Visible‐Light‐Driven Palladium‐Catalyzed Radical Alkylation of C−H Bonds with Unactivated Alkyl Bromides

Reported herein is a novel visible‐light photoredox system with Pd(PPh₃)₄ as the sole catalyst for the realization of the first direct cross‐coupling of C(sp³)−H bonds in N‐aryl tetrahydroisoquinolines with unactivated alkyl bromides. Moreover, intra‐ and intermolecular alkylations of heteroarenes were also developed under mild reaction conditions. A variety of tertiary, secondary, and primary alkyl bromides undergo reaction to generate C(sp³)−C(sp³) and C(sp²)−C(sp³) bonds in moderate to excellent yields. These redox‐neutral reactions feature broad substrate scope (>60 examples), good functional‐group tolerance, and facile generation of quaternary centers. Mechanistic studies indicate that the simple palladium complex acts as the visible‐light photocatalyst and radicals are involved in the process.     

Highly efficient orthopalladated complexes of second and third benzyl amine for catalyzing the Suzuki cross‐coupling reaction

In this work, ortho‐palladated complexes [Pd(µ‐Cl)(C₆H₄CH₂ NRR′‐κ²‐C,N)]₂ and [Pd(C₆H₄CH₂NH₂‐2‐C,N)Cl(Y)] were tested in the Suzuki–Miyaura cross‐coupling reaction. Cyclopalladated Pd(II) complexes as thermally stable catalysts can activate aryl bromides and chlorides. These complexes were active and efficient catalysts for the Suzuki–Miyaura reaction of aryl bromides and even less reactive aryl chlorides. The cross‐coupled products of a variety of aryl bromides and aryl chloride with phenylboronic acid in methanol as solvent at 60 °C were produced in excellent yields. Copyright © 2010 John Wiley & Sons, Ltd.     

Pd‐Catalyzed Carbonylative α‐Arylation of Aryl Bromides: Scope and Mechanistic Studies

Reaction conditions for the three‐component synthesis of aryl 1,3‐diketones are reported applying the palladium‐catalyzed carbonylative α‐arylation of ketones with aryl bromides. The optimal conditions were found by using a catalytic system derived from [Pd(dba)₂] (dba=dibenzylideneacetone) as the palladium source and 1,3‐bis(diphenylphosphino)propane (DPPP) as the bidentate ligand. These transformations were run in the two‐chamber reactor, COware, applying only 1.5 equivalents of carbon monoxide generated from the CO‐releasing compound, 9‐methylfluorene‐9‐carbonyl chloride (COgen). The methodology proved adaptable to a wide variety of aryl and heteroaryl bromides leading to a diverse range of aryl 1,3‐diketones. A mechanistic investigation of this transformation relying on ³¹P and ¹³C NMR spectroscopy was undertaken to determine the possible catalytic pathway. Our results revealed that the combination of [Pd(dba)₂] and DPPP was only reactive towards 4‐bromoanisole in the presence of the sodium enolate of propiophenone suggesting that a [Pd(dppp)(enolate)] anion was initially generated before the oxidative‐addition step. Subsequent CO insertion into an [Pd(Ar)(dppp)(enolate)] species provided the 1,3‐diketone. These results indicate that a catalytic cycle, different from the classical carbonylation mechanism proposed by Heck, is operating. To investigate the effect of the dba ligand, the Pd⁰ precursor, [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)], was examined. In the presence of DPPP, and in contrast to [Pd(dba)₂], its oxidative addition with 4‐bromoanisole occurred smoothly providing the [PdBr(Ar)(dppp)] complex. After treatment with CO, the acyl complex [Pd(CO)Br(Ar)(dppp)] was generated, however, its treatment with the sodium enolate led exclusively to the acylated enol in high yield. Nevertheless, the carbonylative α‐arylation of 4‐bromoanisole with either catalytic or stoichiometric [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)] over a short reaction time, led to the 1,3‐diketone product. Because none of the acylated enol was detected, this implied that a similar mechanistic pathway is operating as that observed for the same transformation with [Pd(dba)₂] as the Pd source.     

