Transition-metal-catalyzed C–H bond activation as a sustainable strategy for the synthesis of fluorinated molecules: an overview

I. Transition-metal-catalyzed directed C–chalcogen bond formation (C–S, C–Se) by C–H bond activation

In the past decade, particular attention has been paid to the development of new methodologies for the incorporation of sulfur-containing fluorinated groups. Although the transition-metal-catalyzed direct C–H bond functionalization appeared to be a powerful tool for the installation of C–C, C–N, or C–O bonds, the direct formation of a C(sp2)–SRF or a C(sp3)–SRF bond remains a challenging task. In this context, key players in the field have been interested in the design of original methodologies for the trifluoromethylthiolation and more recently the difluoromethylthiolation of various compounds by transition-metal catalysis [78]. Moreover, a recent interest was devoted to the trifluoromethylselenolation reaction as depicted in this section.

I.1) Transition-metal-catalyzed C–H trifluoromethylthiolation of aromatic C(sp2) centers

Thanks to its unique features such as an interesting lipophilicity (Hansch parameter = 1.44) [79,80] and a strong withdrawing character, the development of new methodologies for the incorporation of the SCF3 residue on various molecules has known a tremendous expansion [81-113].

Copper catalysis: In 2012, Daugulis and co-workers reported the copper-promoted trifluoromethylthiolation of benzamide derivatives 1 at the ortho-position by C–H bond activation [114]. Indeed, using a bidentate directing group (amide derived from the 8-aminoquinoline), the mono- and difunctionalized compounds were obtained when Cu(OAc)2 (0.5 equiv) and the toxic and volatile disulfide F3CSSCF3 were employed (Scheme 2, 10 examples, up to 76% yield). With this approach, derivatives bearing an aromatic part substituted at the para-position with electron-donating substituents (1a,b), halogens (1c) as well as electron-withdrawing groups (1d) were difunctionalized in good yields. The substitution pattern on the aromatic ring did not affect the reaction efficiency, the meta-substituted derivative 2e as well as the ortho-substituted derivative 2f were obtained in high yields (70% and 63% yields, respectively). It should be noted that the presence of ortho-substituents on the aryl residue allowed the monofunctionalization to occur selectively. Also, amide 1g bearing a disubstituted arene was successfully functionalized in 59% yield. Finally, the difunctionalized thiophene derivative 2h was obtained in 56% yield.

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Scheme 2: Cu(OAc)2-promoted mono- and ditrifluoromethylthiolation of benzamide derivatives derived from 8-aminoquinoline reported by the group of Daugulis [114].

In 2016, Wang's group developed another methodology for the trifluoromethylthiolation of azacalix[1]arene[3]pyridines by C–H bond activation using a complex of Cu(ClO4)2·6H2O and the shelf-stable Me4NSCF3[115,116] as a nucleophilic source of SCF3 (Scheme 3) [100]. Within these conditions, a set of six azacalix[1]arene[3]pyridines bearing electron-donating groups, halogens or electron-withdrawing groups were functionalized and the expected products were isolated in moderate to high yields (4af, 58–91%).

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Scheme 3: Trifluoromethylthiolation of azacalix[1]arene[3]pyridines using copper salts and a nucleophilic SCF3 source reported by Wang and co-workers [100]. aA mixture of CHCl3/MeOH 1:1 was used as solvent.

Palladium catalysis: Several works have been reported for the palladium-catalyzed trifluoromethylthiolation reaction of various aromatic compounds 5 by C–H bond activation and involved in most cases an electrophilic SCF3 source (R1R2NSCF3). For these transformations, the following working hypothesis was generally suggested (Scheme 4). After coordination of the palladium catalyst to a directing group, the metallacycle A is formed. This latter undergoes an oxidative addition in the presence of an electrophilic source or an oxidation/ligand exchange in the presence of a nucleophilic source (i.e., AgSCF3) and an oxidant (B in Scheme 4). Finally, after a reductive elimination step, the expected functionalized product 6 is obtained and the palladium catalyst is regenerated.

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Scheme 4: Working hypothesis for the palladium-catalyzed C–H trifluoromethylthiolation reaction.

