Long-Lived Fluxional Barbaralyl Cations

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The covalent bonds that hold atoms together to make molecules are, by and large, flexible linkages that are prone to stretching and bending. This flexibility imparts a degree of responsiveness to molecules' three-dimensional shape that is key in determining their physical properties and mediating their interactions with other molecules. For example, the molecular recognition of a pharmaceutical within the binding site of a protein can induce organisation in one, or even both, of the binding partners. A mutually favourable spatial arrangement (conformation) is stabilised.In certain cases, the structural dynamics of molecules can be extended beyond changes in their conformation to include changes in their constitution (i.e., changes in their atomic skeleton). Processes termed 'dynamic covalent' reactions rely upon chemical transformations that are reversible in nature, allowing specific covalent bonds to be broken and formed freely under the same set of conditions. In this way, molecular fragments come together and break apart over and over again, reversibly forming different compounds of varying shapes and sizes. This research project seeks to explore constitutional dynamics in a rather unique class of compounds. The 'barbaralyl cations' that will be developed are built around a core of nine carbon atoms in a cage-like arrangement. Each carbon atom is connected to two or three similar neighbours. As a result of this particular composition and the type of covalent bonds that make up the core, the carbon atoms are able to reversibly trade bonds with one another, switching positions with their neighbours. As one bond breaks, another is made and the outcome is that the barbaralyl cation core is 'fluxional'. When different groups are appended to this fluxional core, the molecule as a whole develops the property of being able to change its shape dramatically, not just in terms of its conformation, but also in terms of its constitution. It becomes a 'shapeshifting molecule'. Unlike the vast majority of dynamic covalent processes, this rearrangement occurs entirely within single molecules, meaning that the rate at which it occurs is not dependent on the presence of other reactants and their concentration. The linkages at the centre of the molecule interchange, causing the positions and orientations of the functional groups around the exterior to be switched and giving rise to hundreds (or even thousands!) of different structural permutations.Importantly, the shapeshifting molecules developed during this research will not require onerous preparation, but rather they will be made using relatively straightforward methods, overcoming one of the major impediments associated with the handful of related shapeshifting molecules that have been studied in the past. Ultimately, constitutional dynamics of this kind, occurring within a single molecule, will offer a new means through which to search for structures with mutually stabilising binding interactions with conceivably any target of interest. Applications, therefore, may lie in the improved design of molecular components for sensing medicinally relevant species.