Recent catalytic syntheses of trifluoromethylthio-containing organic compounds by transition metals, chiral organocatalysts, and photocatalysts
Yong Guo , Mei-Wei Huang , Xiao-Lin Fu , Chao Liu , Qing-Yun Chen , Zhi-Gang Zhao , Ben-Zhong Zeng , Jiong Chen
The trifluoromethylthio group (CF3S) has a high lipophilicity (Hansch's hydrophobic parameter π = 1.44) and strong electronwithdrawing properties. It plays an exceedingly important part in biological and medicinal chemistry [1]. The development of synthetic methodologies for CF3S-containing compounds has attracted much interest. Although several general reviews have described the recent developments in the trifluoromethylthiolation of various organic compounds [1-6], catalytic trifluoromethylthiolation reactions have not been reviewed. As a green and sustainable process, catalytic trifluoromethylthiolation is a highly important category; this includes transition-metal-catalyzed cross-coupling, enantioselective C-S bond formation, and visible-light-induced reaction. The essential concepts in this field have great demand; they are described in the following text.
In 2011, Buchwald et al. reported the Pd– catalyzed synthesis of Ar-SCF3 compounds under mild conditions (Scheme 1a) [7]. They converted various aryl bromides 1 to the corresponding trifluoromethyl sulfides 2 using AgSCF3, a catalytic amount of [(cod)Pd(CH2TMS)2] and ligand 3 or 4, and a quaternary ammonium salt. Heteroaryl bromides such as those containing indoles, pyridines, quinolines, thiophenes, and furans were also viable substrates. Unfortunately, attempts to extend this methodology to the coupling of aryl chlorides or aryl triflates were unsuccessful.
In 2015, Schoenebeck et al. reported the trifluoromethylthiolation of aryl iodides and bromides using a bench-stable and easy-to-recover dinuclear Pd(Ⅰ) catalyst 6 (Scheme 1b) [8]. Using 2 mol% of 6 along with an easily accessible SCF3 source, (Me4N) SCF3, diverse aryl iodides and bromides 5 were successfully trifluoromethylthiolated in toluene at 80 ℃. The air stability of the Pd(Ⅰ) catalyst provides a considerable practical advantage over more sensitive Pd(0)-catalyzed processes.
Ni is an attractive alternative to Pd because of its higher abundance and lower cost. In 2012, Zhang and Vicic reported the Ni-catalyzed synthesis of aryl trifluoromethyl sulfides at room temperature (Scheme 2a) [9]. The Ni(cod)2 catalyst combining with a bipyridine was used to efficiently incorporate the SCF3 functionality into diverse aryl iodides and bromides 5. Interestingly, this system works better for electron-rich aryl halides than electron-poor halides. Aryl chlorides were unreactive in this system.
Another advantage of Ni is its ability to react with bonds generally inert toward Pd or Cu. In 2015, Schoenebeck et al., developed a Ni-catalyzed trifluoromethylthiolation of aryl chlorides (Scheme 2b) [10]. This method utilizes a relatively inexpensive Ni(cod)2/dppf (cod = 1, 5-cyclooctadiene; dppf = 1, 1'-bis(diphenylphosphino)ferrocene) catalyst system and (Me4N)SCF3. Their computational and experimental mechanistic data are consistent with a Ni(0)/Ni(Ⅱ) cycle and inconsistent with Ni(Ⅰ) as the reactive species. Only bidentate ligands with wide bite angles (e.g., dppf) were effective. For more challenging substrates, high conversions were achieved using MeCN as a traceless additive. Medicinally important heterocycles, including quinoxaline, thiophene, acridine, and indole derivatives as well as the pharmaceutically relevant drugs indomethacin and fenofibrate, were converted to their SCF3 counterparts in high yields.
Schoenebeck et al. reported the synthesis of trifluoromethyl sulfides from diverse aryl triflates and several vinyl triflates in 2016 (Scheme 2c) [11]. The oxidative addition of [(dppf)Ni(cod)] to PhSCF3 showed an activation freeenergy barrier of 
In 2016, Love et al. reported a mild protocol for the Ni-catalyzed trifluoromethylthiolation of aryl chlorides and bromides 10 bearing directing groups (Scheme 2d) [12]. The method utilized AgSCF3 as the nucleophilic trifluoromethylthiolating reagent and did not require any ligands or additives. Moreover, ortho-selectivity was achieved using diverse directing groups such as imines, pyridines, and oxazolines.