Ethanol‐promoted reductive homocoupling reactions of aryl halides catalyzed by palladium on carbon (Pd/C)

Homocoupling reactions of aryl bromides or iodides proceeded smoothly with palladium on carbon (Pd/C) catalyst, ethanol and base in dimethyl sulfoxide (DMSO) to afford exclusively symmetric biaryls in good to excellent yields. Ethanol was first used as a reducing agent in situ to reduce the Pd²⁺/C species into Pd⁰/C active species to complete the catalytic redox cycle. It was found that ethanol can promote the Pd/C‐catalyzed reductive homocoupling of aryl iodides and bromides efficiently in the presence of base. A reaction mechanism has been put forward and discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Outstanding Methane Oxidation Performance of Palladium‐Embedded Ceria Catalysts Prepared by a One‐Step Dry Ball‐Milling Method

By carefully mixing Pd metal nanoparticles with CeO₂ polycrystalline powder under dry conditions, an unpredicted arrangement of the Pd‐O‐Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering a core of CeO₂ particles. The robust contact that is generated at the nanoscale, along with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO₂ and creates highly reactive and stable sites constituted by PdO_x embedded into CeO₂ surface layers. This specific arrangement outperforms conventional Pd/CeO₂ reference catalysts in methane oxidation by lowering light‐off temperature by more than 50°C and boosting the reaction rate. The origin of the outstanding activity is traced to the structural properties of the interface, modified at the nanoscale by mechanochemical interaction.     

Preparations of Saturated N‐P‐N Type Secondary Phosphine Oxides and their Applications in Cross‐Coupling Reactions

A series of cyclohexane‐1,2‐diamine ( 3a – 3d ) and benzene‐1,2‐diamine derivatives ( 3e – 3h ) were pre‐ pared. Followed by hydrolysis, the reaction of 3a – 3c with PCl₃ successfully led to the formation of cor‐ responding metastable saturated heteroatom‐substituted secondary phosphine oxides (HASPO 4a – 4c ), a tautomer of the saturated heteroatom‐substituted phosphinous acid (HAPA). Whereas ambient‐stable diamine‐coordinated palladium complexes were obtained, HAPA‐coordinated palladium complexes were not successfully synthesized. The molecular structures of HASPO 4c , Pd(OAc)₂(3a) , PdBr₂(3b) and Pd(OAc)₂(3c) and [Cu(NO₃)(3d)⁺][NO₃ ^? ] were determined by single‐crystal X‐ray diffraction method. Catalysis of in‐situ Suzuki‐Miyaura cross‐coupling reactions for aryl bromides and phenylboronic acid using diamine 3a as ancillary ligand showed that the optimized reaction condition at 60 °C is the combination of 2 mmol % 3a /3.0 mmol KOH/1.0 mL 1,4‐dioxane/1 mmol % Pd(OAc)₂. Moreover, moderate reactivity was observed when using aryl chlorides as substrates (supporting infor‐ mation). When diamine 3d was employed in Heck reaction, good tolerance of functional groups of aryl bromides were observed while using 4‐bromoanisole and styrene as substrates. The optimized condi‐ tion for Heck reaction at 100 °C is 3 mmol % 3d /3.0 mmol CsF/1.0 mL toluene/3 mmol % Pd(OAc)₂. In general, cyclohexane‐1,2‐diamine derivatives exhibited better catalytic properties than those of benzene‐1,2‐diamines.     

