Palladium catalyzed oxidative aminations and oxylations: where are we?

Abstract

International audienceSelective product formation in the Pd-catalyzed cyclization of unsaturated amide and carboxylic acid derivatives is an intriguing and challenging task. We recently discovered that the oxidative intramo-lecular Pd(II)-catalyzed amination or oxylation of unsaturated N-sulfonyl carbamates, N-sulfonyl carbox-amides and carboxylic acids takes place through the involvement of cyclic (usually, 5-or 6-membered) aminopalladated (AmPIs) or oxypalladated (OxPI) intermediates. Such cyclic intermediates can undergo a variety of transformations such as distocyclic β-H elimination, oxidative acetoxylation or intramolecular car-bopalladation, depending upon the substrate and/or the reaction conditions. In the absence of appropriate reaction pathways, the cyclic nucleopalladated intermediates (NuPIs) simply engage in an inconsequential equilibrium with the initial substrate and other transformations occur such as allylic C–H activation or, in the particular case of allyl carbamates, [3,3]-sigmatropic rearrangement

Pd(II)-Catalyzed Bidentate Directing Group-Aided Chemoselective Acetoxylation of Remote ε-C(sp2)–H Bonds in Heteroaryl–Aryl-Based Biaryl Systems

“…Transition metal-catalyzed sp 2 /sp 3 C–H activation/functionalization is one of the remarkable synthetic transformations in organic synthesis. − While the directing group-free C–H activation/functionalization is well documented, the directing group-aided C–H activation/functionalization has become a powerful synthetic strategy for accomplishing the site-selectivity. − Especially, the use of the bidentate directing groups (BDGs) has offered a new zeal for achieving the site-selective C–H functionalization (e.g., arylation, alkylation, and acetoxylation) of organic molecules. − The 8-aminoquinoline (8AQ)-type BDGs have preferentially assisted the functionalization of sp 2 /sp 3 β-C–H bonds of carboxylic acid substrates. − The picolinamide (PA)-type BDGs have assisted the functionalization of sp 2 /sp 3 γ- and δ-C–H bonds of amine systems. − The BDG-aided functionalization of sp 2 /sp 3 β- and γ-C–H bonds of appropriate carboxamide systems was well documented, − and there also have been some outstanding efforts on the BDG-aided functionalization of remote sp 2 /sp 3 δ- and ε-C–H bonds of appropriate carboxamide systems. ,− In particular, to the best of our knowledge, there exist only rare reports dealing on the BDG-aided functionalization of remote sp 2 /sp 3 ε-C–H bonds …”

“…While the Pd­(II)-catalyzed C–H activation strategy has been well explored for the construction of C–C bonds, the Pd­(II)-catalyzed, PhI­(OAc) 2 -promoted C–H acetoxylation/oxygenation tactic comprising the conversion of a C–H bond into a C–O bond has also received substantial attention. ,− In particular, the acetoxylation of sp 2 C–H bonds of arenes is considered a direct and efficient method for synthesizing phenolic compounds, which are important substances in industry and academic research …”

“…The BDG-aided acetoxylation of sp 2 /sp 3 β- and γ-C–H bonds of appropriate carboxamide systems was well documented. ,− A literature survey ,, revealed that the attempts on the Pd­(II)-catalyzed, BDG-aided functionalization of remote δ- or ε-C­(sp 2 )–H bonds with PhI­(OAc) 2 generally gave the cyclized products (e.g., 1b , 1f , 1h , and 1i ). The BDG-aided chemoselective acetoxylation of the remote ε-C­(sp 2 )–H bond over cyclization has not explored well. − , Accordingly, obtaining control on the acetoxylation/substitution over cyclization and chemoselectivity in the BDG-aided functionalization of a remote δ- or ε-C–H bond of a suitable substrate is considered to be a challenging task. With regard to the available reports dealing on the Pd­(II)-catalyzed, PhI­(OAc) 2 -promoted activation/functionalization of a remote ε-C­(sp 2 )–H bond, the Daugulis, Chen, and Zhao groups have independently revealed that the reactions of compounds 1g and 1e exclusively gave the corresponding cyclized products (Scheme ).…”

“…Taking an impetus from the enduring developments on the site-selective acetoxylation of C–H bonds, ,− we envisaged to study the prevailing subject comprising dominant cyclization over acetoxylation/substitution in the Pd­(II)-catalyzed, bidentate ligand picolinamide (PA)-aided functionalization of the ε-C­(sp 2 )–H bond using an appropriate substrate. Accordingly, herein, we report our successful attempt on the Pd­(II)-catalyzed, bidentate directing group-aided chemoselective acetoxylation of the remote ε-C­(sp 2 )–H bond using the heteroaryl–aryl-based biaryl system 1m (Scheme ).…”

Peracetic Acid: An Atom-Economical Reagent for Pd-Catalyzed Acetoxylation of C–H Bonds

“…Acylalkoxylation of a C–H bond of an aromatic ring (Scheme ) is one of the earliest examples of Pd-catalyzed C–H activation reactions and is also one of the best exemplified. − While reports of cheaper metal catalysts such as Cu, , Ru, , and even Co are known, Pd­(II) catalysts, specifically Pd­(OAc) 2 , remain the most versatile, as it can be directly employed without ligands in the acetoxylation of a myriad of substrates. One of the major issues, however, is the limited choice of effective oxidants.…”