Biocatalytic Strategy for the Highly Stereoselective Synthesis of Fluorinated Cyclopropanes

Fluorinated cyclopropanes are highly desired pharmacophores in drug discovery owing to the rigid nature of the cyclopropane ring and the beneficial effects of C-F bonds on the pharmacokinetic properties, cell permeability, and metabolic stability of drug molecules. Herein a biocatalytic strategy for the stereoselective synthesis of mono-fluorinated and gem-difluoro cyclopropanes is reported though the use of engineered myoglobin-based catalysts. In particular, this system allows for a broad range of gem-difluoro alkenes to be cyclopropanated in the presence of diazoacetonitrile with excellent diastereo and enantiocontrol (up to 99 : 1 d.r. and 99 % e.e.), thereby enabling a transformation not currently accessible with chemocatalytic methods. The synthetic utility of the present approach is further exemplified through the gram-scale synthesis of a key gem-difluorinated cyclopropane intermediate useful for the preparation of fluorinated bioactive molecules.

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.

Controllable Fluorocarbon Chain Elongation: TMSCF2Br-Enabled Trifluorovinylation and Pentafluorocyclopropylation of Aldehydes

Controllable fluorocarbon chain elongation (CFCE) is a promising yet underdeveloped strategy for the well-defined synthesis of structurally novel polyfluorinated compounds. Herein, the direct and efficient trifluorovinylation and pentafluorocyclopropylation of aldehydes are described by using TMSCF2Br (TMS = trimethylsilyl) as the sole fluorocarbon source, accomplishing the goals of CFCE from C1 to C2 and from C1 to C3, respectively. The key to the success of these CFCE processes lies in the unique and diversified chemical reactivity of TMSCF2Br, which can serve as two different precursors, namely, a TMSCF2 radical precursor and a difluorocarbene precursor. Various functional groups are amenable to this new synthetic protocol, providing streamlined access to a broad range of alcohols containing trifluorovinyl or pentafluorocyclopropyl moieties from abundantly available aldehydes. The potential utility of these methods is further demonstrated by the gram-scale synthesis, derivatization, and measurement of log P values of the products.

Synthesis of di/trifluoromethyl cyclopropane-dicarbonitriles via [2+1] annulation of fluoro-based diazoethanes with (alkylidene)malononitriles

Herein, we describe a [2+1] annulation reaction of di/trifluorodiazoethane with (alkylidene)malononitriles. This protocol offers a streamlined synthesis of a wide range of stereospecific and densely functionalized difluoromethyl and trifluoromethyl cyclopropane-1,1-dicarbonitriles. Further functional group interconversions or skeletal elaborations afford structurally distinct cyclopropyl variants.

Copper-catalyzed synthesis of β-boryl cyclopropanes via 1,2-borocyclopropanation of aryl olefins with CO as the C1 source

Cyclopropane represents one of the most critical rings and has been found present in various bioactive compounds, especially in clinical medicines. It can be synthesized by the reaction of olefins with diazo-derived carbenoids which are potentially hazardous. Carbonylation is a powerful tool for synthesizing carbonylated or carbon-extended compounds. In this communication, we describe a straightforward approach for synthesizing β-boryl cyclopropane derivatives catalyzed by an inexpensive copper catalyst with CO as the C1 source. This reaction was mediated by an in situ generated carbene intermediate and afforded a wide range of cyclopropane-containing organoboron compounds in moderate to good yields.

Stereospecific Nickel-Catalyzed Cross-Electrophile Coupling Reaction of Alkyl Mesylates and Allylic Difluorides to Access Enantioenriched Vinyl Fluoride-Substituted Cyclopropanes

Cross-electrophile coupling reactions involving direct C-O bond activation of unactivated alkyl sulfonates or C-F bond activation of allylic gem-difluorides remain challenging. Herein, we report a nickel-catalyzed cross-electrophile coupling reaction between alkyl mesylates and allylic gem-difluorides to synthesize enantioenriched vinyl fluoride-substituted cyclopropane products. These complex products are interesting building blocks with applications in medicinal chemistry. Density functional theory (DFT) calculations demonstrate that there are two competing pathways for this reaction, both of which initiate by coordination of the electron-deficient olefin to the low-valent nickel catalyst. Subsequently, the reaction can proceed by oxidative addition of the C-F bond of the allylic gem-difluoride moiety or by directed polar oxidative addition of the alkyl mesylate C-O bond.

gem-Halofluorocyclopropanes via [2 + 1] Cycloadditions of In Situ Generated CFX Carbene with Alkenes

An efficient and operationally simple synthesis of gem-bromofluorocyclopropanes under mild conditions has been developed. The method employs ethyl dibromofluoroacetate (EDBFA) as an accessible and inexpensive source of the bromofluorocarbene (:CFBr) intermediate. The protocol provides the bromofluorocyclopropane products in excellent yields, including examples synthesized in multigram scales. The chlorinated ester, ethyl dichlorofluoroacetate (EDCFA), is also utilized to make the analogous gem-chlorofluorocyclopropanes.