In 2014, Shen and Xu [117] developed a new methodology for the selective functionalization of 2-arylpyridine derivatives using an electrophilic SCF3 reagent, the Haas reagent I (Scheme 5) [118]. A broad range of 2-arylpyridine derivatives were trifluoromethylthiolated in good to high yields (18 examples, from 52 to 91% yields). The substitution pattern of the aromatic ring had no impact on the outcome of the reaction as illustrated with substrates substituted by a methyl group (7b, 7d, and 7f) at the para-, meta- and ortho-positions, which were readily functionalized in 71%, 84% and 78% yields, respectively. This reaction was also tolerant of a 2-naphthyl group, the palladium-catalyzed trifluromethylthiolation afforded the corresponding product 8h in 76% yield. Also, the 2,4-dimethoxylated substrate 7g and the benzothiophene derivative 7i were successfully trifluoromethylthiolated in 76% and 63% yields, respectively. This reaction proved to be compatible with the presence of an ester (8c) or a halogen (8e). Other directing groups, such as substituted pyridines (9a and 9b) and pyrimidine (9c) turned out to be also efficient in this transformation (Scheme 5, 4 examples, up to 84% yield).

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Scheme 5: Trifluoromethylthiolation of 2-arylpyridine derivatives and analogs by means of palladium-catalyzed C–H activation reported by the group of Shen [117]. a6 equiv of I were used at 150 °C for 24 h. bThe reaction was conducted at 150 °C for 24 h.

The same year, the group of Huang reported an elegant and straightforward palladium(II)-catalyzed ortho-selective trifluoromethylthiolation of arenes bearing various directing groups using the nucleophilic trifluoromethylthiolating source AgSCF3 in combination with Selectfluor® as oxidant (Scheme 6, 29 examples, up to 91% yield) [119]. 2-Arylpyridine derivatives bearing electron-donating groups, electron-withdrawing groups or halogen at the para- and meta-positions of the aromatic ring were readily functionalized (11a–g, 58–85% yields). Also 2-(2-methoxyphenyl)pyridine (11h) and 2-(2-naphthyl)pyridine (11i) were found to be suitable substrates leading to the corresponding products 12h and 12i in 91% and 83% yields, respectively. The use of other directing groups was also suitable for this transformation such as methyl and cyano-substituted pyridines 13a,b, pyrimidine (13c), pyrazole (13d), as well as the amide derived from 8-aminoquinoline 13e (Scheme 6, 13 examples, up to 75% yield).

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Scheme 6: C(sp2)–SCF3 bond formation by Pd-catalyzed C–H bond activation using AgSCF3 and Selectfluor® as reported by the group of Huang [119]. a20 equiv of Cl2CHCOOH were used instead of AcOH.

In this study, two mechanisms were reported. The first one suggested that a palladacycle C is formed after the irreversible chelation of the 2-phenylpyridine substrate with palladium, which is the rate-determining step (KIE = 2.7). Subsequently palladacycle C is oxidized by Selectfluor® to form a palladium(IV) complex D. After a ligand exchange with AgSCF3, the intermediate E is obtained, which, after reductive elimination, releases the desired product 12 and regenerates the catalyst. Alternatively, a ligand exchange with AgSCF3 occurs before the oxidation step, generating the palladium(II) complex F. After an oxidative addition in the presence of Selectfluor®, the palladium(IV) intermediate E is generated. Finally, after reductive elimination step, the desired product 12 is released and the catalyst regenerated. Note that, in this process, Selectfluor® is playing a key role. Indeed, using this electrophilic fluorinating source as oxidant generates a Pd(IV)(ppy)F(OAc)2 (ppy = 2-phenylpyridine) complex as intermediate. As the competitive C–F bond formation was disfavored (slow reductive elimination step), the desired trifluoromethylthiolated product 12 is selectively afforded after a F/SCF3 ligand exchange.

In 2015, Ye and Liu reported the palladium-catalyzed trifluoromethylthiolation of 2-arylpyridine derivatives using the Billard reagent II (Scheme 7) [120]. Unlike Shen's methodology (Scheme 5), the use of benzoyl chloride was necessary to activate the trifluoromethylthiolated reagent [120]. Unsubstituted and pyridines substituted derivatives 15 were very efficiently ortho-trifluoromethylthiolated (17 examples, up to 91% yield). This methodology was tolerant to electron-donating and electron-withdrawing groups as well as halogens (16bf, 62–91% yields). The meta-substituted (16f) and disubstituted (16j) products were obtained in high yields (73% and 79%, respectively).

