Copper-catalyzed Atom Transfer Radical Research Articles

Conjugated cross-linked phosphine as broadband light or sunlight-driven photocatalyst for large-scale atom transfer radical polymerization

The use of light to regulate photocatalyzed reversible deactivation radical polymerization (RDRP) under mild conditions, especially driven by broadband light or sunlight directly, is highly desired. But the development of a suitable photocatalyzed polymerization system for large-scale production of polymers, especially block copolymers, has remained a big challenge. Herein, we report the development of a phosphine-based conjugated hypercrosslinked polymer (PPh3-CHCP) photocatalyst for an efficient large-scale photoinduced copper-catalyzed atom transfer radical polymerization (Cu-ATRP). Monomers including acrylates and methyl acrylates can achieve near-quantitative conversions under a wide range (450–940 nm) of radiations or sunlight directly. The photocatalyst could be easily recycled and reused. The sunlight-driven Cu-ATRP allowed the synthesis of homopolymers at 200 mL from various monomers, and monomer conversions approached 99% in clouds intermittency with good control over polydispersity. In addition, block copolymers at 400 mL scale can also be obtained, which demonstrates its great potential for industrial applications.

Solvent Coordination Effect on Copper-Based Molecular Catalysts for Controlled Radical Polymerization

The equilibrium of copper-catalyzed atom transfer radical polymerization was investigated in silico with the aim of finding an explanation for the experimentally observed solvent effect. Various combinations of alkyl halide initiators and copper complexes in acetonitrile (MeCN) and dimethyl sulfoxide (DMSO) were taken into consideration. A continuum model for solvation, which does not account for the explicit interactions between the solvent and metal complex, is not adequate and does not allow the reproduction of the experimental trend. However, when the solvent molecules are included in the coordination sphere of the copper(I,II) species and the continuum description of the medium is still used, a solvent dependence of process thermodynamics emerges, in fair agreement with experimental trends.

Red-Light-Induced, Copper-Catalyzed Atom Transfer Radical Polymerization.

Despite advances in photochemical atom transfer radical polymerization (photoATRP), these systems often rely on the use of UV light for the activation/generation of the copper-based catalytic species. To circumvent the problems associated with the UV light, we developed a dual photoredox catalytic system to mediate photoinduced ATRP under red-light irradiation. The catalytic system is comprised of a Cu catalyst to control the polymerization via ATRP equilibrium and a photocatalyst, such as zinc(II) tetraphenylporphine or zinc(II) phthalocyanine, to generate the activator CuI species under red-light irradiation. In addition, this system showed oxygen tolerance due to the consumption of oxygen in the photoredox reactions, yielding well-controlled polymerizations without the need for deoxygenation processes.

Mapping the Pathway to Organocopper(II) Complexes Relevant to Atom Transfer Radical Polymerization.

The rare organocopper(II) complex [Cu(Me6tren)(CH2CN)]+ (Me6tren = tris(2-(dimethylamino)ethyl)amine) has emerged as an important model of potential byproducts in copper-catalyzed atom transfer radical polymerization. This complex has been generated by controlled potential electrolysis of [Cu(Me6tren)(NCMe)]2+ in the presence of BrCH2CN. Time-resolved UV-vis and continuous wave and pulse electron paramagnetic resonance (EPR) spectra identified [Cu(Me6tren)Br]+ as an intermediate. Hyperfine sublevel correlation and electron nuclear double resonance spectroscopy of samples at different timepoints reveal signals that are assigned to a C-bound cyanomethylate ligand, with distinct 14N and 1H hyperfine coupling constants in comparison with the corresponding N-bound acetonitrile and bromido complexes. The experimental EPR data are supported by density functional theory calculations to understand how the geometries of the species involved produce distinct spectroscopic signatures, and a clear picture of how this unusual organocopper(II) complex is formed has emerged.

Conjugated Cross-linked Phenothiazines as Green or Red Light Heterogeneous Photocatalysts for Copper-Catalyzed Atom Transfer Radical Polymerization.

