Synthesis of Functional Polyolefins Using Cationic Bisphosphine Monoxide–Palladium Complexes

Copolymerisation of ethylene with polar monomers by using palladium catalysts bearing an N-heterocyclic carbene–phosphine oxide bidentate ligand

We report the synthesis and characterisation of palladium complexes bearing an N-heterocyclic carbene–phosphine oxide bidentate ligand and their use as catalysts for ethylene polymerisation and ethylene/polar monomer copolymerisation.

Mechanistic Studies of Pd(II)-Catalyzed Copolymerization of Ethylene and Vinylalkoxysilanes: Evidence for a β-Silyl Elimination Chain Transfer Mechanism

Copolymerizations of ethylene with vinyltrialkoxysilanes are reported using both a "traditional" cationic Pd(II) aryldiimine catalyst, t-1 (aryl = 2,6-diisopropylphenyl), and a "sandwich-type" aryldiimine catalyst, s-2 (aryl = 8-tolylnaphthyl). Incorporation levels of vinyltrialkoxysilanes between 0.25 and 2.0 mol % were achieved with remarkably little rate retardation relative to ethylene homopolymerizations. In the case of the traditional catalyst system, molecular weights decrease as the level of comonomer increases and only one trialkoxysilyl group is incorporated per chain. Molecular weight distributions of ca. 2 are observed. For the sandwich catalyst, higher molecular weights are observed with many more trialkoxysilyl groups incorporated per chain. Polymers with molecular weight distributions of ca. 1.2-1.4 are obtained. Detailed NMR mechanistic studies have revealed the formation of intermediate π-complexes of the type (diimine)Pd(alkyl)(vinyltrialkoxysilane)⁺. 1,2-Migratory insertions of these complexes occur with rates similar to ethylene insertion and result in formation of observable five-membered chelate intermediates. These chelates are rapidly opened with ethylene forming alkyl ethylene complexes, a requirement for chain growth. An unusual β-silyl elimination mechanism was shown to be responsible for chain transfer and formation of low molecular weight copolymers in the traditional catalyst system, t-1. This chain transfer process is retarded in the sandwich system. Relative binding affinities of ethylene and vinyltrialkoxysilanes to the cationic palladium center have been determined. The quantitative mechanistic studies reported fully explain the features of the bulk polymerization results.

Highly Isospecific Polymerization of Silyl-Protected ω-Alkenols Using an [OSSO]-Type Bis(phenolato) Dichloro Zirconium(IV) Precatalyst

The coordination polymerization of silyl-protected ω-alkenols such as ω-alken-α-oxytriisopropylsilanes 1 provides poly(ω-alkenyl-α-oxytriisopropylsilalne)s with a highly isospecific microstructure ([mmmm] > 95%) when a combination of [OSSO]-type bis(phenolato) dichloro zirconium(IV) complex 2 and dried methylaluminoxane is used as the precatalyst and activator, respectively. The resulting siloxy-substituted polymers could be efficiently transformed into the corresponding functionalized polyolefins, which contained up to 90% acetyl groups and ≈7% hydroxy groups in the terminal side chains.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

Ligand-controlled insertion regioselectivity accelerates copolymerisation of ethylene with methyl acrylate by cationic bisphosphine monoxide-palladium catalysts

A new series of palladium catalysts ligated by a chelating bisphosphine monoxide bearing diarylphosphino groups (aryl-BPMO) exhibits markedly higher reactivity for ethylene/methyl acrylate copolymerisation when compared to the first generation of alkyl-BPMO-palladium catalysts that contain a dialkylphosphino moiety. Mechanistic studies suggest that the origin of this disparate catalyst behavior is a change in regioselectivity of migratory insertion of the acrylate comonomer as a function of the phosphine substituents. The best aryl-BPMO-palladium catalysts for these copolymerisations were shown to undergo exclusively 2,1-insertion, and this high regioselectivity avoids formation of a poorly reactive palladacycle intermediate. Furthermore, the aryl-BPMO-palladium catalysts can copolymerise ethylene with other industrially important polar monomers.

