Copper(I)-Catalyzed Atom Transfer Radical Polymerization

α-Halocarbonyls as a Valuable Functionalized Tertiary Alkyl Source

This review introduces the synthetic organic chemical value of α-bromocarbonyl compounds with tertiary carbons. This α-bromocarbonyl compound with a tertiary carbon has been used primarily only as a radical initiator in atom transfer radical polymerization (ATRP) reactions. However, with the recent development of photo-radical reactions (around 2010), research on the use of α-bromocarbonyl compounds as tertiary alkyl radical precursors became popular (around 2012). As more examples were reported, α-bromocarbonyl compounds were studied not only as radicals but also for their applications in organometallic and ionic reactions. That is, α-bromocarbonyl compounds act as nucleophiles as well as electrophiles. The carbonyl group of α-bromocarbonyl compounds is also attractive because it allows the skeleton to be converted after the reaction, and it is being applied to total synthesis. In our survey until 2022, α-bromocarbonyl compounds can be used to perform a full range of reactions necessary for organic synthesis, including multi-component reactions, cross-coupling, substitution, cyclization, rearrangement, stereospecific reactions, asymmetric reactions. α-Bromocarbonyl compounds have created a new trend in tertiary alkylation, which until then had limited reaction patterns in organic synthesis. This review focuses on how α-bromocarbonyl compounds can be used in synthetic organic chemistry.

Ligand Development for Copper-Catalyzed Enantioconvergent Radical Cross-Coupling of Racemic Alkyl Halides

The enantioconvergent cross-coupling of racemic alkyl halides represents a powerful tool for the synthesis of enantioenriched molecules. In this regard, the first-row transition metal catalysis provides a suitable mechanism for stereoconvergence by converting racemic alkyl halides to prochiral radical intermediates owing to their good single-electron transfer ability. In contrast to the noble development of chiral nickel catalyst, copper-catalyzed enantioconvergent radical cross-coupling of alkyl halides is less studied. Besides the enantiocontrol issue, the major challenge arises from the weak reducing capability of copper that slows the reaction initiation. Recently, significant efforts have been dedicated to basic research aimed at developing chiral ligands for copper-catalyzed enantioconvergent radical cross-coupling of racemic alkyl halides. This perspective will discuss the advances in this burgeoning area with particular emphasis on the strategic chiral anionic ligand design to tune the reducing capability of copper for the reaction initiation under thermal conditions from our research group.

Copper-Catalyzed Intermolecular Enantioselective Radical Oxidative C(sp3 )-H/C(sp)-H Cross-Coupling with Rationally Designed Oxazoline-Derived N,N,P(O)-Ligands

The intermolecular asymmetric radical oxidative C(sp³ )-C(sp) cross-coupling of C(sp³ )-H bonds with readily available terminal alkynes is a promising method to forge chiral C(sp³ )-C(sp) bonds because of the high atom and step economy, but remains underexplored. Here, we report a copper-catalyzed asymmetric C(sp³ )-C(sp) cross-coupling of (hetero)benzylic and (cyclic)allylic C-H bonds with terminal alkynes that occurs with high to excellent enantioselectivity. Critical to the success is the rational design of chiral oxazoline-derived N,N,P(O)-ligands that not only tolerate the strong oxidative conditions which are requisite for intermolecular hydrogen atom abstraction (HAA) processes but also induce the challenging enantiocontrol. Direct access to a range of synthetically useful chiral benzylic alkynes and 1,4-enynes, high site-selectivity among similar C(sp³ )-H bonds, and facile synthesis of enantioenriched medicinally relevant compounds make this approach very attractive.

ATRP-ARGET of a Styrene Monomer onto Modified Natural Rubber Latex as an Initiator

Atom transfer radical polymerization with an activator regenerated by electron transfer (ATRP-ARGET) was performed for graft copolymerization of styrene onto natural rubber in the latex stage as a heterogeneous system. Deproteinized and subsequently brominated natural rubber particles in the latex stage were subjected to graft copolymerization of styrene on their surfaces in the presence of an activated ATRP catalyst. ¹H NMR spectroscopy and size exclusion chromatography (SEC) characterized the particles. Ozonolysis was performed to deduct the polyisoprene contribution to SEC. Graft copolymerization in heterogeneous media by extraction with an acetone/2-butanone solution. Both the linear evolution of the molecular weight versus monomer conversion and the high grafting efficiency associated with a narrow molecular weight distribution of the resulting grafted polystyrene confirm a living radical behavior.

Copper-Catalyzed Radical 1,2-Carbotrifluoromethylselenolation of Alkenes under Ambient Conditions

We have described a copper-catalyzed radical 1,2-carbotrifluoromethylselenolation of alkenes using the readily available alkyl halides and (Me₄N)SeCF₃ salt. Critical to the success is the use of a proline-based N,N,P-ligand to enhance the reducing capability of copper for easy conversion of diverse alkyl halides to the corresponding radicals via a single-electron transfer process. The reaction features a broad substrate scope, including various mono-, di-, and trisubstituted alkenes with many functional groups.

