Quantitative synthesis and full characterization of the first isolated and stable pincer palladium(iv) complexes. Quantitative and regioselective synthesis of the C–X (X = Cl, Br) reductive elimination products

An N-heterocyclic carbene-based pincer system of palladium and its versatile reactivity under oxidizing conditions

NHC-based pincers (NHC = N-heterocyclic carbene) have been broadly employed as supporting platforms, and their palladium complexes have found many synthetic applications. However, previous studies mainly focused on the NHC pincers of palladium featuring an oxidation number of +II. In contrast, oxidation of these well-defined Pd(II) species and the study of their fundamental high-valent Pd chemistry remain largely undeveloped. In addition, from a perspective of PdII/PdIV catalysis, the reactivity and degradation of NHC pincers in catalytically relevant reactions have not been well understood. In this work, a series of Pd(II) complexes supported by a well-known NHC^Aryl^NHC pincer platform have been prepared. Their reactivity towards various oxidizing reagents, including halogen surrogates, electrophilic fluorine reagents, and alkyl/aryl halides, has been examined. In some cases, ambient-characterizable high-valent Pd NHCs, which have been scarcely reported, were obtained. The carbenes incorporated into the pincer framework proved to be effective spectator donors. In contrast, the central aryl moiety exhibits versatile reactivity and collapse pathways, allowing it to function either as a spectator or a non-innocent actor.

Synthesis and Reactivity of Stable Alkyl-Pd(IV) Complexes Relevant to Monodentate N-Directed C(sp3)-H Functionalization Processes

Alkyl-Pd(IV) complexes are frequently invoked in the proposed mechanisms of Pd-catalyzed C(sp³)-H functionalization reactions, though few examples of Pd(IV) complexes containing cyclopalladated substrates have been isolated due to the instability of the high-valent Pd(IV) center. Herein, we report the synthesis of stable and isolable OCO pincer-supported alkyl-Pd(IV) complexes containing cyclopalladated alkylamine and oxime frameworks, which represent the first examples of alkyl-Pd(IV) complexes derived from the oxidation of cyclopalladated monodentate N-donor substrates. The aminoalkyl-Pd(IV) complexes reacted efficiently with O- and N-nucleophiles to afford γ-functionalized alkylamine products. A mechanistic study of the nucleophile-mediated reductive elimination was conducted using an oxime-derived Pd(IV) complex, which revealed the intermediacy of a previously unexplored anionic Pd(IV) species.

Synthesis of Well-Defined High-Valent Palladium Complexes by Oxidation of Their Palladium(II) Precursors

The last decade has witnessed the rapid development of high-valent Pd-involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one-electron and two-electron oxidants have been employed to give access to high-oxidation-state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd-mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined Pd^III/IV complexes starting from their Pd^II precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O₂ ; 7) sulfonyl chlorides; and 8) others are covered. A "greener" electrooxidation manner has also been reviewed.

High Oxidation State Organopalladium(IV) Species: Applications to C–H Activation

This highlight article focuses on some seminal discoveries in organopalladium(iv) chemistry and the implications and notable applications of these findings in contemporary palladium catalysis and in particular C–H activation.

A new type of oxidative addition of an iodoarene to a Pd(II) complex

Oxidative addition of N-(2-iodophenyl)formamide to Pd(dba)2 (dba = dibenzylideneacetone) in the presence of 4,4'-ditertbutyl-2,2'-bipyridine ((t)Bubpy) produces [Pd(C6H4NHCHO-2)I((t)Bubpy)] (1) which inserts 2-iodophenyl isocyanide to give [Pd{C(=NC6H4I-2)C6H4NHCHO-2}I((t)Bubpy)] (2). Dehydroiodination of 2 with Tl(acac) (acacH = acetylacetone) gives the stable Pd(IV) complex OC-6-35-[Pd{C,N,N-C(=NC6H4-2)C6H4NCHO-2}I((t)Bubpy)] (4) likely resulting from the spontaneous oxidative addition of the I-Ar moiety present in the unstable intermediate Pd(II) complex [Pd{C,N-C(=NC6H4I-2)C6H4NCHO-2}((t)Bubpy)] (3). The crystal structure of 4 shows various C-H···O hydrogen bonds resulting in chains of dimers stacked along the a axis.

