Photocatalytic Difluoromethylation Reactions of Aromatic Compounds and Aliphatic Multiple C-C Bonds

Photochemical Difluoromethylation of Quinoxalin-2(1H)-ones with Difluoroacetic Anhydride and Pyridine N-Oxide

A novel photochemical difluoromethylation of quinoxalin-2(1H)-ones under catalyst-free conditions was achieved with difluoroacetic anhydride and pyridine N-oxide. The green and mild reaction conditions as well as readily attainable difluoroacetic anhydride provide a useful protocol to prepare C3-difluoromethylated quinoxalin-2(1H)-ones.

Anaerobic photoinduced Cu(0/I)-mediated Glaser coupling in a radical pathway

The reaction mechanism of the historic copper-catalyzed Glaser coupling has been debated to be based on redox cycles of Cu ions in specific oxidation states or on a radical mechanism based on Cu(0)/Cu(I). Here, the authors demonstrate two coexisting Glaser coupling pathways which can be differentiated by anaerobic/irradiation or aerobic reaction conditions. Without O2, copper(I) acetylides undergo a photo-excited pathway to generate highly reactive alkynyl radicals, which combine together to form a homo-coupling product or individually react with diverse X-H (X = C, N, O, S and P) substrates via hydrogen atom transfer. With O2, copper(I) acetylides are oxidized to become a Cu-acetylide/Cu-O merged Cu(I/II) intermediate for further oxidative coupling. This work not only complements the radical mechanism for Glaser coupling, but also provides a mild way to access highly energetic alkynyl radicals for efficient organic transformations.

Photocatalyzed Hydroaminodifluoroalkylation of Alkenes

The synthesis of undescribed β-aminodifluoroethylsulfinates and their uses in the hydroaminodifluoroalkylation of alkenes is reported. This reaction is performed in the presence of a photocatalyst (4CzIPN, Ru complexes) and enables the direct incorporation of a β-difluoroamine moiety into vinylic aryls, unactivated alkenes, and electron-rich, or -deficient alkenes. The mechanism was studied, and the formation of a gem-difluoromethyl radical was observed after the selective oxidation of the sulfinate function.

Understanding and Controlling Fluorinated Diacyl Peroxides and Fluoroalkyl Radicals in Alkene Fluoroalkylations

The demand for practical methods for the synthesis of novel fluoroalkyl molecules is increasing owing to their diverse applications. Our group has achieved efficient difunctionalizing fluoroalkylations of alkenes using fluorinated carboxylic anhydrides as user-friendly fluoroalkyl sources. Fluorinated diacyl peroxide, prepared in situ from carboxylic anhydrides, enables the development of novel reactions when used as a radical fluoroalkylating reagent. In this account, we aim to provide an in-depth understanding of the structure, bonding, and reactivity of fluorinated diacyl peroxides and radicals as well as their control in fluoroalkylation reactions. In the first part of this account, the physical properties and reactivity of diacyl peroxides and fluoroalkyl radicals are described. In the subsequent part, we categorize the reactions into copper-catalyzed and metal-free methods utilizing the oxidizing properties of fluorinated diacyl peroxides. We also outline examples and mechanisms.

Difluoromethylarylation of Alkynes from [Bis(difluoroacetoxy)iodo]benzene: Access to CF2H-Containing Dibenzazepines

A photoinduced radical difluoromethylarylation via tandem addition-cyclization of alkynes with easily available [bis(difluoroacetoxy)iodo]benzene is accomplished, providing a straightforward and practical route for the construction of difluoromethylated dibenzazepines. Various sensitive functional groups can be compatible under photoinduced conditions. Mechanism investigation reveals that this transformation is initiated by the addition of alkyne with difluoromethyl radical, generated in situ from [bis(difluoroacetoxy)iodo]benzene.

