Reactions Diels-Alder dissociation

The mechanism by which Lewis-acids can be expected to affect the rate of the Diels-Alder reaction in water is depicted in Scheme 2.6. The first step in the cycle comprises rapid and reversible coordination of the Lewis-acid to the dienophile, leading to a complex in which the dienophile is activated for reaction with the diene. After the irreversible Diels-Alder reaction, the product has to dissociate from the Lewis-acid in order to make the catalyst available for another cycle. The overall... 

Isoindole itself gives normal Diels-Alder addition products, (107 and 108), with maleic anhydride and A-phcnylmaleimide, these derivatives constituting the main evidence for forma,tion of the parent substance. 2-Alkyl- and 2-arylisoindoles also give normal addition products with these two dienophiles.Although only one product is generally isolated, it seems likely, in view of the known tendency of several Diels-Alder adducts of isoindoles to dissociate to their components (see below), that both exo and endo stereoisomers might be formed in certain cases. The reaction between 2-p-tolyl-isoindole and A-phenylmaleimide has been shown to give both e,xo (109) and endo (110) addition products. ... 

Jorgensen et al. [84] studied how solvent effects could influence the course of Diels-Alder reactions catalyzed by copper(II)-bisoxazoline. They assumed that the use of polar solvents (generally nitroalkanes) improved the activity and selectivity of the cationic copper-Lewis acid used in the hetero Diels-Alder reaction of alkylglyoxylates with dienes (Scheme 31, reaction 1). The explanation, close to that given by Evans regarding the crucial role of the counterion, is a stabilization of the dissociated ion, leading to a more defined complex conformation. They also used this reaction for the synthesis of a precursor for highly valuable sesquiterpene lactones with an enantiomeric excess superior to 99%. 

Diels-Alder reactions, 160-161, 163 Dissociation enthalpy, 229-232, 236, 237, 247 Distibines, 251, 253, 254, 263-265 adducts, 252-255 ligands, 99-106, 124, 125 Dmap-stabilized monomers, 276-291 Dodecatrienediyl-Ni11 complex, 169,... 

Itoh and coworkers111 carried out tandem [2 + 2 + 2]/[4 + 2] cycloadditions catalyzed by a ruthenium catalyst. The reaction of diyne 147 with excess norbomene 148 in the presence of ruthenium catalyst 153, for example, afforded 149. Adduct 150 either dissociated from the catalyst or reacted with another equivalent of norbornene. In the latter case, a ruthenium catalyzed Diels-Alder reaction occurred, affording hexacyclic adduct 152 via 151 (equation 43). Compounds 150 and 152 were obtained in yields of 78% and 10%, respectively. Both cycloaddition reactions proceeded with complete stereoselectivity. When 1,6-heptadiyne was used instead of 147, only trace amounts of a cycloadduct were obtained. Replacing norbornene by norbornadiene, which was expected to result in polymer formation, did not afford any adduct at all. 

The labile cycloadduct 262 of azodibenzoyl to cyclopentadiene rearranges to the fused oxadiazine 263 on heating. The process involves dissociation of 262 into its components, followed by a Diels-Alder reaction in which the azo compound functions as a hetero diene (equation 142)135. 

With one exception, naphthalen-l,4-imines with a double bond between C-2 and C-3 are not known to dissociate thermally by either possible retro-Diels-Alder pathway (the reverse of reactions described in Section III, A, 1 and 2), and the enthalpy requirements for the formation of a benzyne or an acylic acetylene are doubtless unfavorable. However, the mass spectra of compounds 93-99 reveal one important fragmentation of the molecular ions to be loss of dimethyl acetylene-dicarboxylate, and another fragmentation pathway involves the formation of nitrilium ions MeC=NR and PhC=NR from 93-95 and 96-99, respectively. ... 

Discrimination of the racemic aluminum reagent 4 can be carried out using chiral ketone 5, which deactivates one enantiomer of racemic 4. The hetero-Diels-Alder reaction is then catalyzed by the remaining opposite enantiomer of racemic 4 (Scheme 8.4). The combination of racemic 4 and chiral ketone 5 in a 1 1 ratio gives better enantiomeric excess than in a 2 1 ratio, implying that one diastereomer of the 4/5 complex readily dissociates to yield optically pure 4 and the chiral ketone 5. 

After in the foregoing chapter thermodynamic properties at high pressure were considered, in this chapter other fundamental problems, namely the influence of pressure on the kinetic of chemical reactions and on transport properties, is discussed. For this purpose first the molecular theory of the reaction rate constant is considered. The key parameter is the activation volume Av which describes the influence of the pressure on the rate constant. The evaluation of Av from measurement of reaction rates is therefor outlined in detail together with theoretical prediction. Typical value of the activation volume of different single reactions, like unimolecular dissociation, Diels-Alder-, rearrangement-, polymerization- and Menshutkin-reactions but also on complex homogeneous and heterogeneous catalytic reactions are presented and discussed. 

