Dipalladium(I) complexes of ortho- and para-functionalized 1,3-bis(aryl)triazenide ligands: Synthesis, structure and catalytic activity

Transannular Functionalization of Multiple C(sp3)-H Bonds of Tropane via an Alkene-Bridged Palladium(I) Dimer

This communication describes the Pd-catalyzed C(sp3)-H functionalization of a tropane derivative to generate products with functionalization at two (β/γ) or three (β/γ/β) different sites on the alicyclic amine core. These reactions proceed via an initial dehydrogenation to generate an alkene product that can react further to form a Pd(I) alkene-bridged dimer. Functionalization of this dimer affords β/γ/β-functionalized allylic arylation and allylic acetoxylation products.

Changes in ligand coordination mode induce bimetallic C-C coupling pathways

Carbon-carbon coupling is one of the most powerful tools in the organic synthesis arsenal. Known methodologies primarily exploit monometallic Pd⁰/Pd^II catalytic mechanisms to give new C-C bonds. Bimetallic C-C coupling mechanisms that involve a Pd^I/Pd^II redox cycle, remain underexplored. Thus, a detailed mechnaistic understanding is imperative for the development of new bimetallic catalysts. Previously, a Pd^II-Me dimer (1) supported by L1, which has phosphine and 1-azaallyl donor groups, underwent reductive elimination to give ethane, a Pd^I dimer, a Pd^II monometallic complex, and Pd black. Herein, a comprehensive experimental and computational study of the reactivity of 1 is presented, which reveals that the versatile coordination chemistry of L1 promotes bimetallic C-C bond formation. The phosphine 1-azaallyl ligand adopts various bridging modes to maintain the bimetallic structure throughout the C-C bond forming mechanism, which involves intramolecular methyl transfer and 1,1-reductive elimination from one of the palladium atoms. The minor byproduct, methane, likely forms through a monometallic intermediate that is sensitive to solvent C-H activation. Overall, the capacity of L1 to adopt different coordination modes promotes the bimetallic C-C coupling channel through pathways that are unattainable with statically-coordinated ligands.

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.