Activation volume, pressure effects, high

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

While many observations are well understood, e.g. those dealing with the reaction rate or with the selectivity, there are some factors which cannot be generalized. Many transformations of particular reactants or under unusual reaction conditions led to unexpected results. There are often singular explanations for such reactions but no overall concept. For instance, computations on Diels-Alder transition structures and thermodynamics of retro-Diels-Alder reactions confirmed that the activation volume of these [4 + 2]-cycloadditions is negative80. This result, pointing to the compact character of the transition structure, is used to explain the dependence of reactivity and selectivity on internal as well as external pressure81-83. These effects are only observed at relatively high external pressures (Table 5). 

Another contribution to AjF was observed to arise in some reactions involving high steric hindrance (see references cited in Ref. 275) this is indicated as A V, the steric volume of activation. According to observation, high pressure promotes the most hindered process. This effect has been ascribed to a displacement of the most hindered transition states toward the more compact products in the reaction profiles [285]. 

Eontanella and co-workers studied the effect of high pressure variation on the conductivity as well as the H, H, and O NMR spectra of acid form Nafionl 17 membranes that were exposed to various humidities. Variation of pressure allows for a determination of activation volume, A V, presumably associated with ionic and molecular motions. Conductivities (a) were obtained from complex electrical impedance diagrams and sample geometry, and A V was determined from the slope of linear isothermal In a versus p graphs based on the equation A E = —kJ d In a/d/j] t, where p is the applied pressure. At room temperature, A Ewas found to be 2.9 cm mol for a sample conditioned in atmosphere and was 6.9 cm mol for a sample that was conditioned in 25% relative humidity, where the latter contained the lesser amount of water. 

The chemical effect of high pressure is to stimulate the selectivity and the rate of reaction together with better product properties and quality as well as improved economy. This is based on better physico-chemical and thermodynamic reaction conditions such as density, activation volume, chemical equilibria, concentration and phase situation. Many successful reactions are basically enhanced by catalysis. 

A typical unimolecular reaction is the decomposition of organic peroxides for which always positive activation volumes of up to 15 cm3/mol have been observed. The decomposition of di(t-butyl)peroxide, an effective initiator for the high pressure polymerisation of ethylene, into two t-butoxyradicals, exhibits a positive activation volume of 13 cm3/mol (Table 3.2-1, a). When new bonds are formed as in the association... 

Organic peroxides, which readily decompose into free radicals under the effect of thermal energy, are used under high pressures as initiators for radical polymerizations. The measurement of the influence of pressure on the rate of decomposition gives rise to the determination of the activation volume, which, in turn, allows conclusions to be drawn on the decomposition mechanism and the transition state. 

Pressure effect on the product distribution in supercritical media would resolve the problem. If the reaction proceeds via the competitive concerted/ stepwise mechanism, the reaction under a higher pressure is expected to give more exo isomer because the activation volume is considered to be smaller for concerted process than the stepwise one and hence more concerted reaction is expected under a higher pressure. If, on the other hand, bimodal lifetime distribution of trajectories is the origin of the stereoselection, the product ratio is expected to approach to unity under high-pressure conditions, since energy randomization is more effective under a high pressure. 

An effect of high pressure on the diastereoselectivity is also observed for intramolecular hetero Diels-Alder reactions as found for the cycloaddition of the benzylidene-isoxazolone 8-23 to afford the cis-annulated 8-24 as the major product together with the frans-diastereomer 8-25 (Fig. 8-8) [549], However, the difference in activation volume with AAV =1.6 0.2 cm3 mol"1 is rather small. The activation volume with AV = 19.6 0.5 cm3 mol-1 at 343 K lies significantly below the usual values found for intermolecular cycloadditions of 1-oxa-1,3-butadienes, indicating that this reaction may be on the border line to a two-step reaction but see also below. 

Intermolecular [4 + 2] cycloadditions exhibit strongly negative activation volumes and reaction volumes. High pressure, therefore, can be applied to accelerate Diels-Alder reactions and to shift the reaction equilibrium towards the cycloadducts. These effects are of particular advantage to (1) promote odierwise slow [4 + 2] cycloadditions involving heat or Lewis acid sensitive educts or products (2) suppress cycloreversion processes which are eidier thermodynamically favored or would interfere with a kinetically controlled stereochemistry. In view of a recent review (1985) only a few examples are presented here. 

Additions of allylic stannanes to aldehydes can also be effected imder high pressure without an added catalyst or promoter [12]. Interestingly, and in apparent contradiction to the thermal additions, mixtures of syn and anti adducts are formed from both (E)- and (Z)-allylic stannanes (Table 3). Moreover, the proportion of (E) syn and (Z) — anti products actually increases as the pressure is increased in contrast with what might have been expected from the thermal reactions. These findings have been interpreted as reflecting reduced activation volumes in the boat vs chair transition states, as illustrated in Eq. (10). The effect is also observed with propanal, 2-ethylbu-tanal, and 2-phenylpropanal. 

Acceleration of some chemical reactions is possible when high-pressure techniques are employed. The effects on a given reaction can be predicted to a certain extent because the thermodynamic properties of solutions are well known. The rate of a reaction can be expressed in terms of the activation volume,... 

Reactions in supercritical media utilize high pressures. Therefore, the effect of pressure on reaction equilibrium as well as reaction rate plays an important role in supercritical phase reactions. Supercritical fluids that exhibit very high negative activation volumes for certain reactions will improve the rate and equilibrium conversion of the reaction. 

Furyl derivatives 76, with an allylether or allylamine-type linkage to a methylenecyclopropane framework, readily undergo high pressure-promoted intramolecular cycloaddition" to give spirocyclopropane tricyclic products 77. No cycloaddition reaction occurred at ambient pressure and the products were mostly tar and polymers. Lewis acid catalysis was only marginally successful (Scheme 7.18). At 1.0 GPa and a slightly elevated temperature (60-70 °C) the intramolecular Diels-Alder reaction occurs readily and is exo-diastereo-selective. To quantify the pressure effect on the kinetics the volumes of activation were determined. 

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