There are exceptions to favored endo stereochemistry of Diels-Alder additions. Some of these exceptions arise because the addition reaction is reversible, dissociation being particularly important at high temperature. The exo configuration is generally more stable than the endo and, given time to reach equilibrium (cf. Section 10-4A), the exo isomer may be the major adduct. Thus endo stereospecificity can be expected only when the additions are subject to kinetic control. 

This reaction is more readily reversible than most Diels-Alder reactions, and the product largely dissociates to the starting materials on heating to 120°. The cycloadduct is an unsaturated cyclic sulfone, which can be hydrogenated to give the saturated cyclic sulfone known as sulfolane ... 

Thermal cycloreversion of the adducts can be accomplished at a convenient rate when heated in toluene under reflux. If a new diene is present in the reaction mixture, the thioaldehyde thus generated in the retro-Diels-Alder reaction may give a new adduct. Therefore, adducts 81 and 82 act as thioaldehyde or thioketone transfer reagents. These adducts dissociate reversibly on heating, thus ensuring that the concentration of the labile species remains very low. For this reason, polymerization is not a serious problem especially in the case of thioaldehydes224. The transient thiocarbonyl compounds can be trapped not only by dienes but also by 1,3-dipolar cycloadditions332 (equation 85). 

The reactions of 109 with furan catalyzed by ZnCl2 gave different results depending on the reaction temperatures [95,96] (Scheme 55). At 0°C the 7r-fa-cial selectivity for both endo and exo addition modes is complete (only endo(t) and exo(t) adducts are obtained) but the exo/endo ratio is very low (de = 36%), endo(t) being the major adduct. At room temperature, the reactions exclusively afforded endo c) and endo t) adducts by dissociation (retro-Diels-Alder) and... 

It is the reaction of the "dissociating dimer or monomer that is of most interest in polymerization. Whether or not Reaction 6 proceeds to afford products II, III, or IV depends on the dienophile, the diene, and the reaction conditions. Cyclopentadienones which are "dissociating dimers can be driven to IV in most cases by the appropriate dienophile at elevated temperatures. This is, in fact, the course of the polymerization reaction shown in Reaction 4. Monomeric cyclopentadienones, on the other hand, will often stop at the intermediate III, even under quite severe conditions. It is this reaction stage that is of particular interest to us for the study of Diels-Alder step-growth polymerizations. 

Fig. 9a4) between cyclopentadiene and a C=C bond of the dumbbell-shaped part of the rotaxane. The dumbbell-shaped part contains two dicarbonyl stations (Fig. 9a3), one derived from fumaric acid (tram -CO-C H=CH-CO-. station 1), the other derived from succinic acid (—CO-CH2-CH2-CO-, station 2). The two diamide sites of the macrocycle can form four H-bonds with the two carbonyl groups of a given station (Fig. 9al for the interaction of the two carbonyl groups of fumaric-acid-derived station 1 with the four NH groups of the macrocycle through four H-bonds, see Fig. 9a2). Station 1 (derived from fumaric acid) has a tram C=C double bond due to its preorganization, this station interacts with the macrocycle better than the station 2. Consequently, the macrocycle is initially located at station 1 (Fig. 9a5). The Diels-Alder cycloaddition (80° C, 90% yield) of cyclopentadiene to the double bond of station 1 results in a mixture of diastereomers (Fig. 9a4) and causes displacement of the macrocycle from station 1 to station 2 (Fig. 9a6). The cycloaddition is reversible and the retro-Diels-Alder reaction occurs quantitatively (250°C, reduced pressure) when cyclopentadiene dissociates from the axle of the rotaxane this produces a displacement of the macrocycle from station 2 back to station 1. 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

Attempts to take into account the retro-Diels-Alder reaction in the treatment of the experimental data did not 0ve a constant kinetic order as observed in the case of the reaction of (V,f) with 1,4-naphthoquinone (see below). On the other hand, the dissociation of cyclopentadienedimeric derivatives is practically ne igible in the range of temperature explored, i. e. 50—90 °C. This condudon is indirectly supported by the absence of information in the literature about this possibility. Tlu results obtained are reported in Table 21 and are plotted in Fig. 24 according to an Arrhenius diagram in the case of (V,f). The value of k = 2.0 10 e i ... 

The Diels-Alder reaction is reversible, and many adducts, particularly those formed from cyclic dienes, dissociate into their components at higher temperatures. Indeed, a refro-Diels-Alder reaction is the principal method for preparing cyclopenta-diene prior to its use in cycloaddition reactions. 

Diels-Alder reactions can sometimes reverse themselves through Retro-Diels-Alder reactions. For example, dicyclopentadiene can be cracked to form 1,3-cyclopentadiene by thermal dissociation. Retro reactions occur under situations where the fragments are stable by themselves. 

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