Update 1 of: Asymmetric fluorination, trifluoromethylation, and perfluoroalkylation reactions

Creation of a Chiral All-Carbon Quaternary Center Induced by CF3 and CH3 Substituents via Cu-Catalyzed Asymmetric Conjugate Addition

Cu-catalyzed asymmetric construction of a chiral quaternary center bearing CH3 and CF3 groups was achieved with high to excellent enantioselectivity using our originally developed ligands. The asymmetric conjugate addition of Me3Al to β-CF3-substituted enones and unsaturated ketoesters proceeded efficiently. The use of unsaturated ketoesters gives optically active furanones in high yields with high enantioselectivities. The perfluoroalkyl-substituted enone does not seem to be favorable in the present reaction.

Recent advances in stereoselective construction of fluorinated quaternary carbon centers from fluorinated compounds

There are many fluorinated quaternary carbon structural units in pharmaceuticals and bioactive compounds. Organic chemists are interested in the stereoselective synthesis of fluorinated quaternary carbon structural units. Constructing a fluorinated quaternary carbon stereocenter can be achieved directly and efficiently by the asymmetric catalytic reaction of fluorinated compounds as substrates. This approach aims to construct fluorinated quaternary carbon stereocenters while diversifying the types of fluorinated compounds. This review introduces a series of reactions for synthesizing fluorinated quaternary carbon chiral centers through asymmetric organic catalysis and transition metal catalysis, including alkylation, arylation, Mannich, Michael addition, and allylation reactions. This work will contribute to the development of the synthesis of fluorinated quaternary carbon chiral center-containing compounds in the future.

Silver-Catalyzed Carbofluorination of Olefins and α-Fluoroolefins with Carbamoyl Radicals

The reactivity of carbamoyl radicals, generated in situ from sodium oxamate salts, has been investigated in the context of radical carbofluorination reactions of olefins and α-fluoroolefins, respectively. Both transformations are catalyzed by silver salts and required the presence of potassium persulfate (K2S2O8) and SelectfluorTM as a radicophilic fluorine source. The reported methods provide a direct access to β-fluoroamides and β,β-difluoroamides.

Enantioselective α-heterofunctionalization reactions of catalytically generated C1-Lewis base enolates

Chiral Lewis base (LB) organocatalysis has emerged as a powerful covalent catalysis concept which allows for highly selective asymmetric C-C and C-heteroatom bond formations. Considering significant recent progress in the development of strategies to access α-heterofunctionalized carboxylic acid derivatives under chiral LB catalysis, we wish to summarize the most significant concepts and advances in this field within this mini review now.

Synthesis of Fluorine-Containing Multisubstituted Oxa-Spiro[4,5]cyclohexadienones via a Fluorinated Alcohol-Catalyzed One-Pot Sequential Cascade Strategy

In recent years, the application of fluorinated alcohols as solvents, cosolvents, or additives has become important in modern organic synthesis. However, their potential as efficient catalysts in organic synthesis has not been well-explored. In this article, we report on the development of a one-pot sequential cascade reaction of p-quinone methides with difluoroenoxysilanes using hexafluoroisopropanol as catalyst. This reaction allows for the preparation of fluorinated multisubstituted oxa-spiro[4,5]cyclohexadienones. By using 50 mol % 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP), the reaction proceeds smoothly to yield 1,6-conjugated products, which are then subjected to oxidative dearomatization/hemiacetalization using PhI(OAc)2. The overall process affords moderate to high yields and excellent diastereoselectivities.

Asymmetric Fluorocyclization of Difluoroalkenes with Concomitant Formation of a Trifluoromethyl Group

Asymmetric electrophilic fluorination of difluoroalkenes remains undeveloped because of the poor reactivity of difluoroalkenes with electrophiles. However, such reactions, if feasible, are expected to be useful for the synthesis of chiral heterocyclic compounds with a trifluoromethyl group at the stereogenic center. In this Letter, we disclose the first example of asymmetric fluoroamide cyclization of difluoroalkenes using our dianionic phase-transfer catalyst. This reaction provides unique chiral trifluoromethylated oxazoline derivatives at a synthetically useful level.