In 2014, Liu et al. reported a Cu-catalyzed trifluoromethylthiolation of aryl bromides and iodides using versatile directing groups such as pyridyl, methyl ester, amide, imine, and oxime (Scheme 3) [13]. CuBr was used as the catalyst, and 1, 10-phenanthroline was used as the ligand. A Cu(Ⅰ/Ⅲ) mechanism was proposed for the catalytic cycle of the aryl-SCF3 bond formation. Ag also played an important role in this transformation, because other SCF3 sources such as (Me4N)SCF3 exhibited a much lower reactivity than AgSCF3.
The development of efficient C-H direct trifluoromethylthiolation has been reported recently. In 2014, Huang et al. reported a Pd-catalyzed ortho-trifluoromethylthiolation reaction of arenes bearing a directing group 14 in the presence of an electrophilic fluorinating reagent as the oxidant (Scheme 4a) [14]. The reaction is proposed to involve a Pd(Ⅱ)/Pd(Ⅳ) cycle.
In the same year, Qing et al. reported a Cu catalyzed direct trifluoromethylthiolation of benzylic C-H bonds via nondirected oxidative C(sp3)-H activation (Scheme 4b) [15]. In the reaction, the combination of AgSCF3 and KCl were used as the active trifluoromethylthiolation source and limiting reagent in a large excess toluene-type substrates 15. Copper(Ⅰ) thiophene-2-carboxylate (CuTC, 40 mol%) was used as the metal catalyst, and (3-CF3)BzOOt-Bu was used as the optimal oxidant. Different electron donating or -withdrawing substituents such as alkyl, alkoxyl, halogen, and aryl groups at different positions of the aromatic ring were well tolerated, and the desired products were obtained in moderate-to-good yields. This reaction provides a novel and straightforward method to prepare various benzyl trifluoromethyl sulfides 16.
In 2014, You et al. reported a Ru-catalyzed regioselective allylic trifluoromethylthiolation reaction (Scheme 5) [16]. The reactions of allylic carbonates gave the corresponding linear allylic trifluoromethyl thioethers in moderate-to-good yields. Mechanistic investigation showed that this reaction proceeds via a double allylic trifluoromethylthiolation sequence.
Anbarasan et al. reported a Cu-catalyzed trifluoromethylthiolation of di(hetero)aryl-λ3-iodanes (Scheme 6) [17]. The reaction showed a wide substrate scope and tolerance to various reactive functional groups such as nitrile, enolizable ketone, ester, nitro, and even free carboxylic acid. The reaction possibly involves a Cu(Ⅰ)/Cu(Ⅲ) catalytic cycle.
In 2016, Goossen et al. reported the trifluoromethylthiolation of α-diazo esters 22 using a Cu catalyst (Scheme 7a)[18]. The reactions were tolerant to air and moisture. These methodologies enrich the transition-metal-catalyzed synthesis of diverse trifluoromethyl sulfides. In 2014, they achieved the Sandmeyer trifluoromethylthiolation of arenediazonium salts 24 using a substoichiometric amount of CuSCN, sodium thiocyanate, and Ruppert–Prakash reagent (Scheme 7b) [19a]. The diazonium salt was first converted to the thiocyanate via a Sandmeyer process. In the presence of Cs2CO3, the nucleophilic trifluoromethylation reagent TMSCF3 did not interfere in the above reaction steps, but efficiently converted the newly formed aryl thiocyanate to the trifluoromethyl thioether. One-pot procedures were developed for Sandmeyer-type trifluoromethylthiolations starting from widely available (hetero)aromatic amines; however, a stoichiometric amount of CuSCN was needed [19b].
In 2014, Ding et al. reported a facile synthesis of 1-[(trifluoromethyl)thio]isoquinolines 26 by the reactions of 2-alkynylbenzaldoximes 25 with silver (trifluoromethyl)thiolate in the presence of p-methoxybenzenesulfonyl chloride and a catalytic amount of silver triflate (Scheme 8) [20]. Silver triflate was thought to facilitate the in-situ formation of isoquinoline-N-oxide intermediate 27, and sulfonyl chloride was essential for the activation of the obtained N-oxide. Nucleophilic attack on the activated N-oxide by AgSCF3 gave CF3S-substituted isoquinoline.