Copper-Catalyzed Asymmetric Radical 1,2-Carboalkynylation of Alkenes with Alkyl Halides and Terminal Alkynes

A copper-catalyzed intermolecular three-component asymmetric radical 1,2-carboalkynylation of alkenes has been developed, providing straightforward access to diverse chiral alkynes from readily available alkyl halides and terminal alkynes. The utilization of a cinchona alkaloid-derived multidentate N,N,P-ligand is crucial for the efficient radical generation from mildly oxidative precursors by copper and the effective inhibition of the undesired Glaser coupling side reaction. The substrate scope is broad, covering (hetero)aryl-, alkynyl-, and aminocarbonyl-substituted alkenes, (hetero)aryl and alkyl as well as silyl alkynes, and tertiary to primary alkyl radical precursors with excellent functional group compatibility. Facile transformations of the obtained chiral alkynes have also been demonstrated, highlighting the excellent complementarity of this protocol to direct 1,2-dicarbofunctionalization reactions with C(sp²/sp³)-based reagents.

Can We Push Rapid Reversible Deactivation Radical Polymerizations toward Immortality?

All reversible deactivation radical polymerization (RDRP) processes require a compromise between the rate of polymerization, which requires a high radical concentration, and retention of chain end functionality, which requires a low radical concentration. Here, we demonstrate that this compromise may be partially averted where fast deactivation of the propagating radical occurs. It is shown that, contrary to the predictions of classical reaction kinetics, when the probability density functions of the termination reactions are adjusted to take into account the time needed for radical diffusion, a reduction in the extent of termination can be expected if chain deactivation is rapid. We subsequently use this framework to explain experimental results in the copper(0)-mediated polymerization of acrylamide. The main concept put forward in the paper questions the commonly held assumptions of the limitations of RDRP processes and suggests the ability for a seemingly impossible level of control of radical reactions.

Copper(I)-Catalyzed Asymmetric Reactions Involving Radicals

Asymmetric functionalization of alkyl radicals represents a robust yet underdeveloped method for efficient construction and decoration of carbon skeletons in chiral organic molecules. In this field, we have been inspired by the excellent redox, alkyl radical trapping, and Lewis acidic properties of copper to develop several catalytic modes for asymmetric reactions involving alkyl radicals. At the beginning, we discovered tandem radical hydrotrifluoromethylation of unactivated alkenes and enantioselective alkoxylation of remote C(sp³)-H bonds by copper/chiral phosphate relay catalysis. This success has stimulated us to develop an asymmetric three-component 1,2-dicarbofunctionalization of 1,1-diarylalkenes using a similar strategy via radical intermediates. Meanwhile, we also discovered a copper/chiral secondary amine cooperative catalyst for asymmetric radical intramolecular cyclopropanation of alkenes using α-aldehyde methylene groups as C1 sources. The trapping of alkyl radical intermediates by Cu^II species during the reaction was essential for the chemoselectivity toward cyclopropanation. Encouraged by the efficient enantiocontrol with chiral phosphate and the effective trapping of alkyl radicals with Cu^II species, we then sought to develop copper/chiral phosphate as a single-electron-transfer catalyst for asymmetric reactions involving alkyl radicals. Subsequently, we successfully achieved a series of highly enantioselective 1,2-aminofluoroalkylation, -aminoarylation, -diamination, -aminosilylation, and -oxytrifluoromethylation of unactivated alkenes. The key for high enantioinduction was believed to be the effective trapping of alkyl radicals by Cu^II/chiral phosphate complexes. Besides, an achiral pyridine ligand was found to be indispensable for achieving high enantioselectivity, presumably via stabilization of Cu^III species in the 1,2-alkoxytrifluoromethylation reaction. This discovery reminded us of tuning the redox properties and chemoreactivity of copper centers with an ancillary ligand. As a result, we subsequently identified cinchona alkaloid-derived sulfonamides as novel neutral-anionic hybrid ligands for simultaneous chemo- and enantiocontrol. We thus accomplished highly enantioselective 1,2-iminoxytrifluoromethylation of unactivated alkenes under the catalysis of copper/cinchona alkaloid-derived sulfonamide ligand, affording trifluoromethylated isoxazolines in high enantiomeric excess. Our copper-catalyzed asymmetric reactions with alkyl radicals provide expedient access to a diverse range of valuable chiral molecules with broad application potential in areas of organic synthesis, medicine, agrochemical, and material sciences.

Transition Metal Mediated Living Radical Polymerisation

Living radical polymerisation has witnessed an unprecedented interest from polymer and materials scientists. Traditionally, polymers tended to replace natural materials such as wood, cotton and glass, and were used primarily for their structural features and performance and cost advantages. New functional polymers are essential for the manufacture of cell phones, lap-top computers, new cosmetics, and many pharmaceuticals. It is important to be able to control how monomers are put together within the macromolecule for the design at the molecular level for specific applications. Living polymerisation allows for end group control, polymer chain length and relatively narrow polydispersity polymers. In nature, the ability to control monomer distribution and chain length is obvious with approximately 20 amino acids being the monomers for polymers as diverse as hair, insulin and haemoglobin. Living radical polymerisation solves many of the problems in the use of monomers that contain heteroatoms and functional groups. These tend to be reactive towards strong nucleophiles and electrophiles which are required in ionic polymerisation. Protecting group chemistry as used in small molecule organic synthesis is not practical in polymer synthesis. Thus radicals that are inert to most functional groups and in particular protic species seem to be the answer. The mechanism of the transition metal mediate systems is extremely complicated with a range of organometallic species present in the reaction mixture. Solvents and coordinating monomers drastically affect the ideal reaction conditions and it is impossible to predict the optimum conditions for each synthesis without certain experiments being carried out. Nevertheless, catalyst systems are available which are acceptable and work well enough to be able to make a plethora of different macromolecules for a diverse range of applications /properties.

