Photoinduced Specific Acylation of Phenolic Hydroxy Groups with Aldehydes

Abstract

A convenient method is reported to specifically acylate phenolic hydroxyl groups through a radical pathway. When a mixture of an aldehyde and a phenol in ethyl acetate is irradiated with blue light in the presence of iridium and nickel bromide catalysts at ambient temperature, phenoxyl and acyl radicals are transiently generated in situ and cross‐couple to furnish an ester. Aliphatic hydroxy groups remain untouched under the reaction conditions.

Asymmetric benzylic C(sp3)−H acylation via dual nickel and photoredox catalysis

“…It has been reported that single-electron oxidation of bromide anion by photoexcited photocatalyst can generate bromine radical ( E 1/2 [Ir(III*/II)] = +1.21 V vs SCE in CH 3 CN; E 1/2 ox [Br − /Br·] = +0.80 V vs SCE in DME) 51 – 54 . According to the literature precedent and our previous mechanistic experiments 38 , 51 – 54 , we hypothesize that the catalytic reaction is initiated by oxidative addition of Ni(0) catalyst I to an in situ-activated carboxylic acid to afford Ni(II) species II . Subsequent trapping of prochiral benzylic radicals generated from the bromine-radical-mediated HAT process provides Ni(III) complex III , which undergoes reductive elimination to yield the desired product and Ni(I) species IV .…”

A cross-dehydrogenative C(sp3)−H heteroarylation via photo-induced catalytic chlorine radical generation

“…Using chloride (Cl − ) in Cl• generation brings several benefits in organic syntheses because it is innocuous and abundant in diverse salt forms. Nevertheless, the unfavourable chloride-to-chlorine oxidation (E = 1.36 V vs NHE) 23 and untamed reactivity of Cl• compared with other halide analogues 17,18,[24][25][26] make chlorine radical-promoted alkylation rarely explored. In this endeavour, few strategies have been disclosed for the efficient usage of Cl•, including (a) the direct singleelectron transfer (SET) from Cl − to photocatalyst under photothermal conditions [27][28][29][30] ; (b) the ligand-to-metal charge transfer (LMCT), which has been employed for the coupling of alkanes and organohalides by metallophotoredox catalysis [31][32][33][34][35][36] ; (c) the photolysis of in situ generated Cl 2 via electrooxidation of HCl 37 ; (d) the bimolecular homolytic substitution (S H 2) between chloroborate and an oxy radical for the alkane borylation 38 .…”

Visible-Light-Driven Dehydrogenative Coupling of Primary Alcohols with Phenols Forming Aryl Carboxylates

“…The generation of dihydrogen gas was confirmed by a gas chromatography (GC) analysis of the headspace of the reaction vessel. We recently reported a photoinduced selective acylation reaction of phenols with aldehydes . We next examined whether a similar acylation reaction took place when a primary alcohol was dehydrogenated in the presence of phenol.…”

“…Chromatographic isolation with silica gel afforded phenyl octanoate 6 in 64% yield. The production of 6 suggested that octanal was generated from 4 through dehydrogenation and that the generated octanal underwent dehydrogenative cross-coupling with 5 . The effect of the substituents of phenols was examined in the reaction of equimolar amounts of 4 and a phenol derivative (Figure ).…”