Reductive N−O bond cleavage has been widely explored for providing either N or O radical species for various coupling processes. Despite significant advances, this photoredox pathway is less appealing due to poor atom economy owing to the loss of one fragment during the transformation. In this regard, the homolytic N−O bond cleavage by an energy-transfer pathway to provide two key radicals would be highly desirable for overcoming the limitations of the use of one fragment. We report an exclusive energy-transfer approach for the development of a challenging radical−radical C(sp 3 )− N cross-coupling process by reactivity-tuning of the catalytic system. The homolytic N−O bond cleavage of oxime esters in the presence of an Ir complex produces acyloxy and iminyl radicals, which undergo decarboxylative cross-coupling to yield valuable imines (typically 0.25 mol % cat. and 1 h reaction time). Extensive photophysical and electrochemical measurements, as well as density functional theory studies, were carried out to probe the mechanism and the operation of a Dexter-type energy-transfer pathway was revealed. The choice of solvent (EtOAc) and reaction concentration were critical for achieving the selectivity and reactivity in this cross-coupling process. The synthetic utility of this method was explored by studying highly functionalized oxime esters, including derivatives of biologically active natural products and drug molecules. Furthermore, in situ transformations of the imine products into pharmaceutically important amines were also demonstrated, showcasing the utility of the imine products as valuable amine building blocks.

An unprecedented approach to the generation of an Ncentered 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 doublefunctionalized 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.

An unconventional approach for intermolecular direct C(sp 3 )−N radical coupling has been developed by photocatalytic C(sp 3 )−H activation of simple alkyl substrates using O-benzoyl oximes. The selective photocatalytic energytransfer-driven homolysis followed by decarboxylation generates the persistent iminyl radical and aryl radical, which would undergo an unprecedented intermolecular hydrogen atom abstraction from the alkyl substrate to provide the key C(sp 3 ) radical. Selective radical−radical C−N cross-coupling furnishes imines which are valuable amine building blocks.

Irradiation of N-enoxybenzotriazoles with blue LEDs in the presence of photosensitizers productively rendered two reactive radical intermediates, that is, benzotriazolyl- and α-carbonyl radicals. The formation of these radicals as discrete species was suggested in the cross-over experiment, and this N–O fragmentation initiated synthetically useful transformations, such as an atom-economical [1,3]-shift and group transfer radical addition, leading to biologically interesting benzotriazolyl derivatives. The facility of these transformations (<5 min) was rationalized by the triplet state energy transfer and the presence of a very long-lived radical chain (Φ = 210 and 131, respectively, for [1,3]-shift and carboamination), which was initiated by the addition of electrophilic benzotriazolyl radicals to the olefin moieties. The N–O homolysis was further characterized by electrochemical and photophysical studies, as well as density functional theory computation.

A new type of sp3-like N-centered radical has been generated by selective energy transfer catalysis. Upon photoexcitation, homolytic N–O bond cleavage of N-indolyl carbonate in the presence of an Ir complex produced N- and O-centered radicals. The high spin density at the C3 position of indole led to radical recombination with the O-centered radical, affording valuable 3-oxyindole derivatives without decarboxylation. Transformations of the desired products into various molecules were also demonstrated.