Silver-Catalyzed para-Selective Sulfenyl Group Transfer Reactions of N-Sulfenylanilides

Silver-catalyzed reactions of N-sulfenylanilides afforded the corresponding p-sulfenylanilides in good to high yields with good para selectivity. The transformation has a high compatibility of functional groups, such as ester, bromo, and iodo groups. Mechanistic studies indicate that the rearrangement reaction proceeds through intermolecular transfer of the sulfenyl group.

Electrochemical Trifluoromethoxylation of (Hetero)aromatics with a Trifluoromethyl Source and Oxygen

Trifluoromethoxylated aromatics (ArOCF₃ ) are valuable structural motifs in the area of drug discovery due to the enhancement of their desired physicochemical properties upon the introduction of the trifluoromethoxy group (CF₃ O). Although significant progress has been made recently in the introduction of CF₃ O group into aromatics, current methods either require the use of expensive trifluoromethoxylation reagents or require harsh reaction conditions. We present a conceptually new and operationally simple protocol for the direct C-H trifluoromethoxylation of (hetero)aromatics by the combination of the readily available trifluoromethylating reagent and oxygen under electrochemical reaction conditions. This reaction proceeds through the initial generation of CF₃ radical followed by conversion to CF₃ O radical, addition to (hetero)aromatics and rearomatization. The utility of this electrochemical trifluoromethoxylation is illustrated by the direct incorporation of CF₃ O group into a variety of (hetero)aromatics as well as bio-relevant molecules.

Radical C-H Trifluoromethoxylation of (Hetero)arenes with Bis(trifluoromethyl)peroxide

Trifluoromethoxylated (hetero)arenes are of great interest for several disciplines, especially in agro- and medicinal chemistry. Radical C-H trifluoromethoxylation of (hetero)arenes represents an attractive approach to prepare such compounds, but the high cost and low atom economy of existing . OCF3 radical sources make them unsuitable for the large-scale synthesis of trifluoromethoxylated building blocks. Herein, we introduce bis(trifluoromethyl)peroxide (BTMP, CF3 OOCF3 ) as a practical and efficient trifluoromethoxylating reagent that is easily accessible from inexpensive bulk chemicals. Using either visible light photoredox or TEMPO catalysis, trifluoromethoxylated arenes could be prepared in good yields under mild conditions directly from unactivated aromatics. Moreover, TEMPO catalysis allowed for the one-step synthesis of valuable pyridine derivatives, which have been previously prepared via multi-step approaches.

Redox-Neutral Selenium-Catalysed Isomerisation of para-Hydroxamic Acids into para-Aminophenols

A selenium-catalysed para-hydroxylation of N-aryl-hydroxamic acids is reported. Mechanistically, the reaction comprises an N-O bond cleavage and consecutive selenium-induced [2,3]-rearrangement to deliver para-hydroxyaniline derivatives. The mechanism is studied through both 18 O-crossover experiments as well as quantum chemical calculations. This redox-neutral transformation provides an unconventional synthetic approach to para-aminophenols.

Redox-Neutrale Selen-katalysierte Isomerisierung von para-Hydroxamsäuren zu para-Aminophenolen

Über eine Selen‐katalysierte para‐Hydroxylierung von N‐Arylhydroxamsäuren wird berichtet. Mechanistisch verläuft diese über N‐O‐Bindungsbruch und nachfolgende Selen‐induzierte [2,3]‐Umlagerungen um para‐Hydroxylanilinderivate zu erzeugen. Der Mechanismus wurde sowohl mittels 18O‐Überkreuzungsexperimenten als auch quantenchemischen Berechnungen untersucht. Diese redox‐neutrale Transformation ermöglicht einen ungewöhnlichen synthetischen Zugang zu para‐Aminophenolen.

Synthesis of Tri- and Difluoromethoxylated Compounds by Visible-Light Photoredox Catalysis

Trifluoromethoxy (OCF3 ) and difluoromethoxy (OCF2 H) groups are fluorinated structural motifs that exhibit unique physicochemical characteristics. Incorporation of these substituents into organic molecules is a highly desirable approach used in medicinal chemistry and drug discovery processes to alter the properties of a parent compound. Recently, tri- and difluoromethyl ethers have received increasing attention and several innovative strategies to access these valuable functional groups have been developed. The focus of this Minireview is the use of visible-light photoredox catalysis in the synthesis of tri- and difluoromethyl ethers. Recent photocatalytic strategies for the formation of O-CF3 , C-OCF3, O-CF2 H, and C-OCF2 H bonds as well as other transformations leading to the construction of ORF groups are discussed herein.

