Kinetic Control in the Synthesis of a Möbius Tris((ethynyl)[5]helicene) Macrocycle Using Alkyne Metathesis

Abstract

The synthesis of conjugated Möbius molecules remains elusive since twisted and macrocyclic structures are low-entropy species sporting their own synthetic challenges. Here we report the synthesis of a Möbius macrocycle in 84% yield via alkyne metathesis of 2,13-bis(propynyl)[5]helicene. MALDI-MS, NMR spectroscopy, and X-ray diffraction indicated a trimeric product of twofold symmetry with PPM/MMP configurations in the helicene subunits. Alternatively, a threefold-symmetric PPP/MMM structure was determined by DFT calculations to be more thermodynamically stable, illustrating remarkable kinetic selectivity for this alkyne metathesis cyclooligomerization. Computational studies provided insight into the kinetic selectivity, demonstrating a difference of 15.4 kcal/mol between the activation barriers for the PPM/MMP and PPP/MMM diastereodetermining steps. Computational (ACID and EDDB) and experimental (UV-vis and fluorescence spectroscopy and cyclic voltammetry) studies revealed weak conjugation between the alkyne and adjacent helicene groups as well as the lack of significant global aromaticity. Separation of the PPM and MMP enantiomers was achieved via chiral HPLC at the analytical scale.