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Scheme 7: Palladium-catalyzed ortho-trifluoromethylthiolation of 2-arylpyridine derivatives reported by the group of Ye and Liu [120].

In 2018, Anbarasan and co-workers described a palladium-catalyzed trifluoromethylthiolation of arenes by C–H bond activation bearing several directing groups (Scheme 8) [121]. With this methodology, the functionalization of 2-phenylpyridine derivatives was possible (10 examples, up to 81% yield) using the Billard reagents III or IV [122]. The trifluoromethylthiolation of 2-arylpyridines substituted by electron-donating groups such as methyl, methoxy or methylthio groups (17b–d) or by halogen (17e) was achieved (Scheme 8, up to 77% yield). Note that in case of disubstituted 2-(4-ethoxy-3-fluorophenyl)pyridine (17h), the expected product 18h was isolated in 31% yield. Moreover, selective oxidation of the SCF3 residue into the corresponding sulfoxide and the sulfone was possible and the corresponding products were obtained in 98% and 95% yields, respectively.

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Scheme 8: Palladium-catalyzed ortho-trifluoromethylthiolation of 2-arylpyridine and analogs reported by Anbarasan [121]. aThe reaction was conducted at 120 °C using 1.1 equiv of Billard reagent IV (R = Me).

This reaction was successfully expanded to the trifluoromethylthiolation of derivatives bearing various directing groups such as substituted pyridines (4 examples, up to 84% yield, 19a, 84% yield), substituted pyrazole derivatives (6 examples, up to 55% yield, 19b, 55% yield), pyrimidine (19c, 31% yield) as well as quinoline and isoquinoline (19d and 19e, 65% and 66% yields, respectively). In addition, the trifluoromethylthiolated benzo[h]quinoline 20f was obtained in good yield (63%).

The same year, Besset and co-workers reported a palladium-catalyzed C(sp2)–SCF3 bond formation on amides derived from 8-aminoquinoline as a cleavable directing group in the presence of the Munavalli reagent V (Scheme 9, 12 examples, up to 71% yield) [106]. Depending on the substitution pattern on the aromatic ring, the amides were mono- or difunctionalized. Indeed, meta- and ortho-substituted derivatives (21ad) were selectively trifluoromethylthiolated while para-substituted substrates led to the difunctionalized products 22e and 22f. Within these reaction conditions, the polysubstituted derivative 21g was also functionalized in high yield (71%).

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Scheme 9: Mono- and ditrifluoromethylthiolation of benzamide derivatives derived from 8-aminoquinoline using PdCl2 as catalyst reported by Besset and co-workers [106]. a2.2 equiv of the fluorinated source V were used.

Pleasingly, other metals have been also successfully applied for the trifluoromethylthiolation of aromatic derivatives by C(sp2)–H bond activation such as Rh(III) and Co(III)-based catalysts as depicted below.

Rhodium catalysis: In 2015, the group of Li disclosed the Cp*Rh(III)-catalyzed regioselective trifluoromethylthiolation of N-substituted indoles with (substituted) pyridines or pyrimidine as the directing groups (Scheme 10) [123]. The selective trifluoromethylthiolation of indoles at the C2 position was achieved in the presence of N-(trifluoromethylthio)saccharine (VI, Shen’s reagent) as both oxidant and electrophilic source (18 examples, up to 91% yield). Indoles bearing various electron-donating and electron-withdrawing groups as well as halogens at the C5-position and at the C6-position were functionalized in high yields (24af, 82–87% yields). The substitution of the indole at the C3-position did not impact the reaction and the product 24g was obtained in 91% yield. Substituted pyridines and pyrimidine (24h and 24i) were also used as directing groups (7 examples, up to 86% yield). This methodology was extended to the functionalization of other heteroaromatic derivatives (24j, 87% yield). It should be noted that the presence of zinc triflate, a Lewis acid, was used for the activation of the electrophilic source VI.