Using the power of light to drive controlled radical polymerizations has provided significant advances in synthesis of well-defined polymers. Photoinduced atom transfer radical polymerization (ATRP) systems often employ UV light to regenerate copper activator species to mediate the polymerization. Taking full advantage of long-wavelength visible light for ATRP would require developing appropriate photocatalytic systems that engage in photoinduced electron transfer processes with the ATRP components to generate activating species. Herein, we developed conjugated microporous polymers (CMP) as heterogeneous photocatalysts to exploit the power of visible light in promoting copper-catalyzed ATRP. The photocatalyst was designed by cross-linking phenothiazine (PTZ) as a photoactive core in the presence of dimethoxybenzene as a cross-linker via the Friedel-Crafts reaction. The resulting PTZ-CMP network showed photoactivity in the visible region due to the extended conjugation throughout the network because of the aromatic groups connecting the PTZ units. Therefore, photoinduced copper-catalyzed ATRP was performed with CMPs that regenerated activator species under green or red light irradiation to start the ATRP process. This resulted in efficient polymerization of acrylate and methacrylate monomers with high conversion and well-controlled molecular weight. The heterogeneous nature of the photocatalyst enabled easy separation and efficient reusability in subsequent polymerizations.

Light-Mediated Dual Phosphine-/Copper-Catalyzed Atom Transfer Radical Addition Reaction.

The atom transfer radical addition reaction catalyzed by triphenylphosphine and copper(I) halide is described. The reaction proceeds under irradiation with 365 nm light using a light-emitting diode and was performed in regular glassware. The proposed mechanism involves the formation of quaternary phosphonium salt, which undergoes single electron reduction by copper(I) salt via photo-induced electron transfer. The method works well for terminal alkenes and activated organic halides such as esters of bromo- and iodoacetic acid and bromoacetonitrile. gem-Difluorinated styrenes, for which atom transfer reactions are rare, also proved to be good substrates for this phosphine-/copper-catalyzed protocol.

Impact of Organometallic Intermediates on Copper-Catalyzed Atom Transfer Radical Polymerization

In atom transfer radical polymerization (ATRP), radicals (R•) can react with CuI/L catalysts forming organometallic complexes, R–CuII/L (L = N-based ligand). R–CuII/L favors additional catalyzed ra...

Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP).

Copper-catalyzed atom transfer radical polymerization (Cu-ATRP) is one of the most widely used controlled radical polymerization techniques. Notwithstanding the extensive mechanistic studies in the literature, the transition states of the activation/deactivation of the growing polymer chain, a key equilibrium in Cu-ATRP, have not been investigated computationally. Therefore, the understanding of the origin of ligand and initiator effects on the rates of activation/deactivation is still limited. Here, we present the first computational analysis of Cu-ATRP activation transition states to reveal factors that affect the rates of activation and deactivation. The Br atom transfer between the polymer chain and the Cu catalyst occurs through an unusual bent geometry that involves pronounced interactions between the polymer chain end and the ancillary ligand on the Cu catalyst. Therefore, the rates of activation/deactivation are determined by both the electronic properties of the Cu catalyst and the ligand-initiator steric repulsions. In addition, our calculations revealed the important role of ligand backbone flexibility on the activation. These theoretical analyses led to the identification of three chemically meaningful descriptors, namely HOMO energy of the catalyst ( EHOMO), percent buried volume ( Vbur%), and distortion energy of the catalyst (Δ Edist), to describe the electronic, steric, and flexibility effects on reactivity, respectively. A robust and simple predictive model for ligand effect on reactivity is thereby established by correlating these three descriptors with experimental activation rate constants using multivariate linear regression. Validation using a structurally diverse set of ligands revealed the average error is less than ±2 kcal/mol compared to the experimentally derived activation energies. The same approach was also applied to develop a predictive model for reactivity of different alkyl halide initiators using R-X bond dissociation energy (BDE) and Cu-X halogenophilicity as descriptors.

Synthesis and Characterization of the Most Active Copper ATRP Catalyst Based on Tris[(4-dimethylaminopyridyl)methyl]amine.