Enantioselective Desymmetrization of Trifluoromethylated Tertiary Benzhydrols via Hydrogen-Acceptor-Free Ir-Catalyzed Dehydrogenative C-H Silylation: Decisive Role of the Trifluoromethyl Group

Although the trifluoromethyl (CF3) group is one of the most important fluorinated groups owing to its significant ability to modulate pharmacological properties, constructing trifluoromethylated stereogenic centers in an enantioselective manner has been a formidable challenge. Herein, we report the development of the enantioselective desymmetrization of trifluoromethylated benzhydrols via intramolecular dehydrogenative silylation using Ir catalysts with chiral pyridine-oxazoline (PyOX) ligands. The produced benzoxasilol was transformed into several unsymmetrical benzhydrols via iododesilylation and subsequent transition-metal-catalyzed cross-coupling reactions. Moreover, the same Ir catalyst system was used for the kinetic resolution of unsymmetrical trifluoromethylated benzhydrols.

Asymmetric Construction of Carbon-Fluorine Quaternary Stereogenic Centers via Synergistic Pd/Cu Catalysis

We developed an asymmetric decarboxylative allylic alkylation of vinylethylene carbonates with α-fluoro pyridinyl acetates through a synergistic palladium/copper catalysis. This protocol provides chiral allylic alcohol with carbon-fluorine quaternary stereogenic centers in good yield with good enantioselectivities and excellent regioselectivities. The utility of this approach was further demonstrated via a gram-scale experiment and derivatizations of the product.

Copper-Catalyzed Regio- and Enantioselective Hydroboration of Difluoroalkyl-Substituted Internal Alkenes

Catalytic asymmetric hydroboration of fluoroalkyl-substituted alkenes is a straightforward approach to access chiral small molecules possessing both fluorine and boron atoms. However, enantioselective hydroboration of fluoroalkyl-substituted alkenes without fluorine elimination has been a long-standing challenge in this field. Herein, a copper-catalyzed hydroboration of difluoroalkyl-substituted internal alkenes with high levels of regio- and enantioselectivities is reported. The native carbonyl directing group, copper hydride system, and bisphosphine ligand play crucial roles in suppressing the undesired fluoride elimination. This atom-economic protocol provides a practical synthetic platform to obtain a wide scope of enantioenriched secondary boronates bearing the difluoromethylene moieties under mild conditions. Synthetic applications including functionalization of biorelevant molecules, versatile functional group interconversions, and preparation of difluoroalkylated Terfenadine derivative are also demonstrated.

Desymmetrization of Geminal Difluoromethylenes using a Palladium/Copper/Lithium Ternary System for the Stereodivergent Synthesis of Fluorinated Amino Acids

Fluorinated amino acids and related peptides/proteins have been found widespread applications in pharmaceutical and agricultural compounds. However, strategies for introducing a C-F bond into amino acids in an enantioselective manner are still limited and no such asymmetric catalysis strategy has been reported. Herein, we have successfully developed a Pd/Cu/Li ternary system for stereodivergent synthesis of chiral fluorinated amino acids. This method involves a sequential desymmetrization of geminal difluoromethylenes and allylic substitution with amino acid Schiff bases via Pd/Li and Pd/Cu dual activation, respectively. A series of non-natural amino acids bearing a chiral allylic/benzylic fluorine motif are easily synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities (up to >20 : 1 dr and >99 % ee). A density functional theory (DFT) study revealed the F-Cu interaction of the allylic substrate and the Cu catalyst significantly influence the stereoselectivity.

