Asymmetric Synthesis of Chiral Cyclopropanes from Sulfoxonium Ylides Catalyzed by a Chiral-at-Metal Rh(III) Complex

An enantioselective cyclopropanation reaction of sulfoxonium ylides with β,γ-unsaturated ketoesters catalyzed by a chiral rhodium catalyst has been realized. A variety of optically pure 1,2,3-trisubstituted cyclopropanes was synthesized in 48-89% yields, with up to 99% ee, and with dr >20:1. Furthermore, research shows that a weak coordination between the chiral rhodium catalyst and β,γ-unsaturated ketoesters was responsible for the high diastereoselectivity and enantioselectivity of the corresponding products.

Synthesis of Optically Active Helical Polycarbenes through Helix-Sense-Selective Polymerization Strategy and Their Application in Chiral Separation

In this work, helical polycarbenes with optical activity were designed and facilely synthesized through the helix-sense-selective polymerization (HSSP) of the diazoacetate monomer with a dimethylbenzyl ester pendant catalyzed by π-allylPdCl with chiral phosphine ligands at room temperature. The polymerization was carried out in a living and controlled style, and a range of helical polycarbenes with the desired number-average molecular weights and narrow molecular weight distributions were obtained. Circular dichroism and UV-vis analyses revealed that these polycarbenes exhibited a stable helical conformation with a preferred handedness, and their helical directions were dependent on the chirality of the chiral phosphine ligands. Further studies showed that the helical conformation of the obtained polycarbenes was from the polymeric backbone rather than the intermolecular aggregation in the solutions. Moreover, the prepared, optically active, helical polycarbenes possessed excellent enantioselective crystallization ability for threonine racemates. The enantiomeric excess (e.e.) of the induced crystals could be up to 83% via utilizing the prepared helical polycarbenes as a chiral separation agent.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Bottlebrush Polymers Based on RAFT and the "C1" Polymerization Method: Controlled Synthesis and Application in Anticancer Drug Delivery

In this work, we reported a strategy to synthesize well-defined bottlebrush polymers. Diazoacetate macromonomers of polystyrene (1-PS_n) with controlled molecular weights were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The diazo can tolerate the RAFT polymerization conditions and remained on the chain end of the yielded PS macromonomer. The terminal diazo groups of the macromonomer were polymerized by the allyl PdCl/L catalyst to afford well-defined bottlebrush polymers ((1-PS_n)_ms) carrying a side chain on each backbone atom. Meanwhile, an amphiphilic bottlebrush polymer containing brush-shaped PS and polyethylene glycol (PEG) was synthesized by polymerization of the diazoacetate macromonomer of PEG (2-PEG) using Pd(II)-terminated (1-PS_n)_m as the macroinitiator. The yielded amphiphilic (1-PS₃₀)₅₀-b-(2-PEG)₁₀₀ could self assemble into a well-defined core-shell micelle in aqueous solutions. The hydrodynamic diameter of the micelle was ca. 146 nm and had good biocompatibility. These results indicate the micelles have great potential in drug delivery.

Nickel(II)-catalyzed living polymerization of diazoacetates toward polycarbene homopolymer and polythiophene-block-polycarbene copolymers

Diazoacetate polymerization has attracted considerable research attention because it is an effective approach for fabricating carbon–carbon (C–C) main chain polymers. However, diazoacetate polymerization based on inexpensive catalysts has been a long-standing challenge. Herein, we report a Ni(II) catalyst that can promote the living polymerization of various diazoacetates, yielding well-defined C–C main chain polymers, polycarbenes, with a predictable molecular weight (M_n) and low dispersity (M_w/M_n). Moreover, the Ni(II)-catalyzed sequential living polymerization of thiophene and diazoacetate monomers affords interesting π-conjugated poly(3-hexylthiophene)-block-polycarbene copolymers in high yields with a controlled M_n, variable compositions, and low M_w/M_n, although the structure and polymerization mechanism of the two monomers differ. Using this strategy, amphiphilic block copolymers comprising hydrophobic poly(3-hexylthiophene) and hydrophilic polycarbene blocks are facilely prepared, which were self-assembled into well-defined supramolecular architectures with tunable photoluminescence.

Diazoacetate polymerizations using inexpensive catalysts has been a long-standing challenge to achieve. Here the authors report a Ni(II) catalyst that can promote living polymerization of various diazoacetates, yielding well-defined carbon–carbon main chain polymers, polycarbenes.