The tris[(4-dimethylaminopyridyl)methyl]amine (TPMANMe2) as a ligand for copper-catalyzed atom transfer radical polymerization (ATRP) is reported. In solution, the [CuI(TPMANMe2)Br] complex shows fluxionality by variable-temperature NMR, indicating rapid ligand exchange. In the solid state, the [CuII(TPMANMe2)Br][Br] complex exhibits a slightly distorted trigonal bipyramidal geometry (τ = 0.89). The UV-vis spectrum of [CuII(TPMANMe2)Br]+ salts is similar to those of other pyridine-based ATRP catalysts. Electrochemical studies of [Cu(TPMANMe2)]2+ and [Cu(TPMANMe2)Br]+ showed highly negative redox potentials (E1/2 = -302 and -554 mV vs SCE, respectively), suggesting unprecedented ATRP catalytic activity. Cyclic voltammetry (CV) in the presence of methyl 2-bromopropionate (MBrP; acrylate mimic) was used to determine activation rate constant ka = 1.1 × 106 M-1 s-1, confirming the extremely high catalyst reactivity. In the presence of the more active ethyl α-bromoisobutyrate (EBiB; methacrylate mimic), total catalysis was observed and an activation rate constant ka = 7.2 × 106 M-1 s-1 was calculated with values of KATRP ≈ 1. ATRP of methyl acrylate showed a well-controlled polymerization using as little as 10 ppm of catalyst relative to monomer, while side reactions such as CuI-catalyzed radical termination (CRT) could be suppressed due to the low concentration of L/CuI at a steady state.

An efficient, heterogeneous, reusable atom transfer radical polymerization catalyst

AbstractMelamine based porous polymer (MPP) was prepared as a template solid to incorporate Cu(I) cations into the framework through chelating with nitrogen groups of the melamine. The copper integrated porous material (Cu(I)@MPP) was used as a heterogeneous catalyst and displayed high activity in copper catalyzed atom transfer radical polymerization of methyl methacrylate. The characterization of the Cu(I)@MPP was performed using nitrogen adsorption experiments and wide‐angle X‐ray and Fourier transform infrared spectroscopies. The atomic absorption spectroscopy measurements confirmed that the catalyst is practically non‐leaching and Cu(I) was found to be below 20 ppb after each atom transfer radical polymerization. Moreover, the catalyst showed reusability without any significant change in its activity. © 2017 Society of Chemical Industry

Copper Catalyzed Atom Transfer Radical Cyclization Reactions

AbstractAtom transfer radical cyclization (ATRC) is a powerful technique to prepare functionalized 4–10 membered ring systems using transition metal catalysts. In this review recent advances in the use of copper complexes are described. In particular the effect of different ligands on reactivity, selectivity and product outcome are illustrated with application to heterocycle and natural product synthesis. New approaches that increase catalyst efficiency with supplemental reducing agents are reviewed and examples using green solvents, benign reducing agents (e.g. ascorbic acid) and with catalyst loadings as low as 0.05 mol‐% are highlighted. Radical addition to aromatic rings and rearrangement reactions under ATRC conditions as well as application to radical‐polar crossover reactions are also described.

Towards the development of highly active copper catalysts for atom transfer radical addition (ATRA) and polymerization (ATRP)‡

Fundamentals of copper catalyzed atom transfer radical addition (ATRA) and mechanistically similar polymerization (ATRP) were discussed. Special emphasis was placed on structural characterization and electrochemical properties of copper complexes. Recent advances in the development of highly active copper complexes for both processes were also reviewed. It was found that electron-donating groups (methoxy and methyl in the 4 and 3,5 positions, respectively) of the pyridine rings in tris(2-pyridylmethyl)amine (TPMA) ligand, significantly increase the catalytic activity in copper mediated ATRA/ATRP.

