Abstract
The kinetic pressure effect on some Michael-like and [4 + 2] cycloadditions in aqueous solution was studied. This effect is complex and variable, in contrast to the pressure dependence of rate constants in organic solvents. In order to determine the origin of this kinetic behavior, systematic solvent studies were made. As a general observation, for a given reaction, the pressure effect is weaker in water than in hydrocarbon solvents. For Michael reactions, the smaller effect is certainly due, at least in part, to the reduction in the volume of electrostriction. For Diels–Alder reactions, pressure alters the intensity of hydrophobic interactions. In the cycloaddition of isoprene with methyl vinyl ketone, the solvent dependence of the volume of activation suggests operation of electrostatic effects in conjunction with hydrophobic interactions and hydrogen bonding effects when the reaction is carried out in water. It is concluded that in aqueous Diels–Alder reactions the pressure effect is dependent on both solvophobic and polarity parameters, the weight of each being dependent on the reaction partners involved. Copyright © 1999 John Wiley & Sons, Ltd.
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