Kinetics retro-Diels-Alder reaction

Retro-Diels-Alder reactions of anthracenedione (51a) have been shown to proceed faster in aqueous solution than in organic solvents, apparently as a consequence of enhanced hydrogen bonding of water to the activated complex,30 since hydrophobic interactions with (51a) are of negligible importance. The results have been compared with previous kinetic data for bimolecular and intramolecular Diels-Alder reactions and the corresponding hydrogen bond and hydrophobic interactions have been discussed. 

Experimental and theoretical results have revealed the influence of a neighbouring cyclopropane ring in a retro-Diels-Alder reaction at lower temperature.14 The lower kinetic barrier for this reaction has been attributed to stabilization of the transition state by orbital interactions involving the cyclopropane bond orbitals. 

The isolation of 61, as well as the expected syn/endo-Hlsyn/endo-H isomer 59 (Scheme 13), illustrates, for the first time, evidence for the existence of a second reaction pathway operating during these repetitive Diels-Alder reactions. HPLC analysis of the reaction mixture, isolated after a high pressure reaction between 38 and 57, revealed the product ratio of 59 61 to be 18 1. The question of whether 61 owes its origin to the existence of a second, less favorable reaction pathway in a kinetically-controlled process, or is, in fact, the outcome of a degree of thermodynamic control, involving a retro Diels-Alder reaction, followed by recombination, will be discussed in Section 3.2. 

Whereas cyclic secondary enaminones and nitroolefins mainly yield indoles in which the enamine nitrogen is incorporated into the heterocyclus (equation 242), linear tertiary a-ketoenamines are shown to react with nitroolefines at low temperature under kinetic control to give 1,2-oxazine N-oxides as [4 + 2]-cycloadducts, followed by retro-Diels-Alder reaction or rearrangement under thermodynamic control which leads diastereo-selectively to aminocyclopentenes. The reaction is called [3 + 2]-carbocyclization, apparently because the ketoenamine is reacting as a 1,3-dipole. The products are hydrolysable to polysubstituted nitrocyclopentanones with retained configuration325 (equation 243). 

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 Durham route to polyacetylene 103, 104) involves the metathesis polymerization of 1 to give a soluble but thermally unstable high polymer (Scheme 5.2). Slowly at room temperature, or more rapidly at 80 °C, the polymer undergoes a retro-Diels-Alder reaction. This reaction results in elimination of a substituted benzene and formation of amorphous polyacetylene. An enormous weight loss accompanies the conversion, but high-density films were produced with no apparent voids. The kinetics of the transformation reaction were extensively studied (JOS). 

Endo selectivity is a kinetic phenomenon. If an equilibrium is established between the higher energy endo and lower energy exo products, then predominantly exo products will be seen. The cycloaddition of maleic anhydride and furan proceeds very rapidly, even at very low temperatures, to give only the exo product. The unusually low energy of furan (an aromatic compound) allows the retro-Diels-Alder reaction of the endo product to proceed at a reasonable rate. Even though the rate of formation of the endo product is faster than the rate of formation of the exo product, establishment of an equilibrium between starting materials and products leads to a thermodynamic ratio that favors the exo product. 

Durst has shown that a Diels-Alder adduct is in fact the kinetic product of reactions of sulfur dioxide with conjugated dienes, but is thermally unstable and readily undergoes a retro-Diels-Alder reaction to butadiene and sulfur dioxide. A thermodynamically more favorable addition then ensues, producing a stable five-membered ring adduct. Thus, the apparent inability of sulfur dioxide to form Diels-Alder adducts is not one of reactivity, but of product instability. 

The outstanding SM behavior was restored after recychng of the corresponding system. Note that recycling means cycloreversion here, that is, onset of the retro Diels-Alder reaction at T > 105 C (Figure 6.4). Note that the anthracene end functionalization, instead of furan, was foreseen to influence the kinetics of the adduct formation and its temperature stabihty. 

Laser-induced and thermal retro-Diels-Alder reactions of norbomene have been investigated with B3LP, CASSCF, and CASPT2 methods. Kinetic studies show that the cycloaddition of l-amino-3-siloxy-1,3-dienes proceeds via a concerted 4-1-2-mechanism, with a relatively low activation barrier. The Diels-Alder reaction between hexachlorocyclopentadiene and ethyl vinyl ether has been shown to be stereo-speciflc since the variation in cis versus trans deuterium content in the natural abundance ethyl vinyl ether is retained in the cycloadduct product. ... 

Scheme 1 Double hydrogen transfer in the molecular ion of the endo isomer [2] of 8,9-disubsituted-tricyclo[5.2.1.02 ]decene (not possible In the molecular Ion of the exo Isomer [1] followed by elimination of CsHe via a retro-Diels-Alder reaction. Reproduced with permission of the American Chemical Society from Kluft E, Nibbering NMM, KQhn H and Herzschuh R (1986) Dyotropic hydrogen rearrangement In the molecular Ions of 8,9-disubsti-tuted tricycio [5.2.10 ] decenes as supported by field ionization kinetics. Journal of the American Chemical Society 06 . 7201-7203.

Photolysis of the oxadiazabicyclo[2.2.3]nonadiene 285 gave a rearranged bicyclic system 286 plus 1-ethoxycarbo-nyl-177-azepine 287 (Equation 42) < 1999H(51)141 The latter was shown to form on photolysis of 285, but it was suggested on the basis of kinetic evidence, that some may also be formed directly from 285 by what is formally a retro-Diels-Alder type reaction. 

The Stereoselectivity of Diels-Alder Reactions. One of the most challenging stereochemical findings is Alder s endo rule for Diels-Alder reactions. The favoured transition structure 6.180 has the electron-withdrawing substituents in the more hindered environment, under the diene unit, giving the kinetically more favourable but thermodynamically less favourable adduct 6.181. Heating eventually equilibrates the adducts in favour of the exo adduct 6.182, by a retro-cycloaddition re-addition pathway. 

Pd/C gave ( )-24 in 99% yield. For comparison, at ambient pressure and temperature reaction of furan and 31 failed (32 was obtained in yield 1.5%), and addition of LiC104 catalyst gave 32 in 10% yield. The kinetic studies showed that 32 reverted to furan and 31 with ti/2=52 h (20 °C) and fi/2=3.3 h (40 °C), showing that the increase of temperature accelerates the retro-Diels-Alder process, and its high susceptibility to tirermal cycloreversion. 

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