Recent advances in synthesis of organosilicons via radical strategies

Organosilicon compounds play an important role in the fields of materials science,pharmacy,and organic synthesis.The development of effective approaches for the preparation of these compounds have also become a research focus in organic synthesis.In recent years,free radical synthesis of organosilicons has been vigorously developed,which generally has the advantages of milder synthesis conditions,higher yields and selectivity,and free of precious metal catalysts compared with traditional strategies.This article reviews research progresses in the synthesis of organosilico n compounds by free radical pathways since 2016.In most cases,the radical silylation is achieved based on the reaction of silyl radicals,which are triggered by four routes including peroxide,transition-metal-induced peroxide decomposition,alkali,photocatalysis.The alkyl radicals can also initiate the radical silylation for the generation of C(sp~3)—Si bonds.     

Visible light-promoted metal-free aerobic oxyphosphorylation of olefins: A facile approach to β-ketophosphine oxides

A metal-free direct aerobic oxyphosphorylation of alkenes with H-phosphine oxides has been developed utilizing visible light photoredox catalysis. A variety of β-ketophosphine oxides have been obtained in good yields from simple olefins under air with inexpensive rhodamine B as the non-metallic photocatalyst. This method provides a mild, green, and practical synthetic approach to valuable β-ketophosphine oxides.     

1,4-Phenylene-bis-((1-methyl-1H-pyrazol-5-yl)borinic 8-oxyquinolinate) as a photoredox catalyst in the atom transfer radical addition of iodoperfluoroalkanes to alkenyl groups bearing organoboron compounds

The key photocatalytic properties of a transition-metal-free heteroleptic complex derived from 1,4-phenyldiboronic acid were evaluated. This compound was utilized in the atom transfer radical addition of iodoperfluoroalkanes to a series of alkenyl group bearing organoboron compounds. The products of these reactions retained the BC bond, and Suzuki-Miyaura coupling of one product gave biphenyl derivatives. The X-ray analysis of one of the obtained perfluoroalkylated boronic acids revealed the presence of open star-shaped channels.     

Developments in visible-light-mediated copper photocatalysis

The scope, applications, and understanding of photoinduced, copper-catalysis have advanced considerably in recent times. Many of these transformations exploit the distinctive properties of photoactive copper species to establish original synthetic methodology and reveal novel reactivity. In this regard, the bifunctionality of copper photocatalysts has often allowed these species to direct and influence synthetic processes in new, interesting, and exciting ways. This article summarizes recent advances in the development of copper photoredox catalysts and provides an overview of the emerging applications of these, and related systems, in organic synthesis.     

Visible-light mediated trifluoromethylation of p-quinone methides by 1,6-conjugate addition using pyrylium salt as organic photocatalyst

A transition metal free visible light mediated organo photoredox catalyzed trifluoromethylation of p-quinone methides (p-QMs) to construct fluoro-analogs of dichlorodiphenyltrichloroethane (DDT) is reported using a bench stable, inexpensive Langlois reagent as a trifluoromethyl radical source. This protocol could generate a benzylic C(sp³)-CF₃ bond with excellent yield under mild reaction conditions using 1,6-conjugate addition/aromatization of trifluoromethyl radical in the absence of any external additives. Further, we demonstrate di-trifluoromethylation and gram scale synthesis of this reaction.     

Visible light photoredox-catalyzed phosphorylation of quinoxalin-2(1H)-ones

A visible light photoredox-catalyzed C-3 phosphorylation of quinoxalin-2(1H)-ones with diphenylphosphine oxide has been developed. The reaction was effectively accelerated using an inexpensive eosin B as a photoredox catalyst under ambient air at room temperature without any other metal, oxidant, or additive. This approach offers a facile way to prepare 3-diphenylphosphorylated quinoxalin-2(1H)-one derivatives.     

Synthesis of 1-(hydroxyaryl)furo[3,4-c]pyridines from 1-amino(alkoxy)furo[3,4-c]pyridines and (poly)phenols

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Preparation and reactivity of sterically encumbered organocatalysts and their use in the preparation of (S)-Pregabalin precursors

Herein we describe the preparation of a new class of sterically demanding organocatalysts derived from d-fructose and their use, alongside hitherto unreported doubly quaternarised Cinchona ammonium salts, in the Michael reaction of nitromethane with 4-nitro-5-alkenyl-isoxazoles.     

