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Pericyclic Reactions: Introduction

Pericyclic Reactions: Introduction

For a long time the assumption was that all chemical reactions proceeded either through cationic, anionic or radical intermediates. However, over time, more and more cases were observed in which the experimental data were not compatible with this supposition. For example, little or no dependency on solvents or pH was found as would have been expected of polar reactions. Furthermore, no radical intermediates could be trapped. These reactions, therefore, were called "no-mechanism" reactions.

Furthermore, the high stereoselectivity of many reactions involving C-C bond formations was remarkable. For instance, in connection with his work on the synthesis of steroids, R. B. Woodward observed in a ring closure reaction the formation of either one or the other of the stereoisomers, depending whether the reaction was thermally or photochemically induced. Using an approach based on quantum mechanics, R. Hoffmann, a young theoretician in Woodward's research group, found a general solution to the problem. K. Fukui arrived at the same results using another approach. Woodward and Fukui shared the Nobel price for Chemistry in 1965 for their work. The prediction of stereochemistry of pericyclic reactions is known as Woodward-Hoffmann rules. Besides the theoretical significance as fundamental reaction mechanism, pericyclic reactions are also extremely important in organic synthesis. As a rule, these reactions are stereoselective and well suited for C-C bond formations (construction of carbon frameworks). The Diels-Alder reaction, for example, simultaneously generates 2 C-C bonds and up to 4 stereocenters.

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