Iron-Catalyzed Fluoromethylene Transfer from a Sulfonium Reagent

Herein, we report the first example of an iron porphyrin catalyzed fluoromethylene transfer from (2,4-dimethylphenyl)(fluoromethyl)(phenyl)sulfonium tetrafluoroborate to unactivated alkenes. The fluorocarbene or fluoromethylene synthon is the smallest "organic" node in a molecular graph of the organofluorine compounds. In this work, we present alternative solution to unavailable fluorodiazomethane (CHFN₂), a missing one-carbon C1 piece in fluorine chemistry, by using a fluoromethylsulfonium reagent.

Recent advances in transition-metal catalyzed directed C–H functionalization with fluorinated building blocks

This review focuses on the advances in transition-metal catalyzed reactions with fluorinated building blocks via directed C–H bond activation for the construction of diverse organic molecules with an insight into the probable mechanistic pathway.

Synthetic Applications of Monofluoromethylsulfonium Salts

Monofluoromethylsulfonium salts are emerging reagents for the fluoromethylation and fluoromethylenation or fluoromethylene transfer. Using this type of reagent is a simple approach for the introduction of the fluoromethyl group into a wide range of nucleophiles using mild basic conditions. Recently, fluoromethylsulfonium salts have been demonstrated to act as a synthetic equivalent for the challenging fluoromethylene synthon. For instance, these reagents can be used for the direct synthesis of monofluoroepoxides and fluorocyclopropanes from activated alkenes via a sulfur fluoromethylide intermediate. Sulfonium salts are an alternative, easy-to-handle option to volatile and environmentally concerning freons for achieving monofluorinated compounds. This review focuses on synthetic application of these reagents known to date.

1 Introduction

2 Fluoromethylation of O-, N-, S-, P-, and C-Nucleophiles

3 Sulfonium Salts for Radical Monofluoromethylation of Alkenes

4 Sulfonium Salts for Fluoromethylene Transfer

5 Conclusions

Fluorinated Rings: Conformation and Application

The introduction of fluorine atoms into molecules and materials across many fields of academic and industrial research is now commonplace, owing to their unique properties. A particularly interesting feature is the impact of fluorine substitution on the relative orientation of a C-F bond when incorporated into organic molecules. In this Review, we will be discussing the conformational behavior of fluorinated aliphatic carbo- and heterocyclic systems. The conformational preference of each system is associated with various interactions introduced by fluorine substitution such as charge-dipole, dipole-dipole, and hyperconjugative interactions. The contribution of each interaction on the stabilization of the fluorinated alicyclic system, which manifests itself in low conformations, will be discussed in detail. The novelty of this feature will be demonstrated by presenting the most recent applications.

Optimized Monofluoromethylsulfonium Reagents for Fluoromethylene-Transfer Chemistry

An investigation of the properties and reactivity of fluoromethylsulfonium salts resulted in the redesign of the reagents for fluoromethylene transfer chemistry. The model reaction, fluorocyclopropanation of nitrostyrene, turned out to be a suitable platform for the discovery of more streamlined fluoromethylene transfer reagents. The incorporation of halides on one aryl ring increased the reactivity, and 2,4-dimethyl substitution on the other aryl ring provided a balance between the reactivity/crystallinity of the reagent as well as the atom economy. The utility of new reagents was demonstrated by the development of an efficient fluorocyclopropanation protocol to access a range of monofluorinated cyclopropane derivatives.

