Nickel-Catalyzed Aromatic Cross-Coupling Difluoromethylation of Grignard Reagents with Difluoroiodomethane

New synthetic approaches for the construction of difluoromethylated architectures

Fluorinated compounds play a vital role in the fields of agrochemicals, pharmaceuticals, and materials science because of their unique lipophilicity, permeability, and metabolic stability. Among all such appealing fluorine-containing functional groups, the difluoromethyl group has attracted considerable attention owing to its outstanding chemical and physical properties. It has been used as a lipophilic hydrogen bond donor and a bioisostere of thiol, hydroxy, or amino groups. The excellent properties of the CF2H group have motivated many chemists to develop effective strategies for the selective incorporation of the CF2H group into target molecules. Over the past decades, a variety of efficient, atom-economical, and facile methods have been discovered for the difluoromethylation of organic substrates. This review summarizes the developments in different types of difluoromethylations, which could be classified into the following categories: radical difluoromethylation, transition metal-catalyzed difluoromethylation, and nucleophilic and electrophilic difluoromethylation.

Nickel-catalysed reductive C-N bond cross-coupling between aryl halides and N-chloroamides

A method for the direct synthesis of N-aryl lactams and amides with aryl halides and N-chloroamides through a Ni-catalyzed reductive C-N coupling reaction has been developed. The reaction features the advantages of mild conditions, good functional group tolerance and broad substrate scope including drug-derived substrates, and also provided direct access to the key synthetic intermediates for some bioactive molecules, suggesting the practicability of this method. Finally, DFT calculations were performed to shed further light on the reaction mechanism and it was found that an amidyl radical might be involved.

A Difluoromethylation Reagent: Access to Difluoromethyl Arenes through Palladium Catalysis

A new radical difluoromethylation was developed by using inexpensive and readily available difluoroacetic anhydride and N-phenyl-4-methylbenzenesulfonamide for the first time. The reaction of arylboronic acids with the new difluoromethylation reagent, N-phenyl-N-tosyldifluoroacetamide, proceeded smoothly in the presence of palladium catalyst to provide difluoromethylarenes in satisfactory to excellent yields. The electronic property (electron-donating or electron-withdrawing) of the substituent linked to the aromatic ring did not considerably influence the reactivity of arylboronic acid. Various groups, including the synthetically useful functional groups Cl, CN, and NO2, were tolerated well under the current reaction conditions.

Sulfone Electrophiles in Cross-Electrophile Coupling: Nickel-Catalyzed Difluoromethylation of Aryl Bromides

Fluoroalkyl fragments have played a critical role in the design of pharmaceutical and agrochemical molecules in recent years due to the enhanced biological properties of fluorinated molecules compared to their non-fluorinated analogues. Despite the potential advantages conferred by incorporating a difluoromethyl group in organic compounds, industrial adoption of difluoromethylation methods lags behind fluorination and trifluoromethylation. This is due in part to challenges in applying common difluoromethyl sources towards industrial applications. We report here the nickel-catalyzed cross-electrophile coupling of (hetero)aryl bromides with difluoromethyl 2-pyridyl sulfone, a sustainably sourced, crystalline difluoromethylation reagent. The scope of this reaction is demonstrated with 24 examples (67 ± 16% average yield) including a diverse array of heteroaryl bromides and precursors to difluoromethyl-containing preclinical pharmaceuticals. This reaction can be applied to small-scale parallel synthesis and benchtop scale-up under mild conditions. As sulfone reagents are uncommon electrophiles in cross-electrophile coupling, the mechanism of this process was investigated. Studies confirmed the formation of •CF2H instead of difluorocarbene. A series of modified difluoromethyl sulfones revealed that sulfone reactivity does not correlate exclusively with reduction potential and that coordination of cations or nickel to the pyridyl group is essential to reactivity, setting out parameters for matching the reactivity of sulfones in cross-electrophile coupling.

