Redox Reactions in Palladium Catalysis: On the Accelerating and/or Inhibiting Effects of Copper and Silver Salt Additives in Cross-Coupling Chemistry Involving Electron-rich Phosphine 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.

Dynamic PdII /CuI Multimetallic Assemblies as Molecular Models to Study Metal-Metal Cooperation in Sonogashira Coupling

Cooperation between two different metals plays a crucial role in many synergistic catalytic reactions, such as the Sonogashira C-C cross-coupling reaction, where an interaction between the Pd and Cu centers is proposed in the transmetalation step. Although several heterobimetallic Pd/Cu complexes were proposed as structural models of the active species in Sonogashira coupling, the detailed understanding of the metal-metal cooperation in transmetalation is still lacking in current systems. In this work, we report a stepwise and systematic approach to building heteromultimetallic Pd/Cu assemblies as a tool to study metal-metal cooperativity. We obtained fully characterized Pd/Cu multimetallic assemblies that show reactivity in alkyne activation, formation of catalytically relevant aryl/acetylide species, and C-C elimination, serving as functional models for Sonogashira reaction intermediates. The combined experimental and DFT studies highlight the importance of ligand-controlled coordination geometry, metal-metal distances and dynamics of the multimetallic assembly for transmetalation step.

Silver(I) Complexes of Two Flexible Bis-phospholane Ligands: Metallamacrocycles, Polymeric Chains, and Metallacryptands

In a 2:2 reaction with silver(I) chloride or bromide, 1,5-bis(1-phospholano)pentane (1a) afforded frame-like macrocyclic structures, with intra- (2, Cl) or intermolecular (3, Br) halido bridges. In contrast, 1,7-bis(1-phospholano)heptane (1b) formed coordination polymers 4a (Cl) and 4b (Br) with bridging bis-phospholane and halido ligands. A unique paddle wheel-type metallacryptand structure 5 was obtained from 1a and silver(I) bromide in a 2:3 reaction (M:L). All complexes were fully characterized by NMR, IR spectroscopy, mass spectrometry, and X-ray crystallography.

Designing Homogeneous Copper-Free Sonogashira Reaction through a Prism of Pd-Pd Transmetalation

Simultaneous introduction of two different palladium (pre)catalysts, one tuned to promote oxidative addition to (hetero)aryl bromide and another to activate terminal alkyne substrate, leads to productive Pd–Pd transmetalation, subsequent reductive elimination, and formation of disubstituted alkyne. This conceptually novel rational design of copper-free Sonogashira reaction enabled facile identification of the reaction conditions, suitable for the synthesis of alkyl, aryl, and heteroaryl substituted alkynes at room temperature with as low as 0.125 mol % total Pd loading.

Formation of Transient Anionic Metal Clusters in Palladium/Diene-Catalyzed Cross-Coupling Reactions

Despite their considerable practical value, palladium/1,3‐diene‐catalyzed cross‐coupling reactions between Grignard reagents RMgCl and alkyl halides AlkylX remain mechanistically poorly understood. Herein, we probe the intermediates formed in these reactions by a combination of electrospray‐ionization mass spectrometry, UV/Vis spectroscopy, and NMR spectroscopy. According to our results and in line with previous hypotheses, the first step of the catalytic cycle brings about transmetalation to afford organopalladate anions. These organopalladate anions apparently undergo S_N2‐type reactions with the AlkylX coupling partner. The resulting neutral complexes then release the cross‐coupling products by reductive elimination. In gas‐phase fragmentation experiments, the occurrence of reductive eliminations was observed for anionic analogues of the neutral complexes. Although the actual catalytic cycle is supposed to involve chiefly mononuclear palladium species, anionic palladium nanoclusters [Pd_nR(DE)_n]⁻, (n=2, 4, 6; DE=diene) were also observed. At short reaction times, the dinuclear complexes usually predominated, whereas at longer times the tetra‐ and hexanuclear clusters became relatively more abundant. In parallel, the formation of palladium black pointed to continued aggregation processes. Thus, the present study directly shows dynamic behavior of the palladium/diene catalyst system and degradation of the active catalyst with increasing reaction time.

