Recent advances in catalytic enantioselective construction of monofluoromethyl-substituted stereocenters

Chiral organofluorine compounds featuring a monofluoromethyl (CH2F)-substituted stereocenter are often encountered in a number of drugs and bioactive molecules. Consequently, the development of catalytic asymmetric methods for the enantioselective construction of CH2F-substituted stereocenters has made great progress over the past two decades, and a variety of enantioselective transformations have been accordingly established. According to the types of fluorinated reagents or substrates employed, these protocols can be divided into the following major categories: (i) enantioselective ring opening of epoxides or azetidinium salts by fluoride anions; (ii) asymmetric monofluoromethylation with 1-fluorobis(phenylsulfonyl)methane; (iii) asymmetric fluorocyclization of functionalized alkenes with Selectfluor; and (iv) asymmetric transformations involving α-CH2F ketones, α-CH2F alkenes, or other CH2F-containing substrates. This feature article aims to summarize these recent advances and discusses the possible reaction mechanisms, advantages and limitations of each protocol and their applications. Synthetic opportunities still open for further development are illustrated as well. This review article will be an inspiration for researchers engaged in asymmetric catalysis, organofluorine chemistry, and medicinal chemistry.

Chemoenzymatic synthesis of optically active α-cyclopropyl-pyruvates and cyclobutenoates via enzyme-catalyzed carbene transfer with diazopyruvate

Cyclopropanes are recurrent structural motifs in natural products and bioactive molecules. Recently, biocatalytic cyclopropanations have emerged as a powerful approach to access enantioenriched cyclopropanes, complementing chemocatalytic approaches developed over the last several decades. Here, we report the development of a first biocatalytic strategy for cyclopropanation using ethyl α-diazopyruvate as a novel enzyme-compatible carbene precursor. Using myoglobin variant Mb(H64V,V68G) as the biocatalyst, this method afforded the efficient synthesis of α-cyclopropylpyruvates in high diastereomeric ratios and enantiomeric excess (up to 99% ee). The ketoester moiety in the cyclopropane products can be used to synthesize diverse optically pure cyclopropane derivatives. Furthermore, the enzymatically obtained α-cyclopropylpyruvate products could be converted into enantiopure cyclobutenoates via a metal-free photochemical ring expansion without loss of optical activity.

Experimental and Computational Studies for the Synthesis of Functionalized Cyclopropanes from 2-Substituted Allylic Derivatives with Ethyl Diazoacetate

The catalytic asymmetric synthesis of highly functionalized cyclopropanes from 2-substituted allylic derivatives is reported. Using ethyl diazo acetate, the reaction, catalyzed by a chiral ruthenium complex (Ru(II)-Pheox), furnished the corresponding easily separable cis and trans cyclopropanes in moderate to high yields (32-97 %) and excellent ee (86-99 %). This approach significantly extends the portfolio of accessible enantioenriched cyclopropanes from an underexplored class of olefins. DFT calculations suggest that an outer-sphere mechanism is operative in this system.

Fluorocyclopropane-Containing Proline Analogue: Synthesis and Conformation of an Item in the Peptide Chemist's Toolbox

Over the years, numerous modifications to the structure of proline have been made in order to tune its effects on bioactive compounds. Notably, the introduction of a cyclopropane ring or a fluorine atom has produced interesting results. Herein, we describe the synthesis of a proline containing fluorocyclopropane. This modified amino acid was inserted into a tripeptide, whose conformation was studied by nuclear magnetic resonance and density functional theory calculations.

Asymmetric Synthesis of Fluoro, Fluoromethyl, Difluoromethyl, and Trifluoromethylcyclopropanes

