Studies into the Synthesis of Cyclacenes

Abstract

Cyclacene is a unique macrocyclic hydrocarbon composed of laterally-fused benzenoid rings that has proven to be a challenging synthetic target. It is recognized as the shortest zigzag carbon nanotube (CNT) and could serve as a well-defined model to study the electronic and photophysical properties. Even prior to the discovery of CNTs, organic, physical organic, and computational chemists debated the consequences of the conjugated macrocyclic structure on the electronic and physical properties of cyclacene. This curiosity further drives the desire for experimental studies which are only possible after the successful synthesis of this molecule. Previous synthetic attempts successfully constructed the macrocyclic framework through Diels-Alder reactions, but late stage dehydrogenation and or oxidation proved problematic. Computational studies predict that one of the major challenges at the late-stage of synthetic efforts is the possibility of an open-shell singlet ground state of cyclacene, which would give the molecule a highly reactive diradical character. Another synthetic challenge associated with forming this macrocycle is its high strain energy. With these obstacles in mind, we have proposed a strategy for the synthesis of cyclacenes that should overcome these challenges because: (1) the intermediates are of approximately equal strain as the final target; (2) conjugation is completed through extrusion of a gas; (3) it allows the incorporation of substituents on the zigzag edge to increase the persistence of the final a cyclacene ring. A more detailed examination of the proposed synthetic route as well as current progress towards the synthesis of a cyclacene molecule is detailed in this dissertation.