Synthetic Applications of the Carbanion with a Fluoroalkyl Group Generated by Palladium(0) Catalyst under Neutral Conditions

Transition-Metal-Catalyzed Monofluoroalkylation: Strategies for the Synthesis of Alkyl Fluorides by C-C Bond Formation

Alkyl fluorides modulate the conformation, lipophilicity, metabolic stability, and p K a of compounds containing aliphatic motifs and, therefore, have been valuable for medicinal chemistry. Despite significant research in organofluorine chemistry, the synthesis of alkyl fluorides, especially chiral alkyl fluorides, remains a challenge. Most commonly, alkyl fluorides are prepared by the formation of C-F bonds (fluorination), and numerous strategies for nucleophilic, electrophilic, and radical fluorination have been reported in recent years. Although strategies to access alkyl fluorides by C-C bond formation (monofluoroalkylation) are inherently convergent and complexity-generating, they have studied less than methods based on fluorination. This Review provides an overview of recent developments in the synthesis of chiral (enantioenriched or racemic) secondary and tertiary alkyl fluorides by monofluoroalkylation catalyzed by transition-metal complexes. We expect this contribution will illuminate the potential of monofluoroalkylations to simplify the synthesis of complex alkyl fluorides and suggest further research directions in this growing field.

Syntheses of α-CF3-α-quaternary ketones via p-quinone methides and their derivatization to compounds with successively congested stereogenic centers

NHC-catalyzed 1,6-conjugate additions of a variety of aldehydes to δ-CF₃-δ-substituted p-quinone methides and their highly diastereoselective derivatization to compounds with successively congested stereogenic centers were performed.

Cooperative bimetallic catalysis in asymmetric allylic substitution

Strategies using a chiral metal catalyst and the cooperative effect of a second achiral or chiral metal catalyst for asymmetric allylic substitution reactions are discussed.

Direct Catalytic Asymmetric Mannich-Type Reaction of α- and β-Fluorinated Amides

The last two decades have witnessed the emergence of direct enolization protocols providing atom-economical and operationally simple methods to use enolates for stereoselective C-C bond-forming reactions, eliminating the inherent drawback of the preformation of enolates using stoichiometric amounts of reagents. In its infancy, direct enolization relied heavily on the intrinsic acidity of the latent enolates, and the reaction scope was limited to readily enolizable ketones and aldehydes. Recent advances in this field enabled the exploitation of carboxylic acid derivatives for direct enolization, offering expeditious access to synthetically versatile chiral building blocks. Despite the growing demand for enantioenriched fluorine-containing small molecules, α- and β-fluorinated carbonyl compounds have been neglected in direct enolization chemistry because of the competing and dominating defluorination pathway. Herein we present a comprehensive study on direct and highly stereoselective Mannich-type reactions of α- and β-fluorine-functionalized 7-azaindoline amides that rely on a soft Lewis acid/hard Brønsted base cooperative catalytic system to guarantee an efficient enolization while suppressing undesired defluorination. This protocol contributes to provide a series of fluorinated analogs of enantioenriched β-amino acids for medicinal chemistry.

Organocatalytic reactions of α-trifluoromethylated esters with terminal alkenes at room temperature

CF3-containing esters smoothly reacted with electron-deficient alkenes in the presence of a phosphine (2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl) organocatalyst at room temperature in an aerobic atmosphere. These Michael reactions efficiently provided products with a CF3 quaternary carbon center.