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Scheme 10: Regioselective Cp*Rh(III)-catalyzed directed trifluoromethylthiolation reported by the group of Li [123]. Cp* = pentamethylcyclopentadienyl.

Cobalt catalysis: In 2017, Wang described the Cp*Co(III)-catalyzed trifluoromethylthiolation of 2-phenylpyridine derivatives using AgSCF3 (Scheme 11) [124]. This methodology allowed the functionalization of several aromatic compounds bearing a pyridine or a pyrimidine as a directing group (20 examples, up to 65% yield). The reaction proceeded smoothly with substrates bearing an electron-donating group (25b,c), halogen (25d) or withdrawing group (25e) and the desired SCF3-containing products were obtained in moderate to good yields. The functionalization of trisubstituted arene 25g and heteroarene 25h was also possible leading to the corresponding products 26g and 26h in moderate yields (23% and 42% yields, respectively). Regarding the reaction mechanism, the active Co(III) complex G was obtained from the dimeric catalyst [Cp*CoI2]2 in the presence AgSCF3 and/or NaOPiv·H2O. Then, the reversible formation of the metallacycle H occurs, which after a ligand exchange in the presence of AgSCF3 leads to the formation of adduct J. The product 26 is released via a reductive elimination step, generating at the same time the reduced cobalt Cp*Co(I), which is converted to the active catalyst after oxidation.

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Scheme 11: Cp*Co(III)-catalyzed ortho-trifluoromethylthiolation of 2-phenylpyridine and 2-phenylpyrimidine derivatives reported by Wang and co-workers [124]. aCH2Cl2 was used as solvent.

The same year, Yoshino and Matsunaga described a similar methodology, using the cobalt(III) complex [Cp*Co(CH3CN)3](SbF6)2 and N-trifluoromethylthiodibenzenesulfonimide VII as electrophilic SCF3 source (Scheme 12) [125]. Under these reaction conditions, 2-arylpyridines (9 examples, up to 94% yield) or 6-arylpurines (10 examples, up to 72% yield) were ortho-trifluoromethylthiolated. The substitution pattern of the aryl part did not impact the outcome of the reaction. This methodology was also tolerant to a large range of functional groups (ester, halogen) as illustrated by the products 28c, 28j, 28d, 28k, and 28g.

[1860-5397-19-35-i12]

Scheme 12: Cp*Co(III)-catalyzed ortho-trifluoromethylthiolation of 2-phenylpyridine and 6-phenylpurine derivatives described by Yoshino and Matsunaga [125]. aWithout AgSbF6. b2.0 equiv of VII, 10 mol % of AgOAc and 30 mol % of Gd(OTf)3 were used instead of AgSbF6.

I.2) Transition-metal-catalyzed C–H trifluoromethylthiolation of vinylic C(sp2) centers

Several research groups have been interested in the development of strategies for the formation of vinylic C(sp2)–SCF3 bonds, offering an efficient tool towards the synthesis of challenging Z-isomers.

Palladium catalysis: In 2018, Bouillon and Besset described the first example of a selective palladium-catalyzed trifluoromethylthiolation of α-arylacrylamides derived from 8-aminoquinolines 29 by C–H bond activation (Scheme 13) [104]. Using the Munavalli reagent V as an electrophilic SCF3 source, this diastereoselective method selectively led to the formation of Z-isomers and turned out to be robust (not air or moisture sensitive). Under these mild reaction conditions, a panel of α-(hetero)arylacrylamides were trifluoromethylthiolated in good to high yields. Acrylamides substituted at the α-position by an aryl bearing an electron-donating group (OMe) or halogen (Cl) at the para-position were readily functionalized (30b and 30c, 70% and 75% yields, respectively). The substitution of the arene with a CF3 residue at the meta-position or with a methoxy group at the ortho-position did not have any impact to the outcome of the reaction (30e and 30f, 62% and 78% yields, respectively). It should be noted that acrylamides bearing a disubstituted arene (29g,h) and an heteroaryl (29i) at the α-position were also suitable substrates for this reaction. Finally, acrylamides bearing a methyl group at the α-position (29j) or the α,β-dimethylated acrylamide (29k) were suitable substrates albeit the corresponding products were obtained in 30% and 20% yields, respectively. Several mechanistic experiments revealed that the C(sp2)–H bond activation step was reversible and represented the rate-determining step (KIE = 2.4). First, the chelation of the palladium(II) catalyst with the bidentate directing group, followed by the C(sp2)–H bond activation involving a concerted metalation–deprotonation pathway affords the metallacycle K. After an oxidative addition in the N-SCF3 bond of the Munavalli reagent V, the palladium(IV) species L is obtained. Finally, the reductive elimination affords the product and regenerates the catalyst. The same year, Besset and co-workers extended this methodology to a larger class of acrylamides [106].

