Reactions in microdroplets
can be accelerated and can present unique
chemistry compared to reactions in bulk solution. Here, we report
the accelerated oxidation of aromatic sulfones to sulfonic acids in
microdroplets under ambient conditions without the addition of acid,
base, or catalyst. The experimental data suggest that the water radical
cation, (H2O)+•, derived from traces
of water in the solvent, is the oxidant. The substrate scope of the
reaction indicates the need for a strong electron-donating group (e.g., p-hydroxyl) in the aromatic ring. An analogous oxidation
is observed in an aromatic ketone with benzoic acid production. The
shared mechanism is suggested to involve field-assisted ionization
of water at the droplet/air interface, its reaction with the sulfone
(M) to form the radical cation adduct, (M + H2O)+•, followed by 1,2-aryl migration and C–O cleavage. A remarkably
high reaction rate acceleration (∼103) and regioselectivity
(∼100-fold) characterize the reaction.
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