Programmable synthesis of difluorinated hydrocarbons from alkenes through a photocatalytic linchpin strategy

The introduction of difluoromethylene moieties into organic molecules has garnered significant attention due to their profound influence on the physicochemical and biological properties of compounds. Nonetheless, the existing approaches for accessing difluoroalkanes from readily available feedstock chemicals remain limited. In this study, we present an efficient and modular protocol for the synthesis of difluorinated compounds from alkenes, employing the readily accessible reagent, ClCF2SO2Na, as a versatile "difluoromethylene" linchpin. By means of an organophotoredox-catalysed hydrochlorodifluoromethylation of alkenes, followed by a ligated boryl radical-facilitated halogen atom transfer (XAT) process, we have successfully obtained various difluorinated compounds, including gem-difluoroalkanes, gem-difluoroalkenes, difluoromethyl alkanes, and difluoromethyl alkenes, with satisfactory yields. The practical utility of this linchpin strategy has been demonstrated through the successful preparation of CF2-linked derivatives of complex drugs and natural products. This method opens up new avenues for the synthesis of structurally diverse difluorinated hydrocarbons and highlights the utility of ligated boryl radicals in organofluorine chemistry.

Enantioselective Synthesis of Highly Substituted Fluoroalkylated Benzopyranones and 3-Coumaranones via N-Heterocyclic Carbene-Catalyzed Intramolecular Annulations

A highly enantioselective intramolecular NHC-catalyzed approach for the synthesis of fluoroalkylated benzopyranones and 3-coumaranones with all-carbon quaternary stereocenters is presented. This reaction is catalyzed by N-heterocyclic carbenes (NHCs) and involves annulation reactions between in situ generated acyl anion intermediates and highly substituted trifluoromethyl-β,β-disubstituted Michael acceptors. The method can also be extended to perfluoroalkyl homologues.

Fluoroalkyl Sulfoximines for Versatile Photocatalytic Radical Fluoroalkylations

Fluoroalkyl sulfoximines, which serve as electron-accepting fluoroalkyl radical sources, are easy-to-handle, solid, and bench-stable chemicals. Fluoroalkyl radicals can be generated from sulfoximine reagents using strong one-electron injectors, such as a highly reducing photoredox catalyst in the excited state. Our group has developed photocatalytic radical di- and mono-fluoromethylation and α-monofluoroalkylation of olefins with the corresponding fluoroalkyl sulfoximines. In this personal account, appropriate combinations of fluoroalkyl sulfoximines and photoredox catalysts, leading to successful radical fluoroalkylation, have been discussed.

Current State of Microflow Trifluoromethylation Reactions

The trifluoromethyl group is a powerful structural motif in drugs and polymers; thus, developing trifluoromethylation reactions is an important area of research in organic chemistry. Over the past few decades, significant progress has been made in developing new methods for the trifluoromethylation of organic molecules, ranging from nucleophilic and electrophilic approaches to transition-metal catalysis, photocatalysis, and electrolytic reactions. While these reactions were initially developed in batch systems, more recent microflow versions are highly attractive for industrial applications owing to their scalability, safety, and time efficiency. In this review, we discuss the current state of microflow trifluoromethylation. Approaches for microflow trifluoromethylation based on different trifluoromethylation reagents are described, including continuous flow, flow photochemical, microfluidic electrochemical reactions, and large-scale microflow reactions.

The Revival of Enantioselective Perfluoroalkylation - Update of New Synthetic Approaches from 2015-2022

Over the last years, methods devoted to the synthesis of asymmetric molecules bearing a perfluoroalkylated chain have been limited in number. Among them, only a few can be used on a large variety of scaffolds. This microreview aims at summarizing these recent advances in enantioselective perfluoroalkylation (-CF3 , -CF2 H, -Cn F2n+1 ) and highlights the need for new enantioselective methods to easily synthesize chiral fluorinated molecules which would be useful for the pharmaceutical and agrochemical industries. Some perspectives are also mentioned.

N-Fluoro Ammonium Salts of Cinchona Alkaloids in Enantioselective Electrophilic Fluorination

From 2000, our two research groups independently and simultaneously designed and developed a novel family of electrophilic fluorinating reagents based on the use of Cinchona alkaloids. The chiral N-fluoro ammonium salts demonstrated the highest efficiency compared to prior art in enantioselective electrophilic fluorination for a wide range of substrates. In this account, we tell our respective stories, how the same idea germinated in our laboratories, the characterization of the chiral reagents, the use in stoichiometric quantity then the development of a catalytic version, the application to the synthesis of chiral fluorinated molecules of pharmaceutical interest, and finally the exploitation of our reagents by other teams and for other applications.

