Precise Synthesis of Ester-Functionalized Cyclo[6]- and Cyclo[7]furans

Shape-persistent
conjugated macrocycles have attracted
interest
for their unique optoelectronic and self-assembly properties, but
the syntheses to obtain these structures can be laborious. In this
work, we describe the straightforward synthesis of a recently discovered
class of macrocycle, the cyclo[n]furan, using Suzuki–Miyaura
cross-coupling of a simple aromatic monomer. We demonstrate that the
combination of hexyl 2-bromo-5-(boronic acid pinacol ester)furan-3-carboxylate
with tris(dibenzylidene­acetone)dipalladium(0), tri-tert-butylphosphonium tetrafluoroborate and cesium fluoride leads to
cyclo[6]- and cyclo[7]furan esters in 45% yield (28% and 17%, respectively).
Crude 1H NMR spectroscopy revealed that total conversion
to macrocycles was 52 ± 6% over 3 runs, highlighting the robustness
of this protocol for cyclofuran synthesis. The oligomerizations are
rapid, and model compound studies suggest that a chain-growth mechanism
may be operative. The hexyl-substituted cyclo[n]furan
esters (n = 6 and 7) are separable via column chromatography.
The unique optical and electronic features for each cycle can be partially
explained by the size difference for the two systems, as well as the
increased conformational flexibility for the larger, ester-functionalized
cyclo[7]furan.