Generation of N‑Centered Radicals via a Photocatalytic Energy Transfer: Remote Double Functionalization of Arenes Facilitated by Singlet Oxygen

An unprecedented approach to the
generation of an N-centered radical
via a photocatalytic energy-transfer process from readily available
heterocyclic precursors is reported, which is distinctive of the previous
electron transfer approaches. In combination with singlet oxygen,
the in-situ-generated nitrogen radical from the oxadiazoline substrate
in the presence of fac-Ir­(ppy)3 undergoes
a selective ipso addition to arenes to furnish remotely double-functionalized
spiro-azalactam products. The mechanistic studies provide compelling
evidence that the catalytic cycle selects the energy-transfer pathway.
A concurrent activation of molecular oxygen to generate singlet oxygen
by energy transfer is also rationalized. Furthermore, the occurrence
of the electron transfer phenomenon is excluded on the basis of the
negative driving forces for one-electron transfer between oxadiazoline
and the excited state of fac-Ir­(ppy)3 with
a consideration of their redox potentials. The necessity of singlet
oxygen as well as the photoactivated oxadiazoline substrate is clearly
supported by a series of controlled experiments. Density functional
studies have also been carried out to support these observations.
The scope of substrates is explored by synthesizing diversely functionalized
cyclohexadienone moieties in view of their utility in complex organic
syntheses and as potential targets in pharmacology.