Kinetic Control in the Synthesis of a Möbius Tris((ethynyl)helicene) Macrocycle Using Alkyne Metathesis
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 from the alkyne metathesis of 2,13-bispropynylhelicene. MALDI-MS, NMR, and X-ray diffraction indicated a trimeric product of two-fold symmetry with PPM/MMP configurations in the helicene subunits. Alternatively, a three-fold symmetric, PPP/MMM structure was determined by DFT calculation 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 in activation barriers between the PPM/MMP vs. 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. The separation of PPM/MMP enantiomers was achieved via chiral HPLC at the analytical scale.