Linear CuI2 Pd0 , CuI Pd02 , and AgI2 Pd0 Metal Chains Supported by Rigid N,N'-Diphosphanyl N-Heterocyclic Carbene Ligands and Metallophilic Interactions

Selective copper(I) to palladium(0) transmetallation of P-donors from the rigid N,N'-diphosphanyl-imidazol-2-ylidene C₃ H₂ [NP(tBu)₂ ]₂ (PC^NHC P) was observed when known [Cu₃ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₃ was reacted with [Pd(PPh₃ )₄ ]. When 1.2 equivalents of [Pd(PPh₃ )₄ ] was used, the product [Cu₂ Pd(μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (2(OTf)₂ ) was obtained, which features a Cu^I -Cu^I -Pd⁰ chain and appears to be the first linear heterotrinuclear complex with d¹⁰ -d¹⁰ interactions between Pd⁰ and Cu^I . When the Cu₃ precursor was reacted with 3.0 equivalents of [Pd(PPh₃ )₄ ], the complex [CuPd₂ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (3(OTf)₂ ) was obtained, which, on the basis of magnetic measurements, DFT calculations, and computed nuclear shieldings, was formulated as containing a Pd⁰ -Cu^I -Pd⁰ chain with an electron hole delocalized over the whole cation, including the metal chain. Similarly, selective transmetallation of the P-donors in [Ag₃ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₃ from silver to palladium (originating from [Pd(PPh₃ )₄ ]) gave the linear chain [Ag₂ Pd(μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (5(OTf)₂ ), which on the basis of NMR spectroscopy comprises an Ag^I -Ag^I -Pd⁰ metal core. However, X-ray diffraction data collected on various samples of 5(OTf)₂ were modeled with 50:50 metal disorder at the terminal positions, corresponding to a (Ag^I /Pd⁰ )-Ag^I -(Ag^I /Pd⁰ ) formulation. Upon standing in solution, 5(OTf)₂ transformed to 6(OTf)₂ , the regioisomer of 5(OTf)₂ in which the Pd center has migrated to the central position of an Ag^I -Pd⁰ -Ag^I chain. Prolonged standing in CH₂ Cl₂ or by reaction with [PtCl₂ (NCMe)₂ ] converts complex 6(OTf)₂ to the Ag^I /Pd^II complex [Ag₂ PdCl₂ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (7(OTf)₂ ). The structural data of 2(OTf)₂ , 3(OTf)₂ , and 7(OTf)₂ establish significant heterometallophilic interactions.

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.

Palladium(II)-catalyzed sequential C-H arylation/aerobic oxidative C-H amination: one-pot synthesis of benzimidazole-fused phenanthridines from 2-arylbenzimidazoles and aryl halides

Starting from 2-arylbenzimidazoles and aryl halides, an efficient palladium-based catalytic method for the synthesis of benzimidazole-fused phenanthridines has been developed. This reaction sequence comprises intermolecular C-H arylation and intramolecular aerobic oxidative C-H amination, involving the rupture of two C-H bonds, one C-X bond, and one N-H bond in one pot. The Pd(II)-Pd(IV)-Pd(II) and Pd(II)-Pd(0)-Pd(II) catalytic cycles work together under the reported conditions to generate phenanthridines with diverse substituents.

Pd-tetrahydrosalan-type complexes as catalysts for sonogashira couplings in water: efficient greening of the procedure

New sulfonated tetrahydrosalen-type ligands and their water-soluble palladium(II) complexes have been synthesized. The palladium(II) complexes catalyze the Sonogashira coupling (23 examples) of various aryl halides (including chloroarenes) with terminal alkynes, with good to excellent conversions under mild conditions (80°C, air, no Cu(I) cocatalyst) in aqueous-organic mixtures and turnover frequencies of up to 2790 h(-1) . Under optimized reaction conditions to minimize environmental contamination, diphenylacetylenes can be isolated in 76-98% yield. The aqueous catalyst solution can be recycled four times with decreasing activity; however, yields between 93 and 98% can still be achieved with extended reaction times. Several water-insoluble products can be isolated in excellent yield by simple filtration and purification by washing with water; this method is used, for the first time, for this type of C-C coupling procedure.

Computational ligand design for the reductive elimination of ArCF₃ from a small bite angle Pd(II) complex: remarkable effect of a perfluoroalkyl phosphine

To date only three ligands are known to trigger the challenging reductive elimination of ArCF3 from Pd(II). We report the computational design of a bidentate trifluoromethylphosphine ligand that although exhibiting a generally ineffective small bite angle is predicted to give facile reductive elimination. Our experimental verification gave quantitative formation of ArCF3 at 80 °C within 2 h. This highlights the distinct effect of P-CF3 in organometallic reactivity and constitutes a proof-of-principle study of computational reactivity design.