Α-diimine Palladium Research Articles

Ligand Steric Effects of α-Diimine Nickel(II) and Palladium(II) Complexes in the Suzuki-Miyaura Cross-Coupling Reaction.

Nickel catalysts represent a low cost and environmentally friendly alternative to palladium-based catalytic systems for Suzuki–Miyaura cross-coupling (SMC) reactions. However, nickel catalysts have suffered from poor air, moisture, and thermal stabilities, especially at high catalyst loading, requiring controlled reaction conditions. In this report, we examine a family of mono- and dinuclear Ni(II) and Pd(II) complexes with a diverse and versatile α-diimine ligand environment for SMC reactions. To evaluate the ligand steric effects, including the bite angle in the reaction outcomes, the structural variation of the complexes was achieved by incorporating iminopyridine- and acenaphthene-based ligands. Moreover, the impact of substrate bulkiness was investigated by reacting various aryl bromides with phenylboronic acid, 2-naphthylboronic acid, and 9-phenanthracenylboronic acid. Yields were the best with the dinuclear complex, being nearly quantitative (93–99%), followed by the mononuclear complexes, giving yields of 78–98%. Consequently, α-diimine-based ligands have the potential to deliver Ni-based systems as sustainable catalysts in SMC.

"Bulky-Yet-Flexible" α-Diimine Palladium-Catalyzed Reductive Heck Cross-Coupling: Highly Anti-Markovnikov-Selective Hydroarylation of Alkene in Air.

To pursue a highly regioselective and efficient reductive Heck reaction, a series of moisture- and air-stable α-diimine palladium precatalysts were rationally designed, readily synthesized, and fully characterized. The relationship between the structures of the palladium complexes and the catalytic properties was investigated. It was revealed that the"bulky-yet-flexible"palladium complexes allowed highly anti-Markovnikov-selective hydroarylation of alkenes with (hetero)aryl bromides under aerobic conditions. Further synthetic application of the present protocol could provide rapid and straightforward access to functional and biologically active molecules.

Benchmark study of density functionals for the insertions of olefin and polar monomers catalyzed by α–diimine palladium complexes

The performance of density functionals has been tested for the insertions of ethylene, methyl acrylate (MA), and vinyl bromide (VB) catalyzed by α-diimine palladium complexes. By screening 67 density functionals and comparing with experimental data, 7 hybrid GGA and 4 hybrid meta-GGA functionals showed better performance in the insertions of both ethylene and MA, and could be therefore suitable for ethylene-MA copolymerization. Meanwhile, three GGA (PW91PW91, HCTH, HCTH407), two meta-GGA (TPSSTPSS, tHCTH), 4 hybrid GGA and 6 hybrid meta-GGA functionals perform well in ethylene-VB copolymerization. Besides, nine D3 or D3BJ augmented functionals are found to be suitable for both copolymerization systems. The D2 dispersion correction overestimated insertion energy barriers of these monomers and is unsuitable for such copolymerization. In addition, the double-zeta basis set is found to be sufficient for solvation single-point calculation of these systems.

Bulky α-diimine palladium complexes supported graphene oxide as heterogeneous catalysts for Suzuki-Miyaura reaction

To explore high efficiency of the Suzuki-Miyaura reactions, an easily prepared heterogeous palladium precatalysts bearing sterically bulky α-diimine was covalently supported on graphene oxide (GO). The characterization of supported palladium complex was discussed in detail. A simple and efficient protocol for Pd-catalyzed cross coupling was described, which revealed that the bulky α-diimine ligand is crucial to promote the CAr-CAr transformation. This immobilized system provides a straightforward access to a wide range of aryl bromides with excellent functional groups tolerance.