A rapid development of electrophilic trifluoromethylthiolating reagents and their reactions occurred from 2012 to 2016. In 2012, Daugulis et al. developed a method for the direct auxiliary-assisted trifluoromethylthiolation of the β-C(sp2)-H bonds of benzoic acid derivatives 28 (Scheme 9a) [21]. The reactions were carried out with 0.5 equiv. Cu(OAc)2, excess disulfide reagent CF3S–SCF3 (30), and DMSO as the solvent at high temperatures. Later, Xu and Shen reported the Pd-catalyzed trifluoromethylthiolation of aryl C (sp2)-H bonds using electrophilic N-(trifluoromethylthio)succinimide (31) (Scheme 9b) [22]. In 2015, Li et al. reported a Rh(Ⅲ)-catalyzed trifluoromethylthiolation of indoles 32 via C-H activation using N-(trifluoromethylthio)saccharin (33) (Scheme 9c) [23]. In the same year, Besset et al. reported the synthesis of trifluoromethylthiolated aliphatic acid derivatives by Pd-catalyzed C(sp3)-H bond functionalization using 33 (Scheme 9d) [24].
Organic boronic acids have been extensively used as the reaction components in catalytic trifluoromethylthiolation (Scheme 10). In 2013, Lu et al. first reported the Cu-catalyzed synthesis of trifluoromethyl sulfides from aryl and vinyl boronic acids using trifluoromethanesulfenate 39 [25]. Later, they examined the reactions of 4-biphenylboronic acid with a series of trifluoromethanesulfenates 39–46; 39 gave trifluoromethyl sulfide in 99% yield, higher than other trifluoromethanesulfenates (52%– 81% yields) [26]. In 2014, Rueping and Shen groups independently developed coupling reactions of aryl and vinyl boronic acids using the same reagent N-(trifluoromethylthio)phthalimide (47) (Scheme 10a, b) [27, 28]. In 2015, N-methyl-N-tosyl-trifluoromethanesulfenamide (48) was used in the above reactions by Billard et al.; an unprecedented crucial role of water was observed in the trifluoromethylthiolation reaction [29]. The coupling reactions were also applied to primary and secondary alkylboronic acids [30] and terminal alkynes [25-27, 31] (Scheme 10c, d).
Cyclopropanols have been widely used as the starting materials in various transition-metal-mediated or -catalyzed ring-opening cross-coupling reactions. In 2015, Dai et al. reported Cu(Ⅰ)-catalyzed ring-opening electrophilic trifluoromethylthiolation of cyclopropanols 55, affording β-SCF3-substituted carbonyl compounds 56 (Scheme 11) [32].
A Ag-catalyzed decarboxylative trifluoromethylthiolation of secondary and tertiary alkyl carboxylic acids 57 using electrophilic trifluoromethylthiolating reagents 39 under mild conditions was reported (Scheme 12) [33]. The reactions were dramatically accelerated in an aqueous emulsion formed by the addition of sodium dodecyl sulfate to water. The results of radical-clock and radical-cyclization experiments indicate that the reaction proceeded through a free-radical process.
Since 2013, Shibata group has published a series of Cu-catalyzed trifluoromethylthiolation using a newly developed hypervalent iodonium ylide 59 (Scheme 13) [34]. Under Cu(Ⅰ) catalysis, ylide 59 transformed to a thioperoxoate intermediate 60, the actual reactive trifluoromethylthiolation species for various substrates [34a]. They firstly achieved the trifluoromethylthiolation of several enamines 61 such as N-aliphatic substituted enamines, N-arylsubstituted enamines, β-enamine esters, β-enamine ketones, and cyclic enamines in the presence of a catalytic amount of CuF2 in dioxane, normally in 5-15 min [34a]. In the same study, they also used various indoles 73 as the substrates, and an additional catalytic amount of PhNMe2 afforded the desired products in moderate-to-high yields in 2 h [34a]. β-Keto esters 69 and 71 were also trifluoromethylated in the presence of a catalytic amount of 2, 4, 6-collidine and Cu(Ⅰ) chloride [34a].
Many compounds with alkene functionality were employed. The electrophilic trifluoromethylthiolation of allylsilanes 63 and silyl enol ethers 65 with trifluoromethanesulfonyl hypervalent iodonium ylide 59 was conducted. In the presence of a catalytic amount of CuF2, the reaction proceeded in moderate-to-high yields under mild conditions using DMAc as the solvent for 10 h [34b]. Allyl alcohols 67 provided trifluoromethylsulfinyl compounds 68 instead of trifluoromethylthio compounds in good yields via a [2, 3]-sigmatropic rearrangement [34c].
The Cu-catalyzed trifluoromethylthiolation of pyrroles 75 with 59 was developed [34d]. Diverse pyrroles were transformed to the corresponding products in good-to-excellent yields.
