Diels kinetic studies

Fortunately, azachalcone derivatives (2.4a-g, Scheme 2.4) turned out to be extremely suitable dienophiles for Lewis-add catalysed Diels-Alder reactions with cyclopentadiene (2.5). This reaction is outlined in Scheme 2.4 and a large part of this thesis will be devoted to the mechanistic details of this process. The presence of a chromophore in 2.4 allows kinetic studies as well as complexation studies by means of UV-vis spectroscopy. Furthermore, the reactivity of 2.4 is such that also the... 

In the kinetic runs always a large excess of catalyst was used. Under these conditions IQ does not influence the apparent rate of the Diels-Alder reaction. Kinetic studies by UV-vis spectroscopy require a low concentration of the dienophile( 10" M). The use of only a catalytic amount of Lewis-acid will seriously hamper complexation of the dienophile because of the very low concentrations of both reaction partners under these conditions. The contributions of and to the observed apparent rate constant have been determined by measuring k pp and Ka separately. ... 

Engberts [3e, 9] has extensively investigated the Diels Alder reaction in aqueous medium. Recently Engberts and colleagues reported [9c] a kinetic study of a Diels Alder reaction of N-alkyl maleimides with cyclopentadiene, 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene in different solvents. The reaction rates of the cycloadditions with the open-chain diene relative to w-hexane are reported in Table 6.3. The aqueous medium greatly accelerates the Diels Alder reaction and the acceleration increases as the hydrophobic character of the alkyl group of the dienophile increases. These and other kinetic data [3e, 9], along with the observation that the intramolecular Diels-Alder reaction is also accelerated in... 

The hetero-Diels-Alder reaction can also employ dienes containing heteroatoms. Cycloaddition of substituted styrenes with di-(2-pyridyl)-1,2,4,5-tetrazine was investigated by Engberts (Eq. 12.56).127 Again, the rate of the reaction increased dramatically in water-rich media. Through kinetic studies, they showed that the solvent effects on the... 

In a subsequent paper [32], however, Berlan himself cast doubt on the existence of nonthermal effects, attributing the observed rate increases to localized hot-spots in the reaction mixture or to superheating of the solvent above its boiling point. He also mentioned the difficulty of measuring the temperature accurately in MW cavities. Furthermore, kinetic studies by Raner et al. [33], showed that the Diels-Alder reaction of 3 with 23 (Scheme 4.12) occurred at virtually the same rate under MW and conventional heating at the same temperature. 

Also, as mentioned earlier, Berlan et al. [28] had observed that the use of a nonpolar solvent resulted in a more significant rate enhancement in a Diels-Alder reaction (vide supra, Scheme 4.12), although later kinetic studies by Raner et al. [33] cast some doubt on this result. 

The first kinetic study of acceleration of some Diels-Alder reactions in water by Breslow et al. has set the stage for worldwide interest in organic... 

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

Density functional theory computational studies have been used to determine die importance of secondary orbital interactions for the stability of transition-state structures for die 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of die 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by die reaction of allyl chloride with sodium bis(bimediyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first tune.177... 

It is worth noting that the dimer and trimer generated in reactions (8) and (9) can react with polymeric radicals as a chain transfer agent, and therefore their effect on the polymer molecular weight should not be neglected the quantitative estimation of the concentration of these byproducts depends on the fact that whether the rate of thermal initiation is a second- or third-order reaction of monomer concentration. More kinetic information for such transfer reactions can be found in a number of publications [14-19]. Nevertheless, detailed kinetic studies on such Diels-Alder byproducts remain scarce. Katzenmayer [20], Olaj et al. [21,22], and Kirchner and Riederle [23] have published some quantitative results on this matter. 

Careful product and kinetic studies for selected electron-deficient alkenes, electron-rich dienes and vinyl-substituted aromatic systems have provided some clarification of the [2 + 2] versus [2 + 2] cycloaddition issue. The thermodynamically favored product can often be anticipated on structural grounds. Reactions of TCNE with vinyl-substituted benzenoid aromatics, protoporphorins or heteroaromaticsgive [2 + 2] products, but for some styrenes the [2 + 4] addition may be kinetically favored. p-Methoxystyrene and TCNE react to form a charge-transfer complex which leads reversibly to the Diels-Alder product, and eventually to the finally isolated [2 + 2] adduct. An isomer of di-cyclopentadiene shows the same pattern, with the initially formed Diels-Alder adduct giving rise to a [2 + 2] adduct. 

Scheme 33. Kinetic studies of the cation radical Diels-Alder reactions of aryl vinyl sulfides with cyclopentadiene and of aryl propenyl ethers with 2,3-dimethyl 1,3-butadiene. In both solvents, both extended reaction series have rates which correlate excellently with the Hammett a parameters and poorly with the Hammett-Brown o-+ values. The preference is statistically significant at or above the 95 % confidence level.

MTO proves to be an efficient and effective catalyst in this reaction when the dienophile is an a, -unsaturated ketone or aldehyde. It is especially active in water, usually with isolated yields >90%. Kinetic studies show that the reaction rate is proportional to the catalyst concentration. The desirability of 1 as a Diels-Alder catalyst stems from a combination of favorable properties the tolerance for many substrates, the inertness to air and oxygen, the use of aqueous medium, and the absence of product inhibition. The initial step appears to be the coordination of the carbonyl oxygen to the rhenium center. Steric crowding around rhenium inhibits reactions of the larger dienophiles [27]. 