Selective Synthesis of Monofluorinated Compounds Applying Amine/HF Reagents

For nucleophilic monofluorination, amine/HF reagents such as Et3 N⋅3HF, Pyr⋅9HF (Olah's reagent) and similar combinations belong to the most frequently used fluoride sources, whereupon the selectivity of these reagents can be very different depending of its acidity, the nucleophilicity of the fluoride equivalent, and the structure of the particular substrate. These reagents can be used safely in ordinary chemistry laboratories for nucleophilic substitution reactions by fluoride at sp3 -hybridized carbon centers. For ring opening reactions of epoxides, the regio- and stereoselectivity is very much depending of the nature of the epoxide and the acidity of the HF reagent favoring either SN 1 or SN 2 type reactions. Similarly, the outcome of halofluorination and similar reactions with sulfur or seleno electrophiles can be controlled by the particular combination of the electrophile and the fluoride source. Examples for the application of these reaction types for the synthesis of fluorine-containing analogues of natural products or biologically relevant compounds are in the focus of this personal account.

1,2-Aminoxyalkylation of alkenes with alkyl iodides and TEMPONa through SET- and XAT-processes

1,2-Aminoxyalkylation of alkenes with alkyl iodides and TEMPONa in combination with an aryldiazonium salt as an XAT mediator is reported. Various primary, secondary and tertiary alkyl iodides engage as C-radical precursors in the 1,2-aminoxyalkylation with electrophilic alkenes as radical acceptors. The product alkoxyamines are readily transformed to the corresponding alcohols or ketones upon reduction or oxidation, respectively. Mechanistic investigations reveal that aryl radicals, generated through SET-reduction of the aryl diazonium salt with TEMPONa, engage in XAT from unactivated alkyl halides to give alkyl radicals that can add to alkenes. Trapping of the adduct radicals with TEMPO provides the 1,2-aminoxyalkylation products. Transition metals are not required for these transformations that are conducted under mild conditions. Perfluoroalkyl halides directly react with TEMPONa and an aryldiazonium salt as XAT-mediator is not required for alkene 1,2-aminoxyperfluoroalkylation.

Perfluoroalkoxylation reaction via dual concurrent catalysis

A catalytic amount of CsI enables dual concurrent activation of poorly reactive perfluoroalkoxide and alkyl halides, especially alkyl chlorides, leading to the formation of diverse perfluoroalkoxylated organic compounds. Installation of perfluoroalkoxy groups by this methodology is cost-effective, circumventing the need for over-stoichiometric cesium or silver salts. This methodology also provides high functional group compatibility and tolerance of sterically hindered substrates.

Cobalt-catalysed nucleophilic fluorination in organic carbonates

The novel P-N ligand 1-((diphenylphosphaneyl)methyl)-1H-benzo-1,2,3-triazole (1), based on a benzotriazole scaffold, has been prepared. The reaction of 1 with [CoCp*(CH₃CN)₃][BF₄]₂ and [CoCp*(I)₂]₂ (Cp* = pentamethylcyclopentadienyl) affords the chelate complexes [CoCp*(CH₃CN)(P-N)][BF₄]₂ (2) and [CoCp*(I)(P-N)]I (3), respectively. Complexes 2 and 3 were studied as catalysts in the fluorination of aromatic and aliphatic acyl chlorides in CH₂Cl₂, with 3 showing notably higher activities than 2. Subsequently, organic carbonates (dimethyl carbonate and propylene carbonate) were also employed as solvents, which led to shorter reaction times and to the broadening of the substrate scope to a variety of aliphatic halides. Comparative studies between 3 and the analogous complex [CoCp*(I)₂(PMePh₂)], which features a monodentate phosphane ligand, showed that higher yields were obtained in the case of the former. DFT calculations and experimental studies were performed in order to shed light on the reaction mechanism, which entails the formation of a cobalt fluoride species that reacts via nucleophilic attack with the substrate to afford the corresponding fluorinated compounds.

