Catalytic Synthesis of Trifluoromethyl Cyclopropenes and Oligo-Cyclopropenes

Nitrogen-Based Organofluorine Functional Molecules: Synthesis and Applications

Fluorine and nitrogen form a successful partnership in organic synthesis, medicinal chemistry, and material sciences. Although fluorine-nitrogen chemistry has a long and rich history, this field has received increasing interest and made remarkable progress over the past two decades, driven by recent advancements in transition metal and organocatalysis and photochemistry. This review, emphasizing contributions from 2015 to 2023, aims to update the state of the art of the synthesis and applications of nitrogen-based organofluorine functional molecules in organic synthesis and medicinal chemistry. In dedicated sections, we first focus on fluorine-containing reagents organized according to the type of fluorine-containing groups attached to nitrogen, including N-F, N-RF, N-SRF, and N-ORF. This review also covers nitrogen-linked fluorine-containing building blocks, catalysts, pharmaceuticals, and agrochemicals, underlining these components' broad applicability and growing importance in modern chemistry.

Recent advances in metal carbene-induced semipinacol rearrangements

As has been well-recognized, the semipinacol rearrangements (SPRs) function as a powerful and versatile tool for the construction of all-carbon and heteroatom-containing quaternary stereocenters, which are present in various natural products and bioactive molecules. In recent years, considerable attention has been paid to exploring the metal carbene-induced semipinacol rearrangements, providing an attractive and powerful strategy for obtaining various important carbonyl compounds. However, to date, no review has been published that summarizes the significant advances in the preparation of functionalized carbonyl compounds using these migration rearrangement reactions. In this review article, we have summarised the recent advances in the field of metal carbene-induced SPR reactions according to different metal classifications. Mechanistic insights, synthetic applications, and their limitations are discussed. The challenges and opportunities in this field are also outlined.

Unlocking catalytic potential: a rhodium(II)-based coordination polymer for efficient carbene transfer reactions with donor/acceptor diazoalkanes

Herein, we report the use of a molecular-defined rhodium(II) coordination polymer (Rh-CP) as a heterogeneous, recyclable catalyst in carbene transfer reactions. We showcase the application of this heterogeneous catalyst in a range of carbene transfer reactions and conclude with the functionalization of natural products and drug molecules.

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.

Gold(I) α-Trifluoromethyl Carbenes: Synthesis, Characterization and Reactivity Studies

Aryl trifluoromethyl diazomethanes 2-R (R=Ph, OMe, CF3 ) are readily decomposed by the (o-carboranyl)diphosphine gold(I) complex 1. The resulting α-CF3 substituted carbene complexes 3-R have been characterized by multi-nuclear NMR spectroscopy as well as X-ray crystallography (for 3-Ph). The bonding situation was thoroughly assessed by computational means, showing stabilization of the electrophilic carbene center by π-donation from the aryl substituent and backdonation from Au, as enhanced by the chelating P^P ligand. Reactivity studies under stoichiometric and catalytic conditions substantiate typical carbene-type behavior for 3-Ph.

Application of Aromatic Substituted 2,2,2-Trifluoro Diazoethanes in Organic Reactions

Abstract:

This review provides an overview of metal-, nonmetal-, light-, or catalyst free-promoting reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with organic molecules for the synthesis of trifluoromethyl-substituted compounds. Several approaches will be reviewed and divided into (i) copper-, iron-, Trop(BF4)-, B(C6F5)3-, light-, or rhodium-promoted reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with silanes, amines, mercaptans, phosphonates, p-cyanophenol, benzoic acid, diphenylphosphinic acid, boranes and nBu3SnH, (ii) rhodium-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with amides and phenylhydroxylamine, (iii) copper-, rhodium-, silver-, and light-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with alkynes, (iv) palladium-, copper-, rhodium- and iron-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with alkenes, (v) BF3·OEt2-, copper-, tin- or TBAB-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with HF·Py, (difluoroiodo)toluene (p-TolIF2), TMSCF3, AgSCF3, TMSCF2Br or 1,3-dicarbonyl compounds, (vi) palladium-, copper-, gold/silver- or rhodium-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with indoles, benzene compounds or pyridines, and (vii) palladium-catalyzed reaction of aromatic substituted 2,2,2-trifluoro diazoethanes with benzyl or allyl bromides.

Emerging Applications of Aryl Trifluoromethyl Diazoalkanes and Diazirines in Synthetic Transformations

Aryl trifluoromethyl diazoalkanes and diazirines have become unique as reactants in synthetic methodology. As privileged compounds containing CF₃ groups and ease of synthetic access, aryl trifluoromethyl diazoalkanes and diazirines have been highlighted for their versatility in applications toward a wide range of synthetic transformations. This Perspective highlights the synthetic applications of these reactants as precursors of stabilized metal carbenes, i.e., donor-acceptor-substituted ones.

Photochemical Cyclopropenation of Alkynes with Diazirines as Carbene Precursors in Continuous Flow

An efficient synthesis of 3-trifluoromethyl-3-aryl-cyclopropenes via the cyclopropenation reaction of alkynes with photolytically generated carbenes from diazirine compounds is described. This reaction is performed in continuous flow using readily available LEDs under mild reaction conditions. This new and efficient method describes the synthesis of 25 examples of 3-trifluoromethyl-3-aryl-cyclopropenes with yields up to 97%, achieved in continuous flow with a 5 min residence time. Control experiments highlighted that diazirines are more efficient than diazo compounds for this transformation.