On the mechanism of activation of copper-catalyzed atom transfer radical polymerization

The mechanism of activation of atom transfer radical polymerization (ATRP) has been analyzed by investigating the kinetics of dissociative electron transfer (ET) to alkyl halides (RX) in acetonitrile. Using a series of alkyl halides, including both bromides and chlorides, the rate constants of ET (kET) to RX by electrogenerated aromatic radical anions (A−) acting as outer-sphere donors have been measured and analyzed according to the current theories of dissociative ET. This has shown that the kinetic data fit very well the “sticky” dissociative ET model with the formation of a weak adduct held together by electrostatic interactions. The rate constants of activation, kact, of some alkyl halides, namely chloroacetonitrile, methyl 2-bromopropionate and ethyl chloroacetate, by [CuIL]+ (L=tris(2-dimethylaminoethyl)amine, tris(2-pyridylmethyl)amine, 1,1,4,7,7-pentamethyldiethylenetriamine) have also been measured in the same experimental conditions. Comparisons of the measured kact values with those predicted assuming an outer-sphere ET for the complexes have shown that activation by Cu(I) is 7–10 orders of magnitude faster than required by outer-sphere ET. Therefore, the mechanism of RX activation by Cu(I) complexes used as catalysts in ATRP occurs by an inner-sphere ET or more appropriately by a halogen atom abstraction.

Preparation of phosphine-functionalized polystyrene stars by metal catalyzed controlled radical copolymerization and their application to hydroformylation catalysis

Well defined star copolymers have been prepared by copper-catalyzed atom transfer radical copolymerization of styrene and styryldiphenylphosphine starting from a modified Boltorn™ H30 multifunctional initiator. These polymers and an analogue obtained by debromination of the arm ends with nBu3SnH have been used in combination with [Rh(acac)(CO)2] for the homogeneous phase hydroformylation of 1-octene.

Substituted Tris(2-pyridylmethyl)amine Ligands for Highly Active ATRP Catalysts.

The synthesis and application of a very active catalyst for copper-catalyzed atom transfer radical polymerizations (ATRP) with tris([(4-methoxy-2,5-dimethyl)-2-pyridyl] methyl)amine (TPMA*) ligand is reported. Catalysts with TPMA* ligands are approximately 3 orders of magnitude more active than those with tris(2-pyridylmethyl)amine (TPMA). Catalyst activity was evaluated by cyclic voltammetry, stopped-flow, and ATRP kinetics. Catalysts with TPMA* ligands perform better than those with TPMA ligands, especially at low catalyst concentrations.

Transesterification of functional methacrylate monomers during alcoholic copper-catalyzed atom transfer radical polymerization: formation of compositional and architectural side products

Neutral methacrylate monomers undergo transesterification in methanol to produce multiple by-products under copper-catalyzed atom transfer radical polymerization (ATRP) conditions. The side-reactions occur for a range of hydrophilic methacrylates but some may be reduced by employing higher alcohols. This study demonstrates that care must be taken when using alcohol solvents for the synthesis of architecturally and compositionally pure methacrylate (co)polymers via copper-catalyzed conventional ATRP.

Photoinitiated ambient temperature copper-catalyzed atom transfer radical addition (ATRA) and cyclization (ATRC) reactions in the presence of free-radical diazo initiator (AIBN)

The use of UV light in copper-catalyzed atom transfer radical addition (ATRA) and cyclization (ATRC) reactions of various (poly)halogenated compounds to highly active alkenes in the presence of AIBN is reported. Radicals generated from photodecomposition of AIBN efficiently regenerated the copper(I) complex at ambient temperature enabling ATRA of CCl(4) and CBr(4) with catalyst loadings as low as 0.05 mol-%. The desired monoadduct was obtained in lower yields in the ATRA of less active halogenated compounds, which was mostly due to incomplete alkene conversions. Ambient temperature ATRA of CCl(4) to various 1,6-dienes followed by sequential ATRC was also performed in the presence of UV light using [Cu(II)(TPMA)Cl][Cl] complex and AIBN. High yields of the 5-exo-trig cyclic product were obtained for all dienes with preferential formation of the cis isomer.