Synthesis of Billard–Langlois reagents and their derivatives by the Cu-catalyzed N-trifluoromethylthiolation of arylamines 77 using 59 was reported [34e]. A series of primary and secondary arylamines were directly NH trifluoromethylthiolated using 59 under mild, Cu-catalyzed reaction conditions in high yields. Heteroaromatic amines were also successfully trifluoromethylthiolated under the same reaction conditions. An arylamine with an NH-pyrrole moiety was N-trifluoromethylthiolated chemoselectively.
Carbonyl compounds have been intensively investigated as the building blocks in organic synthesis. Numerous fine chemicals, pharmaceuticals, and natural products contain at least one carbonyl functional group. Asymmetric α-trifluoromethylthiolated carbonyl compounds have recently gained increasing interest.
In 2013, Shen et al. reported a cinchona-alkaloid-catalyzed asymmetric trifluoromethylthiolation of cyclic β-ketoesters (Scheme 14) [35]. When 39 was used, indanone-derived β-ketoesters 79 afforded the corresponding products in high yields and with excellent enantioselectivities using quinine 89 as the catalyst. The cyclopentanone-derived β-ketoester 81 was successfully converted to the corresponding product 82 in 95% yield and with 94% ee. Interestingly, when more enolizable tetralone-and 1-benzosuberone-derived β-ketoesters with sixand seven-membered rings, respectively, were subjected to the reaction conditions as indanone-derived β-ketoesters, less than 5% of the β-ketoesters were converted to the corresponding trifluoromethylthiolated compounds at 40 ℃ after 36 h. They next studied the reactions of these substrates mediated by cinchona alkaloid-based chiral phase-transfer catalysts (PTCs) 90 to study the reactivity and selectivity. Tetralone-derived adamantyl β-ketoesters 83 generated the corresponding trifluoromethylthiolated products 84 with moderate-to-good enantioselectivities using a catalytic amount of 90, whereas the 1-benzosuberonederived adamantyl β-ketoester 87 generated the product with excellent enantioselectivity. The cyclohexanone-derived adamantyl ester 85 formed the corresponding product 86 with moderate enantioselectivity.
In the same year, Rueping et al. independently reported the cinchona-alkaloid-catalyzed asymmetric trifluoromethylthiolation of cyclic β-ketoesters using N-trifluoromethylthiophthalimide 47 as the electrophilic SCF3 source and quinidine 97 as the chiral catalyst, giving products with good-to-excellent enantioselectivities (Scheme 15) [36].
In 2014, Gade et al. reported a Cu-catalyzed asymmetric trifluoromethylthiolation of cyclic β-ketoesters using reagent 39 (Scheme 16) [37]. Cu-boxmi complexes were found to be highly enantioselective catalysts for electrophilic trifluoromethylthiolations. The "boxmi" ligands are the chiral pincers used in the asymmetric Cu-catalyzed trifluoromethylation of β-ketoesters.
Oxindole is among one of the privileged structural motifs in biological systems and biologically active natural products. It is highly beneficial for drug development to develop new asymmetric methods to prepare optically pure trifluoromethylthiolated oxindoles. Rueping et al., independently reported the asymmetric trifluoromethylthiolation of 3-aryloxindoles 103 using a cinchona alkaloid catalyst and electrophilic trifluoromethylthiolating reagents 47 and 39 or an in-situ-generated electrophilic trifluoromethylthiolating reagents from AgSCF3 and trichloroisocyanuric acid (TCCA) (Scheme 17) [38-40]. Especially, Shen group also reported the asymmetric trifluoromethylthiolation of 3-alkyloxindoles 107 [39]. All the reactions used hydroquinidine-2, 5-diphenyl-4, 6-pyrimidinediyl diether (DHQD)2Pyr (108) or quinine (89) under the optimal conditions to achieve the chiral control.
Notably, an unsuccessful example of the α-asymmetric trifluoromethylthiolation of an aldehyde using proline-derived organocatalyst 111 was reported in 2016 (Scheme 18) [41].
In 2016, Zhao et al. reported enantioselective trifluoromethylthiolating lactonization catalyzed using a bifunctional indane-based chiral sulfide 114 (Scheme 19) [42]. The difunctionalization of various E-configured 4-aryl-substituted but-3-enoic acid 112 using electrophilic 113 generated asymmetric SCF3-containing products 115 with > 99:1 dr and 79%–91% ee. The highly reactive, shelf-stable electrophilic SCF3 reagent 113 was developed independently by Shen [43] and Zhao [42].