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Scheme 13: Diastereoselective trifluoromethylthiolation of acrylamide derivatives derived from 8-aminoquinoline using PdCl2 reported by Bouillon and Besset [104,106]. a20 mol % of PdCl2 and 2.0 equiv of SCF3 source V were used for 36 h. bThe reaction was conducted with 30 mol % of PdCl2 and 2.0 equiv of reagent V were used.

I.3) Transition-metal-catalyzed trifluoromethylthiolation of aliphatic C(sp3)–H bonds

Despite the important progresses presented in the previous section, some limitations of the trifluoromethylthiolation persist. In particular, the functionalization of a C(sp3)–H bond with a trifluoromethylthiolated moiety by transition-metal-catalyzed C–H activation remains a challenging task both in terms of reactivity and selectivity.

In 2015, Besset reported the first C(sp3)–SCF3 bond formation of unactivated primary C(sp3) centers by transition-metal-catalyzed C–H activation with the Munavalli or the Billard reagents as the trifluoromethylthiolation source (Scheme 14) [126]. Using a bidentate directing group, this methodology allowed the functionalization of a large range of aliphatic amides with a primary β-C(sp3)–H bond (21 examples, up to 53% yield). The methodology was applied to the functionalization of a series of amides having an α-quaternary center (α,α-dialkyl (31a), α-alkyl,α-benzyl derivatives 31cf) as well as to an amide with an α-tertiary center (31b) and pleasingly, the presence of α-C–H bonds did not have a significant impact on the outcome of the reaction. It should be noted that this methodology afforded the products with a high regioselectivity, and no incorporation of the SCF3 moiety on the benzylic or at the C5 position of the quinoline part of the directing group was observed. Note that in 2018, Besset and Lebel developed a more efficient process for the palladium-catalyzed trifluoromethylthiolation by CH bond activation under continuous flow conditions [127].

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Scheme 14: C(sp3)–SCF3 bond formation on aliphatic amide derivatives derived from 8-aminoquinoline by palladium-catalyzed C–H bond activation described by Besset and co-workers [126]. Product 32d was contaminated with 10% of an inseparable impurity.

I.4) Difluoromethylthiolation of aromatic and vinylic C(sp2)–H bonds (C–SCF2H and C–SCF2CO2Et bonds)

More recently, researchers became interested in the synthesis of molecules substituted with original and functionalized fluorinated moieties such as SCF2FG [128-136] (FG = functional group). In sharp contrast with the trifluoromethylthiolation reaction, only a handful of reports dealt with the incorporation of such high value-added fluorinated moieties onto C(sp2) centers by transition-metal-catalyzed C–H bond activation.

In 2022, He and Pan reported the first example of a difluoromethylthiolation achieved by transition-metal-catalyzed C–H bond activation (Scheme 15) [137]. Using the reagent IX and a catalytic amount of PdCl2, they succeeded in the functionalization of acrylamides 33 derived from 8-aminoquinoline. Within these mild conditions, α-arylacrylamides substituted at para-, meta-, and ortho-positions were readily difluoromethylthiolated (34ag, 81–95% yields). This reaction was also tolerant to a large class of functional groups (halogens, cyano, trifluoromethyl), affording products 34c,d and 34e in high yields (86%, 89% and 81%, respectively). The α-methyl-substituted acrylamide 33j also underwent difluoromethylthiolation to give product 34j in 81% isolated yield. The α,β- and β-substituted acrylamides were functionalized in high yields (34h,i and 34k–m, 71–86%). The plausible mechanism is similar as the one reported by Besset for the trifluoromethylthiolation of acrylamides derived from 8-aminoquinoline (Scheme 13).

