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Kinetic Control in the Synthesis of a Möbius Tris((ethynyl)[5]helicene) Macrocycle Using Alkyne Metathesis
Version 2 2020-12-29, 12:35
Version 1 2020-03-31, 01:29
journal contribution
posted on 2020-12-29, 12:35 authored by Xing Jiang, Joshua D. Laffoon, Dandan Chen, Salvador Pérez-Estrada, Andrew S. Danis, Joaquín Rodríguez-López, Miguel A. Garcia-Garibay, Jun Zhu, Jeffrey S. MooreThe 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.
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Keywords
helicene subunitscyclic voltammetryComputational studiesMMP enantiomershelicene groupsMALDI-MSAlkyne Metathesisalkyne metathesis cyclooligomerizationselectivityX-ray diffractionchiral HPLCalkyne metathesisMomacrocyclic structuresACIDEDDBsynthesisfluorescence spectroscopyMMMUVKinetic ControlPPMPPPNMR spectroscopyactivation barriersDFT calculationstrimeric productlow-entropy species
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