Promoting Difficult Carbon–Carbon Couplings: Which Ligand Does Best?

A Pd complex, cis‐[Pd(C₆F₅)₂(THF)₂] ( 1 ), is proposed as a useful touchstone for direct and simple experimental measurement of the relative ability of ancillary ligands to induce C?C coupling. Interestingly, 1 is also a good alternative to other precatalysts used to produce Pd⁰L. Complex 1 ranks the coupling ability of some popular ligands in the order P^tBu₃>o‐TolPEWO‐F≈tBuXPhos>P(C₆F₅)₃≈PhPEWO‐F>P(o‐Tol)₃≈THF≈tBuBrettPhos?Xantphos≈PhPEWO‐H?PPh₃ according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked tBuXPhos≈P^tBu₃≈o‐TolPEWO‐F>PhPEWO‐F>P(C₆F₅)₃?tBuBrettPhos>THF≈P(o‐Tol)₃>Xantphos>PhPEWO‐H?PPh₃. This “meter” also detects some other possible virtues or complications of ligands such as tBuXPhos or tBuBrettPhos.     

Direct Synthesis of Functionalized High‐Molecular‐Weight Polyethylene by Copolymerization of Ethylene with Polar Monomers

The introduction of even a small amount of polar functional groups into polyolefins could excise great control over important material properties. As the most direct and economic strategy, the transition‐metal‐catalyzed copolymerization of olefins with polar, functionalized monomers represents one of the biggest challenges in this field. The presence of polar monomers usually dramatically reduces the catalytic activity and copolymer molecular weight (to the level of thousands or even hundreds Da), rendering the copolymerization process and the copolymer materials far from ideal for industrial applications. In this contribution, we demonstrate that these obstacles can be addressed through rational catalyst design. Copolymers with highly linear microstructures, high melting temperatures, and very high molecular weights (close to or above 1 000 000 Da) were generated. The direct synthesis of polar functionalized high‐molecular‐weight polyethylene was thus achieved.     

Synthesis of Stable Diarylpalladium(II) Complexes: Detailed Study of the Aryl–Aryl Bond‐Forming Reductive Elimination

The synthesis of diarylpalladium(II) complexes by twofold aryl C?H bond activation was developed. These intermediates of oxidative cyclization reactions are stabilized by chelation with acetyl groups while still maintaining sufficient reactivity to study their reductive elimination. Four distinct triggers were found for the reductive elimination of these complexes to dibenzofurans and carbazoles. Thermal elimination occurs at very high temperatures, whereas ligand‐promoted and oxidatively induced reductive eliminations proceed readily at room temperature. Under these conditions, no isomerization occurs. In contrast, weak Brønsted acids, such as acetic acid, lead to a sequence of proto‐demetalation, isomerization to a κ³‐diarylpalladium(II) complex, and reductive elimination to non‐symmetrical cyclization products.     

Palladium(0)‐Catalyzed Intermolecular Allylic Dearomatization of Indoles by a Formal [4+2] Cycloaddition Reaction

Bridged indoline derivatives were synthesized by an intermolecular Pd‐catalyzed allylic dearomatization reaction of substituted indoles. The reaction between indoles and allyl carbonates bearing a nucleophilic alcohol side‐chain proceeds in a cascade fashion, providing bridged indolines in excellent enantioselectivity.     

Product‐Derived Bimetallic Palladium Complex Catalyzes Direct Carbonylation of Sulfonylazides

A novel product‐derived bimetallic palladium complex catalyzes a sulfonylazide‐transfer reaction with the σ‐donor/π‐acceptor ligand CO, and is advantageous given its broad substrate scope, high efficiency, and mild reaction conditions (atmospheric pressure of CO at room temperature). This methodology provides a new approach to sulfonylureas, which are present in both pharmaceuticals and agrochemicals. The synthesis of Glibenclamide on a gram scale further revealed the practical utility of this procedure. Mechanistically, the generation of a bridged bimetallic palladium species derived from the product sulfonylurea is disclosed as the crucial step for this catalytic cycle.     

