Pericyclic Reactions in Water. Towards Green Chemistry

Abstract

Abstract This chapter provides an overview of the recent progress in performing synthetically important pericyclic reactions in (and “on”) water. In the past, these reactions were almost exclusively carried out in organic solvents to avoid problems arising from the limited solubility of the reagents in aqueous media. In the early eighties of last century, however, it was found that, rather unexpectedly, many types of pericyclic transformations possess greatly enhanced rates and often more favorable stereochemistry in water. Detailed mechanistic studies have now revealed how these advantages of aqueous reaction media depend on the type of the reaction and on the structural features of the initial state (IS) and activated complex (AC) of the various cycloadditions. This work is reviewed in some detail. Enforced hydrophobic interactions of the IS as well as hydrogen‐bond stabilization of the AC, characteristic for water as the reaction medium, were, in most cases, identified as major reasons for the observed kinetic and stereochemical phenomena. More recently it was found that there are also good possibilities for catalyzing pericyclic reactions in water. Particularly enantioselective Lewis acid‐catalyzed Diels‐Alder reactions have been investigated in detail. Green chemistry greatly benefits from these developments since water is the ultimate green solvent for enviromentally friendly chemical transformations. The future for aqueous organic chemistry looks bright.