AROMATICITY IN MACROCYCLIC POLYPYRROLIC RING SYSTEMS

ABSTRACT The aromatic character of 18(annulene) is maintained in porphin, which is stabilized by the presence of the four bridging nitrogen atoms. Numerous variations on the porphin ring structure are described and the aromatic properties of the new ring systems are assessed by their spectral characteristics and by their chemical behaviour. Convincing experimental verification of the application of Hückel's rules to conjugated ring systems larger than benzene has been provided by Sondheimer1 in his elegant studies on annulenes. Many of these compounds are conformationally mobile as well as chemically unstable which tends to complicate the interpretations of physical data and severely limits the exploitation of chemical reactivity. In this lecture, I shall discuss the stabilization of annulenes by the use of bridging atoms and the consequent effects on the physical and chemical properties, particularly aromaticity, of the resulting ring systems. In the medium rings, especially the [10] an