Visible-Light Photoredox-Catalyzed and Copper-Promoted Trifluoromethoxylation of Arenediazonium Tetrafluoroborates

We report the development of photoredox-catalyzed and copper-promoted trifluoromethoxylation of arenediazonium tetrafluoroborates, with trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation reagent. This new method takes advantage of visible-light photoredox catalysis to generate the aryl radical under mild conditions, combined with copper-promoted selective trifluoromethoxylation. The reaction is scalable, tolerates a wide range of functional groups, and proceeds regioselectively under mild reaction conditions. Furthermore, mechanistic studies suggested that a Cs[Cu(OCF₃ )₂ ] intermediate might be generated during the reaction.

Redox-Active Reagents for Photocatalytic Generation of the OCF3 Radical and (Hetero)Aryl C-H Trifluoromethoxylation

The trifluoromethoxy (OCF3 ) radical is of great importance in organic chemistry. Yet, the catalytic and selective generation of this radical at room temperature and pressure remains a longstanding challenge. Herein, the design and development of a redox-active cationic reagent (1) that enables the formation of the OCF3 radical in a controllable, selective, and catalytic fashion under visible-light photocatalytic conditions is reported. More importantly, the reagent allows catalytic, intermolecular C-H trifluoromethoxylation of a broad array of (hetero)arenes and biorelevant compounds. Experimental and computational studies suggest single electron transfer (SET) from excited photoredox catalysts to 1 resulting in exclusive liberation of the OCF3 radical. Addition of this radical to (hetero)arenes gives trifluoromethoxylated cyclohexadienyl radicals that are oxidized and deprotonated to afford the products of trifluoromethoxylation.

Silver-Promoted Oxidative Benzylic C-H Trifluoromethoxylation

A silver-promoted oxidative benzylic C-H trifluoromethoxylation has been reported for the first time. With trifluoromethyl arylsulfonate as the trifluoromethoxylation reagent, various arenes, having diverse functional groups, undergo trifluoromethoxylation of their benzylic C-H bonds to form trifluoromethyl ethers under mild reaction conditions. In addition, the trifluoromethoxylation and the fluorination of methyl groups of electron-rich arenes have been achieved to prepare α-fluorobenzyl trifluoromethyl ethers in one step.

Radical Trifluoromethoxylation of Arenes Triggered by a Visible-Light-Mediated N-O Bond Redox Fragmentation

A simple trifluoromethoxylation method enables non-directed functionalization of C-H bonds on a range of substrates, providing access to aryl trifluoromethyl ethers. This light-driven process is distinctly different from conventional procedures and occurs through an OCF₃ radical mechanism mediated by a photoredox catalyst, which triggers an N-O bond fragmentation. The pyridinium-based trifluoromethoxylation reagent is bench-stable and provides access to synthetic diversity in lead compounds in an operationally simple manner.

Transition-Metal-Free Synthesis of N-Hydroxy Oxindoles by an Aza-Nazarov-Type Reaction Involving Azaoxyallyl Cations

A novel transition-metal-free method to construct N-hydroxy oxindoles by an aza-Nazarov-type reaction involving azaoxyallyl cation intermediates is described. A variety of functional groups were tolerated under the weak basic reaction conditions and at room temperature. A one-pot process was also developed to make the reaction even more practical. This method provides alternative access to oxindoles and their biologically active derivatives.

Catalytic C-H Trifluoromethoxylation of Arenes and Heteroarenes

The intermolecular C-H trifluoromethoxylation of arenes remains a long-standing and unsolved problem in organic synthesis. Herein, we report the first catalytic protocol employing a novel trifluoromethoxylating reagent and redox-active catalysts for the direct (hetero)aryl C-H trifluoromethoxylation. Our approach is operationally simple, proceeds at room temperature, uses easy-to-handle reagents, requires only 0.03 mol % of redox-active catalysts, does not need specialized reaction apparatus, and tolerates a wide variety of functional groups and complex structures such as sugars and natural product derivatives. Importantly, both ground-state and photoexcited redox-active catalysts are effective. Detailed computational and experimental studies suggest a unique reaction pathway where photoexcitation of the trifluoromethoxylating reagent releases the OCF3 radical that is trapped by (hetero)arenes. The resulting cyclohexadienyl radicals are oxidized by redox-active catalysts and deprotonated to form the desired products of trifluoromethoxylation.