Tetraphosphine/palladium‐catalyzed Suzuki–Miyaura coupling of heteroaryl halides with 3‐pyridine‐ and 3‐thiopheneboronic acid: an efficient catalyst for the formation of biheteroaryls

An easily prepared tetraphosphine N,N,N′,N′‐tetra(diphenylphosphinomethyl)‐1,2‐ethylenediamine (L1) associated with [Pd(η³‐C₃H₅)Cl]₂ affords an efficient catalyst for Suzuki–Miyaura coupling of 3‐pyridineboronic acid with heteroaryl bromides. Reaction could be performed with as little as 0.02 mol% catalyst and a high turnover number of 2500 is obtained. A wide range of substrates is investigated with satisfactory yields, and good compatibility with aminogroup‐substituted pyridines and unprotected indole is exhibited. This protocol can also be applied successfully to the reaction of heteroaryl bromides with 3‐thiopheneboronic acid. This Pd‐tetraphosphine catalyst efficiently restrains the poisoning effect from heteroaryls, and shows good stability and longevity. Copyright © 2013 John Wiley & Sons, Ltd.     

Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck arylation of olefins

Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been investigated. The scope of a coupling process using Pd(II) sources and an α-diimine as ligand in the presence of Cs₂CO₃ as base was tested using various substrates. The Pd(OAc)₂/DAB-Cy (1, DAB-Cy=1,4-dicyclohexyl-diazabutadiene) system presents the highest activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another Pd(II) source, Pd(acac)₂ (acac=acetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when the appropriate concentration of ⁿBu₄NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst loadings and high temperatures. Pd(acac)₂ and Pd(acac)₂/DAB-Cy precatalysts were very effective for the arylation of various olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance.     

A highly efficient memantine-modified palladium catalyst for Suzuki–Miyaura cross-coupling reaction

A new simple Pd(memantine)₂Cl₂ complex was synthesized and characterized by ¹H NMR, ¹³C NMR and X-ray single crystal structure determination. The Suzuki–Miyaura reaction of aryl bromides catalyzed by Pd(memantine)₂Cl₂ complex was investigated in air with different temperature. The high turnover numbers of 650,000 have been obtained in the reaction of 4-bromonitrobenzene with phenylboronic acid at 80 °C. At room temperature, the complex also showed high activity for Suzuki–Miyaura cross-coupling reaction of aryl bromides with a wide range of functional groups under air, and the turnover number of up to 99,000 was achieved. The catalytic system also gives good yields toward the reaction of several heteroaryl bromides with thiophenylboronic acid.     

NH3‐Driven Benzene C−H Activation with O2 that Opens a New Way for Selective Phenol Synthesis

Catalytic benzene C?H activation toward selective phenol synthesis with O₂ remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three‐steps cumene process in liquid phase, which is energy‐intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas‐phase single‐path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O₂ under coexisting NH₃ by the new inter‐ligand concerted mechanism. Among alkali and alkaline‐earth metal ions and transition and precious metal ions, single Cs⁺ and Rb⁺ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas‐phase single‐path reaction process. The single Cs⁺ and Rb⁺ sites that possess neither significant Lewis acidic?basic property nor redox property, cannot activate benzene, O₂, and NH₃, respectively, whereas when they coadsorbed together, the reaction of the inter‐coadsorbates on the single alkali‐metal ion site proceeds concertedly (the inter‐ligand concerted mechanism), bringing about the benzene C?H activation toward phenol synthesis. The NH₃‐driven benzene C?H activation with O₂ was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH₃ on Pt₆ metallic cluster/β and Ni₄O₄ oxide cluster/β. The NH₃‐driven selective oxidation mechanism observed with the Cs⁺/β and Rb⁺/β differs from the traditional redox catalysis (Mars‐van Krevelen) mechanism, simple Langmuir‐Hinshelwood mechanism, and acid?base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.     

Alkene Carboboration Enabled by Synergistic Catalysis

A synergistic Pd/Cu system for the coupling of alkenes, (Bpin)₂ (pin=pinacolate), and aryl/vinyl bromides is disclosed. This method allows for the catalytic generation of secondary Csp³?Cu nucleophiles in situ and subsequent Pd‐catalyzed cross‐coupling.     