Photo-Thermo-Mechanochemical Approach to Synthesize Quinolines via Addition/Cyclization of Sulfoxonium Ylides with 2-Vinylanilines Catalyzed by Iron(II) Phthalocyanine

A novel photo-thermo-mechanochemical approach to assembling quinolines catalyzed by iron(II) phthalocyanine has been realized for the first time. This transformation features a cost-efficient catalytic system and operational simplicity, is free of solvent, and shows good substrate tolerance, providing a green alternative to existing thermal approaches. Mechanistic experiments demonstrate that the in-situ-formed secondary amine may be the key intermediate for the further cyclization/aromatization process.

C1 Polymerization of Fluorinated Aryl Diazomethanes

A library of fluorinated aryl diazomethanes were polymerized using BF₃·OEt₂ as a catalyst. The polymerization of 2,3,4,5,6-pentafluorophenyl diazomethane was found to be controlled, permitted chain extensions, and facilitated access to a series of block copolymers. Moreover, the polymer chains grew in one carbon increments (so-called "C1 polymerizations") and, as such, afforded highly substituted polymers that featured aryl units pendant to every carbon atom of the backbone. The polymers were characterized using size exclusion chromatography, various spectroscopic techniques, and a series of static and dynamic contact angle measurements. Compared to less-substituted analogues that were prepared using typical C2 polymerization methodologies, the C1 fluorinated polymers were found to be more hydrophobic while maintaining a sufficient solubility to be processed into robust films.

Ruthenium-Catalyzed Diazoacetates/Cyclooctene Metathesis Copolymerization

As a powerful synthetic tool, ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) has been widely utilized to prepare diverse polymers. In this contribution, we demonstrated the polymerization of cyclooctene in the presence...

Synthesis and electrochromic properties of polyamines containing a 4,4′-diaminotriphenylamine-N,N′-diyl unit in the polymer backbone: Ru-catalyzed N–H insertion polycondensation of 1,4-phenylenebis(diazoacetate) with 4,4′-diaminotriphenylamine derivatives

Polycondensation of a bis(diazocarbonyl) compound and diaminotriphenylamine derivatization via Ru-catalyzed N–H insertion to afford electroactive polyamine.

Barbier single-atom polymerization induced emission as a one-pot approach towards stimuli-responsive luminescent polymers

A one-pot strategy for the design of stimuli-responsive luminescent polymers has been demonstrated through Barbier PIE, where the N,N-dimethyl moiety endows the polymers with both stimuli-responsive and red-shifted nonconjugated emission properties.

Room-temperature Barbier single-atom polymerization induced emission as a versatile approach for the utilization of monofunctional carboxylic acid resources

Barbier polymerization is realized at room-temperature with single-atom polymerization and polymerization-induced emission characteristics, which exhibits capability on sensitive explosive detection and artificial light-harvesting system fabrication.

Polymethylene Brushes via Surface-Initiated C1 Polyhomologation

Surface-initiated polymerization reactions are a powerful tool to generate chain-end-tethered polymer brushes. This report presents a synthetic strategy that gives access to structurally well-defined hydrocarbon polymer brushes of controlled molecular weights, which can be further modified to generate more complex surface-attached polymer architectures. The hydrocarbon brushes reported in this study are polymethylene brushes that are obtained via surface-initiated C1 polyhomologation of dimethylsulfoxonium methylide. The strategy outlined here is based on the use of an alkylboronic acid pinacol ester initiator, which allows for controlled, unidirectional chain growth by monomer insertion into only the C-B bond of the initiator and which presents the polymerization active group at the growing polymer chain end. This surface-initiated C1 polyhomologation methodology is compatible with photopatterning strategies and can be used to generate micropatterned polymethylene brush films. Furthermore, conversion of the boronic ester chain-end functionalities to hydroxyl groups allows for selective chain-end modification and enables access to a variety of surface-anchored block copolymer architectures by chain extension via, for example, ring-opening or atom transfer radical polymerization chemistries.

Reactivity of poly(alkoxycarbonylmethylene)s under basic conditions: alkylation of main chain carbon atoms via a ketene silyl acetal-type intermediate and cleavage of the carbon–carbon main chain

We report the alkylation of main chain carbon atoms and cleavage of the carbon–carbon main chain of poly(alkoxycarbonylmethylene)s.

