Kenji Uneyama

α-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.

Fragrant sesquiterpenes from agarwood

Abstract Two new sesquiterpene aldehydes, (−)-selina-3,11-dien-14-al and (+)-selina-4,11-dien-14-al, methyl ester derivatives of three new sesquiterpene carboxylic acids, (−)-methyl selina-3,11-dien-14-oate, (+)-methyl selina-4.11-dien-14-oate, and (+)-methyl 9-hydroxyselina-4,11-dien-14-oate, and a new nor-sesquiterpene ketone, (+)-1,5-epoxy-nor-ketoguaiene, have been isolated from Aquilaria agallocha (agarwood). Their structures have been established on the bases of detailed spectroscopic analyses and synthesis. The occurrences of dehydrojinkoh-eremol and neopetasane in agarwood were also confirmed by comparison of their mass spectral data and R inn , on capillary GC with those of synthesized samples. The odoriferous properties of some natural and synthesized sesquiterpenes are also described.

Guaiane sesquiterpenes from agarwood

Abstract Seven new sesquiterpenes, all of which have a guaiane skeleton, i.e. (−)-guaia-1(10),11-dien-15-ol, (−)-guaia-1(10),11-diene-15-carboxylic acid, methyl guaia-1(10),11-diene-15-carboxylate, (+)-guaia-1(10),11-dien-9-one, (−)-1,10-epoxyguai-11-ene, (−)-guaia-1(10),11-dien-15,2-olide and (−)-rotundone, have been isolated from Aquilaria agallocha (agarwood). Their structures have been established on the bases of detailed spectroscopic analyses and synthesis.

Three sesquiterpenes from agarwood

Abstract Three new sesquiterpenes, (−)-guaia-1(10),11-dien-15-al, (−)-selina-3,11-dien-9-one, and (+)-selina-3,11-dien-9-ol, have been isolated from Aquilaria agollocha (agarwood). Their structures have been established on the bases of detailed spectroscopic analyses and synthesis. The odoriferous properties of these compounds are also described.

First synthesis of 3,3-difluoroserine and cysteine derivatives via Mg(0)-promoted selective CF bond cleavage of trifluoromethylimines

Abstract 3,3-Difluoroserine and cysteine derivatives were synthesized via Mg(0)-promoted defluorination of trifluoromethylimines as a key step, followed by addition of alcohols and sulfenyl chloride, respectively.

CHEMISTRIES OF TRIFLUOROACETIMIDOYL HALIDES : PREPARATION, REACTIONS, AND APPLICATIONS FOR THE SYNTHESIS OF FLUORINATED HETEROCYCLES

Abstract Chemistries of trifluoroacetimidoyl halides concerning one-pot preparation, nucleophilic, electrophilic, and radical reactions, and synthetic application for fluorinated heterocycles and amino acids, are described.

Trifluoroacetimidoyl chlorides as a new trifluoromethyl building block for fluorinated nitrogen heterocycles

Abstract N-Substituted trifluoroacetimidoyl chlorides have been prepared and allowed to react with various carbon nucleophiles, fording trifluoromethyl ketimines.

A new sequential defluorination route to α-fluoro-α,β-unsaturated ketones from trifluoromethyl ketones

α-Fluoro-α,β-unsaturated ketones were prepared from trifluoromethyl ketones via a sequence involving Mg metal promoted successive double defluorination. Trifluoromethyl ketones were transformed to β,β-difluoroenol silyl ethers which were then coupled with aldehydes and ketones to β-hydroxy-α,α-difluoroketones. A second Mg-promoted defluorination of the hydroxyketones followed by acid-catalyzed hydrolysis of γ-hydroxy-β-fluoroenol silyl ethers provided α-fluoro-α,β-unsaturated ketones as a final product.

A review of Mg metal-promoted CF bond activation; a reliable synthetic approach to difluorinated organic compounds

Abstract The carbon–fluorine bond of a trifluoromethyl group attached to a π system can be activated by metallic magnesium in the system TMSCl–THF or DMF providing a variety of difluorinated compounds via a selective CF bond cleavage. Thus, trifluoromethyl ketones, α,β-unsaturated ketones, esters, imines, and aromatics can be transformed into the corresponding difluoroenol silyl ethers, enamines, and difluoro(trimethylsilyl)methyl aromatics in good to excellent yields, respectively. This protocol enabled us to synthesize β,β-difluoroamino acids, difluoroheterocycles, and functionalized difluoroacetates.

PALLADIUM-CATALYZED COUPLING REACTIONS OF TRIFLUOROACETIMIDOYL IODIDES WITH OLEFINS AND 1-ALKYNES

Abstract Palladium-catalyzed coupling of trifluoroacetimidoyl iodides with olefins and 1-alkynes affords trifluoromethylated α, β-unsaturated imines, which are transformed into nitrogen heterocycles bearing Cf 3 -group.