Rose Bengal-photocatalyzed perfluorohexylation reactions of organic substrates in water. Applications to late-stage syntheses

The Rose Bengal-photocatalyzed perfluorohexylation of olefins, alkynes, and electron-rich aromatic compounds in water was achieved employing perfluorohexyl iodide as fluoroalkyl source and TMEDA as sacrificial donor under green LED irradiation. Alkenes and alkynes rendered products derived from the atom transfer radical addition (ATRA) pathway, and in the case of alkynes, exclusively as E-stereoisomers. These are the first examples of photocatalyzed ATRA reactions carried out excursively in water alone. The reactions of aromatic compounds under the current protocol in water present the advantage of employing a perfluoroalkyl iodide (C₆F₁₃-I) as source of perfluorohexyl radicals. Examples of photocatalytic late-stage incorporations of fluoroalkyl moieties into two commercial drugs of widespread use are reported.

State of knowledge in photoredox-catalysed direct difluoromethylation

The combination of visible light photoredox catalysis with direct difluoromethylation has allowed the synthesis of a large choice of CF2H-containing value-added molecules under very mild reaction conditions.

Multigram Synthesis of Advanced 6,6-Difluorospiro[3.3]heptane-derived Building Blocks

A convenient methodology for constructing 6,6-difluorospiro[3.3]heptane scaffold - a conformationally restricted isostere of gem-difluorocycloalkanes - is developed. Alarge array of novel 2-mono- and 2,2-bifunctionalized difluorospiro[3.3]heptane building blocks was obtained through the convergent synthesis strategy using a common synthetic precursor - 1,1-bis(bromomethyl)-3,3-difluorocyclobutane. The target compounds and intermediates were prepared by short reaction sequences (6-10 steps) on multigram scale (up to 0.47 kg).

Visible-light mediated stereospecific C(sp2)-H difluoroalkylation of (Z)-aldoximes

A visible light mediated stereospecific C(sp²)-H difluoroalkylation of (Z)-aldoximes to (E)-difluoroalkylated ketoximes has been described. In this reaction, (hetero)-aromatic and aliphatic difluoroalkylated ketoximes could be obtained with the retention of the configuration of the starting aldoximes. A preliminary mechanism study showed that a difluoromethyl radical via an SET pathway was involved.

Phosphorus-mediated sp2-sp3 couplings for C-H fluoroalkylation of azines

Fluoroalkyl groups profoundly affect the physical properties of pharmaceuticals and influence almost all metrics associated with their pharmacokinetic and pharmacodynamic profile^1-4. Drug candidates increasingly contain trifluoromethyl (CF₃) and difluoromethyl (CF₂H) groups, and the same trend in agrochemical development shows that the effect of fluoroalkylation translates across human, insect and plant life^5,6. New fluoroalkylation reactions have undoubtedly stimulated this shift; however, methods that directly convert C-H bonds into C-CF₂X groups (where X is F or H) in complex drug-like molecules are rare^7-13. Pyridines are the most common aromatic heterocycles in pharmaceuticals¹⁴, but only one approach-via fluoroalkyl radicals-is viable for achieving pyridyl C-H fluoroalkylation in the elaborate structures encountered during drug development^15-17. Here we develop a set of bench-stable fluoroalkylphosphines that directly convert the C-H bonds in pyridine building blocks, drug-like fragments and pharmaceuticals into fluoroalkyl derivatives. No preinstalled functional groups or directing groups are required. The reaction tolerates a variety of sterically and electronically distinct pyridines, and is exclusively selective for the 4-position in most cases. The reaction proceeds through initial formation of phosphonium salts followed by sp²-sp³ coupling of phosphorus ligands-an underdeveloped manifold for forming C-C bonds.

Photoredox-catalyzed direct C(sp2)–H difluoromethylation of enamides or heterocycles with [bis(difluoroacetoxy)iodo]benzene

A photoredox-catalyzed direct C(sp2)–H difluoromethylation of enamides and heterocycles is accomplished by using easily accessible [bis(difluoroacetoxy)iodo]benzene as the CF2H source.

Electrochemically driven, cobalt–carbon bond-mediated direct intramolecular cyclic and acyclic perfluoroalkylation of (hetero)arenes using X(CF2)4X

A proof-of-concept for synthetically challenging cyclic and acyclic perfluoroalkylation of (hetero)arenes driven by the valence change of a cobalt catalyst with X(CF₂)₄X is demonstrated.