Copper-Mediated Trifluoromethylation of Benzylic Csp3 -H Bonds

Trifluoromethyl-containing compounds play a significant role in medicinal chemistry, materials and fine chemistry. Although direct C-H trifluoromethylation has been achieved on Csp² -H bonds, direct conversion of Csp³ -H bonds to Csp³ -CF₃ remains challenging. We report herein an efficient protocol for the selective trifluoromethylation of benzylic C-H bonds. This process is mediated by a combination Cu^III -CF₃ species and persulfate salts. A wide range of methylarenes can be selectively trifluoromethylated at the benzylic positions. A combination of experimental and theoretical mechanistic studies suggests that the reaction involves a radical intermediate and a Cu^III -CF₃ species as the CF₃ transfer reagent.

A Radical Revolution for Trifluoromethoxylation

The study of OCF₃ -substituted molecules is somewhat hampered by a lack of diverse synthetic methods to access them. By introducing a new mild and practical reaction system for accessing the ^. OCF₃ radical, a recent study by Liu, Ngai, and co-workers has the potential to dramatically expand the scope of direct trifluoromethoxylation. The features of this ground-breaking system are discussed and placed into context with other recent advances on nucleophilic trifluoromethoxylation.

Direct Dehydroxytrifluoromethoxylation of Alcohols

The first example of a direct dehydroxytrifluoromethoxylation of alcohols has been developed. This method generated an alkyl fluoroformate in situ from alcohols, followed by nucleophilic trifluoromethoxylation with trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation reagent. The reaction is operationally simple and scalable, and it proceeds under mild reaction conditions to provide access to a wide range of trifluoromethyl ethers from alcohols. In addition, this method is suitable for the late-stage trifluoromethoxylation of complex small molecules.

Palladium-Catalyzed 2,2,2-Trifluoroethoxylation of Aromatic and Heteroaromatic Chlorides Utilizing Borate Salt and the Synthesis of a Trifluoro Analogue of Sildenafil

A simple and convenient method was developed for the introduction of a 2,2,2-trifluoroethoxy group to various aromatic and heteroaromatic systems. The novel process utilizes aromatic chlorides as substrates, and tetrakis(2,2,2-trifluoroethoxy) borate salt as an inexpensive and readily available fluoroalkoxy source in a palladium-catalyzed cross-coupling reaction. The power of the developed methodology was demonstrated in the synthesis of a fluorous derivative of Sildenafil.

Synthetic Approaches to Trifluoromethoxy-Substituted Compounds

Because of the unique properties of the trifluoromethoxy group, molecules bearing this moiety will find applications in various fields, particularly in the life sciences. However, despite the great interest in this functional group, only a small number of trifluoromethoxylated molecules are currently synthetically accessible. Over the last few years, several innovative and promising strategies for the synthesis of trifluoromethoxylated compounds have been described. This Minireview discusses these existing methods with a particular focus on more recent advances.

Fluorodecarboxylation for the Synthesis of Trifluoromethyl Aryl Ethers

The synthesis of mono-, di-, and trifluoromethyl aryl ethers by fluorodecarboxylation of the corresponding carboxylic acids is reported. AgF2 induces decarboxylation of aryloxydifluoroacetic acids, and AgF, either generated in situ or added separately, serves as a source of fluorine to generate the fluorodecarboxylation products. The addition of 2,6-difluoropyridine increased the reactivity of AgF2 , thereby increasing the range of functional groups and electronic properties of the aryl groups that are tolerated. The reaction conditions used for the formation of trifluoromethyl aryl ethers also served to form difluoromethyl and monofluoromethyl aryl ethers.

Direct Difluoromethylation of Alcohols with an Electrophilic Difluoromethylated Sulfonium Ylide

A general method for the formation of alkyl difluoromethylethers under mild reaction conditions and with good functional-group tolerance was developed. The development of the method was based on the invention of a stable, electrophilic, difluoromethylating reagent, difluoromethyl-(4-nitrophenyl)-bis(carbomethoxy) methylide sulfonium ylide, which was synthesized by reaction of the easily available 4-nitrophenyl (difluoromethyl)thioether and dimethyl diazomalonate in the presence of a rhodium catalyst.

Regioselective Synthesis of N-Heteroaromatic Trifluoromethoxy Compounds by Direct O-CF3 Bond Formation

The first one-step method for the synthesis of ortho-N-heteroaromatic trifluoromethoxy derivatives by site-specific O-CF3 bond formation using hydroxylated N-heterocycles and Togni's reagent is described. The approach enables the unprecedented syntheses of a wide range of six or five-membered N-heteroaromatic trifluoromethoxy compounds containing one or two heteroatoms from most commonly used hydroxylated N-heterocycles. Notable advantages of this method include its simplicity and mild conditions, avoidance of the need for metals or toxic reagents, and compatibility with a variety of functional groups. Furthermore, this method is especially suitable for the larger scale application.