Visible‐Light Photocatalytic Aerobic Benzylic C(sp³)−H Oxygenations with the ³DDQ*/tert‐Butyl Nitrite Co‐catalytic System

Abstract

Photocatalytic aerobic benzylic C(sp3)−H oxygenations of aromatic hydrocarbons and C3‐substituted indoles were studied by employing a co‐catalytic system of 3DDQ* (DDQ=2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone) and tert‐butyl nitrite. The superior efficiency of these reaction conditions was demonstrated by comparison with the analogous thermal protocol, and a range of substrates could be oxidized catalytically and selectively in good yields.

Metal‐Free Photocatalysts for C−H Bond Oxygenation Reactions with Oxygen as the Oxidant

“…This metal-free system proceeded under air and also oxygenated benzylic CÀHb onds of aromatic hydrocarbonsa nd C3substituted indoles efficiently.N otably,t his catalyst system was appliedt ot he late-stage functionalization of B1 receptor ligand and PDE 5i nhibitor tadalafil to obtain the corresponding carbonyl derivatives. [37] Later,Y adav and co-workers developed CBr 4 -catalyzedo xygenationo fm ethyl arenest oa ldehydes. They rationalized that N 16 O 18 Ow as generated by the oxidation of 18 O 2 (Scheme 7; step 1), and then the photoelectron transfer and subsequent Scheme6.Substrate scope and proposed mechanism for the oxygenation of benzylic sp 3 CÀHb onds to obtain keto derivatives.…”

Visible‐Light‐Mediated Oxygenation Reactions using Molecular Oxygen

“…[26] Recently, Brasholz et al reportedas imilar transformationu sing aD DQ/ TBN (2,3-dichloro-5,6-dicyano-1,4-benzoquinone/tert-butyl nitrite) system under air. [27] As for using ab enzyl radical in photochemical synthesis, Yadav et al and Liu et al reported the construction of b-keto sulfoxides 64 and a-acyloxyk etones 67 independently,i n2 014 and 2016. The former reactionw as initiated from the thiophenol 62 and the latter started with the oxidationo ft he alkene.…”

Catalytic Aerobic Oxidation of C(sp3)−H Bonds

“…[129] However, there has been am ajor shift towards visible light photocatalysis.T his term might be somewhat misleading,a sthe photons themselves are not catalytic, but are used to bring ac atalyst to an excited state (called photocatalyst), which in turn activates the target substrate.Catalysts,either organic [130] or metal based, [131] are designed to absorb at wavelengths in the visible range of the spectrum. [133] In the latter case acarbon-centered radical is formed which can react with as econd molecule of O 2 or with superoxide,o btained in the catalyst reoxidation, to generate oxygenated products.While some photocatalysts can directly oxidize as ubstrate by an electron transfer, sometimes secondary catalytic mediators such as NHPI are used which will, after activation, abstract ah ydrogen atom from the substrate and can subsequently be reoxidized by the photocatalyst. Thec atalyst is promoted to an excited state by ap hoton and oxidizes asubstrate by removing an electron, creating aradical cation.…”