Diels-Alder reactions activation enthalpy

Unfortunately, the number of mechanistic studies in this field stands in no proportion to its versatility" . Thermodynamic analysis revealed that the beneficial effect of Lewis-acids on the rate of the Diels-Alder reaction can be primarily ascribed to a reduction of the enthalpy of activation ( AAH = 30-50 kJ/mole) leaving the activation entropy essentially unchanged (TAAS = 0-10 kJ/mol)" . Solvent effects on Lewis-acid catalysed Diels-Alder reactions have received very little attention. A change in solvent affects mainly the coordination step rather than the actual Diels-Alder reaction. Donating solvents severely impede catalysis . This observation justifies the widespread use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis-acid catalysed Diels-Alder reactions. 

It should be noted that the experimental activation enthalpy for the Diels-Alder reaction is 33 kcal/mol (estimated from the reverse reaction and the experimental reaction energy i.e. the MP2/6-31G(d) value is 14kcal/mol too low. Similarly, the calculated reaction energy of —47 kcal/mol is in rather poor agreement with the... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

With application of reasonable values for trapping parameters and AS2, it was possible to bracket the enthalpy and entropy of activation for isomerization of cyclobutadiene. Hence, A/Zj was estimated to fall between 1.6 and lOkcal/mol, where the upper limit was consistent with theoretical predictions for square-planar cyclobutadiene. Most surprising, though, was the conclusion that AS for automeriza-tion must lie between -17 and -32cal/(molK), based on the AS values normally observed for Diels-Alder reactions as a model for AS2. ... 

TABLE 2.4 Activation Enthalpies AW (kcal/mol) for the Ortho I Meta and Endo/Exo Pathways for the Diels-Alder Reactions Between o-QM and MVE, Styrene, and MVK in the Gas Phase"... 

The reversal of the thermal decomposition of 6 to ethylene and vinylacetylene cannot be utilized to generate 6, since, according to a quantum-chemical analysis, the reaction is slightly endergonic and requires a large free activation enthalpy (0.9 and 42 kcal mol-1, respectively) [59]. The intramolecular variant of this process as well as the addition of typical dienophiles of the normal Diels-Alder reaction to divinylace-tylenes will be discussed at the end of Section 6.3.3. 

In addition to the Hopf cydization of 176, there is a second pericydic reaction leading to 162, that is, the dehydro Diels-Alder reaction of butenyne with acetylene (Scheme 6.47). The theoretical treatment of this process by Johnson et al. [59] predicted a free reaction enthalpy and a free activation enthalpy, both at 25 °C, of -13.4and 42.0kcalmol-1, respectively. Ananikov [116] arrived at a similar result for the intramolecular case of non-l-en-3,8-diyne (202) and calculated the same quantities to be -15.3 and 30.9 kcal mol-1 for the formation of the isoindane 203. As already discussed regarding Scheme 6.40, the conversion of 162 into benzene and likewise that of 203 into indane have to be considered as a sequence of two [1,2]-H shifts 116, 117], whose highest transition state has a significantly lower energy than that for the formation of 162 and 203 by the dehydro Diels-Alder reaction. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

The interesting feature of the cycloadditions is that the ease with which they take place depends on the size of the ring. Whereas the 2 + 4 process (Equation 11.2), known as the Diels-Alder reaction, occurs readily with activation enthalpies of roughly 25-35 kcal mole-1, and has been one of the cornerstones of organic chemistry for many years,7 the 2 + 2 and 4 + 4 additions are accomplished much less easily. One finds, however, that if one of the reacting molecules is in an electronically excited state, the 2 + 2 and 4 + 4 processes occur more readily than the 2 + 4.8... 

Equation 12.35) a low activation enthalpy and highly negative activation entropy (AH = 15.3 kcal mole-1, AS = —35 cal mole-1 °K-1) suggest a concerted mechanism similar to that of the Diels-Alder reaction.70... 

The computed activation energy of 19.6 kcal/mol in vacuum at 298 K is in excellent agreement with the experimental value of 19.7 kcal/mol [70]. In toluene, the experimental activation enthalpy is 15.8 1.4 kcal/mol, with an activation entropy of —38 4 cal/mol K [71]. Therefore the computed 19.1 kcal/mol gas phase activation enthalpy of acrolein and butadiene should be lowered by 3.3 kcal/mol in toluene. In a related Diels-Alder reaction of cyclopentadiene and methyl acrylate, the experimental activation enthalpy is 15.1 kcal/mol in toluene, which is ca. 0.7 kcal/mol lower than that reported for acrolein and butadiene [81],... 

In general, cycloadditions catalyzed by Lewis acids proceed at significantly lower temperatures and with higher selectivities than their uncatalyzed counterparts. Factors that contribute to the increased selectivity of the catalyzed reactions include lower temperatures and more organized transition states. For enthalpy-controlled reactions, lowering temperatures increases selectivity (recall Section 1.4, equation 1.5). Coordination of a Lewis acid to the enone carbonyl not only activates the enone by electron withdrawal, it also restricts conformational motion and thereby reduces the number of competing transition states. Figure 6.12 illustrates several chiral auxiliaries for dienophile modification that have been used in the Diels-Alder reaction. 

Enthalpy of reaction, maleimide, 266 Enthalpy values, MA polymerization, 244 Entropy of activation, Diels-Alder reactions, 138 Entropy values, MA polymerization, 244 Environmental Protection Agency, regulations, in MA production, 17... 

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