TMSCFX2 (X = Cl, Br) as halofluorocarbene sources for the synthesis of halofluorocyclopropanes

TMSCFX2 (X = Cl, Br; TMS = trimethylsilyl) have been developed as halofluorocarbene (CFX, X = Cl, Br) precursors for [2+1] cyclopropanation with alkenes. Structurally diverse halofluorocyclopropanes were obtained in good to excellent yields. It was found that the reactivity order of the three halofluorocarbene reagents (TMSCF2Br, TMSCFCl2, and TMSCFBr2) in halofluorocyclopropanation with 1,1-diphenylethylene can be very different under different reaction conditions.

trans-Fluorine Effect in Cyclopropane: Diastereoselective Synthesis of Fluorocyclopropyl Cabozantinib Analogs

Investigation of the trans-fluorine effect on the hydrolysis rate of diethyl 2-fluorocyclopropane-1,1-dicarboxylate provides synthetic access to both diastereomers of the fluorocyclopropyl analog of cabozantinib, a c-Met and VEGFR-2 inhibitor used as a first-line treatment for thyroid cancer and as a second-line treatment for renal cell carcinoma. Despite some known potent examples, there are only a few drug molecules that contain fluorocyclopropane moieties. Herein, we present a case study in which the monofluoro analog of a known cyclopropane-containing drug molecule displays an improved in vitro profile compared to the parent nonfluorinated structure. The fluorocyclopropane moiety may offer valuable fine-tuning options for lead optimization in drug discovery.

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.

Synthesis of fluorocyclopropanes via the enantioselective cyclopropanation of fluoro-substituted allylic alcohols using zinc carbenoids

Cyclopropanation reactions using zinc carbenoids are a powerful means to access cyclopropanes. Described herein is an enantioselective version of the reaction using zinc reagents and a chiral dioxaborolane ligand in the generation of fluorocyclopropanes. Readily available 2- and 3-fluoroallylic alcohols were efficiently cyclopropanated in high yields and excellent enantioselectivities. This method provides access to a variety of structurally diverse chiral fluorocyclopropanes that can be used as useful chiral building blocks.

Johnson-Corey-Chaykovsky fluorocyclopropanation of double activated alkenes: scope and limitations

Johnson-Corey-Chaykovsky fluorocyclopropanation of double activated alkenes utilizing S-monofluoromethyl-S-phenyl-2,3,4,5-tetramethylphenylsulfonium tetrafluoroborate is an efficient approach to obtain a range of monofluorocyclopropane derivatives. So far, fluoromethylsulfonium salts have displayed the broadest scope for direct fluoromethylene transfer. In contrast to more commonly used fluorohalomethanes or freon derivatives, diarylfluoromethylsulfonium salts are bench stable, easy-to use reagents useful for the direct transfer of a fluoromethylene group to alkenes giving access to the challenging products - fluorocyclopropane derivatives. Interplay between the reactivity of the starting materials and stability of the fluorocyclopropanes formed determines the outcome of the process.

Beyond the Corey-Chaykovsky Reaction: Synthesis of Unusual Cyclopropanoids via Desymmetrization and Thereof

Desymmetrization-based protocols for the synthesis of highly functionalized indeno-spirocyclopropanes and cyclopropa-fused indanes have been established through unexpected reactions triggered by the Corey-Chaykovsky reagent. These structures were further elaborated in one step to privileged scaffolds such as fluorenones, indenones, and naphthaphenones. For instance, an acid-catalyzed transformation of indeno-spirocyclopropanes provided fluorenones via a homo-Nazarov-type cyclization, and naphthaphenones were obtained via an acid-catalyzed cyclopropane ring-opening/retro-Michael sequence.

Diastereoselective Monofluorocyclopropanation Using Fluoromethylsulfonium Salts

Diarylfluoromethylsulfonium salts, alternatives to freons or advanced fluorinated building blocks, are bench stable and easy-to-use sources of direct fluoromethylene (:CHF) transfer to alkenes. These salts enabled development of a trans-selective monofluorinated Johnson-Corey-Chaykovsky reaction with vinyl sulfones or vinyl sulfonamides to access synthetically challenging monofluorocyclopropane scaffolds. The described method offers rapid access to monofluorinated cyclopropane building blocks with further functionalization opportunities to deliver more complex synthetic targets diastereoselectively.

Straightforward chemo- and stereoselective fluorocyclopropanation of allylic alcohols: exploiting the electrophilic nature of the not so elusive fluoroiodomethyllithium

An unprecedented direct fluorocyclopropanation of allylic alcohols is reported. This simple method involves the not so elusive fluoroiodomethyllithium, a carbenoidic intermediate that under the developed conditions discloses its electrophilic nature. Gratifyingly, the reaction turned out to be highly chemo- and stereoselective, and DFT calculations provided insights into the structure and nature of this new type of carbenoid.

Fluoromethylene Transfer from Diarylfluoromethylsulfonium Salts: Synthesis of Fluorinated Epoxides

Diarylfluoromethyl sulfonium salts are efficient fluoromethylene transfer reagents equivalent to fluorocarbene, which is difficult to access. This was demonstrated by the development of a monofluorinated Johnson-Corey-Chaykovsky reaction with ketones and aldehydes, delivering uncommon 2-unsubstituted fluoroepoxides. This is the first evidence for the feasibility of sulfur fluoromethylylide and its action as a reaction intermediate.