Thermally robust α-diimine nickel and palladium catalysts with constrained space for ethylene (co)polymerizations

The axial and equatorial plane model has been widely accepted for α-diimine nickel and palladium catalysts of olefins polymerization. In this paper, dinaphthobarrelene backbone-based α-diimine nickel and palladium complexes with the constrained space were designed and synthesized from the viewpoint of three-dimensional (3D) space. The 3D-constrained microenvironment around the Ni/Pd metal center created by the bulky ligand substituents fully shielded the back and axial sites, which improved catalytic activity, thermal stability, and living fashion of catalysts. Additionally, enhanced tolerance towards polar groups in copolymerization of ethylene and polar monomers was realized by dinaphthobarrelene-derived α-diimine nickel and palladium catalysts.

A comprehensive picture on catalyst structure construction in palladium catalyzed ethylene (co)polymerizations

Late transition metal palladium catalysts chelated by benchmark α-diimine, phosphine-sulfonate, bisphosphine-monoxide ligands are of great interest for ethylene polymerization and challenging copolymerization with polar comonomers. Compared to numerous studies on the classic α-diimine and phosphine-sulfonate palladium catalysts, the new generation bisphosphine-monoxide palladium (BPMO-Pd) system is far less investigated. In this contribution, by skillfully designing a family of BPMO-Pd catalysts bearing differed steric and electronic P(III) phosphine groups, differed steric and electronic O = P(V) phosphine monoxide groups, and different connectivity of them, a comprehensive picture on catalyst structure construction is built for ethylene (co)polymerizations. This constructed catalyst structure-polymerization property relationship paves the way for the development of more efficient late transition metal catalysts for olefin (co)polymerization.

Flexible cycloalkyl substituents in insertion polymerization with α-diimine nickel and palladium species

The investigation of the relationship between the structure of the catalyst and the microstructure of the obtained polymer has attracted much attention and broad interest in the field of transition metal-catalyzed olefin polymerization.

A remote nonconjugated electron effect in insertion polymerization with α-diimine nickel and palladium species

The remote nonconjugated electronic perturbations exert great influence on ethylene polymerization.

Front Cover Picture: Bis‐Alkoxycarbonylation of Acrylic Esters and Amides for the Synthesis of 2‐Alkoxycarbonyl or 2‐Carbamoyl Succinates (Adv. Synth. Catal. 3/2020)

The front cover picture, provided by Carla Carfagna et al., shows the first example of an oxidative bis-alkoxycarbonylation of acrylic esters and acrylic amides, leading to 2-EWG-substituted succinates. Despite the low reactivity of electron-deficient olefins in carbonylation reactions, the aryl α-diimine palladium(II) catalyst is able to promote the carbonylation of both the β- and the generally non-reactive α-positions of electron-poor alkenes, under mild reaction conditions (PCO=4 bar, T=20 °C). Further details of this work can be found in the full paper on pages 533–544 (D. Olivieri, R. Tarroni, N. Della Ca′, R. Mancuso, B. Gabriele, G. Spadoni, C. Carfagna, Adv. Synth. Catal. 2020, 362, 533–544; DOI: 10.1002/adsc.201900918).

π–π interaction effect in insertion polymerization with α-Diimine palladium systems

Control of branching densities of polyolefins by polymerization conditions is a great challenge in the field of α-diimine palladium catalyzed olefin (co)polymerization. In this contribution, a series of acenaphthene-based highly sterically hindered α-diimine ligands bearing bulky diarylmethyl moiety and corresponding palladium catalysts have been designed, synthesized, characterized and investigated for ethylene polymerization and copolymerization with several polar monomers. The π-π interaction between the acenaphthene moiety and the phenyl of diarylmethyl moiety was observed. It was suggested that this π-π interaction could freeze the N-aryl-bond rotation at room temperature, resulting in the enhanced axial steric bulk as well as the low branching densities of the polyethylene products. In contrast, at high temperature, the π-π interaction was weaken and resulted in significantly increased branching densities of polyethylene products, indicating branching densities of polyolefins are adjustable within a wide range by polymerization conditions using this catalytic system.