Cross-linked poly(4-vinylpyridine-N-oxide) as a polymer-supported oxygen atom transfer reagent

Oxygen atom transfer (OAT) reagents are common in biological and industrial oxidation reactions. While many heterogeneous catalysts have been utilized in OAT reactions, heterogeneous OAT reagents have not been explored. Here, cross-linked poly(4-vinylpyridine-N-oxide), called x-PVP-N-oxide, was tested as a heterogeneous OAT reagent and its oxidation chemistry compared to its molecular counterpart, pyridine-N-oxide. The insoluble oxidant x-PVP-N-oxide demonstrated comparable reactivity to pyridine-N-oxide in direct oxidation reactions of phosphines and phosphites in acetonitrile, but x-PVP-N-oxide did not react in other solvents. The polymer backbone of x-PVP-N-oxide, however, allowed for easy filtering and recycling in sequential oxidation reactions. In addition, x-PVP-N-oxide was tested as the stoichiometric oxidant in a copper-catalyzed OAT reaction to α-diazo-benzeneacetic acid methyl ester. The heterogeneous oxidant was much less reactive than pyridine-N-oxide, indicating that interaction with the metal catalyst was challenging. These results demonstrated a proof-of-concept that recyclable, polymer-supported OAT reagents could be a viable OAT reagents in direct oxidation reactions without metal catalysts.     

Effect of the synthetic method and support porosity on the structure and performance of silica‐supported CuBr/pyridylmethanimine atom transfer radical polymerization catalysts. I. Catalyst preparation and characterization

Silica‐supported CuBr/pyridylmethanimine (PMI) complexes that facilitate the atom transfer radical polymerization of methyl methacrylate have been prepared and characterized. Four different synthetic routes, including multistep‐grafting (M1), two‐step‐grafting (M2), one‐pot (M3), and preassembled‐complex (M4) methods, have been evaluated on three different silica supports (mesoporous SBA15 with 48‐ and 100‐Å pores and nonporous Cab‐O‐Sil EH5). The resulting solids have been characterized by a battery of techniques, including thermogravimetric analysis/differential scanning calorimetry, FT‐Raman spectroscopy, ¹³C and ²⁹Si magic‐angle‐spinning and cross‐polarity/magic‐angle‐spinning spectroscopy, low‐temperature nitrogen physisorption, and elemental analysis. The combination of elemental analysis and spectroscopic results has indicated that a variety of different surface species likely exist for most catalysts, including copper species that are both monocoordinated and biscoordinated by PMI ligands, and PMI‐free copper bromide species interacting with the silica surface. M4 appears to give a material that has the smallest amount of the uncomplexed ligand (by FT‐Raman spectroscopy) and is, therefore, the most homogeneous. After M4, the metallation efficiency decreases in the order M2 ≥ M3 > M1, with M1 giving a material with a highly heterogeneous surface composition. The ligand loading on all the catalysts has been determined to be approximately 1 mmol/g of SiO₂, with Cab‐O‐Sil‐supported materials giving much higher ligand densities because of its lower surface area. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1367–1383, 2004     

Effect of the synthetic method and support porosity on the structure and performance of silica‐supported CuBr/pyridylmethanimine atom transfer radical polymerization catalysts. II. Polymerization of methyl methacrylate

A systematic study of the effect of the synthesis method and catalyst structure on the atom transfer radical polymerization (ATRP) performance of copper(I) bromide/pyridylmethanimine complexes supported on silica is described. Four different synthetic routes, including multistep‐grafting (M1), two‐step‐grafting (M2), one‐pot (M3), and preassembled‐complex (M4) methods, have been evaluated on three different silica supports (mesoporous SBA15 with 48‐ and 100‐Å pores and nonporous Cab‐O‐Sil EH5). The resulting solids have been used for ATRP of methyl methacrylate. The catalysts allow for moderate to poor control of the polymerization, with polydispersity indices (PDIs) ranging from 1.46 to greater than 2. The materials made with the preassembled‐complex (M4) and one‐pot (M3) approaches are generally more effective than those prepared with a grafting method (M1 and M2) on porous silica, whereas all the methods provide similarly performing catalysts on the nonporous support. Nonporous Cab‐O‐Sil EH5 is the most effective support because of its small particle size, lack of porosity, and relative compatibility in the reaction media. All the catalysts leach copper into solutions in small amounts. In addition, the catalysts can be effectively recycled, with improved controlled character in recycle runs (PDI ～ 1.2). Control experiments have shown that this improved performance of the used catalysts is likely due to the presence of a soluble Cu(II) complex in the materials that effectively deactivates the growing polymer chain, leading to narrow PDIs and controlled molecular weights. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1384–1399, 2004     

Highly Efficient and Facile Photocatalytic Recycling System Suitable for ICAR ATRP of Hydrophilic Monomers

Photoinduced initiators for continuous activator regeneration atom transfer radical polymerization (ATRP) of hydrophilic monomers in heptane/ethanol latent‐biphasic system for copper catalyst separation and recycling have been realized for the first time at room temperature with different wavelengths of visible light LED (green, blue, purple, and white LED) as external stimulus, using 2‐bromophenylacetate as the ATRP initiator and camphorquinone/triethylamine as the photoinitiator. In this system, hybrid catalyst complex (HCc) is synthesized as a novel nonpolar catalyst, which is preferentially dissolved in heptane. The hydrophilic polymers obtained catalyzed by HCc in heptane/ethanol mixture solvent show typical “living” features, for example, the values of M_n,GPC increase linearly with monomer conversion up to quantitative level (>96%) and the molecular weight distributions were kept narrow (M_w/M_n < 1.20) throughout the polymerization process. It should be noted that the excellent controllability of this novel polymerization system can be achieved even after 5 catalyst recycling experiments under LED irradiation.

