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

Abstract

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.

Reactivity control of a photocatalytic system by changing the light intensity

“…The ndings presented herein on controlled substrate activation through light-power-density regulation might well stimulate a broader application of this concept in the continuouslygrowing eld of photo(redox) catalysis. Possible further directions include the incorporation of our approach into cascadetype reactions, in which triplets 28,38,[60][61][62][63][64][65][66] or intermediate radicals 43,67-69 are trapped by further additives resulting in more Fig. 3 3 Irsppy (left) and e aq c À (right) induced reductive activation of 4-(chloromethyl)benzoic acid resulting in the hydrogen abstraction and the dimerization main product, respectively, together with rate constant determinations of the two substrate activation reactions.…”

Recent Advances in the Construction of Spiro Compounds via Radical Dearomatization

“…From this moment, synthetic chemists are constantly trying to design reasonable substrates or develop novel strategies in the field of intramolecular radical dearomative spirocyclization. To date, several groups have independently explored and established intramolecular radical dearomative spirocyclization with oxygen promotion or visible‐light irradiation (Scheme 24d–f) [40f–j] . In 2013, an efficient and green intramolecular oxidative coupling of phenol for the direct construction of 108 was developed by the group of Wang, in which utilizing environment‐friendly sodium nitrite as the catalyst and air oxygen as the terminal oxidant (Scheme 24e) [40f] .…”

“…In 2013, an efficient and green intramolecular oxidative coupling of phenol for the direct construction of 108 was developed by the group of Wang, in which utilizing environment‐friendly sodium nitrite as the catalyst and air oxygen as the terminal oxidant (Scheme 24e) [40f] . In 2019, another approach to the generation of an N‐centered radical and application in dearomatization via a photocatalytic energy‐transfer process from readily available heterocyclic precursors is reported (Scheme 24f) [40j] …”

Recent Advances in Photocatalytic C–N Bond Coupling Reactions

“…Cho, You and co-workers presented a unique double functionalization of arenes facilitated by singlet oxygen (Scheme 82). 85 Insertion of photoexcited singlet oxygen into the starting material would generate a hydroperoxyl species. Next, homolytic N-O bond cleavage would take place to generate an iminyl radical.…”