Divergent Generation of the Difluoroalkyl Radical and Difluorocarbene via Selective Cleavage of C-S Bonds of the Sulfox-CF2SO2Ph Reagent

A new difluoroalkylation reagent Sulfox-CF2SO2Ph bearing both sulfoximine and sulfone moieties was prepared from commercially available SulfoxFluor and PhSO2CF2H. On one hand, the Sulfox-CF2SO2Ph reagent could act as a (phenylsulfonyl)difluoromethyl radical source under photoredox catalysis, in which the arylsulfoximidoyl group is selectively removed. On the other hand, under basic conditions, Sulfox-CF2SO2Ph could serve as a difluorocarbene precursor for S- and O-difluoromethylations with S- and O-nucleophiles, respectively, in which the phenylsulfonyl group in Sulfox-CF2SO2Ph is selectively removed (followed by α-elimination of the arylsulfoximidoyl group).

Pd-Catalyzed Difluoromethylations of Aryl Boronic Acids, Halides, and Pseudohalides with ICF2 H Generated ex Situ

An expedient ex-situ generation of difluoroiodomethane (DFIM) and its immediate use in a Pd-catalyzed difluoromethylation of aryl boronic acids and ester derivatives in a two-chamber reactor is reported. Heating a solution of bromodifluoroacetic acid with sodium iodide in sulfolane proved to be effective for the generation of near stoichiometric amounts of DFIM for the ensuing catalytic coupling step. A two-step difluoromethylation of aryl (pseudo)halides with tetrahydroxydiboron as a low-cost reducing agent, both promoted by Pd catalysis, proved effective to install this fluorine-containing C1 group onto several pharmaceutically relevant molecules. Finally, the method proved adaptable to deuterium incorporation by simply adding D2 O to the DFIM-generating chamber.

Late-stage difluoromethylation: concepts, developments and perspective

This review describes the conceptual advances that have led to the multiple difluoromethylation processes making use of well-defined CF₂H sources.

Nickel-catalysed fluoromethylation reactions

This review highlights important developments in nickel-catalysed mono-, di- and tri-fluoromethylation, trifluoromethylthiolation and trifluoromethylselenolation.

Recent Progress in Utilization of Functionalized Organometallic Reagents in Cross Coupling Reactions and Nucleophilic Additions

Organometallic compounds have become increasingly important in organic synthesis because of their high chemoselectivity and excellent reactivity. Recently, a variety of organometallic reagents were found to facilitate transition-metal-catalyzed cross-coupling reactions and nucleophilic addition reactions. Here, we have summarized the latest progress in cross-coupling reactions and in nucleophilic addition reactions with functionalized organometallic reagents present to illustrate their application value. Due to the tremendous contribution made by the Knochel group towards the development of novel organometallic reagents, this review draws extensively from their work in this area in recent years.

Introduction

1 Transition-Metal-Catalyzed Cross Couplings Involving Organo­zinc Reagents

2 Transition-Metal-Catalyzed Cross Couplings Involving Organomagnesium Reagents

3 Transition-Metal-Free Cross Couplings Involving Zn and Mg ­Organometallic Reagents

4 Nucleophilic Additions Involving Zn and Mg Organometallic Reagents

5 Cross-Coupling Reactions or Nucleophilic Additions Involving Mn, Al-, La-, Li-, Sm- and In-Organometallics

6 Conclusion

Two Ligands Transfer from Ag to Pd: En Route to (SIPr)Pd(CF2H)(X) and Its Application in One-Pot C-H Borylation/Difluoromethylation

A process for the concurrent transfer of both the NHC ligand and the difluoromethyl group from [(SIPr)Ag(CF₂H)] to PdX₂ (X = Cl, OAc, and OPiv) for the preparation of [(SIPr)Pd(CF₂H)X] complexes is described. These complexes were air-stable and easily underwent transmetalation with aryl pinacol boronate/reductive elimination to generate ArCF₂H in high yields. Based on this discovery, the first one-pot C-H borylation and difluoromethylation process for the preparation of difluoromethylated (hetero)arenes was developed.