Combining Pd(π-allyl)Cp and PPh3 as a unique catalyst for efficient synthesis of alkyliodo indoles via C(sp3)–I reductive elimination

Alkyl iodides were formed via reductive elimination of alkylpalladium iodides possessing syn-β-hydrogens.

O,N,N-Pincer ligand effects on oxidatively induced carbon–chlorine coupling reactions at palladium

The structural properties of a series of ONN-Pd(ii) chloride pincer complexes have been shown to influence the conversion to 4-chlorotoluene upon oxidation with di-p-tolyliodonium triflate.

Mizoroki-heck cross-coupling reactions catalyzed by dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium under mild reaction conditions

Dichloro-bis(aminophosphine) complexes of palladium with the general formula of [(P{(NC₅H₁₀)_3-n(C₆H₁₁)_n})₂Pd(Cl)₂] (where n = 0-2), belong to a new family of easy accessible, very cheap, and air stable, but highly active and universally applicable C-C cross-coupling catalysts with an excellent functional group tolerance. Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium [(P(NC₅H₁₀)₃)₂Pd(Cl)₂] (1), the least stable complex within this series towards protons; e.g. in the form of water, allows an eased nanoparticle formation and hence, proved to be the most active Heck catalyst within this series at 100 °C and is a very rare example of an effective and versatile catalyst system that efficiently operates under mild reaction conditions. Rapid and complete catalyst degradation under work-up conditions into phosphonates, piperidinium salts and other, palladium-containing decomposition products assure an easy separation of the coupling products from catalyst and ligands. The facile, cheap, and rapid synthesis of 1,1',1"-(phosphinetriyl)tripiperidine and 1 respectively, the simple and convenient use as well as its excellent catalytic performance in the Heck reaction at 100 °C make 1 to one of the most attractive and greenest Heck catalysts available.

We provide here the visualized protocols for the ligand and catalyst syntheses as well as the reaction protocol for Heck reactions performed at 10 mmol scale at 100 °C and show that this catalyst is suitable for its use in organic syntheses.

Stepwise cyclopalladation of 2-phenacylthiopyridine to give C,C,N-pincer complexes

The synthesis of a family of palladium C,C,N-pincer complexes derived from 2-phenacylthiopyridine, obtained by an unprecedented stepwise double cyclopalladation process, is reported. The pincer moiety behaves as a metallaligand.

C-H bond activation at palladium(IV) centers

This communication describes the first observation and study of C-H activation at a Pd(IV) center. This transformation was achieved by designing model complexes in which the rate of reductive elimination is slowed relative to that of the desired C-H activation process. Remarkably, the C-H activation reaction can proceed under mild conditions and with complementary site selectivity to analogous transformations at Pd(II). These results provide a platform for incorporating this new reaction as a step in catalytic processes.

Mechanistic and computational studies of oxidatively-induced aryl-CF3 bond-formation at Pd: rational design of room temperature aryl trifluoromethylation

This article describes the rational design of first generation systems for oxidatively induced Aryl-CF(3) bond-forming reductive elimination from Pd(II). Treatment of (dtbpy)Pd(II)(Aryl)(CF(3)) (dtbpy = di-tert-butylbipyridine) with NFTPT (N-fluoro-1,3,5-trimethylpyridinium triflate) afforded the isolable Pd(IV) intermediate (dtbpy)Pd(IV)(Aryl)(CF(3))(F)(OTf). Thermolysis of this complex at 80 °C resulted in Aryl-CF(3) bond-formation. Detailed experimental and computational mechanistic studies have been conducted to gain insights into the key reductive elimination step. Reductive elimination from this Pd(IV) species proceeds via pre-equilibrium dissociation of TfO(-) followed by Aryl-CF(3) coupling. DFT calculations reveal that the transition state for Aryl-CF(3) bond formation involves the CF(3) acting as an electrophile with the Aryl ligand serving as a nucleophilic coupling partner. These mechanistic considerations along with DFT calculations have facilitated the design of a second generation system utilizing the tmeda (N,N,N',N'-tetramethylethylenediamine) ligand in place of dtbpy. The tmeda complexes undergo oxidative trifluoromethylation at room temperature.