Pericyclic reactions, high pressure

An analogy can be drawn between pericyclic reactions in water and under high pressure. Water s internal pressure on hydrophobic substrates acts on the volume of activation of a reaction in the same way as an externally applied pressure does. Thus, the internal pressure of water influences the rates of pericyclic reactions in water in the same direction as external pressures. The use of salting-out salts will further increase the rate of pericyclic reactions. Recently, Kumar quantified the relationship between internal pressure and the rate of the aqueous Diels-Alder reaction. A linear relationship between the two was observed.5... 

The first chapter, High Pressure Synthesis of Heterocycles Related to Bioactive Molecules by Kiyoshi Matsumoto, presents a unique high-pressure synthetic methodology in heterocyclic chemistry. Basic principles and fruitful examples for pericyclic reactions, such as Diels-Alder reactions, 1,3-dipolar reactions, and also for ionic reactions, such as Sn and addition reactions, are discussed. The review will be of considerable interest to heterocyclic chemists and synthetic chemists. 

The rate of Diels-Alder reactions can be increased by the application of high pressure. An early example by Dauben and Kozikowski demonstrated that methyl 1-pentadienoate reacted with 126 to give 127 in 88% yield. 27 Applications of high-pressure techniques to organic synthesis are enumerated in an excellent review by Matsumoto 28 as well as in his monograph. 29 jiie second part of this review 29 deals with high-pressure pericyclic reactions, including Diels-Alder reactions. 

Fxirther evidence for a highly ordered transition state conies from the observation that entropies of activation (-125 to -170 J/moUK) are comparable to that of the Diels-Alder reaction or other pericyclic processes. Additionally, high pressure chemistry studied on Alder-Ene reactions with mesoxalate showed a large negative volume of activation (-36 to -48 mL/mol), similar to the Diels-Alder reaction. One would expect the biradical process to be about 10 mL/mol less than the pericyclic process. 

Pericyclic reactions are characterized by a large negative activation volume, and the reactivity and selectivity are influenced by high pressure. 

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. 

---