Diels activation parameters

B. l,3>2>Dioxaphospholens.—The kinetics of the addition of trialkyl phosphites to benzil have been investigated spectrophotometrically. The second-order reaction of trimethyl phosphite in dioxan has activation parameters of A// = 8.4 kcal mol and AS = — 47.5 e.u. In benzene the rate constant increases linearly with low concentrations of added organic acid and decreases linearly with low concentrations of added base. The Diels-Alder mechanism is considered unlikely on the basis of these data, and the slow step is considered to be nucleophilic addition of the phosphite to the carbon of the carbonyl group (see Scheme). 

The pseudothermodynamic analysis of solvent elfects in 1-PrOH-water mixtures over the whole composition range (shown in Figure 7.3) depicts a combination of thermodynamic transfer parameters for diene and dienophile with isobaric activation parameters that allows for a distinction between solvent elfects on reactants (initial state) and on the activated complex. The results clearly indicate that the aqueous rate accelerations are heavily dominated by initial-state solvation effects. It can be concluded that for Diels-Alder reactions in water the causes of the acceleration involve stabilization of the activated complex by enforced hydrophobic interactions and by hydrogen bonding to water (Table 7.1, Figure 7.4). °... 

The racemization of Diels-Alder adducts in the solid state appears to proceed via diradical or complete cycloreversion. For example, (-l-)-96 racemizes in the solid state from 130 to 155 °C (AH =40.0 kcal mol AS =14 cal mol K ) to give (-)-96 (Scheme 9), whereas the melt reaction (eutectic temperature is 165 °C) from 176 to 194 °C has much lower activation parameters (AfJ =... 

By measuring activation parameters (Eq. (2) and Table 2), it has been shown that the cceleration arises from a favorable change of activation entropy, which is an indication of the implication of the hydrophobic effect [32], Concentrated aqueous carbohydrate solutions (glucose and saccharose for instance) have been shown to accelerate the Diels-Alder reaction. The acceleration is even greater than that observed with saturated fi-cyclodextrin solution (Eq. (3) and Table 3) [32],... 

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. 

The author has tried to collect in Table 9 a representative sample of Diels-Alder reactions with different types of reactants, for which the usual activation parameters could be determined with care (in most cases from kinetic runs at four or more different temperatures, in a range of at least 20°C). Other data (E and log A values) have been given in Tables 2 and 3 (pp. 93, 94) the relevant values of the activation entropy can be calculated from log A by the conversion formulae ... 

A less-common activation parameter, the volume of activation (AF ), has been determined for a few Diels-Alder reactions carried out under pressure in liquid phase. The processes are cyclopentadiene dimerisation , isoprene dimerisation , addition of 2,3-dimethylbutadiene to butyl acrylate , addition of cyclopentadiene to dimethyl acetylene dicarboxylate , and addition of maleic anhydride to 1,3-cyclohexadiene, /rans-l-methoxybutadiene and isoprene . Activation volumes are negative, i.e. the reacting systems contract on passing from the initial to the transition state. In some cases the transition state appears to be even smaller than the adduct, independently of the solvent . Some of these experimental results gave rise to controversial interpretations however, the most recent ones favour a concerted four-center mechanism for the reaction. 

In the field of 1,3-cycloaddition a wealth of data about activation parameters comparable with that for the Diels-Alder reaction is not available. Published values were obtained from experiments rarely carried out at more than three, sometimes at only two, different temperatures, and this may be a source of error. 

Bloomfield found that log k = 13.14 — 27 %10/13RT for this transformation. Also observed was a mixture of Diels-Alder adducts with dimethylacetylene dicarboxylate identical to that derived from CT when BOE was heated at 110°C. Further, Bloomfield observed the formation of CT from BOE and found Aif = 5.60 kcal/mol and A5 = -0.7 e.u. for the BOE, CT equilibrium. Because of this pre-equilibrium, the activation parameters for the 1,5-hydrogen shift in CT must be roughly A/f = 37 kcal/mol and = 2 e.u. 

The vrater structure is modified by the presence of solutes, with effects referred to as structure-making or breaking, salting-out or salting-in, antichaotropic or chaotropic, pro- or antihydrophobic. " Such effects modify the kinetics and selectivity outcome of hydrophobic effect driven reactions, as shown by Breslow who reported the acceleration of the aqueous Diels-Alder reaction due to the presence of lithium chloride and by Lubineau and Queneau who later proved the entropic origin of this activation by determining the activation parameters. ... 

Table 4 Influence of additives on the rate constants and activation parameters of the aqueous Diels-Alder reaction of diene 2 with methylvinylketone at 25 °C.

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