Synthesis of α-CF3 Amides via Palladium-Catalyzed Carbonylation of 2-Bromo-3,3,3-trifluoropropene

Amide compounds are important organic compounds, which play an important role in biomedical chemistry, materials science, life science, and other fields. The synthesis of α-CF₃ amides, especially compounds containing 3-(trifluoromethyl)-1,3,4,5-tetrahydro-2H-benzo[b][1,4]diazepine-2-one, has long been a challenge due to the tensile properties and instability of the rings. Here, we report an example of palladium-catalyzed carbonylation of CF₃-containing olefin to form α-CF₃ acrylamide. By controlling the ligands, we can get different amide compounds as products. This method has good substrate adaptability and functional group tolerance.

Asymmetric Fluorofunctionalizations with Carboxylate-Based Phase-Transfer Catalysts

Fluorine is an attractive element in the field of pharmaceutical and agrochemical chemistry due to its unique properties. Considering the chiral environment in nature, where enantiomers often show different biological activities, the introduction of fluorine atom(s) into organic molecules to make chiral fluorinated compounds is an important subject. Herein, we describe the story of the development of our chiral carboxylate-based phase-transfer catalysts and their applications for asymmetric fluorocyclizations of alkenes bearing a carboxylic acid, an amide, and an oxime as an internal nucleophile with a dicationic fluorinating reagent, Selectfluor. We also describe dearomative fluorinations of indole derivatives, 2-naphthols, and resorcinols.

KHMDS/Triglyme Cryptate as an Alternative to Phosphazene Base in Stereodivergent Pentafluoroethylation of N-Sulfinylimines Using HFC-125

A protocol for the stereodivergent pentafluoroethylation of N-sulfinylimines using HFC-125 with KHMDS/triglyme has been developed. Both diastereomers of the pentafluoroethylated amines can be selectively synthesized based on the presence or absence of triglyme. This additive-controlled protocol allows the KHMDS/triglyme cryptate to be a straightforward and cheap alternative to previously reported base-controlled stereodivergent trifluoromethylation using potassium hexamethyldisilazide (KHMDS) versus P₄-^tBu.

Difluoromethylation of Unactivated Alkenes Using Freon-22 through Tertiary Amine-Borane-Triggered Halogen Atom Transfer

The application of abundant and inexpensive fluorine feedstock sources to synthesize fluorinated compounds is an appealing yet underexplored strategy. Here, we report a photocatalytic radical hydrodifluoromethylation of unactivated alkenes with an inexpensive industrial chemical, chlorodifluoromethane (ClCF₂H, Freon-22). This protocol is realized by merging tertiary amine-ligated boryl radical-induced halogen atom transfer (XAT) with organophotoredox catalysis under blue light irradiation. A broad scope of readily accessible alkenes featuring a variety of functional groups and drug and natural product moieties could be selectively difluoromethylated with good efficiency in a metal-free manner. Combined experimental and computational studies suggest that the key XAT process of ClCF₂H is both thermodynamically and kinetically favored over the hydrogen atom transfer pathway owing to the formation of a strong boron-chlorine (B-Cl) bond and the low-lying antibonding orbital of the carbon-chlorine (C-Cl) bond.

Mechanism-Dependent Selectivity: Fluorocyclization of Unsaturated Carboxylic Acids or Alcohols by Hypervalent Iodine

To understand the unprecedented difference between 6-endo and 5-exo selectivity in hypervalent iodine (III) promoted fluorocyclization of unsaturated carboxylic acids or alcohols by difluoroiodotoluene, density functional theory (DFT) studies have been performed to systematically compare both the previous proposed “fluorination first and cyclization later” mechanism and the alternative “cyclization first and fluorination later” mechanism. Our results revealed that the selectivity is mechanism-dependent. The unsaturated alcohol prefers the fluorination first and the 6-endo-tet cyclization later pathway, leading to the experimentally observed 6-endo ether product. In contrast, the unsaturated carboxylic acid plausibly undergoes the 5-exo-trig cyclization first and the fluorination later to the experimentally observed 5-exo lactone product. The pKa property of the functional group of the substrate is found to play a key role in determining the reaction mechanism. The provided insights into the mechanism-dependent selectivity should help advance the development of fluorocyclization reactions with hypervalent iodine reagents.