     

Effect of ligand on the synthesis of star polymers by resorcinarene‐based ATRP initiators

The effect of the steric hindrance on the initiating properties of two multifunctional resorcinarene‐based initiators in atom transfer radical polymerization (ATRP) was studied by using Cu(I)‐complexes of three multidentate amine ligands in the polymerization of tert‐butyl acrylate and methyl methacrylate. These ligands are less sterically hindered and have higher activities in the catalysis of ATRP of (meth)acrylates than 2,2′‐bipyridine. The polymerizations were faster and more controlled than with the 2,2′‐bipyridyl catalyst, but the tendency for bimolecular coupling increased. Even though the initiator was octafunctional, the resulting star polymers had only four arms. This indicates that the steric hindrance arising from the conformations of the initiators determines the structure of the polymer, but the ligand noticeably affects the controllability of the polymerization © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3349–3358, 2005     

Recent advances in direct catalytic asymmetric transformations under proton-transfer conditions

Cooperative catalysis has proven to be a particularly powerful strategy for promoting stereoselective organic transformations under mild reaction conditions. The specific interactions between the catalyst components and substrates are precisely orchestrated to elicit high catalytic efficiency and excellent control of the stereochemical course. By harnessing the power of cooperativity, various sets of stereoselective reactions proceed under mild proton-transfer conditions with perfect atom economy. This Minireview summarizes our recent contributions to several C-N and C-C bond-forming reactions in this field and related transformations.     

Visible light-driven acridone catalysis for atom transfer radical polymerization

Acridone as a new kind of visible light photocatalyst has been developed to catalyze metal free atom transfer radical polymerization (ATRP). The photocatalyst possess low excited state potential as can undergo an oxidative quenching pathway to initiate ATRP of vinyl monomers. Kinetic study and light on/off reaction demonstrate the “living”/controlled nature of the polymerization by light. Block copolymers can be achieved by using PMMA as macroinitiator to reinitiate polymerization of other vinyl monomers, which shows highly preserved Br chain-end functionality in the synthesized polymers. Moreover, the polymerization can be conducted under air atmosphere as most photocatalysts need anaerobic condition, which may give inspiration of further application of this kind of photocatalyst.     

Atom‐Transfer Radical Addition Reactions Catalyzed by RuCp* Complexes: A Mechanistic Study

Kinetic and spectroscopic analyses were performed to gain information about the mechanism of atom‐transfer radical reactions catalyzed by the complexes [RuCl₂Cp*(PPh₃)] and [RuClCp*(PPh₃)₂] (Cp*=pentamethylcyclopentadienyl), in the presence and in the absence of the reducing agent magnesium. The reactions of styrene with ethyl trichloroacetate, ethyl dichloroacetate, or dichloroacetonitrile were used as test reactions. The results show that for substrates with high intrinsic reactivity, such as ethyl trichloroacetate, the oxidation state of the catalyst in the resting state is +3, and that the reaction is zero‐order with respect to the halogenated compound. Furthermore, the kinetic data suggest that the metal catalyst is not directly involved in the rate‐limiting step of the reaction.     

Competitive Equilibria in Atom Transfer Radical Polymerization

With the recent development of new initiation techniques in atom transfer radical polymerization (ATRP) that allow catalysts to be employed at unprecedented low concentrations (∼10 ppm), a thorough understanding of competitive equilibria that can affect catalyst performance is becoming increasingly important. Such mechanistic considerations are discussed herein, including i) factors affecting the position of the ATRP equilibrium; ii) dissociation of the ATRP catalyst at high dilution and loss of deactivator due to halide dissociation; iii) conditional stability constants as related to competitive monomer, solvent, and reducing agent complexation as well as ligand selection with respect to protonation in acidic media; and iv) competitive equilibria involving electron transfer reactions, including the radical oxidation to carbocations or reduction to carbanions, radical coordination to the metal catalyst, and disproportionation of the Cu^I-based ATRP activator.     

A hydrogen pump: Transfer of four hydrogens from a cyclohexane ring to a triple bond during a menthofuran/bromoacetylene adduct rearrangement

The cycloadducts of menthofuran with acylbromoacetylenes, (3-bromo-1,6-dimethyl-5,6,7,8-tetrahydro-2H-2,4a-epoxynaphthalen-4-yl)(aryl)methanones, rearrange (CHCl₃, reflux, 1 h) to 2-(2-acylethyl)benzofurans (along with the expected 2-bromo-3-hydroxy-4,7-dimethyl-5,6,7,8-tetrahydronaphthalene-1-yl)(aryl)methanones) via 2-acylethynylmenthofurans, thus indicating the exceptionally mild and rapid transfer of four hydrogens from a cyclohexane ring to a triple bond through the furan moiety in the key intermediate 2-acylethynylmenthofuran.