Nickel(0)-Mediated Transformation of Tetrafluoroethylene and Vinylarenes into Fluorinated Cyclobutyl Compounds

What's Next for First Row Fluorometallacycles?

Hydrocarbon-derived metallacycles have been identified as key intermediates in a host of catalyzed transformations of unsaturated organic substrates. In contrast, our knowledge of analogous reactivity of fluorometallacycles is underdeveloped and largely confined to first row metals. Our summary of recent advances aims to inform young investigators of the exciting challenges offered by this pursuit.

Electrochemical Reductive Cross-Coupling of Alkyl or Alkenyl Halides with gem-Difluoroalkenes

Herein, we describe an electroreductive cross-electrophile coupling protocol for the construction of valuable monofluoroalkenes from easily accessible alkyl or alkenyl halides with gem-difluoroalkenes. The reaction can be conducted under sustainable and mild conditions delivering valuable and functionalized monofluoroalkenes with excellent Z-selectivity. The protocol's most notable advantage is the in situ release of nickel catalyst from the inexpensive electrodes without the addition of extra hazardous metal catalyst and superstoichiometric reductant.

N-Heterocyclic Carbenes with Polyfluorinated Groups at the 4- and 5-Positions from [3 + 2] Cycloadditions between Formamidinates and cis-1,2-Difluoroalkene Derivatives

We herein report the formation of fluorinated N-heterocyclic carbenes (NHCFs) that bear fluorine atoms at the 4- and 5-positions of the imidazol-2-ylidene ring. Treatment of sodium N,N'-bis(aryl)formamidinates with tetrafluoroethylene followed by the addition of LiBF4 induced a [3 + 2] cycloaddition to afford 4,5-difluorinated imidazolium salts, which served as the precursors for 4,5-difluorinated NHCs. A key feature of this procedure is its applicability to other perfluorinated compounds, which enabled us to incorporate polyfluorinated functional groups at 4- and 5-positions on the imidazol-2-ylidene skeleton. Thus, employing octafluorocyclopentene and hexafluorobenzene led to the formation of 4,4,5,5,6,6-hexafluoro-1,3-diaryl-3,4,5,6-tetrahydrocyclopenta[d]imidazolium (CypIPrF·HBF4) and 4,5,6,7-tetrafluoro-1,3-diarylbenzimidazolium (BIPrF·HBF4) salts, respectively. A thorough NMR analysis of these NHCFs, their selenium adducts, and their tricarbonyl nickel complexes, (NHCF)Ni(CO)3, demonstrated that the fluorine substituents, contrary to expectations, tend to act as electron donors owing to the considerable positive mesomeric effect, while the perfluorocyclopentene-fused and tetrafluorobenzo-fused rings are pure electron acceptors due to their strong negative inductive effect. The unique and increased π-accepting character of the perfluorocyclopentene-fused and tetrafluorobenzo-fused NHCFs in both stoichiometric and catalytic reactions is further demonstrated by employing (NHCF)Ni(CO)3 and (NHCF)AuCl species, respectively. Moreover, an analysis of the % buried volume (%Vbur) values clearly suggests that the modification of the NHC backbone with polyfluorinated groups can drastically alter the electronic properties of the NHC ligand without substantially changing its steric properties. Our experimental results were further corroborated by a series of computational calculations.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Modern approaches towards the synthesis of geminal difluoroalkyl groups

This review will cover the importance of and most recent approaches toward geminal difluoroalkyl groups. Transition metal-mediated, photochemical, organocatalytic, and other methods as well as their mechanistic implications will be discussed, with special emphasis on applications to biologically-relevant compounds.

Cobalt-catalyzed selective hydroacylation of alkynes

An efficient protocol for alkyne hydroacylation using a catalytic system incorporating Co(acac)3, dppp and AlMe3 is disclosed.

Ni-catalyzed migratory fluoro-alkenylation of unactivated alkyl bromides with gem-difluoroalkenes

We describe a nickel-catalyzed highly regio- and stereoselective migratory fluoro-alkenylation of unactivated alkyl bromides. A unique catalytic cycle merging alkyl nickel chain-walking and defluorinative coupling enables the introduction of a broad array of fluoroalkenyl moieties into carbon chains. Control experiments with other halogenated alkenes demonstrated the essential role of fluorine atoms in this reaction. Notably, the reaction proceeds under mild conditions and allows for the synthesis of a variety of valuable monofluoroalkenes.

Selective Catalytic Formation of Cross-Tetramers from Tetrafluoroethylene, Ethylene, Alkynes, and Aldehydes via Nickelacycles as Key Reaction Intermediates

In the presence of a catalytic amount of Ni(cod)₂ (cod = 1,5-cyclooctadiene) and PCy₃ (Cy = cyclohexyl), the cross-tetramerization of tetrafluoroethylene (TFE), ethylene, alkynes, and aldehydes leads to a variety of fluorine-containing enone derivatives. This reaction is the first example of a highly selective cross-tetramerization between four different unsaturated compounds. Stoichiometric reactions revealed that the present reaction involves partially fluorinated five- and seven-membered nickelacycles as key reaction intermediates.

Unexpected reactivity of cyclic perfluorinated iodanes with electrophiles

We have found that cyclic perfluorinated iodanes react with electrophiles (E+ = Br, Cl, F, I) to afford perfluorinated E-RF compounds. This reactivity is unexpected since cyclic perfluorinated iodanes are considered as electrophilic reagents that normally react with nucleophiles (e.g. Nu- = SR, OR) to afford Nu-RF products. The utility of this new transformation is demonstrated for a [18F]CF3CF2-containing compound which was prepared from [18F]XeF2 obtained from cyclotron produced [18F]fluoride.

Synthesis of tetrafluoroethylene- and tetrafluoroethyl-containing azides and their 1,3-dipolar cycloaddition as synthetic application

Tetrafluoroethylene-containing azides are accessed in two steps (one pot) from tetrafluoroalkyl bromides by metalation and reaction with electrophilic azides. Subsequent copper(i)-catalyzed azide-alkyne cycloaddition afforded N-tetrafluoroethyl and N-tetrafluoroethylene 4-substituted 1,2,3-triazoles. In addition, the protocol for the synthesis of 4,5-disubstituted 1,2,3-triazoles is presented.

Nickel-Catalyzed Formation of 1,3-Dienes via a Highly Selective Cross-Tetramerization of Tetrafluoroethylene, Styrenes, Alkynes, and Ethylene

In the presence of a catalytic amount of Ni(cod)₂ (cod = 1,5-cyclooctadiene) and PCy₃ (Cy = cyclohexyl), the cross-tetramerization of tetrafluoroethylene (TFE), alkynes, and ethylene occurred in a highly selective manner to afford a variety of 1,3-dienes with a 3,3,4,4-tetrafluorobutyl chain. In addition, a Ni(0)-catalyzed cross-tetramerization of TFE, alkynes, ethylene, and styrenes was developed. These catalytic reactions might proceed via partially fluorinated five- and seven-membered nickelacycle key intermediates.

Cobalt Catalysis for Enantioselective Cyclobutanone Construction

Over the past 40 years, intramolecular hydroacylation has favored five-membered rings, in preference to four membered rings. Herein, we report a catalyst derived from earth-abundant cobalt that enables preparation of cyclobutanones, with excellent regio-, diastereo-, and enantiocontrol, under mild conditions (2 mol % catalyst loading and as low as 50 °C).