Photochemical Unmasking of Polyyne Rotaxanes

Bulky photolabile masked alkyne equivalents
(MAEs) are needed for
the synthesis of polyyne polyrotaxanes, as insulated molecular wires
and as stabilized forms of the linear polymeric allotrope of carbon,
carbyne. We have synthesized a novel MAE based on phenanthrene and
compared it with an indane-based MAE. Photochemical unmasking of model
compounds was studied at different wavelengths (250 and 350 nm), and
key products were identified by NMR spectroscopy and X-ray crystallography.
UV irradiation at 250 nm leads to unmasking of both MAEs. Irradiation
of the phenanthrene system at 350 nm results in quantitative dimerization
via [2 + 2] cycloaddition to form a [3]-ladderane; irradiation of
this ladderane at 250 nm generates a dihydrotriphenylene, which can
be oxidized easily to a triphenylene. Irradiation of the indane-based
MAE at 350 nm in the presence of traces of oxygen forms an endoperoxide
and a bisepoxide. Both MAEs have been incorporated into rotaxanes
via copper-mediated active metal template Glaser or Cadiot–Chodkiewicz
coupling. The identity of the rotaxanes was confirmed by NMR spectroscopy
and mass spectrometry. The phenanthrene rotaxane decomposes during
attempted photochemical unmasking, whereas photolysis of the indane
rotaxane results in unmasking of the polyyne thread to form a rotaxane
with a chain of 16 sp-hybridized carbon atoms. This
approach opens avenues toward the synthesis of encapsulated carbon
allotropes.