Synthesis of Well-Defined High-Valent Palladium Complexes by Oxidation of Their Palladium(II) Precursors

The last decade has witnessed the rapid development of high-valent Pd-involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one-electron and two-electron oxidants have been employed to give access to high-oxidation-state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd-mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined Pd^III/IV complexes starting from their Pd^II precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O₂ ; 7) sulfonyl chlorides; and 8) others are covered. A "greener" electrooxidation manner has also been reviewed.

Palladium-Catalyzed C(sp3 )-H Arylation of Primary Amines Using a Catalytic Alkyl Acetal to Form a Transient Directing Group

C-H Functionalization of amines is a prominent challenge due to the strong complexation of amines to transition metal catalysts, and therefore typically requires derivatization at nitrogen with a directing group. Transient directing groups (TDGs) permit C-H functionalization in a single operation, without needing these additional steps for directing group installation and removal. Here we report a palladium catalyzed γ-C-H arylation of amines using catalytic amounts of alkyl acetals as transient activators (e.g. commercially available (2,2-dimethoxyethoxy)benzene). This simple additive enables arylation of amines with a wide range of aryl iodides. Key structural features of the novel TDG are examined, demonstrating an important role for the masked carbonyl and ether functionalities. Detailed kinetic (RPKA) and mechanistic investigations determine the order in all reagents, and identify cyclopalladation as the turnover limiting step. Finally, the discovery of an unprecedented off-cycle free-amine directed ϵ-cyclopalladation of the arylation product is reported.

N-Heterocyclic Carbene Ligand-Enabled C(sp(3))-H Arylation of Piperidine and Tetrahydropyran Derivatives

Pd(II)-catalyzed C(sp(3))-H arylation of saturated heterocycles with a wide range of aryl iodides is enabled by an N-heterocyclic carbene (NHC) ligand. A C(sp(3))-H insertion step by the Pd(II)/NHC complex in the absence of ArI is demonstrated experimentally for the first time. Experimental data suggests that the previously established NHC-mediated Pd(0)/Pd(II) catalytic manifold does not operate in this reaction. This transformation provides a new approach for diversifying pharmaceutically relevant piperidine and tetrahydropyran ring systems.

Synthetic studies towards putative yuremamine using an iterative C(sp3)–H arylation strategy

An overview of synthetic efforts toward the initially proposed structure of yuremamine using iterative C(sp³)–H arylations is described.