Fluxional Carbon Cages and their 'Shapeshifting' Properties

Abstract

Fluxional carbon cages1 such as bullvalene, the barbaralyl cation and barbaralane are molecules which possess ‘shapeshifting’ properties. These shapeshifting properties arise through rapid and reversible Cope rearrangements, which allow for every carbon atom to exchange with every other carbon atom giving rise to thousands of degenerate valence isomers. This rare phenomenon is crucial in the study of the fundamental concept of valence isomerisation and can manifest as a dynamic structural library arising from a single, rigid molecule.2
The possibility of accessing a large number of nondegenerate isomers from functionalised bullvalene derivatives has been utilised to design shapeshifting sensors,3 where the equilibrium distribution is shifted through specific noncovalent bonding interactions or dynamic covalent bonds when interacting with different guests. Applications of these molecules, however, are restricted by their lengthy and low-yielding syntheses as well as their structural complexity, which hinders a full understanding over equilibria and rearrangements.
This Thesis will discuss investigations into barbaralane structures – an attractive alternative fluxional scaffold which can be readily synthesised and exists as just two isomers in rapid flux at room temperature. New classes of tractable fluxional carbon cages have been explored with their rearrangements and equilibria being fully understood and controlled, in both the solution and solid state. These bistable nondegenerate systems are later elaborated into highly fluxional cations – the stability of which is tuned by substituent effects.
1. (a) W. von E. Doering and W. R. Roth, Tetrahedron, 1963, 19, 175; (b) P. Alhberg, D. L. Harris and S. Winstein, J. Am. Chem. Soc. 1993, 115, 7445; (c) J. G. Henkel and J. T. Hane, J. Org. Chem. 1983, 48, 3858.
2. P. R. McGonigal, C. de León, Y. H. Wang, A. Homs, C. R. Solorio-Alvardo and A. M. Echavarren, Angew. Chem. Int. Ed. 2012, 51, 13093.
3. (a) K. K. Larson, M. He, J. F. Teichert, A. Naganawana and J. W. Bode, Chem. Sci. 2012, 3, 1825; (b) J. F. Teichert, D. Mazunin and J. W. Bode, J. Am. Chem. Soc. 2013, 135, 1134.