Synthesis of Cyclopropanoids via Substrate-Based Cyclization Pathways

A series of unexpected reactions triggered by the dimethyloxosulfonium methylide led to the discovery of unconventional approaches for the synthesis of cyclopropa-fused tetralones and indeno-spirocyclopropanes. These highly functionalized structures were further elaborated in one step to privileged scaffolds such as tetralones, indenones, and fluorenones. As a whole, the results presented herein establish new diversity-oriented folding pathways.

Rhodium catalysed enantioselective synthesis of mono-(halo)-methyl-cyclopropanes

The catalytic asymmetric synthesis of mono-fluoro-, -chloro- and -bromomethyl-1,2-diaryl ester cyclopropanes is described.

General Catalytic Enantioselective Access to Monohalomethyl and Trifluoromethyl Cyclopropanes

An efficient catalytic enantioselective access to chiral functionalized trifluoromethyl cyclopropanes from two classes of diazo compounds and α-trifluoromethyl styrenes using Rh₂ ((S)-BTPCP)₄ as a catalyst is described. This method provides an efficient and practical strategy for the synthesis of highly functionalized CF₃ -cyclopropanes with excellent diastereoselectivities (up to 20:1) and enantioselectivities (up to 99 % ee). The depicted methodology represents, to date, the most efficient catalytic enantioselective method to access highly decorated chiral CF₃ -cyclopropanes. Extension to chiral monohalomethyl cyclopropanes in high ee is also reported.

Synthesis of Trifluoroacetyl-Substituted Cyclopropanes Using Onium Ylides

The use of carbonyl-stabilized ammonium- and sulfonium ylides allows for the synthesis of highly-functionalized trifluoroacetyl-substituted cyclopropanes. It turned out that the diastereoselectivities strongly depend on the nature of the chosen ylide and the employed base. The products could be obtained in good yields under operationally simple conditions.

Catalytic enantioselective synthesis of cyclopropanes featuring vicinal all-carbon quaternary stereocenters with a CH2F group; study of the influence of C–F⋯H–N interactions on reactivity

A highly diastereo- and enantioselective synthesis of cyclopropanes with adjacent all-carbon quaternary stereocenters featuring a CH₂F group is reported.

Divergent Synthesis of Cyclopropane-Containing Lead-Like Compounds, Fragments and Building Blocks through a Cobalt Catalyzed Cyclopropanation of Phenyl Vinyl Sulfide

Cyclopropanes provide important design elements in medicinal chemistry and are widely present in drug compounds. Here we describe a strategy and extensive synthetic studies for the preparation of a diverse collection of cyclopropane-containing lead-like compounds, fragments and building blocks exploiting a single precursor. The bifunctional cyclopropane (E/Z)-ethyl 2-(phenylsulfanyl)-cyclopropane-1-carboxylate was designed to allow derivatization through the ester and sulfide functionalities to topologically varied compounds designed to fit in desirable chemical space for drug discovery. A cobalt-catalyzed cyclopropanation of phenyl vinyl sulfide affords these scaffolds on multigram scale. Divergent, orthogonal derivatization is achieved through hydrolysis, reduction, amidation and oxidation reactions as well as sulfoxide-magnesium exchange/functionalization. The cyclopropyl Grignard reagent formed from sulfoxide exchange is stable at 0 °C for > 2 h, which enables trapping with various electrophiles and Pd-catalyzed Negishi cross-coupling reactions. The library prepared, as well as a further virtual elaboration, is analyzed against parameters of lipophilicity (ALog P), MW and molecular shape by using the LLAMA (Lead-Likeness and Molecular Analysis) software, to illustrate the success in generating lead-like compounds and fragments.

Recent advances in the syntheses, transformations and applications of 1,1-dihalocyclopropanes

gem-Dihalocyclopropanes have wide-spread applications in organic synthesis due to their versatile chemistry. They can serve as substrates for a large range of useful materials such as natural products, alkaloids, cyclopropanes, heterocycles, aromatic ring systems etc. Normally the dihalocyclopropanes are prepared by the addition of dihalocarbene to alkene, but due to the great synthetic efficacy of gem-dihalocyclopropanes a number of methods have been developed for their synthesis. Generally gem-dihalocyclopropanes exist as strained cyclic systems with astonishing kinetic stability. They are capable of undergoing transformations leading to a variety of products which have potential applications in various synthetic organic chemistry fields.