Gradient helical copolymers: synthesis, chiroptical properties, thermotropic liquid crystallinity, and self-assembly in selective organic solvents

A series of novel gradient copolymers R-(−)-poly(StN-grad-C8) were synthesized through atom transfer radical copolymerization of an achiral styrenic monomer, N,N-dimethyl-4-ethenylbenzamide (M-StN), and a chiral bulky vinylterphenyl monomer, (−)-2,5-bis{4′-[(R)-sec-octyloxycarbonyl]phenyl}styrene (R-(−)-M-C8).

Stabilization of Two Radicals with One Metal: A Stepwise Coupling Model for Copper-Catalyzed Radical-Radical Cross-Coupling

Transition metal-catalyzed radical-radical cross-coupling reactions provide innovative methods for C-C and C-heteroatom bond construction. A theoretical study was performed to reveal the mechanism and selectivity of the copper-catalyzed C-N radical-radical cross-coupling reaction. The concerted coupling pathway, in which a C-N bond is formed through the direct nucleophilic addition of a carbon radical to the nitrogen atom of the Cu(II)-N species, is demonstrated to be kinetically unfavorable. The stepwise coupling pathway, which involves the combination of a carbon radical with a Cu(II)-N species before C-N bond formation, is shown to be probable. Both the Mulliken atomic spin density distribution and frontier molecular orbital analysis on the Cu(II)-N intermediate show that the Cu site is more reactive than that of N; thus, the carbon radical preferentially react with the metal center. The chemoselectivity of the cross-coupling is also explained by the differences in electron compatibility of the carbon radical, the nitrogen radical and the Cu(II)-N intermediate. The higher activation free energy for N-N radical-radical homo-coupling is attributed to the mismatch of Cu(II)-N species with the nitrogen radical because the electrophilicity for both is strong.

Star Polymers

Recent advances in controlled/living polymerization techniques and highly efficient coupling chemistries have enabled the facile synthesis of complex polymer architectures with controlled dimensions and functionality. As an example, star polymers consist of many linear polymers fused at a central point with a large number of chain end functionalities. Owing to this exclusive structure, star polymers exhibit some remarkable characteristics and properties unattainable by simple linear polymers. Hence, they constitute a unique class of technologically important nanomaterials that have been utilized or are currently under audition for many applications in life sciences and nanotechnologies. This article first provides a comprehensive summary of synthetic strategies towards star polymers, then reviews the latest developments in the synthesis and characterization methods of star macromolecules, and lastly outlines emerging applications and current commercial use of star-shaped polymers. The aim of this work is to promote star polymer research, generate new avenues of scientific investigation, and provide contemporary perspectives on chemical innovation that may expedite the commercialization of new star nanomaterials. We envision in the not-too-distant future star polymers will play an increasingly important role in materials science and nanotechnology in both academic and industrial settings.

The influence of surface grafting on the growth rate of polymer chains

MWs of polymers synthesized simultaneously on a surface and in solution by ATRP differ, depending on the surface grafting density.

Operando X-ray absorption and EPR evidence for a single electron redox process in copper catalysis

A single electron redox process between Cu(ii) and a sulfinic acid, and characterization of the formed Cu(i) are clearly shown using operando X-ray absorption and EPR evidence.

An unprecedented single electron redox process in copper catalysis is confirmed using operando X-ray absorption and EPR spectroscopies. The oxidation state of the copper species in the interaction between Cu(ii) and a sulfinic acid at room temperature, and the accurate characterization of the formed Cu(i) are clearly shown using operando X-ray absorption and EPR evidence. Further investigation of anion effects on Cu(ii) discloses that bromine ions can dramatically increase the rate of the redox process. Moreover, it is proven that the sulfinic acids are converted into sulfonyl radicals, which can be trapped by 2-arylacrylic acids and various valuable β-keto sulfones are synthesized with good to excellent yields under mild conditions.

Amide group-containing polar solvents as ligands for iron-catalyzed atom transfer radical polymerization of methyl methacrylate

Iron-catalyzed ATRP of MMA using polar solvents based on amide groups as ligands is reported.

Synthesis of thermally cleavable multisegmented polystyrene by an atom transfer nitroxide radical polymerization (ATNRP) mechanism

Based on the common features of well-defined NRC reaction, ATRP and NMRP mechanisms, an atom transfer nitroxide radical polymerization (ATNRP) mechanism was presented, and further used to construct multisegmented PS_m embedded with multiple alkoxyamine linkages.