Solution‐Based Healing of Polymer Coatings Using the Diels–Alder Reaction

AbstractThe healing of a furan‐functional epoxy coating using bismaleimide (BMI) solutions is demonstrated. Corrosion studies of scratched coated carbon steel show that solution healing of damaged coatings inhibits corrosion. The healing relies on (i) the diffusion of solution components into the polymer, causing the system to swell, thus bringing crack surfaces into contact, and (ii) Diels–Alder adduct formation between furan moieties of the polymer network and the maleimide, resulting in covalent bonding across the crack interface. For the system investigated, crack healing is reaction‐limited for small crack widths. The results provide a guideline for the design and implementation of self‐healing systems based on BMI solution encapsulation.

Mayr electrophilicity predicts the dual Diels–Alder and σ-adduct formation behaviour of heteroaromatic super-electrophiles

We report on the dual reactivity, i.e. anionic Meisenheimer sigma adduct formation and Diels-Alder adduct formation, of a series of heteroaromatic super-electrophiles, including 4,6-dinitro-benzofuroxan, -N-arylbenzotriazoles (4), -benzothiadiazole and -benzoselenadiazole. Measured pK(a)(H(2)O) values for sigma adduct formation provide a quantitative measure of super-electrophilic reactivity with a satisfactory correlation between the Mayr E electrophilicity parameter and pK(a)(H(2)O): E = -0.662 pK(a)(H(2)O) (or pK(R+) -3.20 (r(2) = 0.987). The most highly electrophilic, pre-eminent super-electrophile is 4,6-dinitrotetrazolopyridine (E = -4.67, pK(a)(H(2)O) = 0.4), which supercedes the reference Meisenheimer super-electrophile, 4,6-dinitrobenzofuroxan (E = -5.06, pK(a) = 3.75), having itself an E value superior by 8 orders of magnitude compared to 1,3,5-trinitrobenzene as the benchmark normal Meisenheimer electrophile (E = -13.19, pK(a)(H(2)O) = 13.43). (For relevant kinetic parameters as well as E and pK(a) values, see .) In a parallel study we have investigated Diels-Alder (normal and inverse electron demand) reactivity of this series of heteroaromatic electrophiles and have shown that Mayr E values are valid predictors of whether DA adducts will form and how rapidly. The observed order of pericyclic reactivity corresponds to E = -8.5 as the demarcation E value, in close agreement with sigma complexation; thus pointing to a common origin for the two processes, i.e. an inverse relationship between the degree of aromaticity of the carbocyclic ring and ease of sigma complexation, or DA reactivity, respectively.

Reaction Pathway of the [4 + 2] Diels−Alder Adduct Formation on Si(100)-2×1

Despite a long history of experimental and theoretical investigation, the mechanism of the Diels-Alder (DA) reaction has been controversial since its discovery 80 years ago. From these investigations, two schools of thought have emerged, namely that the reaction can proceed via a concerted, symmetric or asymmetric mechanism or via a nonconcerted mechanism involving a zwitterion or diradical as an intermediate. Here, we employ finite temperature ab initio molecular dynamics simulations, employing forces computed "on the fly" from electronic structure calculations, to investigate the microscopic mechanism of DA adduct formation between 1,3-butadiene and the Si(100)-2x1 surface. Free energy profiles and nonequilibrium trajectories strongly suggest a nonconcerted mechanism that forms a zwitterionic intermediate state. This mechanism, which begins with a nucleophilic attack of the C=C double bond on the positive member of a charge-asymmetric buckled Si-Si dimer, was previously shown to be common to the formation of a wide range of adducts that can form on the surface.

A Versatile Method for the Preparation of Enantiomeric Benzene-1,2-bis(alanine) Derivatives

A versatile method for the preparation of dicarba analogues of cystine as substituted benzene-, dihydrobenzene-, and tetrahydrobenzene-1,2-bis(alanine) derivatives is described. The partially saturated products resulted from Diels-Alder adduct formation with stereoselectively prepared (2R,7R)-2,7-diacetamido-4,5-bis (methylene)octane-1,8-dioic acid dimethyl ester. Aromatization of the dihydro adducts by manganese dioxide provided the parent benzene-1,2-bis(alanine) derivatives.

Intramolecular site selectivity in cation radical Diels-Alder cycloadditions of difunctional and trifunctional dienophiles

The cation radical Diels–Alder reactions of several difunctional and trifunctional dienophiles with 1,3-cyclopentadiene, catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate, were determined. When both of the reactive sites are of the 4-methoxystyrene type, reaction occurs smoothly at both styrene moieties to give the bis-adducts. However, when one reactive moiety is of the styrene type and one is of the enol ether type, reaction occurs exclusively at the styrene moiety to give a mono-adduct, even in the presence of a large excess of cyclopentadiene. When three potentially reactive moieties are included, two of them being of the styrene type and one of the 1,2-diaryloxyethene type, the reaction is again specific for the styrene moieties, giving only the bis-adduct. The high site selectivity observed in these reactions is especially noteworthy in view of the observation that monofunctional molecules containing all three of these moieties are found to be reactive toward Diels–Alder adduct formation under the same reaction conditions and that these adducts are relatively stable under the reaction conditions. Copyright ­© 2000 John Wiley & Sons, Ltd.

