Ring-Opening Functionalization of Simple gem-Difluorocyclopropanes by Single-Electron Oxidants

Selective 1,1- and 1,2-dibromination of phenylethanes in the presence of NaBr/NaBrO3/H2SO4 as the bromination reagent

Selective 1,1- and 1,2-dibromination of phenylethanes by simply adjusting the reaction conditions has been developed. Mixtures of NaBr/NaBrO3/H2SO4 are employed as green bromination reagents, which can release Br2 or BrOH in situ as required without polluting the environment. Both the resulting 1,1- and 1,2-dibromoethyl arenes can be easily transformed to phenylacetylenes via elimination under basic conditions, demonstrating great potential for industrial applications.

Selectivity in Rh-catalysis with gem-difluorinated cyclopropanes

Small-ring chemistry is a fascinating field in organic chemistry. gem-Difluorinated cyclopropanes, a unique class of cyclopropanes, have garnered significant interest due to their intrinsic high reactivity. In this context, gem-difluorinated cyclopropanes have been extensively investigated as fluoroallylic synthons in Pd-catalyzed ring-opening/cross-coupling reactions for the synthesis of monofluoroalkenes with linear or branched selectivity. In contrast, Rh-catalysis has revealed diverse selectivity in the reaction of gem-difluorinated cyclopropanes, such as regioselectivity, enantioselectivity, and chemoselectivity. This feature article aims to summarize our efforts towards developing Rh-catalyzed reactions of gem-difluorinated cyclopropanes, briefly discussing the design, selectivity, reaction mechanisms and future research prospects.

Cobalt-Catalyzed Fluoroallyllation of Carbonyls via C-C Activation of gem-Difluorocyclopropanes

A new Co-catalyzed sequential C-C and C-F activation of gem-difluorinated cyclopropanes (gem-FCPs) to form nucleophilic fluoroallylcobalt, followed by addition to aldehydes, is reported. The protocol features the regioselective cleavage of dual chemical bonds of readily available gem-FCPs to prepare easily separable linear (Z)- and (E)-fluorinated homoallylic alcohols with a broad scope. This discovery established a new strategy for the efficient transformation of gem-FCPs as well as the synthesis of challenging fluorinated homoallylic alcohols.

(2-Fluoroallyl)pyridinium tetrafluoroborates: novel fluorinated electrophiles for Pd-catalyzed allylic substitution

An efficient two-step approach to 2-fluoroallyl amines was developed that involves the synthesis of (2-fluoroallyl)pyridinium tetrafluoroborates from readily available gem-bromofluorocyclopropanes and the application of the former as novel and stable 2-fluoroallyl electrophiles for Pd-catalyzed allylic substitution.

A General Method for the Dibromination of Vicinal sp3 C-H Bonds Exploiting Weak Solvent-Substrate Noncovalent Interactions

A general procedure of 1,2-dibromination of vicinal sp³ C-H bonds of arylethanes using N-bromosuccinimide as the bromide reagent without an external initiator has been established. The modulation of the strength of the intermolecular noncovalent interactions between the solvent and arylethane ethanes, quantitatively evaluated via quantum chemical calculations, allows us to circumvent the fact that arylethane ethane cannot be dibrominated through traditional methods. The mechanism was explored by both experiments and quantum chemical calculations, revealing a radical chain with HAA process.

The preparation and properties of 1,1-difluorocyclopropane derivatives

Recently, the functionalization of organic molecules with fluorine substituents has grown rapidly due to its applications in such fields as medicine, agriculture or materials sciences. The aim of this article is to review the importance of 1,1-difluorocyclopropane derivatives in synthesis. It will examine the role of the fluorine substituents in both ring-forming and ring-opening reactions, as well as methods for obtaining difluorocyclopropanes as single enantiomers. Several examples are provided to highlight the biological importance of this class of compounds.

Visible-Light-Promoted Regioselective 1,3-Fluoroallylation of gem-Difluorocyclopropanes

A strategically novel protocol for ring-opening functionalization of aryl gem-difluorocyclopropanes (F₂CPs), which allows an expedient construction of CF₃-containing architectures via visible-light-promoted F-nucleophilic attack manifold, was disclosed. Single electron oxidation of F₂CPs was ascribed as the critical step for the success of this transformation by prompting F-nucleophilic attack, as well as the ensuing C-C bond scission. The observed intriguing regioselectivity for fluoroincorporation in this reaction was rationalized by invoking the cation-stabilization property of gem-difluorine substituents and also the thermodynamic gains acquired from forming CF₃ functionality. By using cost-effective fluorination reagent and readily available substrates, a broad collection of structurally diversified α-allyl-β-trifluoromethyl ethylbenzene derivatives could be obtained in generally good yields. Further mechanistic investigations proved the engagement of a benzylic radical intermediate in this transformation.

Asymmetric Transfer Hydrogenation of gem-Difluorocyclopropenyl Esters: Access to Enantioenriched gem-Difluorocyclopropanes

Catalytic enantioselective access to disubstituted functionalized gem-difluorocyclopropanes, which are emerging fluorinated motifs of interest in medicinal chemistry, was achieved through asymmetric transfer hydrogenation of gem-difluorocyclopropenyl esters, catalyzed by a Noyori-Ikariya (p-cymene)-ruthenium(II) complex, with (N-tosyl-1,2-diphenylethylenediamine) as the chiral ligand and isopropanol as the hydrogen donor. The resulting cis-gem-difluorocyclopropyl esters were obtained with moderate to high enantioselectivity (ee=66-99 %), and post-functionalization reactions enable access to valuable building blocks incorporating a cis- or trans-gem-difluorocyclopropyl motif.