Shape-selective fluxional carbon cages Seminar

Dynamic covalent rearrangements of fluxional carbon cages, such as bullvalenes and barbaralanes, impart ‘shapeshifting’ molecular properties.1 This seminar will describe a barbaralanes that exhibit fluxional constitutions and fluxional sp3-C stereochemistry. Derivatives bearing two aryl groups (example below) interconvert dynamically between two constitutional isomers in solution, but resolve to single isomers upon crystallisation. Unexpectedly, the minor solution-phase isomers are resolved in two instances. Through dynamic NMR, crystallographic and DFT analyses, we show that the isomer observed in the solid state is not a direct consequence of the equilibrium distribution in solution or any specific noncovalent interactions. Rather, the dynamic preferential crystallisation is dictated by differences in molecular size and shape.2 By changing the substitution pattern of the barbaralanes, the fluxional rearrangement gives rise to rapidly interconverting mixture of enantiomers – formally inverting sp3-C stereocentres. The fluxional carbon stereocentres can be controlled by covalent modification or noncovalent bonding interactions.

1 A. N. Bismillah, B. M. Chapin, B. A. Hussein, P. R. McGonigal, Chem. Sci. in press. 2 A. N. Bismillah, J. Sturala, B. M. Chapin, D. Yufit, P. Hodgkinson, P. R. McGonigal, Chem. Sci. 2018, 9, 8631.