Compared to the transition-metal-catalyzed cross-coupling reactions of various organic substrates with electrophilic or nucleophilic trifluoromethylthiolation reagents, radical methodologies have been much less studied for the incorporation of the SCF3 group. On the other hand, photocatalysis has experienced a resurgence in prominence in recent years as a mild and environmentally attractive method to activate organic molecules that harness energy from abundant visible light.
To expand the scope of radical trifluoromethylthiolation, Hopkinson et al. adopted visible-light photoredox catalysis as an alternative approach to generate SCF3 radicals [44]. They reported a dual photoredox/halide catalytic system that was applied to synthesize vinyl-SCF3 compounds from simple styrenes 116 in a formal C-H functionalization process (Scheme 20a). Moreover, this new visible-light-promoted strategy is also capable of delivering alkyl-SCF3 compounds containing an all-carbon quaternary stereocenter in the form of trifluoromethylthiolated cyclic ketones 119 (Scheme 20b) and oxindoles 121 (Scheme 20c) through radical–polar crossover ring-expansion and cyclization processes, respectively.
Recently, visible-light-promoted decarboxylation has emerged as a very mild approach for the generation of alkyl radicals from carboxylic acids. In 2016, Glorius et al. reported a light-mediated decarboxylative trifluoromethylthiolation protocol (Scheme 21) [45]. The reaction proceeded well when tertiary, secondary, and benzylic carboxylic acids were used in the presence of 2 mol% of photocatalyst [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (124). Especially, the trifluoromethylthiolation of primary acids also proceeded smoothly when 3 mol% of photocatalyst 124 was used, providing the products in moderate-to-good yields. Photoexcitation of Ir(Ⅲ) photocatalyst 124 produces a strong oxidant (E1/2Ⅲ*/Ⅱ=+1.21V vs. SCE in CH3CN) that promotes a thermodynamically favorable single-electron oxidation of the alkyl carboxylate (hexanoate ion E 1/2=+1.16V vs. SCE in CH3CN). A metal-free trifluoromethylthiolation of secondary and tertiary carboxylic acids 122 was achieved by using a strongly oxidizing organic dye 9-mesityl-10-methylacridinium perchlorate (125), which is an inexpensive photosensitizer used in several visible-light-promoted decarboxylative processes.
Selective functionalization of ubiquitous C(sp3)-H bonds using visible light is a highly challenging yet desirable goal in organic synthesis. In 2016, Glorius et al., reported the direct catalytic activation of unactivated C(sp3)-H bonds and subsequent trifluoromethylthiolation under photoredox activation using a hydrogen atom transfer (HAT) reagent (Scheme 22) [46]. The direct trifluoromethylthiolation of 5-methylhexan-2-yl benzoate using Ir complex 124 as the photocatalyst, tetrabutylammonium benzoate (Bu4N+PhCO2-) as the HATcatalyst, and electrophilic reagent 47 in acetonitrile under irradiationusing 5-W blue light-emitting diodes (LEDs) at room temperature gave tertiary C-H bond trifluoromethylthiolated product 129. The inherent reactivity and selectivity of this C-S bond-forming reaction in substrates with multiple tertiary C-H bonds were investigated. For example, the C-H trifluoromethylthiolation took place at the distal C-H bond, giving product 130. The reactions tolerated diverse functional groups such as protected alcohols, amines, esters, and amides. Substrates with secondary C-H bonds generally reacted sluggishly. However, the methylene C-H bonds adjacent to heteroatoms were easily functionalized. For example, ambroxide, a naturally occurring terpenoid responsible for the odor of Ambergris, was selectively trifluoromethylthiolated, giving 131 in a high yield. Substrates containing thiophene, pyridine, thiazole, and quinoline cores smoothly afforded the corresponding trifluoromethylthiolated derivatives with a high selectivityand in good isolated yields, such as product 132.
A mechanism was proposed (Scheme 22). The strongly oxidizing excited state of the Ir-based photoredox catalyst 124 was quenched by tetrabutylammonium benzoate in an acetonitrile solution at room temperature. Under catalytic conditions, the benzoyloxy radical (PhCO2·) was generated upon the reductive quenching of 124 by PhCO2-. The electrophilic PhCO2· radical performed fast yet selective hydrogen atom abstraction from the electron-rich hydridic C(sp3)-H bonds of an alkane substrate R-H. The generated nucleophilic alkyl radical R· then reacts with the shelf-stable electrophilic trifluoromethylthiolating reagent Phth-SCF3 (47), forming the product R-SCF3 along with the phthalimide radical (Phth·). Oxidation of the reduced photoredoxcatalyst with Phth· via single-electron transfer regenerates the photocatalyst and Phth-. The latter then readily deprotonates the benzoic acid, regenerating the HATcatalyst and completing the catalytic cycle.