Novel urea derivative-loaded PLGA nanoparticles to inhibit caries-associated Streptococcus mutans

Caries is the most common chronic infectious disease in the human oral cavity and the existing anti-caries agents may lead to drug resistance and microecological imbalance. A novel urea derivative, 1,3-bis[3,5-bis(trifluoromethyl)phenyl]urea, has a potentially prominent antibacterial effect on cariogenic bacterial strain Streptococcus mutans UA159. In this study, we encapsulated the water-insoluble urea derivative in poly(lactic-co-glycolic acid) (PLGA) nanoparticles, performed physicochemical characterizations and explored its potential as a caries-preventive agent. The results showed that the drug-loaded PLGA nanoparticles exhibited satisfying surface morphology, particle size, size distribution and stability. With an optimized theoretical drug loading (10%), the drug-loaded PLGA nanoparticles exhibited negligible cytotoxicity against human oral squamous cell carcinoma cells. We noticed a biphasic drug release in vitro and the rate and cumulative release was higher in an acidic environment (pH 4.5) compared to a neutral environment (pH 7.4). The drug-loaded PLGA nanoparticles significantly inhibited the growth and lactic acid production of planktonic S. mutans as well as S. mutans biofilms. Our results indicate that the novel urea derivative-loaded PLGA nanoparticles serve as a promising anti-caries agent with remarkable pharmaceutical characteristics, low cytotoxicity, and satisfying antimicrobial effect.

Perfluoroalkylation of Terminal Alkynes with Perfluoroalkyl Iodides Catalyzed by an Iron Salt

The one-step, direct perfluoroalkylation of terminal alkynes with perfluoroalkyl iodides has been developed in which a simple ligandless iron salt is employed as the catalyst. Various perfluoroalkylated alkynes could be afforded in good to excellent yields with good functional group compatibility. Preliminary mechanistic studies suggest the involvement of the perfluoroalkyl radical in the catalytic cycle and the perfluoroalkylated alkenyl iodides as intermediates. The method provides straight, streamlined, and sustainable access to perfluoroalkylated acetylenes.

New trifluoromethylated sesquiterpenoids: synthesis, rearrangement, and biological activity

The tandem isomerization and trifluoromethylation reactions are a simple, convenient, and atom-economical strategy for the synthesis of various products in high yields from simple substrates.

Water-promoted regio-selective trifluoromethylation of vinyl conjugated diazoacetates

An efficient trifluoromethylation reaction of vinyldiazoacetates under mild reaction conditions was depicted.

Copper-Catalysed Synthesis of Trifluoromethyl Allenes via Fluoro-carboalkynylation of Alkenes

Allenes and trifluoromethyl motifs are considered as important building blocks in materials and pharmaceuticals. A copper-catalysed synthesis of trifluoromethyl allenes utilizing readily available feedstocks under mild and environmentlly friendly conditions...

Water facilitated photolysis of perfluoroalkyl iodides via halogen bonding

A strategy for the efficient water facilitated photolysis of perfluoroalkyl iodide has been established for the synthesis of various perfluoroalkylated products.

