Continuous Flow α-Trifluoromethylation of Ketones by Metal-Free Visible Light Photoredox Catalysis

Visible-Light Promoted Radical Fluoroalkylation of O- and N-Substituted Alkenes

Interaction of enol ethers enol acetates, enamides and enamines with fluorinated reagents may be considered as a reliable method for the synthesis of organofluorine compounds. While classic nucleophile/electrophile substitution or addition mechanisms cannot be realized for coupling of these components, their intrinsic reactivities are revealed with the aid of photoredox catalysis. A combination of these electron donating and accepting components provides a perfect balance needed for individual redox steps, which in some cases may proceed even without a photocatalyst. The same electronic factors also support the key C,C-bond forming event involving addition of fluorinated radical at the electron rich double bond.

Current State of Microflow Trifluoromethylation Reactions

The trifluoromethyl group is a powerful structural motif in drugs and polymers; thus, developing trifluoromethylation reactions is an important area of research in organic chemistry. Over the past few decades, significant progress has been made in developing new methods for the trifluoromethylation of organic molecules, ranging from nucleophilic and electrophilic approaches to transition-metal catalysis, photocatalysis, and electrolytic reactions. While these reactions were initially developed in batch systems, more recent microflow versions are highly attractive for industrial applications owing to their scalability, safety, and time efficiency. In this review, we discuss the current state of microflow trifluoromethylation. Approaches for microflow trifluoromethylation based on different trifluoromethylation reagents are described, including continuous flow, flow photochemical, microfluidic electrochemical reactions, and large-scale microflow reactions.

Pyridinium-Masked Enol as a Precursor for Constructing Alpha-Fluoromethyl Ketones

We present herein a pyridinium-masked enol as a versatile platform to produce ketones bearing tri-, di-, and monofluoromethyl in the presence of [Ir(dF(Me)ppy)]2(dtbbpy)]PF6 under blue light (455 nm) irradiation. By simply changing the F-source, α-trifluoromethyl ketones, α-difluoromethyl ketones, and α-monofluoromethyl ketones could be easily prepared in moderate to excellent yields in one step, making it a practical tool for the synthesis of fluorine-containing ketones. In addition, the pyridinium-masked enol could also be extended to the synthesis of sulfonyl ketones. The findings of the present protocol contribute to the arsenal of fluorine chemistry and might have potential applications in the pharmaceutical and agrochemical industries.

Polydopamine-Coated Polyurethane Foam as a Structured Support for the Development of an Easily Reusable Heterogeneous Photocatalyst Based on Eosin Y

An easy-to-handle eosin Y-based heterogeneous photocatalyst was prepared by post-functionalization of a polydopamine-coated open cell polyurethane foam (PDA@PUF) via the silanization of the adhesive layer with 3-(triethoxysilyl)propan-1-amine (APTES) and the subsequent EDC-mediated coupling of the resulting amino-functionalized foam with eosin Y. The obtained macroscopic material, EY-APTES@PDA@PUF, showed good efficiency and excellent reusability, in an easy-to-carry “dip-and-play” mode for at least six runs as photocatalyst for the aerobic oxidation of 2-methyl-5-nitroisoquinolin-2-ium iodide to the corresponding isoquinolone. Subsequent investigation of the catalytic efficiency of EY-APTES@PDA@PUF for the oxidation of sulfides to sulfoxides, however, evidenced non-negligible eosin Y leaching, leading to a progressive deactivation of the catalytic foam in this case. Two alternative synthetic protocols for the preparation of the macroscopic photocatalyst were next explored to avoid eosin Y leaching. In both cases however, cycling tests also highlighted a progressive deactivation of the catalytic foams in sulfide-to-sulfoxide oxidation reactions.

Remote Radical Trifluoromethylation: A Unified Approach to the Selective Synthesis of γ-Trifluoromethyl α,β-Unsaturated Carbonyl Compounds

Site-selective trifluoromethylation of silyl dienol ethers derived from α,β-unsaturated aldehydes, ketones, and amides was achieved for the first time in the remote γ position. This photoredox catalyzed process is quite general to compounds bearing many functionalities and is applicable to the late-stage functionalization of biorelevant molecules. The use of S-perfluoroalkyl sulfoximines as ·R_F radical sources enables the generalization of the reaction to other perfluoroalkyl groups (R_F = CF₂H, C₄F₉). Importantly, an unprecedented enantioselective C(sp³)-H perfluoroalkylation process is disclosed.

Eosin Y-Catalyzed Visible-Light-Induced Hydroperfluoroalkylation of Electron-Deficient Alkenes

The eosin Y-catalyzed hydroperfluoroalkylation of electron-deficient alkenes is described herein. The reaction proceeded smoothly under visible light irradiation and selectively afforded a hydroperfluoroalkylated product. Various perfluoroalkyl bromides and electron-deficient olefins can be used in this reaction, and all chemicals required for this reaction are safe and readily available.