Synthetic aspects of electrophilic ipso-reactions of polyfluoroarenes

Interaction with electrophiles at the ipso-position with respect to fluorine is the general property of polyfluoroarenes, including perfluoroarenes. Depending on nucleophilicity and basicity of a medium, the reaction may yield polyfluorinated cyclohexadienes and -dienones or stop on the stage of formation of intermediates of these reactions — polyfluorinated arenium ions. Synthetic application of these reactions includes also further transformations of initially formed compounds that are reactive towards nucleophilic attacks, isomerization, Diels-Alder adduct formation and aromatization to yield modified fluoroarenes. The universal character of these reactions forms a basis for the general approach to fluoroarene synthesis according to the scheme: ▪ The theoretical basis for synthetic application of this strategy is the regioselectivity of ipso-reactions of polyfluoroarenes with electrophiles, which is shown to be consistent with the regularities of substituent influence on the relative stability of polyfluorinated arenium ions.

Oxokohlenstoffe und verwandte Verbindungen; 17. Mitteilung.13-Chlor-3-cyclobuten-1,2-dion, ein hochreaktives Dienophil; einfache Synthese von Bicyclo[4.2.0]octa-1(6),3-dien-7,8-dionen und Benzocyclobutendionen

3-Chloro-3-cyclobutene-1,2-dione (1c) reacts with the dienes 3a-e on careful heating to 110 o C to afford the bicyclo[4.2.0]octa-1(6), 3-diene-7,8-dione 4a-e. The reaction pathway is explained by Diels Alder adduct formation and subsequent elimination of hydrogen chloride. 4a-e are readily oxidized by activated manganese(IV) oxide to give the benzocyclobutenediones 5a-e in good yields. The parent benzocyclobutenedione (5a) has been prepared even more conveniently in a one-pot Diels-Alder route by heating a mixture of 3-chloro-3-cyclobutene-1,2-dione (1c) and 1-acetoxy-1,3-butadiene (3f)

The preparation and thermal fragmentation of 2-Acyl-5-azidofurans

The reaction of the nitrofurans (1)-(4) with sodium azide in Me2SO gave the thermally unstable azidofurans (5)-(8), which decompose to give nitrogen and the (Z)-dioxoalkenenitriles (32) and(15)-(17) respectively. The azides (6)-(8), the (Z)-nitriles (15)-(17) and their (E)-isomers (18)-(20)were characterized by spectroscopic techniques. The nitriles were also characterized by formation of Diels-Alder adducts and quinoxaline derivatives. The nitriles (17) and (20) gave the adducts(21a,b) with 2,3-dimethylbuta-1,3-diene, and the nitriles (16) and (19) gave the adducts (23), (24)and (25) (or (26)) with diene (22). The quinoxalines (27)-(31) were prepared by condensation of o-phenylenediamine with nitriles (17), (20), (18) and the unstable compounds (32) and (33) respectively. The first-order rate constants for ring-opening of azides (5)-(8) were determined by u.v. spectroscopy in cyclohexane and activation parameters were also evaluated. The trends in activation energies and entropies were rationalized by consideration of valence-bond contributors to the azides and the presumed nitrene intermediates. The acid-catalysed decomposition of azide (5) in methanol was also studied.

Synthesis and photolysis of 1,2,3,8-tetrakis(trifluoromethyl)-cyclo-octatetraene

The title compound was derived from tetrakis-(trifluoromethyl)-5-thiabicyclo[2.1.0]pent -2-ene via Diels–Alder adduct formation with butadiene; its photolysis gave [2 + 2] reaction products.

Regiochemical aspects of Diels-Alder reactions of cyclopentadienone

The regiochemistry of the Diels-Alder reaction between cyclopentadienone and 6,6-dimethylfulvene (isopropylidenecyclopentadiene) has been studied. Two adducts (in addition to cyclopentadienone dimer) were isolated and their structures were shown to be endo-10-isopropylidenetricyclo[5,2,1,02,6]-deca-4,8- dien-3-one (6a) and endo-3-isopropylidenetricyclo[5,2,1,02,6]deca-4,8- dien-10-one (7a). Kinetic parameters for the Cope rearrangement (6a) ↔ (7a) have been determined. The adducts (6a) and (7a) were formed in the ratio 45 : 55 respectively. Thus, in contrast to FMO predictions, and to its reaction with cyclopentadiene, cyclopentadienone tends to behave as a diene towards the fulvene. A tentative explanation of these results, based on angle strain in the transition states for the formation of Diels-Alder adducts, is proposed.