Fluorine-containing cyclopropanes are a subclass of cyclopropane derivatives that have generated considerable interest in medicinal chemistry for several decades. The replacement of a cyclopropane C-H or C-CH₃ bond with fluorine or a fluorinated group (such as CF₃ or CF₂H) can lead sometimes to synergistic effects in terms of biological activity and improved metabolic profile of a cyclopropane containing bioactive compound. In this context, the preparation of fluoro-, difluoromethyl-, or trifluoromethyl-cyclopropane is particularly attractive and important but quite challenging considering the unique electronic properties that result from the incorporation of a fluorine atom into a substrate or a reagent. In the past decade, we have sought to develop new routes for the stereoselective synthesis of these building blocks using the most reliable cyclopropanation methods and convenient and readily available starting materials. The challenge that had to be undertaken was how we could use the unique properties of the fluorine atom to improve upon the efficiency of a given process rather than shutting it down. This could be overcome by defining new substrate/reagent reactivity guidelines and carefully selecting whether the fluorinated group was introduced on the electrophilic or nucleophilic partner for a given reaction. In this Account, we describe our contributions in this area that take advantage of diazo-derived rhodium carbenes, zinc carbenoids, ring closure processes, and biocatalytic methods to access these important potential drug subunits. Our initial investigation relied on the development of a Michael-initiated ring closure reaction using the Reformatsky enolate derived from readily available ethyl dibromofluoroacetate and α,β-unsaturated electrophiles. The reaction proceeded extremely well but with modest to good diastereoselectivities with ester acrylates. Further extension to various fluorinated nucleophiles such as oxazolidinone based and DABCO ylides led to similar selectivities.In order to access enantioenriched fluorocyclopropanes, we then investigated the chiral dioxaborolane mediated zinc carbenoid based approaches using the fluoroiodomethylzinc carbenoid/allylic alcohol combination or the iodomethylzinc carbenoid/fluoroallylic alcohol combination. Quite surprisingly, both approaches were equally successful at providing the corresponding fluorocyclopropanes with excellent diastereo- and enantioselectivities.To broaden the scope of fluorinated cyclopropane building blocks that could be prepared with good enantiocontrol, we then investigated the rhodium-catalyzed cyclopropanation of fluoro-, difluoromethyl-, and trifluoromethyl-substituted alkenes with acceptor-acceptor and donor-acceptor diazo reagents. Depending on the substrate/reagent combination, Hashimoto's Rh₂((S)-TCPTTL)₄ or Davies' Rh₂((S)-BTPCP)₄ catalyst proved be the most efficient catalysts providing the cyclopropane derivatives with the highest enantioselectivities.More recently, a collaboration with Fasan's group led to the use of engineered myoglobins to catalyze the reaction of ethyl diazoacetate and difluoromethyl-substituted alkenes. This biocatalyzed process led to high turnover number and high enantioselectivities.Although our work has significantly increased the number of tools in the organic chemist's toolbox, continuous efforts in this area would be beneficial to the development of diastereo- and enantioselective approaches to allow the preparation of any elusive isomers of these valuable chiral building blocks.

Synthesis of Fluoro-, Monofluoromethyl-, Difluoromethyl-, and Trifluoromethyl-Substituted Three-Membered Rings

This Minireview describes recent advances toward the synthesis of fluoro-, monofluoromethyl-, difluoromethyl-, and trifluoromethyl-substituted three-membered rings such as cyclopropanes, aziridines, epoxides, episulfides, cyclopropenes, and 2 H-azirines. The main synthetic methodologies since 2016 for cyclopropanes and since 2010 for the other three-membered rings are reported.

Comparison of 1,2-Diarylcyclopropanecarboxylates with 1,2,2-Triarylcyclopropanecarboxylates as Chiral Ligands for Dirhodium-Catalyzed Cyclopropanation and C-H Functionalization

Dirhodium triarylcyclopropanecarboxylate catalysts (Rh₂TPCP₄) are sterically demanding and capable of controlling the site selectivity of C-H functionalization by means of C-H insertion with donor/acceptor carbenes. This study compares the structures and reactivity profiles of dirhodium triarylcyclopropanecarboxylates with dirhodium diarylcyclopropanecarboxylates. The absence of the third aryl group makes the catalysts less sterically demanding and lacks a well-defined preferred conformation. The catalysts have a greater tendency for inducing C-H functionalization at tertiary C-H bonds versus their triaryl counterparts but are generally not capable of achieving high levels of asymmetric induction. These studies confirm the critical requirement of having at least three substituents on the cyclopropanecarboxylate ligands to have well-defined sterically demanding catalysts capable of high levels of asymmetric induction.

Stereoselective Synthesis of Terminal Monofluoroalkenes from Trifluoromethylated Alkenes

Herein we report the hydrodefluorination reaction of trifluoromethylated alkenes to access terminal monofluoroalkenes. The use of LiAlH₄ allowed the stereoselective synthesis of the terminal monofluoroalkenes in good to excellent yields with good to excellent diastereoselectivities. Mechanistic studies suggested a hydroalumination reaction followed by a stereoselective fluoride elimination.