Palladium‐Catalyzed Intramolecular Carbene Insertion into C(sp3)−H Bonds

A palladium‐catalyzed carbene insertion into C(sp³)?H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd⁰ and Pd^II, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium‐catalyzed C(sp³)?C(sp³) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp³)?H bond functionalization reaction involves an unprecedented concerted metalation–deprotonation step.     

New Copper,Palladium and Nickel Catalytic Systems: An Evolution towards More Efficient Procedures

Metal‐catalysed reactions are a fundamental tool in synthetic chemistry. Increasingly challenging transformations can be accomplished only by means of certain metal catalysts. However, there still remains the need for a substantial decrease of the amount of catalyst, for better reuse or recycling of such active species, and for the avoidance of relatively toxic solvents in favour of environmentally friendly media. These facts apply to copper‐, palladium‐, and nickel‐catalysed cross‐coupling reactions, direct arylations, and oxidative processes. This account summarises our research on the last reactions, featuring an evolution towards more sustainable procedures in this field.     

Synthesis of Hollow Platinum–Palladium Nanospheres with a Dendritic Shell as Efficient Electrocatalysts for Methanol Oxidation

Engineering the size, composition, and morphology of platinum‐based nanomaterials can provide a great opportunity to improve the utilization efficiency of electrocatalysts and reinforce their electrochemical performances. Herein, three‐dimensional platinum–palladium hollow nanospheres with a dendritic shell (PtPd‐HNSs) are successfully fabricated through a facile and economic route, during which SiO₂ microspheres act as the hard template for the globular cavity, whereas the triblock copolymer F127 contributes to the formation of the dendritic shell. In contrast with platinum hollow nanospheres (Pt‐HNSs) and commercial platinum on carbon (Pt/C) catalyst, the novel architecture shows a remarkable activity and durability toward the methanol oxidation reaction (MOR) owing to the coupled merits of bimetallic nanodendrites and a hollow interior. As a proof of concept, this strategy is also extended to trimetallic gold–palladium–platinum hollow nanospheres (AuPdPt‐HNSs), which paves the way towards the controlled synthesis of other bi‐ or multimetallic platinum‐based hollow electrocatalysts.     

A Novel Trihybrid Material Based on Renewables: An Efficient Recyclable Heterogeneous Catalyst for C−C Coupling and Reduction Reactions

The generation of organic–inorganic hybrid materials from renewable resources and their utilization in basic and applied areas has been at the forefront of research in recent years for sustainable development. Herein, a novel organic–inorganic trihybrid material was synthesized by in situ generation of palladium nanoparticles (PdNPs) in a hybrid gel matrix based on renewable chemicals. Constituents of the hybrid gel included a pentacyclic triterpenoid arjunolic acid extractable from Terminalia arjuna and the leaf extract of Chrysophyllum cainito rich in flavonoids. We took advantage of the presence of flavonoid molecules in this hybrid gel to generate an advanced trihybrid gel through in situ reduction of doped Pd^II salts to stable PdNPs. The xerogel of this trihybrid material was used as a recyclable heterogeneous catalyst for C?C coupling and reduction reactions in aqueous media. We also demonstrated that the in situ generated PdNPs containing trihybrid material was a more efficient catalyst than the trihybrid material generated with presynthesized PdNPs.     

Pd-catalyzed asymmetric allylic substitution reactions using P-chirogenic diaminophosphine oxides: DIAPHOXs

This paper describes the development of a new class of chiral phosphorus ligand: aspartic acid-derived P-chirogenic diaminophosphine oxides, DIAPHOXs, and their application to several Pd-catalyzed asymmetric allylic substitution reactions. Pd-catalyzed asymmetric allylic alkylation was initially examined in detail using diaminophosphine oxides 1a, resulting in the highly enantioselective construction of quaternary stereocenters. Mechanistic investigations revealed that 1a is activated by N,O-bis(trimethylsilyl)acetamide-induced tautomerization to afford a trivalent diamidophosphite species 12, which functions as the actual ligand. Furthermore, asymmetric allylic amination was examined using Pd-DIAPHOX catalyst systems, providing a variety of chiral allylic amines.