Developing flow photo-thiol–ene functionalizations of cinchona alkaloids with an autonomous self-optimizing flow reactor

Continuous flow photo-thiol–ene reactions on cinchona alkaloids with a variety of organic thiols have been developed using enabling technologies such as a self-optimizing flow photochemical reactor.

Acid-controlled multicomponent selective synthesis of 2,4,6-triaryl pyridines and pyrimidines by using hexamethyldisilazane as a nitrogen source under microwave irradiation

An efficient and general protocol for the synthesis of functionalized 2,4,6-triaryl pyridines and pyrimidines was developed from commercially available aromatic ketones, aldehydes and hexamethyldisilazane (HMDS) as a nitrogen source under microwave irradiation. In this multicomponent synthetic route, Lewis acids play an important role in selectively synthesizing six-membered heterocycles, including pyridines (1N) and pyrimidines (2N), by involving [2 + 1 + 2 + 1] or [2 + 1 + 1 + 1 + 1] annulated processes.

Synthesis of pyrimidines and pyridines from commercially available ketones and aldehydes by using hexamethyldisilazane as a nitrogen source and controlled by acids under microwave irradiation.

NMR relaxation time measurements of solvent effects in an organocatalysed asymmetric aldol reaction over silica SBA-15 supported proline

The behaviour of solvents in solid-supported proline organocatalysts is explored using NMR relaxation measurements coupled with reaction screening. Solvents with a lower affinity for the solid surface lead to a higher reactivity.

Electrochemical trifluoromethylation/cyclization for the synthesis of isoquinoline-1,3-diones and oxindoles

Herein, we describe a protocol for electrochemical cathode reduction to generate trifluoromethyl radicals. The trifluoromethylation reagent (IMDN-SO₂CF₃) used in this strategy is inexpensive and easy to obtain, and the reaction can be conducted efficiently without the addition of additional redox reagents. Using this strategy, we achieved electrochemical trifluoromethylation/cyclization for the synthesis of isoquinoline-1,3-diones and oxindoles. This protocol has good functional group tolerance and a broad substrate scope.

One-pot synthesis of 2-chloro-2-thio/selenocyanato ketones from β-keto acids

The chlorothiocyanato difunctionalization reaction has been achieved, and a variety of α-chlorothio/selenocyanato difunctional ketones are synthesized through one-pot strategy from β-keto acids.

Scissoring Enaminone C═C Double Bond by Free Radical Process for the Synthesis of α-Trifluoromethyl Ketones with CF3SO2Na

The C═C double bond cleavage on tertiary enaminones, enabling the formation of a new C-CF₃ bond, has been realized as a practical method for the synthesis of α-trifluoromethyl ketones with only the promotion of TBHP and ambient heating. Control experiments support that the reactions proceed via a featured free radical process. The deuterium labeling experiment employing D₂O indicates that water participated in the product formation by donating the hydrogen atom for the newly generated α-C-H bond in the product.

Synthesis of Thiocarbamoyl Fluorides and Isothiocyanates Using Amines with CF3SO2Cl

A practical and efficient method to synthesize thiocarbamyl fluorides and isothiocyanates from amines with trifluoromethanesulfonyl chloride was developed. In the presence of the reducing agent triphenylphosphine and sodium iodide, thiocarbamyl fluorides and isothiocyanates were synthesized from secondary/primary amine in moderate to excellent yields, respectively. A broad scope of substrates and good functional group compatibility were observed.

Metal-Free ATRP Catalyzed by Visible Light in Continuous Flow

ATRP of methyl methacrylate catalyzed by Eosin Y, an inexpensive and an environmental benign dye, was performed in a continuous flow reactor made of FEP tubing and irradiated by visible light green LEDs. The reaction under flow conditions was significantly more rapid and controlled compared to that in batch giving 90% of polymerization after only 3 h of irradiation. The formed polymers in flow have M n measured by GPC and DOSY NMR in accordance with the theoretical values and show low dispersities (Ð < 1.5). The livingness of the polymers has been confirmed by LED on and LED off experiments and by the synthesis of block copolymers. The protocol described herein serves as a "proof of concept" of using Eosin Y as a photocatalyst for controlled polymerization and of using 1D and 2D NMR for polymer characterization. The protocol could be replicated in the future for other reversible-deactivation radical polymerizations.

Eosin Y: Homogeneous Photocatalytic In-Flow Reactions and Solid-Supported Catalysts for In-Batch Synthetic Transformations

In this paper, the most recent and significant applications of Eosin Y as an organo-photocatalyst will be discussed, focusing the attention on enabling technological aspects in homogeneous photochemical flow reactions, as well as on recent developments in solid-supported catalyst applications for batch synthetic transformations.

