Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cover Picture (Angew. Chem. Int. Ed. Engl. 3/1994)
The cover picture shows the topology of the transition state of a 1,3-dipolar cycloaddition and of the addition of methylene to bicyclobutane. Although the reactions appear very different at first glance, they both profit from the same type of aromatic stabilization in the transition state. The topology of the carbene reaction can be derived from that of the pericyclic reaction by constriction. Alongside pericyclic and polar reactions (SN2,E2, etc.), reactions with coarctate transition states form a third, hitherto never consistently characterized class of concerted reactions in organic chemistry, which includes several hundred addition, rearrangement, and elimination reactions. Theoretical insight into these reactions allows the prediction of mechanisms and stereochemistry as well as the rational design of new representatives of this class of reactions. R. Herges reports more on this subject on p. 255ff.