Imidazolin‐2‐ylidenaminophosphines as Highly Electron‐Rich Ligands for Transition‐Metal Catalysts

A variety of chemical transformations benefit from the use of strong electron‐donating ancillary ligands, such as alkylphosphines or N‐heterocyclic carbenes when electron‐rich metal centers are required. Herein, we describe a facile and highly modular access to monodentate and bidentate imidazolin‐2‐ylidenamino‐substituted phosphines. Evaluation of the phosphine’s electronic properties substantiate that the formal replacement of alkyl or aryl groups by imidazolin‐2‐ylidenamino groups dramatically enhance their donor ability beyond that of alkylphosphines and even N‐heterocyclic carbenes. The new phosphines have been coordinated onto palladium(II) centers, and the beneficial effect of the novel substitution patterns has been explored by using the corresponding complexes in the palladium‐catalyzed Suzuki–Miyaura cross‐coupling reaction of non‐activated aryl chloride substrates.     

Biological and Catalytic Applications of Pd(II)-Indenyl Complexes Bearing Phosphine and N-Heterocyclic Carbene Ligands

A general synthetic entryway into novel cationic Pd(II) indenyl complexes bearing one alkyl/aryl phosphine and one N-heterocyclic carbene is reported. All metal complexes have been exhaustively characterized by spectroscopic and structural analyses, highlighting that the indenyl fragment has an hapticity intermediate between η³ and η⁵. Most of the target complexes are stable in solid state and in solution for a long time. Two different applications of these organopalladium compounds are proposed. Firstly, they have been tested as antiproliferative agents towards three different ovarian cancer cell lines, showing a cytotoxicity significantly higher than that of cisplatin, with a clear dependence on the nature of the coordinated phosphine. Moreover, the similar cytotoxicity towards cisplatin-sensitive and cisplatin-resistant cell lines suggests that these new palladium derivatives act with a different mechanism of action with respect to classical platinum-based drugs. Finally, the water-soluble palladium complexes bearing 1,3,5-triaza-7-phosphaadamantane (PTA) have demonstrated interesting catalytic performances in Suzuki–Miyaura coupling in aqueous media, being, inter alia, readily and efficiently recyclable.     

Acyclic Palladium(II)‐N‐heterocyclic Carbene Metallacrown Ether Complexes: Synthesis,Structure and Catalytic Activity

Novel acyclic Pd(II)‐N‐heterocyclic carbene (NHC) metallacrown ethers 5a , 5b have been synthesized. Reaction of the imidazolium salts bearing a long polyether chain with Ag₂O afforded Ag‐NHC complexes, which then reacted as carbene transfer agent with PdCl₂(MeCN)₂ to give the desired acyclic Pd(II)‐NHC metallacrown ether complexes 5a and 5b . The ¹H NMR and ¹³C NMR spectra show 5a and 5b exist as mixtures of cis and trans isomers in solution. The trans isomer of 5a was characterized by X‐ray diffraction, which clearly demonstrated two pseudo‐crown ether cavities in trans‐ 5a . Pd(II)‐NHC complexes 5a and 5b have been shown to be highly effective in the Suzuki‐Miyaura reactions of a variety of aryl bromides in neat water without the need of inert gas protection.     

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.     

Catalytic Decarboxylative Cross‐Coupling of Aryl Chlorides and Benzoates without Activating ortho Substituents

The restriction of decarboxylative cross‐coupling reactions to ortho‐substituted or heterocyclic carboxylate substrates was overcome by holistic optimization of a bimetallic Cu/Pd catalyst system. The combination of a CuI/Me₄phen decarboxylation catalyst and a [(MeCN)₄Pd](OTf)₂/XPhos cross‐coupling catalyst enables the synthesis of biaryls from inexpensive aryl chlorides and potassium benzoates regardless of their substitution pattern.     

PEPPSI‐Type Palladium Complexes Containing Basic 1,2,3‐Triazolylidene Ligands and Their Role in Suzuki–Miyaura Catalysis

A series of PEPPSI‐type palladium(II) complexes was synthesized that contain 3‐chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki–Miyaura cross‐coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50 °C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol‐2‐ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N‐heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system.     

Synthesis,characterization, and reactivity of palladium(II) complexes containing piperidoimidazolin‐2‐ylidene

A series of Pd–N‐heterocyclic carbene (Pd‐NHC) complexes with pyrazine ( 1 ) or pyridine ( 2 ) and NHC ( 3 ) were synthesized and characterized by elemental analysis and spectroscopic methods. In addition, the molecular structure of 3 was determined by X‐ray diffraction studies. The effects of these ligands on catalyst activation and the performance of complexes 1 , 2 , 3 were studied on Suzuki–Miyaura reactions of phenylboronic acid with aryl chlorides. Finally, we demonstrated that complex 1 is very adept at re‐forming the Kumada–Tamao–Corriu cross‐coupling reaction. Copyright © 2013 John Wiley & Sons, Ltd.     

