High Pressures - Transition State Volume Effects

High Pressures - Transition State Volume Effects 

Similar effects were found for the stereoselectivity of the Diels-Alder reaction between furylmethanols with maleimides, with pressure changing slightly the endo/ exo ratio of the cydoadducts [46]. 

The esterification reaction of phthalic anhydride with methanol was performed at different temperatures at pressures up to 110 bar using supercritical CO2 as co-solvent [47]. By virtue of high pressure, a 53-fold rate enhancement was achieved at 110 bar and 60 °C (i.e. under non-supercritical conditions) as compared with batch experiments at 1 bar at the same temperature. Using in addition supercritical CO2 gives a 5400-fold increase. 

---

One question that needs to be addressed is why are the activation volumes of pericyclic cycloadditions smaller (more negative) than those of the corresponding stepwise reactions involving diradical intermediates In the past it was assumed that the simultaneous formation of two new n bonds in a pericyclic [4 - - 2] cycloaddition leads to a larger contraction of volume than the formation of one bond in the stepwise process. The interpretation presented [28] is limited by the scope of Eyring transition state theory where the activation volume is related to the transition state volume, as mentioned above, and does not incorporate dynamic effects related to pressure-induced changes in viscosity [41]. An extensive discussion of reaction rates in highly viscous solvents can be found in Chapter 3. 

The influence of the presence of carbohydrate solutes had been previously briefly explored by Lubineau and Scherrmann in the case of the Mukaiyama aldol-type reaction of silylenol ethers with aldehydes, which had been found to be accelerated with effects on the syn-anti selectivity similar to what is observed under high pressure. Favoring the syn aldol product was clearly consistent with its smaller transition state volume as compared to the anti one. When this reaction was studied in the presence of carbohydrate solutes, a moderate yield increase was observed, in the same ty -directed selectivity. This effect was however ascribed to slower competitive hydrolysis of the silylenol ether, probably related to a limitation of water activity in the medium, responsible for this undesired side reaction. ... 

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]. 

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. 

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. 

Faulkner et al. performed surface-confined electrochemistry at high pressures to probe the structure of the transition state during the oxidation of a tethered ferrocene probe (analogous to System 4) [139]. In these studies, the ferrocene-containing SAMs on gold were subjected to pressures between 1 and 6000 atm. The pressure dependence of the anodic peak potential reveals a positive volume of activation for oxidation, which is consistent with a solvent reorganization in the transition state, which allows ion complexation. This study demonstrates the importance of structural and environmental effects on surface-confined electron-transfer processes. 

When a reaction takes place under high pressures, however, the effect of pressure on the reaction rate may not be neglected. The description of the pressure dependence of reaction rates (15) Is based on the assunptlon that the reaction goes through a transition state Involving a volume change between reactants and the transi-... 

The temperature of a polymer s melting, glass transition, crystallization, and its solid-state annealing are all known to have pressure dependencies.High-pressure instruments, such as the Gnomix Inc. (Colar-ado, U.S.A.), have been developed to study PVT relationships in polymers. In these experiments, the sample is placed in an incompressible fluid and then the desired pressure is applied. Full details of this technique, as well as a collection of P-V-T relationships for a wide range of polymers up to 200 MPa ( 30,000 psi) and 400°C, have recently been published. These data have mainly been collected isothermally to report the effects of pressure and temperature on the volume of... 

In these cases the size of the activation volumes obviously depends not only on the effective packing probably caused by the restriction of vibrations and rotations in the transition state, but also on the transition-state polarization enhanced by the polar groups leading to a further decrease in volume which is not observed in the less polarized cycloadducts. Blake and Jorgensen [51] have assumed similar effects to explain the acceleration of Diels-Alder reactions in water. Catalytic and solvophobic promotion of high pressure addition reactions will be discussed in Chapter 11. 

The transition state theory (TST) may be considered to be established in 1941 by publication of a momunental book The Theory of Rate Processes [1. In Chapter VIII of the book, the authors discuss solution reactions and conclude. . that the ratedetermining step in solution is. .. the formation from the reactants of an activated complex which subsequently decomposes . Though the authors pointed out the importance of diffusion in bimolecular reactions, they did not consider a possible break down of their two key assumptions, that is, thermal equilibrium between the initial and the transition state and neglecting recrossing, in imimolecular rate processes. The remarkable success of TST in the interpretation of kinetic effects of pressure [2] turned the attention of high-pressure kineticists away from a possible failure of TST and efforts were concentrated on the interpretation of the activation volume obtained from pressure dependence of a rate constant fe at a constant temperature (Eq. 3.1). 

---