Direct Oxygenation of C–H Bonds through Photoredox and Palladium Catalysis

Abstract

This report presents the oxygenation of C–H bonds via the merger of photocatalysis and Pd catalysis. Herein, we describe the utilization of a photocatalyst to oxidize an organopalladium­(II) intermediate to high-valent PdIII or PdIV intermediates, which promotes the formation of C–O bonds. The demonstrated method works efficiently with various directing groups, such as oxime ether and benzothiazole. The applicability of this direct C–O bond formation method is shown by synthesizing several metal complexes of 2-(benzo­[d]­thiazol-2-yl)­phenol that can be used in organic light-emitting diodes and pharmaceuticals.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

“…Examples include the acetoxylation of oximes (Fig. 102b) 266 or the hydroxylation of arenes (Fig. 102c and 102d).…”

“…104. 266 Oxidative quenching of the excited PC by oxygen generates the superoxide radical anion as well as the oxidised PC. Cyclopalladation of the oxime or arene generates the Pd(II) complex, which is oxidised by both the oxidised PC and the superoxide radical, resulting in a Pd(IV) species.…”

Recent Advances in Palladium-Catalyzed Oxidative Couplings in the Synthesis/Functionalization of Cyclic Scaffolds Using Molecular Oxygen as the Sole Oxidant

“…77 In 2020, Singh and co-workers reported a metalaphotoredox reaction involving oxygenation of the C-H bonds of aromatic rings under visible-light irradiation to construct C(sp 2 )-O and C(sp 3 )-O bonds, occurring through oxidation of the Pd(II) species to high-valent Pd(III) or Pd(IV) intermediates mediated by a photocatalyst and O 2 (Scheme 45). 78 Electron-donating and electron-withdrawing groups on the oxime moiety were explored, giving satisfactory results for monoacetoxylation, except for ortho-substituted substrates due to steric hindrance. A second acetoxylation reaction was observed only in trace quantities for the same steric effect.…”

Recent Advancements on Transition‐Metal‐Catalyzed, Chelation‐Induced ortho‐Hydroxylation of Arenes

“…has recently reported Pd(II)-catalyzed photocatalytic orthohydroxylation of 2-aryl benzothiazoles and 2-aryl benzoxazole where trifluoroacetate anion (TFA À ) has been utilized as the source of hydroxyl group. [56]…”

“…al . has recently reported Pd(II)‐catalyzed photocatalytic ortho ‐hydroxylation of 2‐aryl benzothiazoles and 2‐aryl benzoxazole where trifluoroacetate anion (TFA − ) has been utilized as the source of hydroxyl group [56] . A series of substituted 2‐(benzo[ d ]thiazol‐2‐yl)phenols and 2‐(benzo[ d ]oxazol‐2‐yl)phenols were synthesized in reasonable to good yields by exposing the mixtures of staring material (0.3 mmol), Pd(OAc) 2 (10 mol%), photocatalyst (2 mol%), CF 3 COOH (0.3 mol) and CF 3 COOK (3 eq.)…”