Zweikernige Palladiumkomplexe - Vorstufen oder Katalysatoren?

Catalysis with Palladium(I) Dimers

Dinuclear PdI complexes have found widespread applications as diverse catalysts for a multitude of transformations. Initially their ability to function as pre-catalysts for low-coordinated Pd0 species was harnessed in cross-coupling. Such PdI dimers are inherently labile and relatively sensitive to oxygen. In recent years, more stable dinuclear PdI -PdI frameworks, which feature bench-stability and robustness towards nucleophiles as well as recoverability in reactions, were explored and shown to trigger privileged reactivities via dinuclear catalysis. This includes the predictable and substrate-independent, selective C-C and C-heteroatom bond formations of poly(pseudo)halogenated arenes as well as couplings of arenes with relatively weak nucleophiles, which would not engage in Pd0 /PdII catalysis. This Minireview highlights the use of dinuclear PdI  complexes as both pre-catalysts for the formation of highly active Pd0 and PdII -H species as well as direct dinuclear catalysts. Focus is set on the mechanistic intricacies, the speciation and the impacts on reactivity.

A Next-Generation Air-Stable Palladium(I) Dimer Enables Olefin Migration and Selective C-C Coupling in Air

We report a new air-stable PdI dimer, [Pd(μ-I)(PCy2 t Bu)]2 , which triggers E-selective olefin migration to enamides and styrene derivatives in the presence of multiple functional groups and with complete tolerance of air. The same dimer also triggers extremely rapid C-C coupling (alkylation and arylation) at room temperature in a modular and triply selective fashion of aromatic C-Br, C-OTf/OFs, and C-Cl bonds in poly(pseudo)halogenated arenes, displaying superior activity over previous PdI dimer generations for substrates that bear substituents ortho to C-OTf.

Modular Functionalization of Arenes in a Triply Selective Sequence: Rapid C(sp2 ) and C(sp3 ) Coupling of C-Br, C-OTf, and C-Cl Bonds Enabled by a Single Palladium(I) Dimer

Full control over multiple competing coupling sites would enable straightforward access to densely functionalized compound libraries. Historically, the site selection in Pd0 -catalyzed functionalizations of poly(pseudo)halogenated arenes has been unpredictable, being dependent on the employed catalyst, the reaction conditions, and the substrate itself. Building on our previous report of C-Br-selective functionalization in the presence of C-OTf and C-Cl bonds, we herein complete the sequence and demonstrate the first general arylations and alkylations of C-OTf bonds (in <10 min), followed by functionalization of the C-Cl site (in <25 min), at room temperature using the same air- and moisture-stable PdI dimer. This allowed the realization of the first general and triply selective sequential C-C coupling (in 2D and 3D space) of C-Br followed by C-OTf and then C-Cl bonds.

Site-Selective C-S Bond Formation at C-Br over C-OTf and C-Cl Enabled by an Air-Stable, Easily Recoverable, and Recyclable Palladium(I) Catalyst

This report widens the repertoire of emerging PdI catalysis to carbon-heteroatom, that is, C-S bond formation. While Pd0 -catalyzed protocols may suffer from the formation of poisonous sulfide-bound off-cycle intermediates and lack of selectivity, the mechanistically diverse PdI catalysis concept circumvents these challenges and allows for C-S bond formation (S-aryl and S-alkyl) of a wide range of aryl halides. Site-selective thiolations of C-Br sites in the presence of C-Cl and C-OTf were achieved in a general and a priori predictable fashion. Computational, spectroscopic, X-ray, and reactivity data support dinuclear PdI catalysis to be operative. Contrary to air-sensitive Pd0 , the active PdI species was easily recovered in the open atmosphere and subjected to multiple rounds of recycling.

Defined Palladium-Phthalimidato Catalysts for Improved Oxidative Amination

New palladium(II)-phthalimidato complexes have been synthesized, isolated, and structurally characterized. As demonstrated from over 30 examples, they constitute superior catalysts for oxidative amination reactions of alkenes with phthalimide as the nitrogen source. This work streamlines vicinal difunctionalization of alkenes and provides access to significantly improved and experimentally simplified synthetic protocols.

Highly Efficient C-SeCF3 Coupling of Aryl Iodides Enabled by an Air-Stable Dinuclear Pd(I) Catalyst

Building on our recent disclosure of catalysis at dinuclear Pd(I) sites, we herein report the application of this concept to the realization of the first catalytic method to convert aryl iodides into the corresponding ArSeCF3 compounds. Highly efficient C-SeCF3 coupling of a range of aryl iodides was achieved, enabled by an air-, moisture-, and thermally stable dinuclear Pd(I) catalyst. The novel SeCF3 -bridged dinuclear Pd(I) complex 3 was isolated, studied for its catalytic competence and shown to be recoverable. Experimental and computational data are presented in support of dinuclear Pd(I) catalysis.

Modulation of benzene or naphthalene binding to palladium cluster sites by the backside-ligand effect

The backside-ligand modulation strategy to enhance the substrate binding property of Pd clusters is reported. The benzene or naphthalene binding ability of Pd3 or Pd4 clusters is enhanced significantly by the backside cyclooctatetraene ligand, leading to the formation of the first solution-stable benzene- or naphthalene Pd clusters. The present results imply that the ligand design of the metal clusters, especially for the backside ligand of the metal cluster site, is crucial to acquire a desired reactivity of metal clusters.