Mechanistic Insights into the Decarboxylative Electrophilic Trifluoromethylthiolation of β-Ketocarboxylic Acids: Mechanistic Insights into the Decarboxylative Electrophilic Trifluoromethylthiolation of β-Ketocarboxylic Acids

A Toolbox of Reagents for Trifluoromethylthiolation: From Serendipitous Findings to Rational Design

Two types of electrophilic trifluoromethylthiolating reagents were developed in the past 10 years in our laboratory. The development of the first type of reagent, trifluoromethanesulfenate I, which is highly reactive toward a variety of nucleophiles, was based on an unexpected discovery in the initial design for the development of an electrophilic trifluoromethylthiolating reagent with a hypervalent iodine skeleton. A structure-activity study disclosed that α-cumyl trifluoromethanesulfenate (reagent II) without the iodo substituent is equally effective. Subsequent derivatization let us develop an α-cumyl bromodifluoromethanesulfenate III that could be used for the preparation of [¹⁸F]ArSCF₃. To remediate the low reactivity of the type I electrophilic trifluoromethylthiolating reagent for Friedel-Crafts trifluoromethylthiolation of electron-rich (hetero)arenes, we designed and prepared N-trifluoromethylthiosaccharin IV, which exhibits broad reactivity toward various nucleophiles, including electron-rich arenes. A comparison of the structure of N-trifluoromethylthiosaccharin IV with that of N-trifluoromethylthiophthalimide showed that the replacement of one carbonyl group in N-trifluoromethylthiophthalimide with a sulfonyl group made N-trifluoromethylthiosaccharin IV much more electrophilic. Thus, the replacement of both carbonyls with two sulfonyl groups would further increase the electrophilicity. Such a rationale prompted us to design and develop the current most electrophilic trifluoromethylthiolating reagent, N-trifluoromethylthiodibenzenesulfonimide V, and its reactivity was much higher than that of N-trifluoromethylthiosaccharin IV. We further developed an optically pure electrophilic trifluoromethylthiolating reagent, (1S)-(-)-N-trifluoromethylthio-2,10-camphorsultam VI, for the preparation of optically active trifluoromethylthio-substituted carbon stereogenic centers. Reagents I-VI now constitute a powerful toolbox for the introduction of the trifluoromethylthio group into the target molecules.

Visible-Light-Induced Photocatalytic Trifluoromethylation of Bunte Salts: Easy Access to Trifluoromethylthiolated Synthons

A metal-free visible-light-induced trifluoromethylation of Bunte salts of α-bromoketones/alkyl bromide/benzyl bromides/functionalized allyl (Baylis-Hillman) bromides has been accomplished using Langlois' reagent in the presence of inexpensive eosin Y as a photocatalyst to form the privileged trifluoromethylthiolated synthons. The method is straightforward, operationally simple, and endowed with broad substrate scope and good functional group tolerance.

Wonderful fusion of organofluorine chemistry and decarboxylation strategy

Decarboxylation strategy has been emerging as a powerful tool for the synthesis of fluorine-containing organic compounds that play important roles in various fields such as pharmaceuticals, agrochemicals, and materials science. Considerable progress in decarboxylation has been made over the past decade towards the construction of diverse valuable fluorinated fine chemicals for which the fluorinated part can be brought in two ways. The first way is described as the reaction of non-fluorinated carboxylic acids (and their derivatives) with fluorinating reagents, as well as fluorine-containing building blocks. The second way is dedicated to the exploration and the use of fluorine-containing carboxylic acids (and their derivatives) in decarboxylative transformations. This review aims to provide a comprehensive summary of the development and applications of decarboxylative radical, nucleophilic and cross-coupling strategies in organofluorine chemistry.

One-pot synthesis of 2-chloro-2-thio/selenocyanato ketones from β-keto acids

The chlorothiocyanato difunctionalization reaction has been achieved, and a variety of α-chlorothio/selenocyanato difunctional ketones are synthesized through one-pot strategy from β-keto acids.

