Role of the Medium in High Pressure Organic Reactions. A Review

As a thermodynamical parameter, pressure modifies the physical and physicochemical properties of liquids considered as media for organic synthesis. In a first part, the review details the effect of pressure on solubility, freezing point, and viscosity. The second paragraph examines the pressure effects on solute-medium interactions associated with volume changes. There are two main pressure-sensitive interactions: electrostatic and solvophobic interactions. i) Considering electrostrictive effects, the pressure rate acceleration is highest in apolar media. This is illuminated by many examples taken from unimolecular reactions, [4+2] and [2+2] cycloadditions, Michael reactions, Menshutkin quaternisations, Morita-Baylis-Hillman additions. ii) Pressure reduces the magnitude of solvophobic (hydrophobic, fluorophobic), interactions meaning less pressure sensitivity of reaction rates. In the last part, future high pressure media are outlined i.e. supercritical fluids and ionic liquids. However, the results reported to date do not permit to draw a clear conclusion. High pressure synthetic applications are described in the light of the above emphasized effects. The results presented in the review should allow a judicious choice of the right medium for a rational design of a liquid phase organic synthesis under pressure.