Redox reactions, which take place after photon absorption, enable preparation and
functionalization of heterocyclic compounds under mild reaction conditions. Out of those, we
focused our attention on the transformations of pyrazole-based compounds.
Under visible light irradiation in the presence of eosin, a photoredox catalyst, and oxygen, an
oxidant, 1-substituted pyrazolidin-3-ones were successfully oxidized. So formed azomethine
imines are useful building blocks in the synthesis of heterocyclic compounds, as demonstrated
by a one-pot photocatalyzed oxidation and cycloaddition of ensuing dipoles with ynones. After
optimization of reaction conditions, a plethora of azomethine imines and
pyrazolo[1,2-a]pyrazoles were synthesized. 1-Arylpyrazolidin-3-ones were also successfully
oxidized to 1-arylpyrazol-3-ones. Aldehydes and ketones, which are formed from N1
substituent of azomethine imines, were identified as side products of pyrazolidin-3-one
oxidation.
Pyrazolo[1,2-a]pyrazoles are fluorescent compounds, which can absorb visible light. Some of
the optical properties of these bicyclic compounds were measured. Their instability in solutions
was for the first time attributed to exposure to visible light as pyrazole ring opening takes place.
With optimization of reaction conditions, 6 distinct transformations into substituted pyrazoles
were achieved. The selectivity of the reactions is dependent on the substituents on the
pyrazolo[1,2-a]pyrazole and the added reagents. The combined research proved crucial in the
use of pyrazolo[1,2-a]pyrazoles as fluorescent markers.
Attempts to sulfonate pyrazoles with photoredox catalyzed radical addition with sulfonyl
chlorides, which were synthesized under photoredox catalyzed conditions as well, were
unsuccessful. The research was concluded with an unexpected two-step formation of dialkenyl
sulfones with sequential radical additions between triflyl chloride and arylacetylenes. This
transformation has not yet been explored despite already investigated reactions involving
alkenes.
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