Direct ortho‐Hydroxylation of 2‐Phenylpyridines using Palladium(II) Chloride and Hydrogen Peroxide

Abstract

Direct functionalization of the ubiquitous C À Hb ondi sr eceiving much attentionb ecause complex structures can be formed from simplep recursors.T his paperr eportsauseful method for the direct hydroxylation of 2-phenylpyridines using palladium(II) chloride anda queous hydrogenp eroxide.I nt his method, hydrogenp eroxide,w hich has high atom efficiency,i se mployed as the oxidant and phenol derivatives are generated via C À Ha ctivation.

Synthesis of Phenolic Compounds via Palladium Catalyzed C−H Functionalization of Arenes

“…Accordingly, Itoh et al. demonstrated as method for the direct hydroxylation of 2‐arylpyridine using hydrogen peroxide (H 2 O 2 ) as an oxidant and PdCl 2 as catalyst (Figure ) . However, it was observed that the substrates bearing heteroaromatic ring as well as substituents at meta or ortho position of arenes resulted in the desired hydroxylated product with a much lower yield.…”

“…Accordingly, Itoh et al demonstrateda sm ethodf or the direct hydroxylation of 2-arylpyridine using hydrogen peroxide (H 2 O 2 )a sa no xidanta nd PdCl 2 as catalyst( Figure 31). [21] However,i tw as observed that the substrates bearingh eteroaromatic ring as well as substituents at meta or ortho positiono f arenes resulted in the desired hydroxylated product with a much lower yield. It has been reportedt hat the use of terephthalic acid as OH-radical scavenger did not hamper the reaction outcome.A ccordingly,t he authors discarded the possibility of the formation of hydroxyl radical in this hydroxylation protocol.…”

Transition-metal-catalyzed synthesis of phenols and aryl thiols

“…In 2015, Itoh and co-workers employed PdCl 2 as catalyst and the hydroxylation of arylpyridines was carried out in 4-methyl-2-pentanone at 100 °C in the presence of H 2 O 2 (Scheme 32) [61]. In terms of reactivity, substituent at the para -position of the phenyl ring is more favored than at other positions.…”

Microwave‐Assisted and Base‐Promoted Domino Reaction of Cyclic N‐Sulfonyl Ketimines with α,β‐Disubstituted Nitroalkenes: A Green Access to 2‐Hydroxyarylpyridines

“…On the other hand, the synthesis of functionalized 2‐pyridylphenol derivatives is rapidly growing interest in recent times due to their valuable applications in light‐emitting materials and bioactive molecules . In this context, the direct C−H hydroxylation of 2‐arylpyridines catalyzed by several expensive transition‐metal‐salts (e. g. Pd(II),– Rh(III),– Ru(II) etc) has been greatly followed for preparing those compounds. However, this process requires high temperature, a large amount of oxidants (K 2 S 2 O 8 , TBHP, H 2 O 2 , oxone, NHPI, PIDA etc) and longer reaction time.…”