Effect of pressure on sterically hindered reactions with late transition states

The effect of high pressure is examined in two types of sterically congested reactions presenting late transition states: isopolar Diels-Alder cycloadditions of isoprene and quinones of various steric environment and conjugate additions of tert-butylamine and acrylic compounds. In the Diels-Alder cycloadditions, pressure has no enhanced kinetic sensitivity in sterically demanding reactions compared with unhindered ones. This is due to the structural closeness of the transition state and product. At variance, in the conjugate addition of amines, the effect of pressure is magnified with increasing steric congestion at the reaction centres, This is ascribed to enhanced electrostriction with removal of steric hindrance to ionization. Copyright (C) 2004 John Wiley Sons, Ltd.