Stepwise mechanism, pressure effects, high

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. 

The introduction of an atom other than carbon to either dienes or dienophiles will result in the hetero-Diels-Alder reaction. Owing to the nature (steric effect, electronic effect, etc.) of substituents on dienes and dienophiles, the Diels-Alder reaction might occur through a synchronous concerted, an asynchronous concerted, or a stepwise reaction mechanism. The stepwise and asynchronous concerted Diels-Alder reactions proceed via diradical intermediates, whereas the synchronous concerted mechanisms does not. It should be pointed out that most of the Diels-Alder reactions are concerted as a result, both the rate constants and the stereoselectivities of Diels-Alder reaction are only moderately sensitive to the changes in the nature of organic sol vents. However, it has been clearly shown that the applications of water to the reaction system can greatly accelerate such reactions. Other modifications on this reactions include the application of high pressure, Lewis acid, and ultrasound radiation.More information about this reaction can be easily attained from reviews and relevant books. 

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