Redox-switchable α-diimine palladium catalysts for control of polyethylene topology

A series of palladium complexes bearing ferrocene substituted α-diimine ligands was synthesized and investigated for ethene polymerization at different conditions to modulate the extent of “chain-walking” mechanism and regulate branching and topology of resulting polyethylenes. All ferrocene substituted complexes catalyzed living/controlled ethene polymerization. The ability of ferrocene substituted palladium complexes to provide polyethylenes with dendritic topology was proved by measuring their Mark-Houwink plots. In-situ chemical oxidation of ferrocene moieties via silver triflate was used to affect the catalyst electronic structure and support the “chain-walking” mechanism. Oxidation of palladium catalyst led to its destabilization while the catalytic activity of newly formed sites was substantially increased. It was demonstrated that catalyst oxidation is a powerful tool to regulate the topology of resulting polyethylenes.

Pentiptycenyl Substituents in Insertion Polymerization with α-Diimine Nickel and Palladium Species

Motivated by the need for a new generation of α-diimine Ni(II) and Pd(II) catalysts for tuning the catalytic activity, polymer molecular weight, comonomer incorporation, and branching density in ethylene polymerization and copolymerization with polar monomers, a family of α-diimine Ni(II) and Pd(II) catalysts Ipty-Ni1–4 and Ipty-Pd1–4 derived from sterically demanding and rotationally restricted pentiptycenyl N-aryl substituents were synthesized and fully characterized by NMR, IR, MALDI-TOF, elemental analysis, and X-ray diffraction. Pentiptycenyl-substituted Ni(II) and Pd(II) catalysts were further probed in ethylene (co)polymerization as a comparison with the rotationally free dibenzhydryl substituent reported previously. In the Ni-catalyzed ethylene polymerization (20–80 °C), catalytic activities ((0.64–3.74) × 106 g mol–1 h–1), polymer molecular weights ((1.1–37.7) × 104 g mol–1), branching densities (6–55/1000C), and melting points (94–135 °C) could be tuned over a broad range. In the Pd-catalyzed et...

Synthesis of functional and hyperbranched ethylene oligomers using unsymmetrical α-diimine palladium catalysts

A series of unsymmetrical α-diimine palladium complexes bearing low steric crowdedness on one side and bulky substituents on the other side were prepared and characterized. Activated with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate, these α-diimine Pd(II) complexes were used to catalyze ethylene oligomerization and cooligomerization with polar monomers including methyl acrylate (MA), acrylic acid (AA) and vinyltriethoxysilane (VTEoS). In ethylene oligomerization, the hyperbranched oil products were obtained with high activities (14.2–27.0 × 103 g mol−1 h−1). The remote substituent of bulky diarylmethyl moiety showed a significant effect on catalyst activity, while exhibit little variation on the molecular weight of ethylene oligomers. In ethylene-polar monomer cooligomerization, functional and hyperbranched oils were produced and polar monomer incorporations can be tuned by the perturbations of the remote substituent.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

Here we will summarize some of the recent advances in α-diimine ligand developments, as well as some new and challenging monomers that this class of catalysts can address through these ligand improvements.

Bulky yet flexible substituents in insertion polymerization with α-diimine nickel and palladium systems

The bulky yet flexible substituent on the N-aryl moieties of α-diimine ligands may adopt different conformations and provide dynamic steric hindrance in the process of ethylene (co)polymerization.

Living (co)polymerization of ethylene and bio-based furfuryl acrylate using dibenzobarrelene derived α-diimine palladium catalysts

Living (co)polymerizations of petroleum-based ethylene and bio-based furfuryl acrylate were realized using dibenzobarrelene-derived α-diimine palladium catalysts.