In a kinetic study of the Diels-Alder addition of 24 dienophiles with cyclopenta-diene and 9,10-dimethylanthracene, TCNE was found to be the most reactive. Paquette treated the azepinone (1) with maleic anhydride or N-phenylmaleimide in the temperature range 35-120° but failed to obtain a crystalline adduct. Reaction occurred when (1) was heated with dimethyl acetylenedicarboxylate without solvent at 130°, but the yield of adduct was only 20%. In contrast, a solution of equivalent amounts of (1) and TCNE in tetrahydrofurane at room temperature had an initial violet-brown color which faded completely in 30 min. to pale yellow and the adduct (2) was obtained in qnantitative yield. 

In view of the widespread interest in the Diels-Alder reaction, the results of a kinetic study of the order of reactivity of dienes toward maleic anhydride is of particular interest. The figures under the formulas give values of rate constants, 10 x/ j (liters/mole/sec.) in dioxane at 30°. Where cisoid and transoid forms are both possible, the transoid form is shown, although there is evidence that a bulky group at the 2-position favors the cisoid form required for reaction. ... 

The most intensively studied reactions of 1,2,4,5-tetrazines are cycloaddition reactions with alkenes and alkynes, not only due to theoretical interest but also due to its importance in synthesis. The 1,2,4,5-tetrazines are diene components in this [4 + 2]-cycloaddition reaction while the alkynes and alkenes are the dienophiles. Kinetic studies have shown that these reactions should be classified as Diels Alder reactions with inverse electron demand. Therefore, 1,2,4,5-tetrazines with electron-withdrawing substituents (C02Me, CF3) and dienophiles with electron donating substituents are the most reactive compounds. However, dimethyl... 

The [4 -I- 2] cycloaddition is the prototypical example of a reaction showing a relative insensitivity to solvent. Table 10.17 shows kinetic studies at ambient pressure for some Diels-Alder reactions carried out in different media including water and ethylene glycol. Most reactions are normal electron-demand cycloadditions, one example involves an inverse electron-demand reaction (hexachlorocyclopentadiene (HCCP) -I- styrene) [77]. 

When equimolar amounts of TCNE and 381 reacted, a mixture of dihydro-Dewar benzene 382 and dihydrobenzvalene 383 was formed by cycloaddition". Diels-Alder reactions of TCNE are of both preparative and theoretical interest. So recently the reaction with 1,2-dimethylenecyclopentane has been investigated in detail. Kinetical studies by UV-Vis absorption spectroscopy have shown that the [4 + 2]cycloaddition takes place without preorientation by a charge-transfer complex. ... 

Several additions have been kinetically studied by more than one research group, but most of them were studied in solution and different workers used different solvents, so that the published data cannot be compared for reproducibility, Such a difficulty does not arise for reactions in the gas and in the pure liquid phases. Two popular Diels-Alder reactions, the dimerisation of cyclopentadiene to endo-dicyclopentadiene and that of butadiene to 4-vinyl-cyclohexene are suitable for a comparison of experimental results. Rate coefficients from 8 different sources for the former reaction in the pure liquid phase at various temperatures (in the range where comparison is possible) are given in Table 2. The coefficients are all extrapolated to zero time, as both secondary reactions and variation of the environment during the reaction cause a drift in the observed values of k. Rate coefficients for butadiene dimerisation in the gas phase, from different sources, are collected in Table 3. In this case also the temperature range has been limited to that where comparison is possible. In both Tables 2 and 3 activation parameters, as given by the authors (unless otherwise indicated) are also listed. 

Most of the classical dienes are hydrocarbons, like cyclopentadiene. butadiene, anthracene, 9,10-dimethylanthracene, isoprene, 2,3-dimethylbutadiene. For dienes of this kind it is generally true that electrophilic dienophiles are the most reactive, and actually acrylic derivatives, maleic anhydride, p-benzo-quinone and similar compounds have found a large use as dienophiles both for preparative purposes and for kinetic studies. The latter demonstrated quantitatively the importance of electronegative groups on the dienophiles, and conversely of electron-releasing substituents on the diene, in order to accelerate such type of Diels-Alder reaction. It was also realised later that Diels-Alder additions with inverse electron demand , that is between electrophilic dienes and nucleophilic dienophiles, do occur . ... 

Extensive kinetic studies of normal, neutral, and inverse Diels-Alder reactions observed in [4+2] cycloadditions of some thio- and selenocarbonyl compounds to 3,6-bis(trifluoromethyl)-l,2,4,5-tetrazine prove the highly dienophilic activity of the C=S and C=Se bonds. Studies of the solvent and temperature dependence of the reaction rate indicate a concerted mechanism <1998EJ02875>. 

These data, although they give important information about the hydrogen donor ability of 1,3-dialkylimidazolium ionic hquids, do not exclude the possibility that other factors can influence the rate and selectivity of Diels-Alder reactions. Recently, the kinetic study of reaction between the first excited state of molecular oxygen and... 

The Diels-Alder reaction cannot be explained adequately by the familiar polar reaction mechanism involving electron pair interactions or by a free radical mechanism. In fact in older literature, it was sometimes described as a no-mechanism reaction. There are no intermediates, and kinetic studies show that the reaction is first order in both the diene... 

Mechanistic and kinetic studies have been made of Diels-Alder reactions in SCCO2. For the reaction of maleic anhydride and isoprene in the presence of aluminum chloride in SCCO2, a two-step mechanism in which a <7-bond is formed followed by cyclization was suggested. For the reaction of ethyl acrylate and cyclopentadiene in SCCO2, good correlation was found between the density and the temperature-normalized rate constant. ... 

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