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1. Comparing Copper Catalysts in the Synthesis of 1,2,3-Triazoles via Click Chemistry

   Green Emma; Leitschuh Ethan; Lanorio Jocelyn (November 2022, Transactions of the Illinois State Academy of Science)

   Copper-catalyzed azide-alkyne cycloadditions (CuAAC) produce 1,4-disubstituted 1,2,3-triazoles, molecules that have many applications in pharmaceuticals. Click reactions are atom-efficient and produce 1,4-disubstituted triazoles selectively with high yields at room temperature. Byproducts are rarely observed, and the product is easily separated by washing, eliminating the need for purification measures such as column chromatography. We tested various copper complexes for ease of use as homogeneous catalysts at various conditions. The 1,4-disubstituted triazole products were obtained in moderate to excellent yields. The progress of reaction was determined using TLC and IR spectroscopy, and products were characterized by GC-MS and NMR spectroscopy. We found that there is little that changes the outcome of the reaction upon variations in solvent and temperature conditions. However, preliminary results show that the anion of the copper salt used in preparing the copper complexes affects the kinetics of the triazole formation. A significant finding was that copper(II)-catalyzed reactions appear to form product even in the absence of a reducing agent. 

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2. Iron Catalysts in Atom Transfer Radical Polymerization

   https://doi.org/10.3390/molecules25071648  

   Dadashi-Silab, Sajjad; Matyjaszewski, Krzysztof (April 2020, Molecules)

   Catalysts are essential for mediating a controlled polymerization in atom transfer radical polymerization (ATRP). Copper-based catalysts are widely explored in ATRP and are highly efficient, leading to well-controlled polymerization of a variety of functional monomers. In addition to copper, iron-based complexes offer new opportunities in ATRP catalysis to develop environmentally friendly, less toxic, inexpensive, and abundant catalytic systems. Despite the high efficiency of iron catalysts in controlling polymerization of various monomers including methacrylates and styrene, ATRP of acrylate-based monomers by iron catalysts still remains a challenge. In this paper, we review the fundamentals and recent advances of iron-catalyzed ATRP focusing on development of ligands, catalyst design, and techniques used for iron catalysis in ATRP. 

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3. Recyclable Catalysts for Alkyne Functionalization

   https://doi.org/10.3390/molecules26123525  

   Trigoura, Leslie; Xing, Yalan; Chauhan, Bhanu P. (June 2021, Molecules)

   In this review, we present an assessment of recent advances in alkyne functionalization reactions, classified according to different classes of recyclable catalysts. In this work, we have incorporated and reviewed the activity and selectivity of recyclable catalytic systems such as polysiloxane-encapsulated novel metal nanoparticle-based catalysts, silica–copper-supported nanocatalysts, graphitic carbon-supported nanocatalysts, metal organic framework (MOF) catalysts, porous organic framework (POP) catalysts, bio-material-supported catalysts, and metal/solvent free recyclable catalysts. In addition, several alkyne functionalization reactions have been elucidated to demonstrate the success and efficiency of recyclable catalysts. In addition, this review also provides the fundamental knowledge required for utilization of green catalysts, which can combine the advantageous features of both homogeneous (catalyst modulation) and heterogeneous (catalyst recycling) catalysis. 

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4. Open-air green-light-driven ATRP enabled by dual photoredox/copper catalysis

   https://doi.org/10.1039/D2SC04210J  

   Szczepaniak, Grzegorz; Jeong, Jaepil; Kapil, Kriti; Dadashi-Silab, Sajjad; Yerneni, Saigopalakrishna S.; Ratajczyk, Paulina; Lathwal, Sushil; Schild, Dirk J.; Das, Subha R.; Matyjaszewski, Krzysztof (October 2022, Chemical Science)

   Photoinduced atom transfer radical polymerization (photo-ATRP) has risen to the forefront of modern polymer chemistry as a powerful tool giving access to well-defined materials with complex architecture. However, most photo-ATRP systems can only generate radicals under biocidal UV light and are oxygen-sensitive, hindering their practical use in the synthesis of polymer biohybrids. Herein, inspired by the photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization, we demonstrate a dual photoredox/copper catalysis that allows open-air ATRP under green light irradiation. Eosin Y was used as an organic photoredox catalyst (PC) in combination with a copper complex (X–Cu II /L). The role of PC was to trigger and drive the polymerization, while X–Cu II /L acted as a deactivator, providing a well-controlled polymerization. The excited PC was oxidatively quenched by X–Cu II /L, generating Cu I /L activator and PC˙ + . The ATRP ligand (L) used in excess then reduced the PC˙ + , closing the photocatalytic cycle. The continuous reduction of X–Cu II /L back to Cu I /L by excited PC provided high oxygen tolerance. As a result, a well-controlled and rapid ATRP could proceed even in an open vessel despite continuous oxygen diffusion. This method allowed the synthesis of polymers with narrow molecular weight distributions and controlled molecular weights using Cu catalyst and PC at ppm levels in both aqueous and organic media. A detailed comparison of photo-ATRP with PET-RAFT polymerization revealed the superiority of dual photoredox/copper catalysis under biologically relevant conditions. The kinetic studies and fluorescence measurements indicated that in the absence of the X–Cu II /L complex, green light irradiation caused faster photobleaching of eosin Y, leading to inhibition of PET-RAFT polymerization. Importantly, PET-RAFT polymerizations showed significantly higher dispersity values (1.14 ≤ Đ ≤ 4.01) in contrast to photo-ATRP (1.15 ≤ Đ ≤ 1.22) under identical conditions. 

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5. Tetranuclear and trinuclear copper( i ) pyrazolates as catalysts in copper mediated azide–alkyne cycloadditions (CuAAC)

   https://doi.org/10.1039/d1dt04026j  

   Patterson, Monika R.; Dias, H. V. (December 2021, Dalton Transactions)

   Homoleptic, tetranuclear copper( i ) pyrazolates {[3,5-( t -Bu) 2 Pz]Cu} 4 , {[3-(CF 3 )-5-( t -Bu)Pz]Cu} 4 , and {[4-Br-3,5-( i -Pr) 2 Pz]Cu} 4 are excellent stand-alone catalysts for azide–alkyne cycloaddition reactions (CuAAC). This work demonstrates that a range of pyrazolates, including those with electron donating and electron-withdrawing groups to sterically demanding substituents on the pyrazolyl backbones, can serve as effective ligand supports on tetranuclear copper catalysts. However, in contrast to the tetramers and also highly fluorinated {[3,5-(CF 3 ) 2 Pz]Cu} 3 , trinuclear copper( i ) complexes such as {[3,5-( i -Pr) 2 Pz]Cu} 3 and {[3-(CF 3 )-5-(CH 3 )Pz]Cu} 3 supported by relatively electron rich pyrazolates display poor catalytic activity in CuAAC. The behavior and degree of aggregation of several of these copper( i ) pyrazolates in solution were examined using vapor pressure osmometry. Copper( i ) complexes such as {[3,5-(CF 3 ) 2 Pz]Cu} 3 and {[3-(CF 3 )-5-( t -Bu)Pz]Cu} 4 with electron withdrawing pyrazolates were found to break up in solution to different degrees producing smaller aggregates while those such as {[3,5-( i -Pr) 2 Pz]Cu} 3 and {[3,5-( t -Bu) 2 Pz]Cu} 4 with electron rich pyrazolates remain intact. In addition, kinetic experiments were performed to understand the unusual activity of tetranuclear copper( i ) pyrazolate systems. 

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