Catalysis with Palladium(I) Dimers

Abstract

Dinuclear Pd^I 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 Pd^I dimers are inherently labile and relatively sensitive to oxygen. In recent years, more stable dinuclear Pd^I -Pd^I 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 /Pd^II catalysis. This Minireview highlights the use of dinuclear Pd^I complexes as both pre-catalysts for the formation of highly active Pd⁰ and Pd^II -H species as well as direct dinuclear catalysts. Focus is set on the mechanistic intricacies, the speciation and the impacts on reactivity.

Keywords: catalysis; cross-coupling; dinuclear PdI; palladium; pre-catalyst.

Figure 1

Conceptual types of Pd^I dimers, the general synthetic approaches to single‐atom‐bridged Pd^I dimers, and catalytically relevant examples (with reference to their first preparation). All structures were previously verified by X‐ray crystallographic analysis.

Figure 2

Application of Pd^I dimers as pre‐catalysts in Pd‐catalyzed cross‐coupling.[ ^7b , ¹¹ , ¹² , ¹⁴ , ¹⁵ , ¹⁶ , ^18a , ^18c , ^18d , ^18e , ^18f ]

Figure 3

In situ liberation of catalytically active monoligated palladium from Pd⁰L₂ or Pd^I dimer as pre‐catalysts (top)[ ¹² , ²¹ ] and N‐scale quantification for dimer activation (bottom). ^[22] A suitable nucleophile is required for activation, which is present as a reagent or additive and should not function well as stabilizing bridge.

Figure 4

Generation of Pd^I dimer 1 under cross‐coupling conditions.[ ^9a , ²⁷ , ²⁸ , ²⁹ ]

Figure 5

Dinuclear Pd^I‐mediated halide exchange—experimental and computational support for dinuclear catalysis.[ ³¹ , ³² ]

Figure 6

First dinuclear Pd^I catalysis (top) and proposed mechanism (bottom).[ ³¹ , ³² ]

Figure 7

Application of dinuclear Pd^I catalysis in trifluoromethylthiolations and ‐selenolations: isolation of catalytically competent SCF₃‐ and SeCF₃‐bridged Pd^I dimers (top) and scope of the catalytic transformations (bottom).[ ^8a , ^33a ]

Figure 8

Heteroatom bond formation via dinuclear catalysis using thiolates or selenolates (top) ^[35] and phosphorothioates (bottom). ^[36]

Figure 9

C−C bond formation by dinuclear Pd^I catalysis: site‐selective arylations and alkylations (top),[ ²² , ²⁸ , ³⁹ , ⁴⁰ ] modular, sequential diversification (middle),[ ⁴¹ , ⁴² ] and polymerization (bottom). ^[43]

Figure 10

Isomerization (and metathesis) strategies with Pd^I dimer as pre‐catalyst.[ ⁴⁶ , ^48a , ^48c , ⁴⁹ ]

Figure 11

Isomerization with Pd^I dimer 3 as pre‐catalyst. ^[8c]