A phosphine‐free heterogeneous Suzuki–Miyaura reaction of aryl bromides catalyzed by MCM‐41‐supported tridentate nitrogen palladium complex under air

MCM‐41‐supported tridentate nitrogen palladium(II) complex [MCM‐41‐3 N‐Pd(II)] was conveniently synthesized from commercially available and cheap 3‐(2‐aminoethylamino)propyltrimethoxysilane via immobilization on MCM‐41, followed by reacting with pyridine‐2‐carboxaldehyde and PdCl₂. It was found that this palladium complex is an excellent catalyst for the Suzuki–Miyaura coupling reaction of aryl bromides on two points: (i) the use of 5 × 10⁻⁴ mol equiv. of MCM‐41‐3 N‐Pd(II) under air afforded the coupling products efficiently after easy workup; (2) the catalyst can be reused many times without loss of catalytic activity. Copyright © 2011 John Wiley & Sons, Ltd.     

Pd Nanocubes@ZIF‐8: Integration of Plasmon‐Driven Photothermal Conversion with a Metal–Organic Framework for Efficient and Selective Catalysis

Composite nanomaterials usually possess synergetic properties resulting from the respective components and can be used for a wide range of applications. In this work, a Pd nanocubes@ZIF‐8 composite material has been rationally fabricated by encapsulation of the Pd nanocubes in ZIF‐8, a common metal–organic framework (MOF). This composite was used for the efficient and selective catalytic hydrogenation of olefins at room temperature under 1 atm H₂ and light irradiation, and benefits from plasmonic photothermal effects of the Pd nanocube cores while the ZIF‐8 shell plays multiple roles; it accelerates the reaction by H₂ enrichment, acts as a “molecular sieve” for olefins with specific sizes, and stabilizes the Pd cores. Remarkably, the catalytic efficiency of a reaction under 60 mW cm^?2 full‐spectrum or 100 mW cm^?2 visible‐light irradiation at room temperature turned out to be comparable to that of a process driven by heating at 50 °C. Furthermore, the catalyst remained stable and could be easily recycled. To the best of our knowledge, this work represents the first combination of the photothermal effects of metal nanocrystals with the favorable properties of MOFs for efficient and selective catalysis.     

Mild and Selective Catalytic Hydrogenation of the C=C Bond in α,β‐Unsaturated Carbonyl Compounds Using Supported Palladium Nanoparticles

Chemoselective reduction of the C=C bond in a variety of α,β‐unsaturated carbonyl compounds using supported palladium nanoparticles is reported. Three different heterogeneous catalysts were compared using 1 atm of H₂: 1) nano‐Pd on a metal–organic framework (MOF: Pd⁰‐MIL‐101‐NH₂(Cr)), 2) nano‐Pd on a siliceous mesocellular foam (MCF: Pd⁰‐AmP‐MCF), and 3) commercially available palladium on carbon (Pd/C). Initial studies showed that the Pd@MOF and Pd@MCF nanocatalysts were superior in activity and selectivity compared to commercial Pd/C. Both Pd⁰‐MIL‐101‐NH₂(Cr) and Pd⁰‐AmP‐MCF were capable of delivering the desired products in very short reaction times (10–90 min) with low loadings of Pd (0.5–1 mol %). Additionally, the two catalytic systems exhibited high recyclability and very low levels of metal leaching.     

The Synthesis of (η3‐C3H5)Pd{Si[N(tBu)CH]2}Cl and the Catalytic Property for Heck Reaction

Treatment of N‐heterocyclic silylene Si[N(^tBu)CH]₂ ( 1 ) and [(η³‐C₃H₅)PdCl]₂ in toluene led to the formation of the mononuclear complex (η³‐C₃H₅)Pd{Si[N(^tBu)CH]₂}Cl ( 3 ), the silicon analogue to N‐heterocyclic carbene complex (η³‐C₃H₅)Pd{C[N(^tBu)CH]₂}Cl ( 2 ). Complex 3 was characterized with ¹H NMR and ¹³C NMR. Investigation shows that (η³‐C₃H₅)Pd{Si[N(^tBu)CH]₂}Cl is an active catalyst for Heck coupling reaction of styrene with aryl bromides.     

