Mild and Diazo-Free Synthesis of Trifluoromethyl-Cyclopropanes Using Sulfonium Ylides

Cationic ligands - from monodentate to pincer systems

For a long time, the small group of cationic ligands stood out as obscure systems within the general landscape of coordinative chemistry. However, this situation has started to change rapidly during the last decade, with more and more examples of metal-coordinated cationic species being reported. The growing interest in these systems is not only of purely academic nature, but also driven by accumulating evidence of their high catalytic utility. Overcoming the inherently poor coordinating ability of cationic species often required additional structural stabilization. In numerous cases this was realized by functionalizing them with a pair of chelating side-arms, effectively constructing a pincer-type scaffold. This comprehensive review aims to encompass all cationic ligands possessing such pincer architecture reported to date. Herein every cationic species that has ever been embedded in a pincer framework is described in terms of its electronic structure, followed by an in-depth discussion of its donor/acceptor properties, based on computational studies (DFT) and available experimental data (IR, NMR or CV). We then elaborate on how the positive charge of these ligands affects the spectroscopic and redox properties, as well as the reactivity, of their complexes, compared to those of the structurally related neutral ligands. Among other systems discussed, this review also surveys our own contribution to this field, namely, the introduction of sulfonium-based pincer ligands and their complexes, recently reported by our group.

Taming of Furfurylidenes by Chiral Bismuth-Rhodium Paddlewheel Catalysts. Preparation and Functionalization of Optically Active 1,1-Disubstituted (Trifluoromethyl)cyclopropanes

Although 2-furyl-carbenes (furfurylidenes) are prone to instantaneous electrocyclic ring opening, chiral [BiRh]-paddlewheel complexes empowered by London dispersion allow (trifluoromethyl)furfurylidene metal complexes to be generated from a bench-stable triftosylhydrazone precursor. These reactive intermediates engage in asymmetric [2+1] cycloadditions and hence open entry into valuable trifluoromethylated cyclopropane or -cyclopropene derivatives in optically active form, which are important building blocks for medicinal chemistry but difficult to make otherwise.

Alkyl sulfinates as cross-coupling partners for programmable and stereospecific installation of C(sp3) bioisosteres

In recent years, a variety of cycloalkyl groups with quaternary carbons, in particular cyclopropyl and cyclobutyl trifluoromethyl groups, have emerged as promising bioisosteres in drug-like molecules. The modular installation of such bioisosteres remains challenging to synthetic chemists. Alkyl sulfinate reagents have been developed as radical precursors to prepare functionalized heterocycles with the desired alkyl bioisosteres. However, the innate (radical) reactivity of this transformation poses reactivity and regioselectivity challenges for the functionalization of any aromatic or heteroaromatic scaffold. Here we showcase the ability of alkyl sulfinates to engage in sulfurane-mediated C(sp3)-C(sp2) cross-coupling, thereby allowing for programmable and stereospecific installation of these alkyl bioisosteres. The ability of this method to simplify retrosynthetic analysis is exemplified by the improved synthesis of multiple medicinally relevant scaffolds. Experimental studies and theoretical calculations for the mechanism of this sulfur chemistry reveal a ligand-coupling trend under alkyl Grignard activation via the sulfurane intermediate, stabilized by solvation of tetrahydrofuran.

General and Scalable Approach to Trifluoromethyl-Substituted Cyclopropanes

CF₃-cyclopropanes with aliphatic, aromatic, and even heteroaromatic substituents were prepared on a multigram scale by deoxyfluorination of cyclopropane carboxylic acids or their salts with sulfur tetrafluoride. For labile α-pyridine acetic acids, only the use of their potassium salts allowed to obtain the needed products. Derivatization of CF₃-cyclopropanes into building blocks ready for direct use in medicinal chemistry was performed.

Solvent-controlled base-free synthesis of bis(trifluoromethyl)-cyclopropanes and -pyrazolines via cycloaddition of 2-trifluoromethyl-1,3-enynes with 2,2,2-trifluorodiazoethane

A highly efficient solvent-controlled synthesis of bis(trifluoromethyl)cyclopropanes and bis(trifluoromethyl)pyrazolines via a [2 + 1] or [3 + 2] cycloaddition reaction of 2-trifluoromethyl-1,3-conjugated enynes with CF₃CHN₂ was developed. The reactions of 2-trifluoromethyl-1,3-conjugated enynes with CF₃CHN₂ proceeded smoothly under transition-metal and base-free conditions, affording the expected cycloaddition products in good to excellent yields. When DMAc (N,N-dimethylacetamide) was used as the solvent, bis(trifluoromethyl)pyrazolines were obtained; however, in contrast, bis(trifluoromethyl)cyclopropanes were formed by changing the solvent from DMAc to DCE (1,2-dichloroethane).

