Pericyclic reactions, solvent effects

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

The elucidation of reaction mechanisms is a central topic in organic chemistry that led to many elegant studies emphasizing the interplay of theory and experiment as demonstrated, for example, by the seminal contributions of the Houk group to the understanding of the Diels-Alder and other pericyclic reactions.38 This reaction class is rather typical for the elucidation of reaction mechanisms. On the experimental side, the toolbox of solvent, substituent and isotope effect studies as well as stereochemical probes have been used extensively, while the reactants, products, intermediates and transition structures involved have been calculated at all feasible levels of theory. As a result, these reactions often serve as a success story in physical organic chemistry. 

Most pericyclic reactions, though of course not all, are little influenced by Coulombic forces for example, it is well known that the polarity of the solvent has little effect on the rate of Diels-Alder reactions. We can therefore expect that a major factor influencing reactivity will be the size of the frontier orbital interaction represented by the third term of equation 2-7, p. 27. This is why this chapter is much the largest in this book the most dramatic successes of frontier orbital theory have been the explanations it has given to an amazingly large number of observations in pericyclic chemistry. 

The cycloaddition reaction rate revealed activation parameters which are characteristic of a pericyclic reaction. The entropy of activation is large (ASt = -176.95 Kr1 mor1), and the enthalpy of activation is small (A//t = 30.3 kJ mol-1)- These data imply a highly ordered, early transition state with concerted bond making and breaking. Solvent polarity had only a small effect on the rate of the reaction, indicating little separation of charge in the transition state, consistent with a concerted mechanism. 

This is only found for reactions involving non-polar diradical intermediates and is not observed for polar zwitterionic intermediates. In the latter case the volume of activation of the zwitterionic intermediate can be more negative than that of a competing pericyclic reaction due to the solvent-dependent effect of electro-... 

Investigation of Solvent Effects on Pericyclic Reactions by Computer Simulations. 

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