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Scheme 15: Regio- and diastereoselective difluoromethylthiolation of acrylamides under palladium catalysis reported by He and Pan [137].

The same year, Besset and co-workers reported the first palladium-catalyzed C(sp2)–SCF2CO2Et bond formation by C–H bond activation (Scheme 16) [138]. In the presence of the electrophilic SCF2CO2Et source X, the methodology was successfully applied for the functionalization of 2-arylpyridine derivatives 35ai as well as 2-vinylpyridine derivatives 35jm (35 examples, up to 87% yield). The substitution pattern of the aryl substituent of the 2-phenylpyridine derivatives did not influence the reaction as for instance products 36b, 36h, and 36i were obtained in good yields (70%, 63% and 61% yield, respectively). This reaction was tolerant to a broad range of functional groups such as halogens, ester, aldehyde, cyano, and nitro (36cg, 36–74% yield). It is noteworthy that a disubstituted compound 35j and a thiophene derivative 35k were also efficiently difluoromethylthiolated (36j and 36k, 72% and 65%, respectively). α-Substituted vinylpyridines with electron-donating or electron-withdrawing groups on the aromatic ring were functionalized and 36l and 36m were easily isolated in 73% and 81% yields, respectively. Even an α,β-disubstituted vinylpyridine 35n and the benzoquinoline 35o were smoothly functionalized showing the efficiency of the approach. Of high interest, the modularity of the SCF2CO2Et was highlighted by its conversion into various other fluorinated residues (amide, carboxylic acid) and its selective oxidation into the corresponding sulfoxide and sulfone.

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Scheme 16: Palladium-catalyzed (ethoxycarbonyl)difluoromethylthiolation reaction of 2-(hetero)aryl and 2-(α-aryl-vinyl)pyridine derivatives reported by Besset [138].

I.5) Trifluoromethylselenolation of aromatic and vinylic C(sp2)–H bonds by palladium catalysis

Very recently, the palladium-catalyzed trifluoromethylselenolation of (hetero)aromatic and olefinic derivatives has been investigated by the group of Billard using similar catalytic systems as those depicted for the trifluoromethylthiolation reactions. Indeed, using amides 37 derived from 5-methoxy-8-aminoquinoline, the functionalization of (hetero)aromatic compounds was achieved using 20 mol % of Pd(CH3CN)4Cl2 in the presence of TolSO2SeCF3 as the fluorinating source (14 examples, Scheme 17) [139]. Of note, when the reaction was carried out on derivatives bearing substituents at the meta- (37c and 37d) as well as at the para- (37a and 37b) positions, the corresponding products 38a–d were obtained as a mixture of mono- and disubstituted trifluoromethylselenolated derivatives with global yields ranging from 48% to 70%. When one of the ortho-positions of the aromatic ring was substituted with a Me (37e), a OMe (37f), a Cl (37g) or a CF3 (37h) group, the corresponding compounds 38e, 38f, 38g, and 38h were isolated in moderate to high yields (43% to 80%). The methodology also allowed the trifluoromethylselenolation of the furan derivative 37i, which led to the desired product 38i in 30% yield. A careful monitoring of the reaction unveiled the rapid formation of the CF3SeSeCF3 dimer, which could be the active trifluoromethylselenolating reagent in this transformation.

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Scheme 17: Pd(II)-catalyzed trifluoromethylselenolation of benzamides derived from 5-methoxy-8-aminoquinoline reported by the group of Billard [139]. aThe yields given are the sum of the yields of mono- and ditrifluoromethylselenolated products.

In 2022, Magnier, Billard and co-workers applied the previous methodology to the trifluoromethylselenolation of acrylamide derivatives [140]. Using the same directing group, a panel of α-arylacrylamide derivatives 39a–f was successfully functionalized with a high Z-selectivity (yields up to 98%, Scheme 18). Both, thermal reaction conditions (DMSO at 70 °C for 16 h) and microwave irradiation (100 °C using microwaves in only 1 h) turned out to be efficient in the process. α-Methyl- and α,β-dimethylacrylamides 39g and 39h were also functionalized. Furthermore, a series of β-substituted acrylamides 39i–m with various substituents readily underwent the trifluoromethylselenolation reaction with high selectivity in moderate yields. Finally, the SeCF3-containing β-methylacrylamide 40n was also obtained in 37% yield.

