Synergic Palladium Catalysis for Aerobic Oxidative Coupling

Abstract

Aerobic oxidative coupling is a dehydrogenative organic transformation in which molecular oxygen acts as a re-oxidant for the catalyst system. Pd has emerged as one of the most popular transition metal catalysts for such reactions. Combinations of Pd with other transition metal catalysts can also provide effective catalytic cycles together with highly atom-and step-economical organic transformations. A synergic Pd catalyst system can promote such reactions with a high degree of efficiency based on a re-oxidation step exhibiting improved kinetics in which two electrons are transferred from another transition metal. This occurs in conjunction with a lower energy barrier than that associated with direct re-oxidation by molecular oxygen. This review highlights recent developments in aerobic oxidative coupling reactions catalyzed by synergic combinations of Pd with other transition metals. Highlighted reactions include dehydrogenative CÀ C bond formation such as arylation of arenes, heteroarenes, alkenes and dehydrogenative CÀ N and CÀ O bond formations with unsaturated hydrocarbons such as alkenes, 1,3-dienes and allenes. Moreover, mechanistic insights into these aerobic reactions based on experimental, computational and optical studies are detailed.

Synthesis of Organic Optoelectronic Materials Using Direct C−H Functionalization

“…Because the coupling reactions between CÀ H/CÀ H bonds do not require pre-functionalization on either substrate, the CDC reaction is the ultimate short-process coupling reaction. [16][17][18] However, excess amounts of substrate are often required to achieve high reaction conversion and selectivity. Fagnou et al reported a pioneering CDC reaction between an indole derivative and benzene (Scheme 5).…”

“…Furthermore, cross-dehydrogenative cou-pling (CDC; i. e., CÀ H/CÀ H coupling) reactions do not require the introduction of halogen moieties or organometallic groups, which further shortens the synthesis process (Scheme 1c). [16][17][18] However, this reaction inevitably proceeds with an undesired homo-coupling as a side reaction because both reaction points are CÀ H bonds. Therefore, to establish the CDC reaction as a method for preparing organic optoelectronic materials, high cross-coupling selectivity is required, in addition to the regioselectivity mentioned above.…”

“…Because aromatic compounds have multiple C−H bonds, regioselectivity, in which the reaction proceeds only at the expected C−H bond, is important in direct arylations. Furthermore, cross‐dehydrogenative coupling (CDC; i. e., C−H/C−H coupling) reactions do not require the introduction of halogen moieties or organometallic groups, which further shortens the synthesis process (Scheme 1c) [16–18] . However, this reaction inevitably proceeds with an undesired homo‐coupling as a side reaction because both reaction points are C−H bonds.…”

Recent Progress in the Catalytic Synthesis of Pyrrolo[1,2‐α]quinoxaline

“…In recent years, many efficient synthetic strategies have been proposed by the tireless efforts of synthetic chemists, such as Transfer Hydrogenation, [8a–d] Oxidative coupling, [8e–f] C−H bond activation, [8h–i] Decarboxylative coupling [8j–l] and Domino cascade, [8m–n] many new synthetic methods have been developed based on these strategies, in which catalysts play an indispensable role. The presence of catalysts activates the above strategies in the construction of pyrrolo[1,2‐ α ]quinoxalines, which greatly expands the range of substrates for the construction of pyrrolo[1,2‐ α ]quinoxalines, shortens the synthesis steps, and makes the skeleton construction more efficient.…”

Pd-Catalyzed Oxidative Functionalization of Alkenes, Arenes, and 1,3-Dienes Using Molecular Oxygen as the Terminal Oxidant

“…1,2 Oxidative couplings involve cleavage of ubiquitous bonds such as C-H and heteroatom-H bonds in substrates. 3 This characteristic feature realizes more atom-and step-economical transformations than the other coupling reactions. 4 Oxidative couplings have been inspired by the Wacker process, which is the coupling of ethylene with water, and the process has been developed with a variety of substrates.…”