Self-Assembled Benzophenone Bis-urea Macrocycles Facilitate Selective Oxidations by Singlet Oxygen

This
manuscript investigates how incorporation of benzophenone,
a well-known triplet sensitizer, within a bis-urea macrocycle, which
self-assembles into a columnar host, influences its photophysical
properties and affects the reactivity of bound guest molecules. We
further report the generation of a remarkably stable organic radical.
As expected, UV irradiation of the host suspended in oxygenated solvents
efficiently generates singlet oxygen similar to the parent benzophenone.
In addition, this host can bind guests such as 2-methyl-2-butene and
cumene to form stable solid host–guest complexes. Subsequent
UV irradiation of these complexes facilitated the selective oxidation
of 2-methyl-2-butene into the allylic alcohol, 3-methyl-2-buten-1-ol,
at 90% selectivity as well as the selective reaction of cumene to
the tertiary alcohol, α,α′-dimethyl benzyl alcohol,
at 63% selectivity. However, these products usually arise through
radical pathways and are not observed in the presence of benzophenone
in solution. In contrast, typical reactions with benzophenone result
in the formation of the reactive singlet oxygen that reacts with alkenes
to form endoperoxides, diooxetanes, or hydroperoxides, which are not
observed in our system. Our results suggest that the confinement,
the formation of a stable radical species, and the singlet oxygen
photoproduction are responsible for the selective oxidation processes.
A greater understanding of the mechanism of this selective oxidation
could lead to development of greener oxidants.