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Scheme 18: Pd(II)-catalyzed trifluoromethylselenolation of acrylamide derivatives derived from 5-methoxy-8-aminoquinoline reported by the groups of Magnier and Billard [140]. aMicrowave, 100 °C, 1 h. b70 °C for 16 h. c20 mol % of Pd(CH3CN)2(Cl)2, 100 °C for 24 h.

II. Transition-metal-catalyzed fluoroalkoxylation of (hetero)arenes by C–H bond activation II.1) Fluoroalkoxylation of aromatic C(sp2)–H bonds by transition-metal catalysis

Over the last years, key advances have been made for the formation of a C(sp2)–OCHRCF3 bond by transition-metal-catalyzed C–H bond activation. Indeed, fluorinated ethers [71,141-153] are key compounds, with especially molecules substituted with the 2,2,2-trifluoroethoxy moiety (OCH2CF3), an important fluorinated group found in several bioactive compounds such as flecanide [154,155] and lansoprazole [156], as flagship molecules. Although the transition-metal-catalyzed hydroxylation and alkoxylation have been studied especially under palladium catalysis [157,158], the direct dehydrogenative 2,2,2-trifluoroethoxylation of (hetero)arenes, often using 2,2,2-trifluoroethanol as a readily available, inexpensive, and green fluorination source [159,160], is still underexplored.

In 2004, Sanford and co-workers reported the dehydrogenative 2,2,2-trifluoroethoxylation of benzo[h]quinoline under palladium catalysis in the presence of PhI(OAc)2 as oxidant (Scheme 19) [161]. Since this seminal work and in the course of their investigation towards the development of new methods for the alkoxylation of C(sp2) centers by transition-metal catalysis, few examples of transition-metal-catalyzed dehydrogenative 2,2,2-trifluoroethoxylation reactions have been reported. In 2021, the palladium-catalyzed ortho-2,2,2-trifluoroethoxylation of 3-arylcoumarins was depicted by the group of Kumar (6 examples, up to 69% yield) [162]. Further developments unveiled the use of copper catalysts for such functionalization. In 2013, the group of Daugulis described the copper-catalyzed ortho-2,2,2-trifluoroethoxylation of a 3-trifluoromethylated benzamide derived from 8-aminoquinoline, giving the corresponding product in 73% yield [149]. The group of Baidya showed that the dehydrogenative 2,2,2-trifluoroethoxylation of benzamide with another bidentate directing group was also possible in the presence of Cu(OAc)2 and hexamethyldisilane [163]. Using N,N- and N,O-bidentate directing groups, the construction of C(sp2)–OCH2CF3 bonds by CH bond activation was also reported using Ni [164] and Co catalysis [165-167]. In 2022, Volla and co-workers reported the ortho-2,2,2-trifluoroethoxylation of benzamide using an N,O-bidentate directing group by merging Co- and visible light organophotocatalysis [168].

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Scheme 19: Transition-metal-catalyzed dehydrogenative 2,2,2-trifluoroethoxylation of (hetero)aromatic derivatives by C–H bond activation [149]. aCF3CH2OH was used as solvent. bPd cat. = [Pd(μ-κ1-OAc)(κ2-N,C10-benzo[h]quinoline)]2. c5 equiv of CF3CH2OH. d15 equiv of CF3CH2OH. e50 equiv of CF3CH2OH. fCF3CH2OH or (CF3)2CHOH was used as solvent with o-chlorotoluene with a ratio of 1:1. AdCOOH = adamantanecarboxylic acid, DMAP = 4-(dimethylamino)pyridine, ORF = OCH2CF3.

Palladium catalysis: In 2017, Ji, Li and co-workers reported a thorough study on the Pd-catalyzed 2,2,2-trifluoroethoxylation of N-sulfonylbenzamides in the presence of PhI(OAc)2 as oxidant and an excess of TFA (Scheme 20) [150]. The functionalization of diversely sub

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