The review summarizes the recent developments in direct trifluoromethylthiolation using three catalytic methods, namely, transition-metal catalysis, chiral organocatalysis, and photocatalysis. Significant improvement has been made in the transitionmetal catalysis method, especially in cross-coupling reactions. Besides expensive AgSCF3, inexpensive or easily handled SCF3 sources such as a quaternary ammonium salt (Me4N)SCF3, N-(trifluoromethylthio)saccharin 33, trifluoromethanesulfenate 39, N-(trifluoromethylthio)phthalimide 47, and N-methyl-N-tosyltrifluoromethanesulfenamide 48 have been used in the crosscoupling reactions of aryl halides, aryl or vinyl triflates, organic boronic acids, alkynes, and substrates bearing directing groups for C-H functionalization. In the transition-metal catalysis method, the development of more general and efficient systems using inexpensive, stable, and recyclable metal species and ligands and applicable to more challenging and complex substrates are expected in the future. In the organocatalysis method, a high enantioselectivity was achieved in several cases using carbonyl compounds as the starting substrates. However, the substrate scope is limited; most of the successful chiral control was limited to β-ketoesters and oxindoles. Asymmetric trifluoromethylthiolation should be further extensively investigated for drug discovery; there is immense possibility to develop excellent new reactions as well as new elegant chiral catalysts for many bioinspired substrates. Currently, visible-light-promoted photocatalyzed trifluoromethylthiolation is an emerging field and the future research trend. It will be widely applied to the reasonable design and synthesis of complex molecules by cascade multiple bond formations. The mechanistic study of photoredox trifluoromethylthiolation will be interesting and meaningful. We are sincerely looking forward to more catalytic trifluoromethylthiolation, and hope this short review would generate a wide interest in organofluorine chemistry.
Support of our work by National Basic Research Program of China (973 Program, No. 2012CB821600), National Natural Science Foundation of China (Nos. 21421002, 21172241, 21302207, 21672239) is gratefully acknowledged.
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Scheme 1 Pd-catalyzed trifluoromethylthiolation of aryl halides using nucleophilic trifluoromethylthiolating reagents.
Scheme 2 Ni-catalyzed trifluoromethylthiolation using nucleophilic trifluoromethylthiolating reagents.
Scheme 3 Cu-catalyzed trifluoromethylthiolation of aryl halides bearing directing groups using AgSCF3.
Scheme 6 Cu-catalyzed trifluoromethylthiolation of di(hetero)aryl-λ3-iodanes using AgSCF3.
Scheme 7 Cu-catalyzed trifluoromethylthiolation of a-diazo esters and arenediazonium salts.
Scheme 8 Synthesis of 1-[(trifluoromethyl)thio]isoquinolines by the Ag-catalyzed reaction of 2-alkynylbenzaldoxime with AgSCF3.
Scheme 10 Cu-catalyzed coupling of electrophilic trifluoromethylthiolating reagents with organic boronic acids or alkynes.
Scheme 11 Cu-catalyzed electrophilic ring-opening cross-coupling of cyclopropanols and 39.
Scheme 12 Ag-catalyzed decarboxylative trifluoromethylthiolation with electrophilic trifluoromethylthiolating reagents 39.
Scheme 14 Catalytic asymmetric trifluoromethylthiolation of cyclic β-ketoesters using quinine 89 and cinchona alkaloid based chiral PTC 90.
Scheme 15 Catalytic asymmetric trifluoromethylthiolation of cyclic β-ketoesters using N-trifluoromethylthiophthalimide 47 and quinidine 97.
Scheme 16 Cu-catalyzed asymmetric trifluoromethylthiolation of cyclic β-ketoesters using reagent 39.
Scheme 17 Asymmetric trifluoromethylthiolation of 3-aryl/alkyl substituted oxindoles controlled using a catalytic amount of (DHQD)2Pyr 108 or quinine 89.
Scheme 20 (a) Visible-light-promoted trifluoromethylthiolation of alkenes. (b) Visible-light-promoted trifluoromethylthiolation/semipinacol-type rearrangement. (c) Visible-light-promoted trifluoromethylthiolation/cyclization of N-phenyl acrylamide derivative.
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