Triggering Redox Activity in a Thiophene Compound: Radical Stabilization and Coordination Chemistry

The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L⁻ are presented. This π‐conjugated anion displayed pronounced redox activity, undergoing facile one‐electron oxidation to the acyclic, metal‐free, neutral radical L ^. on reaction with FeBr₂. In contrast, the reaction of L⁻ with CuI formed the unique, neutral Cu₂I₂( L ^.) complex of a ligand‐centered radical, whereas reaction with the stronger oxidant AgBF₄ formed the metal‐free radical dication L ^.2+.     

Synthesis,characterization and catalytic activity of new aminomethyldiphosphine–Pd(II) complexes for Suzuki cross‐coupling reaction

A new range of CF₃‐substituted aminomethyldiphosphine (P―C―N) ligands ((C₆H₅)₂PCH₂)₂NR (R = ―C₆H₄(2‐CF₃) ( 1 ), ―C₆H₄(3‐CF₃) ( 1b ) has been synthesized from 2‐(trifluoromethyl)aniline and 3‐(trifluoromethyl)aniline with diphenylphosphine. The aminomethyldiphosphine ligands were reacted with Pd(cod)Cl₂ to give corresponding metal complexes, PdLCl₂ ( 2a , 2b ). The aminomethyldiphosphine–palladium compounds were characterized by utilizing several methods including NMR (¹H, ¹³C, ³¹P) and elemental analysis. These compounds were used as catalysts in Suzuki cross‐coupling reaction of aryl chlorides and bromides. The effect of base was also investigated in this current project. CF₃‐substituted aminomethyldiphosphine–palladium complexes were found to be efficient catalysts in Suzuki cross‐coupling reaction of activated and deactivated aryl boronic acids. Copyright © 2013 John Wiley & Sons, Ltd.     

Electroactive Composite Pd–Polypyrrole and Its Catalytic Properties in the Reaction of Styryl Bromide Cyanation

A composite material in the form of powder is synthesized by a redox reaction in mixed aqueous solution of Pd(NH₃)₄Cl₂ + pyrrole. The composite consists of polypyrrole globules with palladium nanoparticles uniformly distributed inside the latter. Being applied as a film on the electrode surface, both components of this material exhibit redox activity. Palladium particles inside the composite exhibit catalytic properties in cyanation of styryl bromides, a reaction widely used in fine organic synthesis.     

Suzuki coupling catalyzed by a homoleptic Pd(I)-Pd(I) solvento complex

The Pd^I-Pd^I bonded complex [Pd₂(CH₃CN)₆][SbF₆]₂ is catalytically active towards Suzuki cross-coupling reactions of aryl bromides or chlorides with various arylboronic acids under mild conditions giving good to excellent yields. Its performance is enhanced by the introduction of stoichiometric or limited phosphines. The effects of different ligands, metal oxidation states [Pd(II), Pd(I) Pd(0)], bases and solvents have been examined.     

Facile Synthesis Of Composition‐Controllable PtPdAuTe Nanowires As Superior Electrocatalysts For Direct Methanol Fuel Cells

Multicomponent Pt‐based nanowires (NWs) have attracted widespread attention as eletrocatalysts toward direct alcohol fuel cells because of their unique one‐dimensional structure and high reaction dynamics. Quaternary PtPdAuTe NWs are designed via a facile template method, and NWs with a different composition are obtained by adjusting the feed ratio of metal precursors. The direct displacement reaction of metal precursors with Te NWs and the partial oxidation of Te lead to the formation of quaternary NWs. The rough surface and abundant reactive sites deriving from the rearrangement of metal atoms on the Te NWs surface endow the PtPdAuTe NWs with a superior electrocatalytic property and durability for methanol oxidation. The Pt₂₀Pd₂₀Au₁₀Te₅₀ NWs display the largest mass activity and best stability among all catalysts. The preparation of PtPdAuTe NWs could provide a viable strategy for the preparation of other multicomponent NWs.