Synthesis of α-Trifluoromethylthio-α,β-Unsaturated Carbonyl Compounds by DABCO-Mediated Electrophilic Trifluoromethylthiolation with N-SCF3-Dibenzenesulfonimide

A DABCO-mediated electrophilic α-trifluoromethylthiolation of α,β-unsaturated carbonyl compounds comprising no β-substituents has been achieved using N-trifluoromethylthio-dibenzenesulfonimide as the SCF₃ source. The direct trifluoromethylthiolation provides the corresponding α-trifluoromethylthio-α,β-unsaturated carbonyl products in good yields (up to 88%). Furthermore, the vinyl group in the α-trifluoromethylthio-α,β-unsaturated carbonyl product was successfully transformed into diverse functional groups in good to excellent yields (70-95%) by reactions such as epoxidation, aziridination, hydrocyanation, and hydrogenation.

Establishing Cation and Radical Donor Ability Scales of Electrophilic F, CF3, and SCF3 Transfer Reagents

Because of their unique biological, physical, and chemical properties, organofluorine compounds play an increasingly important role in numerous areas of chemistry and everyday life. However, although fluorine is the most abundant halogen in the earth's crust and ranks 13th in abundance among all elements, naturally occurring organofluorine compounds are rare. Consequently, there is a growing demand for the development of safe and efficient reagents and selective synthetic methodologies for the introduction of fluorine or fluorine-containing groups into organic compounds. A wide variety of shelf-stable electrophilic fluorinating and fluoroalkylating reagents have been developed in the past decades. Some of them have also been shown to act as radical sources. These versatile reagents have promoted revolutionary advances in synthetic fluorine chemistry. These developments of novel reagents and the choice of suitable reagents for new reactions have relied largely on the traditional trial-and-error approach because (i) structure-reactivity relationships and mechanisms of reactions of these reagents are sparse and (ii) the rules that govern the synthesis of non-fluorinated analogues cannot necessarily be transposed to fluorinated compounds ( Cahard , D. ; et al. Chem. Soc. Rev. 2014 , 43 , 135 ), since organic fluorine compounds often exhibit unusual properties. Over the past several years, our studies have aimed at establishing comprehensive cation and radical donor scales of electrophilic F, CF₃, and SCF₃ transfer reagents. We have also developed detailed structure-reactivity relationships. We used density functional theory calculations to systematically investigate the energies required to heterolytically cleave the Y-F/CF₃/SCF₃ bonds to donate electrophilic F/CF₃/SCF₃ groups. We found that these energies can be used as convenient indicators of the relative electrophilic fluorinating/trifluoromethylating/trifluoromethylthiolating strengths of these reagents. We have constructed the first comprehensive cation donor scales for electrophilic F, CF₃, and SCF₃ transfer reagents. In collaboration with Mayr group, we experimentally determined the electrophilicity parameters of SCF₃ transfer reagents and demonstrated the importance of intrinsic barriers for predicting their kinetic reactivity. The recognition of the novel application of a few traditional electrophilic reagents as radical sources prompted us further to construct comprehensive radical donor scales of electrophilic F, CF₃, and SCF₃ transfer reagents. We identified a series of potential new radical F, CF₃, and SCF₃ donors. Single electron transfer was found to exhibit a substantial effect on activation of the Y-CF₃/SCF₃ bonds, significantly facilitating the release of CF₃/SCF₃ radicals. This Account summarizes computational and experimental accomplishments from our group and others to establish the missing links between structure and reactivity for these reagents. Our results pave the way toward the rational optimization, design, and prediction of novel electrophilic fluorinating and fluoroalkylating reagents and new reactions.

Trifluoromethylthiolation-arylation of diazocarbonyl compounds by modified Hooz multicomponent coupling

Multicomponent reaction of diazocarbonyl and dibenzenesulfonimide-SCF₃ reagents with BAr₄ salts in the presence of Zn(NTf₂)₂ gives α,α′-difunctionalized trifluoromethylthio compounds.

A new Zn-mediated trifluoromethylthiolation-based bifunctionalization reaction is developed. In this process, simultaneous C–SCF₃ and C–C bond formation takes place in a multicomponent reaction, in which an aryl and a SCF₃ group arise from different reagents. Our studies show that the reaction mechanism is similar to the Hooz multicomponent coupling. The process involves in situ generation of BAr₃, which reacts with a diazocarbonyl compound, and the reaction is terminated by an electrophilic SCF₃ transfer. The reaction can also be extended to fluorination based bifunctionalization which proceeds with somewhat lower yield than the analogous trifluoromethylthiolation reaction.