Adjustable coordination of a hybrid phosphine–phosphine oxide ligand in luminescent Cu, Ag and Au complexes

The mixed-donor ligand shows variable binding ability with respect to d¹⁰ metal ions to afford a series of mono- and dinuclear complexes with tunable photophysical characteristics.

Influence of ligand second coordination sphere effects on the olefin (co)polymerization properties of α-diimine Pd(ii) catalysts

A series of α-diimine ligands and their corresponding palladium complexes were synthesized and characterized.

Insertion Homo- and Copolymerization of Diallyl Ether

The previously unresolved issue of polymerization of allyl monomers CH2 CHCH2 X is overcome by a palladium-catalyzed insertion polymerization of diallyl ether as a monomer. An enhanced 2,1-insertion of diallyl ether as compared to mono-allyl ether retards the formation of an unreactive five-membered cyclic O-chelate (after 1,2-insertion) that otherwise hinders further polymerization, and also enhances incorporation in ethylene polymers (20.4 mol %). Cyclic ether repeat units are formed selectively (96 %-99 %) by an intramolecular insertion of the second allyl moiety of the monomer. These features even enable a homopolymerization to yield polymers (poly-diallyl ether) with degrees of polymerization of DPn ≈44.

Mono-Oxidation of Bidentate Bis-phosphines in Catalyst Activation: Kinetic and Mechanistic Studies of a Pd/Xantphos-Catalyzed C-H Functionalization

Kinetic, spectroscopic, crystallographic, and computational studies probing a Pd-catalyzed C-H arylation reaction reveal that mono-oxidation of the bis-phosphine ligand is critical for the formation of the active catalyst. The bis-phosphine mono-oxide is shown to be a hemilabile, bidentate ligand for palladium. Isolation of the oxidative addition adduct, with structural elucidation by X-ray analysis, showed that the mono-oxide was catalytically competent, giving the same reaction rate in the productive reaction as the Pd(II)/xantphos precursor. A dual role for the carboxylate base in both catalyst activation and reaction turnover was demonstrated, along with the inhibiting effect of excess phosphine ligand. The generality of the role of phosphine mono-oxide complexes in Pd-catalyzed coupling processes is discussed.

Insertion Copolymerization of Difunctional Polar Vinyl Monomers with Ethylene

A single-step synthesis, structural characterization and application of a neutral, acetonitrile ligated, palladium-phosphinesulfonate complex [{P^∧O}PdMe(L)] (P^∧O = κ²-P,O-Ar₂PC₆H₄SO₂O with Ar = 2-MeOC₆H₄; L = CH₃CN) (3) in coordination/insertion copolymerization of ethylene with difunctional olefin is investigated. In a significant development, complex 3 was found to catalyze insertion copolymerization of industrially relevant 1,1-disubstituted difunctional vinyl monomers for the first time. Thus, insertion copolymerization of ethyl-2-cyanoacrylate (ECA or super glue) and trifluoromethyl acrylic acid (TFMAA) with ethylene produced the corresponding copolymers with 6.5% ECA and 3% TFMAA incorporation. Increasing the concentration of difunctional olefins led to higher incorporation but at the expense of lower activities. These observations indicate that complex 3 tolerates difunctional vinyl monomers and provides direct access to difunctional polyolefins that have not been attempted before.

Synthesis of a 2,4,6,8,10-dodecapentanoic acid thioester as a substrate for biosynthesis of Heat Stable Antifungal Factor (HSAF)

The N-acetylcystamine (SNAC) thioester of dodecapentaenoic acid, an analog of a putative intermediate in the biosynthesis of Heat Stable Antifungal Factor (HSAF), is synthesized. Key steps include sequential Horner-Emmons homologations with the Weinreb amide of diethylphosponoacetic acid, and thioesterification of an aldol-derived 3-hydroxyalkanoate, which serves as a stable precursor of the sensitive polyenoate. The thioester was investigated as a biosynthetic substrate using a purified nonribosomal peptide synthetase and was not incorporated in the observed products.

Ortho-phosphinobenzenesulfonate: a superb ligand for palladium-catalyzed coordination-insertion copolymerization of polar vinyl monomers

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.