Iridium-Catalyzed Hydrogenation of a Phenoxy Radical to the Phenol: Overcoming Catalyst Deactivation with Visible Light Irradiation

Abstract

Piano-stool iridium hydride complexes bearing phenylpyridine ligands are effective precatalysts for promoting the formation of element-hydrogen bonds using H 2 as the stoichiometric H-atom source. Irradiation with blue light resulted in a profound enhancement of catalyst turnover for the iridium-catalyzed hydrogenation of the aryloxyl radical 2,4,6-t Bu 3 -C 6 H 2 O • to the corresponding phenol. Monitoring the progress of the reaction revealed the formation of an iridium 3,3-dimethyl-2,3dihydrobenzofuranyl compound arising from two C−H activation events following the proton-coupled electron transfer (PCET) step. Under thermal conditions, this compound was inactive for catalytic aryloxide hydrogenation, representing a deactivation pathway. Irradiation with blue light under H 2 released the free heterocycle and regenerated the piano-stool iridium hydride precatalyst, establishing a pathway for catalyst recovery and overall enhanced turnover.

Amination of Phenols and Halophenols via Pyridinium–Iridium Dual Photocatalysis

“…The amination of 2-methoxyhydroquinone ( 1h ) resulted in a mixture of 3h (31%) and 3ha (53%) with a 84% yield overall. We hypothesize that a 1,5-hydrogen atom transfer (hydrogen atom transfer) with the phenoxyl radical and adjacent methoxy group resulted in the ketal cyclization product 3ha . Amination of a phenolic analogue of the selective serotonin reuptake inhibitor fluoxetine ( 1j ) proceeded in a 43% yield after only 4 h. Electron-neutral phenols were found to be largely unreactive to the C–H amination, potentially due to a lack of sufficiently electrophilic sites in the radical cation (see Supporting Information for substrate limitations).…”