Regio- and enantioselective umpolung gem-difluoroallylation of hydrazones via palladium catalysis enabled by N-heterocyclic carbene ligand

The enantioselective construction of C-CF₂R (R: alkyl or fluoroalkyl) bonds has attracted the attention of synthetic chemists because of the importance of chiral fluorinated compounds in life and materials sciences. Catalytic asymmetric fluoroalkylation has mainly been realized under organocatalysis and Lewis acid catalysis, with substrates limited to carbonyl compounds. Few examples using transition-metal catalysis exist, owing to side reactions including decomposition and isomerization of fluoroalkylating reagents. Herein we report umpolung asymmetric difluoroallylation of hydrazones with 3-bromo-3,3-difluoropropene (BDFP) under palladium catalysis. Difluoroallylation products having quaternary chiral carbon centers are afforded in good yields with high α/γ- and enantioselectivities. The usefulness of the reaction products is demonstrated and an inner-sphere mechanism of the reaction is proposed. The use of chiral N-heterocyclic carbene as ligand is the key for the selectivities as well as the productivity of the reaction.

Pentafluoroethylation of Carbonyl Compounds Using HFC-125 in a Flow Microreactor System

The protocol of micro-flow nucleophilic pentafluoroethylation using pentafluoroethane (HC₂F₅, HFC-125), a nontoxic, inexpensive, and commercially available greenhouse gas, is described. The micro-flow pentafluoroethylation by HFC-125 proceeded smoothly at room temperature or at -10 °C in DMF or toluene in the presence of a potassium base, namely, t-BuOK or KHMDS. A broad range of ketones, aldehydes, and chalcones with various substituted benzene rings were successfully converted to the corresponding pentafluoroethyl carbinols instantly with good to high yields.

Structure Dependence in Asymmetric Deprotonative Fluorination and Fluorocyclization Reactions of Allylamine Derivatives with Linked Binaphthyl Dicarboxylate Phase-Transfer Catalyst

The asymmetric fluorofunctionalization of γ,γ-disubstituted allylamine derivatives (e.g., 3, 7, and 8) was investigated using our dianionic phase-transfer catalyst. Depending on the substituents on the alkene moiety, the reaction afforded chiral allylic fluorides and fluorinated dihydrooxazines in a highly enantioselective manner (up to 99% ee). The absolute stereochemistry of these products was found to be opposite to that in our previously reported fluorocyclization of γ-monosubstituted allylic amides (e.g., 13 and 14). To probe this interesting phenomenon, we investigated the influence of the substitution pattern of the alkene moiety on the reaction by means of NMR experiments and kinetic studies. The rate laws of the deprotonative fluorination and the fluorocyclization of γ,γ-disubstituted substrates were v = k[cat]^0.6, while that of the fluorocyclization of γ-monosubstituted substrates was v = k[substrate][cat]^0.4. An exponent of less than 1 suggests the involvement of an aggregated state of the catalyst ion pair in the catalytic cycle. Interestingly, a positive nonlinear effect was observed in the reactions of the γ,γ-disubstituted substrates, while a negative nonlinear effect was observed in the case of the γ-monosubstituted substrates. Thus, the reaction pathway depends on the presence or absence of an alkyl substituent at the γ position of the substrates, and on the basis of our mechanistic studies we propose that the active catalytic species for γ,γ-disubstituted substrates is a catalyst ion pair aggregate, whereas that for γ-monosubstituted substrates is the more active monomeric catalyst ion pair species, even though its concentration would be low.

Mechanistic Insight into Copper-Mediated Trifluoromethylation of Aryl Halides: The Role of CuI

The synthesis, characterization, and reactivity of key intermediates [Cu(CF₃)(X)]^-Q⁺ (X = CF₃ or I, Q = PPh₄) in copper-mediated trifluoromethylation of aryl halides were studied. Qualitative and quantitative studies showed [Cu(CF₃)₂]^-Q⁺ and [Cu(CF₃)(I)]^-Q⁺ were not highly reactive. Instead, a much more reactive species, ligandless [CuCF₃] or DMF-ligated species [(DMF)CuCF₃], was generated in the presence of excess CuI. On the basis of these results, a general mechanistic map for CuI-promoted trifluoromethylation of aryl halides was proposed. Furthermore, on the basis of this mechanistic understanding, a HOAc-promoted protocol for trifluoromethylation of aryl halides with [Ph₄P]⁺[Cu(CF₃)₂]^- was developed.