Alpha-trifluoromethylated carbanion synthons

Trifluomethylated organic compounds often have properties that make them suitable for diverse applications, including materials science, agrochemistry, and pharmaceutical industry. But of all the therapeutic drugs currently available, about 10% of them have a partially fluorinated moiety. Thus, a great deal of attention is being paid to the development of reliable methodologies for trifluoromethylation. Introduction of a trifluoromethyl group into the target molecules mostly relies on either trifluoromethylating reagents or trifluoromethylated synthetic blocks. The chemistry of trifluoromethyl carbanions, nucleophilic trifluoromethylating agents such as the Ruppert-Prakash reagent, and organometallic species has been intensively developed for their important synthetic applications. But the chemistry of beta,beta,beta-trifluoroethyl carbanions (alpha-trifluoromethyl carbanions) and organometallic species has remained undeveloped despite their potential usefulness in organic synthesis. The issue needs to be addressed. This Account outlines successful alkylations and useful synthetic applications of alpha-trifluoromethyl carbanions, such as alpha-substituted beta,beta,beta-trifluoroethyl, alpha-trifluoromethylethenyl, trifluoroacetimidoyl, alpha-trifluoromethyloxiranyl, and related alpha-trifluoromethylated carbanions. The strong electron-withdrawing effect of the alpha-trifluoromethyl group may stabilize the carbanion species electronically. But alpha-trifluoromethyl carbanions and their corresponding organometallic species mostly release fluoride spontaneously to produce difluoroalkenes. This notorious decomposition of alpha-trifluoromethylated carbanions and anionoids has hindered the development of these species for organic synthesis. A well-designed device for the generation, stabilization, and acceleration for alkylation of the alpha-trifluoromethylated carbanions is needed for their synthetic application, as well as stabilization by the electron-withdrawing alpha-substituent. The reported alpha-substituted alpha-trifluoromethyl carbanions can be roughly categorized into three classes based on their structures. The first category, A, is pi-conjugation-stabilized carbanions, which are stabilized by ester, nitro, sulfone, carbonyl, or phenyl groups. alpha-Substituents of these carbanions can delocalize the negative charges on their pi-system with large sigma R electron-withdrawing effects; this prevents accumulation of negative charge on the fluorine atoms. The second category, B, consists of carbanions with sp(3) orbitals either of highly halogenated carbanionsexamples include pentafluoroethyl(trimethyl)fluorosilicate, pentafluoroethyllithium, and alpha,alpha-dichloro-beta,beta,beta-trifluoroethylzinc speciesor of cyclic structures such as oxiranyl- and aziridinyllithiums. Both of these carbanions are also stabilized since they reduce molecular orbital (MO) overlapping of the carbanion orbital to C-F bond orbitals. The third category, C, has carbanions with their anion center at the sp(2) orbital, such as alpha-trifluoromethylated alkenyl carbanions and imidoyl carbanions. These sp(2) orbitals of the carbanion center usually have a small overlap with the C-F bonds of trifluoromethyl groups. The small overlap is able to suppress the E2-type eliminations. alpha-Trifluoromethylated carbanions are, in general, unstable. Their stability is largely affected by factors like hybridization of the orbital that accommodates lone pair electrons, the electronic nature of the alpha-substituents, the degree of covalency in a bond between the carbon and metal, the class of countercation, stabilization by chelation of a metal cation, and so on. The stability, therefore, can be sometimes controlled by tuning these factors adequately so that they can be used for organic synthesis. The chemistry of alpha-trifluoromethylated carbanions for organic synthesis has been progressing steadily. However, the simplest trifluoroethyl and trifluoroacetyl carbanions have never been successfully produced and employed for organic synthesis. Elegant generation and synthetic application of these metal species are one of the most attractive and challenging subjects for active investigation in the future.

Formation of quaternary carbon center with a trifluoromethyl group using a Pd-catalyzed allylation reaction

Synthesis of compounds with quaternary carbons is one of the most attractive reactions in the synthetic chemistry. However, there are only a few reports on synthesis of the compounds with a fluoroalkyl group at a quaternary carbon center. Recently, we reported the synthesis of alpha-trifluoromethylated ketones by the reaction of alpha,beta-unsaturated ketones with CF(3)-I using a Rh catalyst. When the alpha-trifluoromethylated ketones and allyl carbonates were treated with a Pd catalyst, the allylation reaction proceeded smoothly at the trifluoromethylated carbon to give the desired compounds with a trifluoromethylated quaternary carbon center in good to excellent yields.

α-Trifluoromethylation of Secondary and Sterically Hindered Carboxylates with Use of BrF3

Secondary esters and those with sterical hindrance at the beta carbon were reacted with base, carbon disulfide, and methyl iodide to produce methyl 2-carboalkoxydithioalkenoate (2). These compounds were reacted with BrF(3), forming the corresponding alpha-trifluoromethyl esters (3) along with 1,1-difluoro-2-trifluoromethyl-2-alkyl ethers (4). The products of type 4 have been transformed to derivatives of type 3, thus raising the overall yields of the target respective alpha-trifluoromethyl esters to 65-80%. The reaction is tolerant to different functional groups such as halogens, protected alcohols, esters, and lactones.

The first generation and stereospecific alkylation of alpha-trifluoromethyl oxiranyl anion

[reaction: see text] Generation and reactions of oxiranyl anion 2 stabilized by a trifluoromethyl group are described. Treatment of (S)-2,3-epoxy-1,1,1-trifluoropropane (75% ee) with n-BuLi followed by electrophiles gave corresponding 2-alkylated epoxide 3 with retention of stereochemistry in moderate to good yields. The reaction is applicable for a general synthesis of optically active trifluoromethylated tertiary alcohols.