Nucleophilic Migratory Cyclopropenation of Activated Alkynes: A Nonmetal Approach to Unbiased Cyclopropenes

An unprecedented reductive [2 + 1] annulation of α-keto esters with alkynones mediated by P(NMe₂)₃ is described. Although this nonmetal cyclopropenation is a nucleophilic process, attributed to the ester migration via a formal [2 + 2] cycloaddition reaction of Kukhtin-Ramirez adducts and alkynones followed by a fragmentation, cyclopropenes with an unbiased alkene scaffold are formed in good to excellent yields, thus providing a promising complementarity to electrophilic metal-catalyzed cyclopropenation.

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.

In Situ Generation of Unstable Difluoromethylphosphonate-Containing Diazoalkanes and Their Use in [3 + 2] Cycloaddition Reactions with Vinyl Sulfones

A [3 + 2] cycloaddition reaction of unstable difluoromethylphosphonate-containing diazoalkanes with vinyl sulfones under simple reaction conditions is developed, which provides an efficient route toward functionalized fluorinated pyrazolines derivatives in good chemical yields. The difluoro diazoalkanes are generated in situ using t-BuONO for the diazotization of (β-amino-α,α-difluoroethyl)phosphonates, and their stabilities and reactivities were carefully investigated.

Multi C-H Functionalization Reactions of Carbazole Heterocycles via Gold-Catalyzed Carbene Transfer Reactions

Herein we describe a multiple C-H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C-H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one-pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization.

Isolable Copper(I) η2-Cyclopropene Complexes

Treatment of bis(pyrazolyl)borate ligand supported [(CF₃)₂Bp]Cu(NCMe) with 1,2,3-trisubstituted cyclopropenes produced thermally stable copper(I) η²-cyclopropene complexes amenable to detailed solution and solid-state analysis. The [(CF₃)₂Bp]Cu(NCMe) also catalyzed [2 + 1]-cycloaddition chemistry of terminal and internal alkynes with ethyl diazoacetate affording cyclopropenes, including those used as ligands in this work. The tris(pyrazolyl)borate [(CF₃)₂Tp]Cu(NCMe) is a competent catalyst for this process as well. The treatment of [(CF₃)₂Tp]Cu with ethyl 2,3-diethylcycloprop-2-enecarboxylate substrate gave an O-bonded rather than a η²-cyclopropene copper complex.

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.

Synthesis of Trifluoromethylated Tetrasubstituted Allenes via Palladium-Catalyzed Carbene Transfer Reaction

Herein, we report on the palladium-catalyzed synthesis of trifluoromethylated, tetrasubstituted allenes from vinyl bromides and trifluoromethylated diazoalkanes in good to excellent yield. This reaction proceeds via oxidative addition of a Pd(0) complex with vinyl bromide. Subsequent base-promoted reductive elimination generates the allene. This methodology provides an efficient strategy even on gram scale to valuable trifluoromethylated, tetrasubstituted allenes under mild reaction conditions. The allene products can be used in acid catalyzed cyclization reactions to give trifluoromethylated indene products.

Access to gem-Difluoro Olefins via C-H Functionalization and Dual Role of Anilines

In this Letter, we describe a simple, practical approach in which cheap CuI was used as a catalyst to introduce a gem-difluoro olefin onto simple electron-rich aniline derivatives in good yield via direct C-H functionalization and a subsequent HF elimination reaction. Detailed mechanistic studies point at a dual role of aniline derivatives in this reaction, which serve as a substrate and a basic promoter to trigger the HF elimination step.

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.

Synthesis of gem-Difluoro Olefins through C-H Functionalization and β-fluoride Elimination Reactions

A palladium catalyzed C-H functionalization and consecutive β-fluoride elimination reaction between indole heterocycles and fluorinated diazoalkanes is reported. This approach provides for the first time a facile method for the rapid synthesis of gem-difluoro olefins using fluorinated diazoalkanes under mild reaction conditions. Cyclopropanation products were obtained when N-arylated rather than N-alkylated indoles were applied in this reaction. Mechanistic studies reveal the importance of the β-fluoride elimination step in this transformation. This method presents a new concept for the simple and direct transfer of a 1-aryl-(2,2-difluorovinyl) group to access gem-difluoro olefins.

Catalytic Synthesis of Trifluoromethyl Cyclopropenes and Oligo-Cyclopropenes

The synthesis of trifluoromethylated cyclopropenes is often associated with important applications in drug discovery and functional materials. In this report, we describe the use of readily available chiral rhodium(II) catalysts for a highly efficient asymmetric cyclopropenation reaction of fluorinated donor–acceptor diazoalkanes with a broad variety of aliphatic and aromatic alkynes. Further studies highlight the unique reactivity of fluorinated donor–acceptor diazoalkanes in the synthesis of oligo‐cyclopropenes. Subsequent C−H functionalization of trifluoromethyl cyclopropenes furnishes densely substituted cyclopropene frameworks and also allows the alternative synthesis of bis‐cyclopropenes.

Photochemical ring expansion reactions: synthesis of tetrahydrofuran derivatives and mechanism studies

We have shown light mediated ring-expansion reactions of 4-membered ring heterocycles. The reaction proceeds via a diradical mechanism and bond length play a key role in the stereodetermining step.

The reaction mechanism of oxygen and sulfur ylide mediated rearrangements is even today a matter of debate. In this report, we describe ring expansion reactions of oxetane and thietane heterocycles that allow probing the underlying reaction mechanism under metal-free, photochemical conditions. This ring expansion proves highly efficient and allows the synthesis of tetrahydrofuran and thiolane heterocycles under mild and operationally simple reaction conditions. These studies reveal marked differences in the stereoselectivity of the ring expansion of oxygen or sulfur ylides, which were further investigated computationally. DFT calculations show that carbenes react under ylide formation and that the corresponding ring expansion reactions proceed via a diradical pathway. The different bond lengths in free oxygen or sulfur ylide intermediates cause the distinctive stereochemical outcome.