Alkylation of polyethyleneimine for homogeneous ligands in ATRP

Ethylated and butylated polyethyleneimine ligands were synthesized and employed in copper catalyzed atom transfer radical polymerization of styrene and methyl methacrylate with suitable initiators in order to obtain homogeneous polymerizations, resulting in well defined polymers with low polydispersities. Linear curves drawn from kinetics and conversion–molecular weight plots indicate that all the polymerizations were successfully controlled. In ATRP reactions of S and MMA, the apparent rate of polymerization, k p app, exhibits a plateau at [Ligand]/[CuBr] ≥ 0.5 ratio for both ligands. The apparent rate constant also decreases by increasing the alkyl chain length of the alkylated polyethyleneimine ligand. Ethylated and butylated polyethyleneimine ligands in ATRP of S and MMA were found to be faster than the existing ATRP ligands.

Direct Synthesis of Well-Defined Poly[{2-(methacryloyloxy)ethyl}trimethylammonium chloride] Brush via Surface-Initiated Atom Transfer Radical Polymerization in Fluoroalcohol

A well-defined cationic polymer and polymer brush were simultaneously prepared by copper-catalyzed atom transfer radical polymerization (ATRP) of 2-(methacryloxy)ethyltrimethylammonium chloride (MTAC) in 2,2,2-trifluoroethanol (TFE) at 60 °C initiated with ethyl 2-bromoisobutylate and the corresponding alkylbromide immobilized on silicon wafer. ATRP of MTAC in TFE as well as aqueous solution proceeded quickly but in a poorly controlled manner, leading to a relatively higher molecular weight distribution (MWD) than Mw/Mn = 1.3, where Mw and Mn are weight-average and number-average molecular weights, respectively. Addition of a small amount of isopropyl alcohol to the TFE-based reaction mixture reduced the polymerization rate and produced a poly(MTAC) with predictable molecular weight and narrower MWD than 1.19. The Mn of poly(MTAC) was controllable in the range 104−105 g mol−1 based on the molar ratio of monomer and initiator in the feed. The Mn of the brush on a flat silicon wafer matched that of the free poly(MTAC). The graft density of poly(MTAC) was 0.20 chains nm−2. The ATRP of MTAC in TFE/1-ethyl-3-methylimidazolium chloride (EMImCl) also proceeded in a controlled manner at 60 °C, yielding polymers with narrow MWDs (Mw/Mn = 1.12−1.13). Analysis of the 13C NMR spectra of the carbonyl region of the resulting poly(MTAC)s revealed that syndiotactic-rich configurations were produced by ATRP of MTAC in aqueous solution, TFE/isopropyl alcohol, and TFE/EMImCl. The solvent effect of TFE and ionic liquid on the tacticity of polymer was negligible.

Kinetic Studies of the Initiation Step in Copper Catalyzed Atom Transfer Radical Addition (ATRA) in the Presence of Free Radical Diazo Initiators as Reducing Agents

Kinetic parameters for the reduction of copper(II) complexes in atom transfer radical addition (ATRA) in the presence of free-radical diazo initiator (AIBN) were determined using both experimental and kinetic modeling techniques. The rate constant of decomposition of AIBN (k(dc)) in various solvents was determined at 60 degrees C using UV-vis spectroscopy. Rate constants of deactivation (k(d,AIBN)) of [Cu(II)(TPMA)Cl][Cl] (TPMA = tris(2-pyridylmethyl)amine), [Cu(II)(Me(6)TREN)Cl][Cl] (Me(6)TREN = tris[2-(N,N-dimethylamino)ethyl]amine), [Cu(II)(PMDETA)Cl(2)] (PMDETA = N,N,N',N'',N''-pentamethyldiethylenetriamine), and [Cu(II)(bpy)(2)Cl][Cl] (bpy = 2,2'-bipyridine) complexes by radicals generated from the decomposition of AIBN were measured using the TEMPO-trapping method in a competitive clock reaction. Activation rate constants (k(a,AIBN)) were finally estimated from kinetic modeling utilizing the experimentally determined rate constants of decomposition of AIBN and deactivation. The effect of k(a,AIBN), k(d,AIBN), k(dc) and initial AIBN concentration on the overall copper(I) and copper(II) concentrations in the initiation step of the ATRA process was also evaluated through kinetic modeling.