Enhancement on Alternating Copolymerization of Carbon Monoxide and Styrene by Dibenzobarrelene-Based α-Diimine Palladium Catalysts

CO/styrene copolymerization by α-diimine palladium catalysts is a promising method for direct synthesis of polyketones. The effect of the catalyst backbone structure on CO/styrene copolymerization has been studied with the aim of developing robust α-diimine palladium catalysts able to improve the polymerization productivity and controllability. Dibenzobarrelene derived α-diimine palladium catalysts without o-aryl substituents were designed and synthesized for CO/styrene alternating copolymerization. Introduction of the rigid and bulky dibenzobarrelene backbone enhanced the thermal stability and the productivity of palladium catalyst. The dibenzobarrelene ligand backbone also improved the polymerization controllability, and living CO/styrene copolymerizations were achieved at 15 °C using α-diimine palladium catalysts in CH2Cl2. The steric hindrance of backbone and the π–π stacking between the dibenzobarrelene backbone and the aniline played crucial roles in stereocontrol and productivity.

Synthesis, characterization, and olefin (co)polymerization behavior of unsymmetrical α-diimine palladium complexes containing bulky substituents at 4-position of aniline moieties

Three novel unsymmetrical α-diimine Pd(II) complexes containing bulky substituents at 4-position of aniline moieties were prepared and characterized. Unfortunately, the introduction of large substituents on para- N-aryl moieties did not slow the exchange between the anti and syn forms for these complexes. These unsymmetrical α-diimine Pd(II) complexes also did not improve thermal stability and broaden molecular weight distribution in ethylene polymerization. However, these α-diimine palladium catalysts could copolymerize ethylene with biorenewable comonomer acrylic acid (AA), with comonomer incorporation in the range of 1.1–2.7% and high copolymer molecular weights. The AA units are incorporated predominately at the end of the branches. In addition, a systematic investigation on the polymerization of 1-hexene, 1-octene and 1-decene using these complexes was also performed. Changes in the ligand sterics and monomer length can influence the branch density and molecular weight of poly(α-olefins). Interestingly, these α-olefin polymers show properties characteristic of thermoplastic elastomers, i.e., good elastomeric recovery and high strain at break. Pervious work has shown that ethylene or α-olefin polymerization using some nickel α-diimine catalysts can generate elastic polyolefin materials. This work may provide an alternative and effective strategy to synthesize thermoplastic elastomers by α-diimine Pd(II) catalysts in one step using only α-olefin as the feedstock.

Direct Synthesis of Polar-Functionalized Linear Low-Density Polyethylene (LLDPE) and Low-Density Polyethylene (LDPE)

Late-transition-metal catalysts have great potentials to incorporate polar comonomers during olefin polymerization. The preparation of polar-functionalized polyolefins with different microstructures and topologies is a highly fascinating concept. In this contribution, we demonstrate this possibility through palladium-catalyzed ethylene copolymerizations with polar-functionalized α-olefins and their terpolymerizations with α-olefins. A phosphine–sulfonate–palladium catalyst (PO-Pd) afforded highly linear polyethylene during ethylene homopolymerization. Linear low-density polyethylene (LLDPE) was obtained from the PO-Pd-catalyzed copolymerizations of ethylene with α-olefins; in these copolymerizations, the partial or complete replacement of α-olefins with polar-functionalized α-olefins led to the formation of polar-functionalized LLDPE (P-LLDPE). A specially designed α-diimine palladium catalyst (NN-Pd) afforded polyethylenes with tunable branching densities (16–37 per 1000 carbon atoms), melting points (10...

Accessing Multiple Catalytically Active States in Redox-Controlled Olefin Polymerization

The majority of work in the field of olefin polymerization catalysis has been focused on ligand modifications. In this work, we describe an alternative strategy for the modulation of olefin polymerization and copolymerization processes. The two ferrocenyl units in an α-diimine palladium catalyst can be oxidized in a stepwise fashion. This stepwise redox control can be used to modulate the catalyst properties during the homopolymerization of ethylene and 1-hexene, as well as the copolymerizations of ethylene with norbornene, methyl acrylate, and 5-norbornene-2-yl acetate. Moreover, polymer microstructure and polydispersity can be controlled during these stepwise oxidation processes.