Heteroaromatic Tosylates as Electrophiles in Regioselective Mizoroki–Heck‐Coupling Reactions with Electron‐Rich Olefins

Easy and direct : Regioselective Mizoroki–Heck‐coupling reactions using heteroaromatic tosylates as electrophiles were achieved, thus providing direct and easy access to highly functionalized α‐heteroarylvinyl amides and ethers.

     

Catalytic Activity of Cationic and Neutral Silver(I)–XPhos Complexes with Nitrogen Ligands or Tolylsulfonate for Mannich and Aza‐Diels–Alder Coupling Reactions

Cationic and neutral silver(I)–L complexes (L=Buchwald‐type biaryl phosphanes) with nitrogen co‐ligands or organosulfonate counter ions have been synthesised and characterised through their structural and spectroscopic properties. At room temperature, both cationic and neutral silver(I)–L complexes are extremely active catalysts in the promotion of the single and double A³ coupling of terminal (di)alkynes, pyrrolidine and formaldehyde. In addition, the aza‐Diels–Alder two‐ and three‐component coupling reactions of Danishefsky’s diene with an imine or amine and aldehyde are efficiently catalysed by these cationic or neutral silver(I)–L complexes. The solvent influences the catalytic performance due to limited complex solubility or solvent decomposition and reactivity. The isolation of new silver(I)–L complexes with reagents as ligands lends support to mechanistic proposals for such catalytic processes. The activity, stability and metal–distal arene interaction of these silver(I)–L catalysts have been compared with those of analogous cationic gold(I) and copper(I) complexes.     

一种经MCM-41负载双齿膦钯(0)配合物催化的羰基化Suzuki偶联合成二芳酮的新途径

在催化量的MCM-41负载双齿膦钯(0)配合物存在下,芳基碘化物和芳基硼酸、一氧化碳在常压下能顺利进行羰基化Suzuki偶联反应,高产率地生成了各种二芳酮化合物。MCM-41负载双齿膦钯(0)配合物具有比PdCl2(PPh3)2 更高的活性和选择性,且可回收再用10次其活性基本不变,为各种功能化二芳酮的合成提供了方便实用的新途径。     

High Catalytic Activity of Palladium(II)‐Exchanged Mesoporous Sodalite and NaA Zeolite for Bulky Aryl Coupling Reactions: Reusability under Aerobic Conditions

Exchange for the better : Mesoporous sodalite and NaA zeolite exchanged with Pd²⁺ exhibit remarkably high activity and reusability in C? C coupling reactions under aerobic atmosphere. It is proposed that the catalytic reactions are mediated by a molecular Pd⁰ species generated in situ within the pores (see picture), which is oxidized back to Pd²⁺ by O₂, preventing the formation of catalytically inactive Pd⁰ agglomerates.

     

Mechanisms in the Reaction of Palladium(II)–π‐Allyl Complexes with Aryl Halides: Evidence for NHC Exchange Between Two Palladium Complexes

A detailed experimental and DFT study (PBE level) of the reaction of [Pd(η³‐C₃H₅)(tmiy)(PR₃)]BF₄ (tmiy=tetramethylimidazolin‐2‐ylidene, PR₃=phosphane), precursors to monoligated Pd⁰ species, with aryl electrophiles yielding 2‐arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over Pd^IV and σ‐bond metathesis pathways, and that transmetalation is the rate‐determining step. Calculations indicate that the key step involves the concerted exchange of NHC and iodo ligands between two different Pd^II complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin‐2‐ylidene).     

Large yet Flexible N‐Heterocyclic Carbene Ligands for Palladium Catalysis

A straightforward and scalable eight‐step synthesis of new N‐heterocyclic carbenes (NHCs) has been developed from inexpensive and readily available 2‐nitro‐m‐xylene. This process allows for the preparation of a novel class of NHCs coined ITent (“Tent” for “tentacular”) of which the well‐known IMes (N,N′‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene), IPr (N,N′‐bis(2,6‐di(2‐propyl)phenyl)imidazol‐2‐ylidene) and IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene) NHCs are the simplest and already known congeners. The synthetic route was successfully used for the preparation of three members of the ITent family: IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene), IHept (N,N′‐bis(2,6‐di(4‐heptyl)phenyl)imidazol‐2‐ylidene) and INon (N,N′‐bis(2,6‐di(5‐nonyl)phenyl)imidazol‐2‐ylidene). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes. Finally the effect of these new ITent ligands in Pd‐catalyzed Suzuki–Miyaura and Buchwald–Hartwig cross‐couplings was investigated.