Palladium-Catalyzed Insertion of Ethylene and 1,1-Disubstituted Difunctional Olefins: An Experimental and Computational Study

Insertion or coordination copolymerization of ethylene with di-substituted olefins is challenging and the choice of di-substituted mono-functional olefin versus di-substituted di-functional olefin (DDO) appears to be decisive. Here we show that DDO-inserted species are amenable to ethylene insertion and polymerization. DDOs such as 2-acetamidoacrylic acid (AAA), methyl 2-acetamidoacrylate (MAAA), and ethyl 2-cyanoacrylate (ECA) were treated with palladium complex [{P∧O}PdMe(L)] (P∧O=κ² -P,O-Ar₂ PC₆ H₄ SO₂ O with Ar=2-MeOC₆ H₄ ; L=C₂ H₆ OS) and the existence of respective insertion intermediates in moderate yield (up to 37 %) was established. These intermediates were exposed to ethylene and corresponding ethylene-inserted products were isolated and characterized. A careful comparison with three model compounds confirmed ethylene insertion and polymerization. Thus, the combined experimental and computational investigations show that DDO-inserted species can undergo ethylene insertion and polymerization.

Effects of solvents, additives, and π-allyl ligand structures on the polymerization behavior of diazoacetates initiated by π-allylPd complexes

We have revealed that the quality of control of the polymerization was improved by adding some compounds and polymer tacticity was dependent on the structures of π-allyl ligands.

A Dinuclear Mechanism Implicated in Controlled Carbene Polymerization

Carbene polymerization provides polyolefins that cannot be readily prepared from olefin monomers; however, controlled and living carbene polymerization has been a long-standing challenge. Here we report a new class of initiators, (π-allyl)palladium carboxylate dimers, which polymerize ethyl diazoacetate, a carbene precursor in a controlled and quasi-living manner, with nearly quantitative yields, degrees of polymerization >100, molecular weight dispersities 1.2-1.4, and well-defined, diversifiable chain ends. This method also provides block copolycarbenes that undergo microphase segregation. Experimental and theoretical mechanistic analysis supports a new dinuclear mechanism for this process.

Enhanced fluorescence quantum yield of syndiotactic side-chain TPE polymers via Rh-catalyzed carbene polymerization: influence of the substitution density and spacer length

The fluorescence quantum yields of the TPE-based C1 polymers also increase with the shortened spacer lengths and further improve by about 20% as compared with the corresponding C2 polyacrylate counterparts.

Palladium-catalyzed carbene coupling of N-tosylhydrazones and arylbromides to synthesize cross-conjugated polymers

Palladium-catalyzed cross-coupling reactions of N-tosylhydrazones and arylbromides have been applied for the first time in the synthesis of cross-conjugated polymers, namely poly(arylene-1,1-vinylidene)s (iso-PAVs).

Thermal Decomposition of Ethyl Diazoacetate in Microtube Reactor: A Kinetics Study

Ethyl diazoacetate (EDA) commonly experiences intensive decomposition as well as complex conversion concerning safety and efficiency. In this work, a careful kinetics study on the thermal decomposition of EDA was isothermally conducted in a microtube reactor to establish a mechanism-based kinetic model. The model parameters were well calibrated with experimental data including the yield of dimmers and the conversion of EDA, confirming the rationality of the proposed three-step reaction route. It allows the model to concisely describe the complex species transformations during EDA decomposition, which is unavailable for an apparent kinetic model. Considering an isothermal reaction system and the tolerance of EDA consumption by thermal decomposition, this work could help reveal the requirement on the kinetic characteristics of the desired catalytic reaction in which EDA is involved, as a reference on reaction process modeling and regulation.

Aqueous Phase Separation Behavior of Highly Syndiotactic, High Molecular Weight Polymers with Densely Packed Hydroxy-Containing Side Groups

Herein we describe the Rh-catalyzed C1 polymerization of silyl-protected diazoacetates of the general formula HC(=N₂)C(=O)O(CH₂) _x OSiR₃, where x = 2-5. After polymerization and subsequent desilylation, syndiotactic polymers bearing a hydroxy-containing side group on every backbone carbon are obtained. The molecular weight of the desired polymers can be controlled via chain transfer with methanol during the polymerization. The produced polymers are compared to atactic analogues formed by [(η³-C₃H₅)PdCl]-catalyzed polymerization of silyl-protected diazoacetates with the same general formula. While the polymers produced by the Rh and Pd catalysts have the same hydrophilic/hydrophobic balance, the stereoregularity of the polymers formed by the Rh catalyst was found to be of influence on the thermoresponsive behavior of the polymer. The effect of this stereoregularity on the thermoresponsive phase separation behavior of the produced polymers in aqueous solution was investigated.