Pushing the boundaries of C–H bond functionalization chemistry using flow technology

C–H functionalization chemistry is one of the most vibrant research areas within synthetic organic chemistry. While most researchers focus on the development of small-scale batch-type transformations, more recently such transformations have been carried out in flow reactors to explore new chemical space, to boost reactivity or to enable scalability of this important reaction class. Herein, an up-to-date overview of C–H bond functionalization reactions carried out in continuous-flow microreactors is presented. A comprehensive overview of reactions which establish the formal conversion of a C–H bond into carbon–carbon or carbon–heteroatom bonds is provided; this includes metal-assisted C–H bond cleavages, hydrogen atom transfer reactions and C–H bond functionalizations which involve an SE-type process to aromatic or olefinic systems. Particular focus is devoted to showcase the advantages of flow processing to enhance C–H bond functionalization chemistry. Consequently, it is our hope that this review will serve as a guide to inspire researchers to push the boundaries of C–H functionalization chemistry using flow technology.

3D designed and printed chemical generators for on demand reagent synthesis

Modern science has developed well-defined and versatile sets of chemicals to perform many specific tasks, yet the diversity of these reagents is so large that it can be impractical for any one lab to stock everything they might need. At the same time, isssues of stability or limited supply mean these chemicals can be very expensive to purchase from specialist retailers. Here, we address this problem by developing a cartridge -oriented approach to reactionware-based chemical generators which can easily and reliably produce specific reagents from low-cost precursors, requiring minimal expertise and time to operate, potentially in low infrastructure environments. We developed these chemical generators for four specific targets; transition metal catalyst precursor tris(dibenzylideneacetone)dipalladium(0) [Pd₂(dba)₃], oxidising agent Dess-Martin periodinane (DMP), protein photolinking reagent succinimidyl 4,4’-azipentanoate (NHS-diazirine), and the polyoxometalate cluster {P₈W₄₈}. The cartridge synthesis of these materials provides high-quality target compounds in good yields which are suitable for subsequent utilization.

Synthetic labs rely on a vast number of chemicals, which are often unstable with time and affected by price fluctuations. Here, the authors report ad hoc developed cartridge reactionware for the synthesis of four different targets in a time- and cost-saving manner.

Progress and prospects in the use of photocatalysis for the synthesis of organofluorine compounds

Data on the synthesis of fluorinated organic compounds by photocatalysis are systematically considered and analyzed. The attention is focused on the mechanisms of photocatalytic reactions and the selectivity problem.

The bibliography includes 173 references.

On the reactivity of anodically generated trifluoromethyl radicals toward aryl alkynes in organic/aqueous media

Two competing pathways have been experimentally observed and the selectivity has been explained by means of DFT calculations.

Visible-light-promoted oxidation/condensation of benzyl alcohols with dialkylacetamides to cinnamides

An oxidative cross-coupling reaction of benzyl alcohols with dialkylacetamides was developed to construct cinnamides under visible-light-enabled photocatalytic conditions.

Micro-flow nanocatalysis: synergic effect of TfOH@SPIONs and micro-flow technology as an efficient and robust catalytic system for the synthesis of plasticizers

The combination of continuous flow technology with immobilizing of only 0.13 mol% of triflic acid (TfOH) on silica-encapsulated superparamagnetic iron oxide nanoparticles (SPIONs) under solvent-free conditions successfully provided a powerful, efficient, and eco-friendly route for the synthesis of plasticizers. The turnover frequency value in micro-flow conditions varied in the range of 948.7 to 7384.6 h^-1 compared to 403.8 to 3099 h^-1 for in-flask. This technique works efficiently, encouraging future applications of micro-flow nano-catalysis in green chemistry.

Highly ( Z)-Diastereoselective Synthesis of Trifluoromethylated exo-Glycals via Photoredox and Copper Catalysis

Highly ( Z)-diastereoselective approaches for the synthesis of trifluoromethylated exo-glycals by copper and photoredox catalysis are described. These complementary reactions are applicable to a wide range of methylene exo-glycals generated from the corresponding pyranoses and furanoses and give trifluoromethylated compounds under mild conditions in moderate to good yields. DFT calculations have allowed a rationalization of the observed ( Z)-stereoselectivity.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

CF3SO2X (X = Na, Cl) as reagents for trifluoromethylation, trifluoromethylsulfenyl-, -sulfinyl- and -sulfonylation and chlorination. Part 2: Use of CF3SO2Cl

The recent progresses of the application of trifluoromethanesulfonyl chloride, CF₃SO₂Cl, in the formation of C–CF₃, C–SCF₃, C–SOCF₃, and C–Cl bonds are summarised in this second part of a two-part review published back-to-back on both sodium trifluoromethanesulfinate, CF₃SO₂Na, (Part 1) and trifluoromethanesulfonyl chloride, CF₃SO₂Cl (Part 2). There are many reactions in common between these two reagents but it should be noted that CF₃SO₂Cl reacts under reductive conditions while CF₃SO₂Na requires oxidative conditions. Electrophilic chlorination is obviously the exclusive preserve of CF₃SO₂Cl that has been exploited with emphasis in enantioselective chlorination.