Applications of tropylium ions in organic chemistry

Cycloheptatrienium (tropylium) is one of three primary members of the non-benzenoid carbocyclic aromatic ion family. Tropylium has a planar cyclic structure with (4n+ 2)π electrons in a fully conjugated system. It fulfils Huckel’s rule for aromaticity and hence possess extraordinary stability. Since the historic discovery of tropylium bromide in the late 19th century, these non-benzenoid aromatic ions have attracted a lot of attention because of their unique combination of stability and reactivity. This thesis has six chapters in total that attempt to delve deeper into understanding what the tropylium ion is capable of, and has two main themes: (i) using the tropylium ion and related species as catalysts for the promotion of synthetic reactions and (ii) incorporating the tropylium moiety as a building block into new structural motifs for the development of advanced functional materials. The first Chapter is a literature review that highlights the recent developments in utilising nonbenzenoid aromatic ions for the promotion of organic reactions and discusses the potential of aromatic ions in promoting synthetically useful organic transformations. In Chapter 2, we demonstrate for the first time that tropylium salts can act as organic Lewis acid catalysts to facilitate acetalization and transacetalization reactions of a wide range of aldehyde substrates. Chapter 3 is the exploration of acyl transfer reactions utilising tropolone salt, a tropyliumrelated species, as a catalyst. Chapter 4 explores the chemical versatility and unique structural properties of the tropylium ion to derive a family of novel push–pull organic dyes. Chapter 5 focuses on the development of a conceptually new method for the formation of biaryl motifs, which was carried out through the oxidative ring contraction of the tropylium ion with mild oxidants