DBN-Mediated Addition Reaction of α-(Trifluoromethyl)styrenes with Diazoles, Triazoles, Tetrazoles, and Primary, Secondary, and Secondary Cyclic Amines

A mild and efficient DBN-mediated addition reaction of α-(trifluoromethyl)styrenes with diazoles, triazoles, tetrazoles, and primary, secondary, and secondary cyclic amines was developed. This practical protocol provided a robust method for the synthesis of various β-trifluoromethyl nitrogen-containing heterocycles and β-trifluoromethyl amines.

Enantioselective Copper-Catalyzed Synthesis of Trifluoromethyl-Cyclopropylboronates

A copper-catalyzed enantioselective cyclopropanation involving trifluorodiazoethane in the presence of alkenyl boronates has been developed. This transformation enables the preparation of 2-substituted-3-(trifluoromethyl)cyclopropylboronates with high levels of stereocontrol. The products are valuable synthetic intermediates by transformation of the boronate group. This methodology can be applied to the synthesis of novel trifluoromethylated analogues of trans-2-arylcyclopropylamines, which are prevalent motifs in biologically active compounds.

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.

Late stage C-H functionalization via chalcogen and pnictogen salts

Late-stage functionalization (LSF) of heteroarenes can dramatically accelerate SAR studies by enabling the installation of functional groups that would otherwise complicate a synthetic sequence. Although heteroaryl halides and boronic esters have well-established chemistries for LSF, alternatives that enable site-selective C-H functionalization are highly attractive. Recently, three unrelated cationic groups (phosphonium, pyridinium, and thianthrenium), which can replace C-H bonds late stage, have been identified as precursors to various functional groups. This review will discuss the synthesis and application of these three salts with an emphasis on their use for LSF and application to medicinal chemistry.

Diastereoselective Synthesis of 1,3-Diyne-Tethered Trifluoromethylcyclopropanes through a Sulfur Ylide Mediated Cyclopropanation/DBU-Mediated Epimerization Sequence

A one-pot synthesis of 1,3-diyne-tethered trifluoromethylcyclopropanes starting from 2-CF₃-3,5-diyne-1-enes and sulfur ylides via a sulfur ylide mediated cyclopropanation and a DBU-mediated epimerization sequence is described in this work. This process is highly diastereoselective with broad substrate scope. Moreover, a series of synthetic transformations based on the diyne moieties were conducted smoothly, affording cyclopropanes featuring trifluoromethyl-substituted all-carbon quaternary centers.

Enantioselective Synthesis of 4-Allyl Tetrahydroquinolines via Copper(I) Hydride-Catalyzed Hydroallylation of 1,2-Dihydroquinolines

CuCl/(R,R)-Ph-BPE-catalyzed asymmetric hydroallylation of 1,2-dihydroquinolines, prepared from readily available quinolines, was developed. The optically active tetrahydroquinolines (THQs) bearing an allylic functionality at position 4 were obtained in good yields and excellent enantioselectivity. The introduced allylic groups are amenable to diverse transformations, thus offering chances to rapidly expand the THQ libraries.

Discovery and Optimization of Salicylic Acid-Derived Sulfonamide Inhibitors of the WD Repeat-Containing Protein 5-MYC Protein-Protein Interaction

The treatment of tumors driven by overexpression or amplification of MYC oncogenes remains a significant challenge in drug discovery. Here, we present a new strategy toward the inhibition of MYC via the disruption of the protein-protein interaction between MYC and its chromatin cofactor WD Repeat-Containing Protein 5. Blocking the association of these proteins is hypothesized to disrupt the localization of MYC to chromatin, thus disrupting the ability of MYC to sustain tumorigenesis. Utilizing a high-throughput screening campaign and subsequent structure-guided design, we identify small-molecule inhibitors of this interaction with potent in vitro binding affinity and report structurally related negative controls that can be used to study the effect of this disruption. Our work suggests that disruption of this protein-protein interaction may provide a path toward an effective approach for the treatment of multiple tumors and anticipate that the molecules disclosed can be used as starting points for future efforts toward compounds with improved drug-like properties.

Ullmann-type N-arylation of anilines with alkyl(aryl)sulfonium salts

A palladium/copper-cocatalyzed Ullmann-type N-arylation of anilines using alkyl(aryl)sulfonium triflates as arylation reagents has been accomplished. The reaction enabled Caryl-S bond cleavage over Calkyl-S bond breakage of alkyl(aryl)sulfoniums by Pd(P(tBu)3)2/CuI and gave the corresponding N-arylated products in good to high yields. It was also significant that the reactions of aniline with asymmetric butyl(mesityl)(aryl)sulfonium triflates showed excellent selectivity, in which the aryl groups other than the bulky and electron-rich mesityl moieties were transformed.

Recent advances in the synthesis and applications of α-(trifluoromethyl)styrenes in organic synthesis

α-Trifluoromethylstyrene derivatives are versatile synthetic intermediates for the preparation of more complex fluorinated compounds.