selective hydro- trifluoromethylation of terminal alkynes

step of migratory insertion of initially formed Ar-C≡C-CF3 into LCu(I)-H intermediate, and subsequent hydrogenation of the vinyl-copper species to g...
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Cite This: J. Org. Chem. 2018, 83, 10908−10915

Cu(III)−CF3 Complex Enabled Unusual (Z)‑Selective Hydrotrifluoromethylation of Terminal Alkynes Song-Lin Zhang* and Chang Xiao Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, China

J. Org. Chem. 2018.83:10908-10915. Downloaded from pubs.acs.org by RENSSELAER POLYTECHNIC INST on 09/21/18. For personal use only.

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ABSTRACT: An efficient and selective hydro-trifluoromethylation of terminal alkynes is developed to enable the synthesis of 1,2-disubstituted trifluoromethylated Z-alkenes from terminal alkynes, a silane and a Cu(III)−CF3 complex in DMF. The unusual Z-selectivity and the compatibility of various functional groups make this reaction complementary to previous vinyl−CF3 bond-forming methods producing dominantly E-products or a mixture of E,Z-isomers. Extensive mechanistic studies indicate that this reaction may involve a key step of migratory insertion of initially formed Ar−CC−CF3 into LCu(I)-H intermediate and subsequent hydrogenation of the vinyl-copper species to give finally the Z-trifluoromethylated styrenes. Deuterium labeling experiments show that DMF acts not merely as the solvent but also as the source of α-H in the Z-trifluoromethylated styrene products. Given the novel syn-difunctionalization of alkynes using Cu(III)−CF3 complexes/a nucleophilic reagent we developed recently that are also in contrast to the previous anti-difunctionalization chemistry, Cu(III)−CF3 complexes are demonstrated to show distinct reactivity properties compared to conventional CF3 reagents (Togni’s reagents and Umemototype reagents) and may thus enable the development of other interesting reactions. photocatalyst)7c and is still unknown from hydro-trifluoromethylation of alkynes. Recently, we developed an efficient fluoro- and oxytrifluoromethylation of aryl acetylenes in a syn mode,11 exploiting the novel reactivity of isolated Cu(III)−CF3 complexes,12,13 which are bench-stable, easy-to-handle, and highly reactive CF3 precursors. These reactions were proposed to involve a key vinyl radical intermediate A (Scheme 1e) formally from regioselective addition of CF3 radical to the alkynes.14 We envisioned whether it is possible to trap this vinyl radical by an external radical species, for instance a hydrogen radical. Remarkably, the attack of the radical species to the vinyl radical A might be side-selective to avoid the stereoelectronic repulsion between external radical and the larger CF3 group. This may thus allow a stereoselective radicaltrifluoromethylation of alkynes. Inspired by this hypothesis, we developed an efficient hydro-trifluoromethylation of terminal alkynes by Cu(III)−CF3 complex in the presence of silane/ base in DMF (Scheme 1e) although the detailed mechanism is later suggested to be much more complicated. Unusual Zisomeric products are selectively obtained using this method.

1. INTRODUCTION Trifluoromethylated alkenes are an important structural motif occurring in pharmaceuticals, agrochemicals, and materials due to the unique properties imparted by the CF3 group.1,2 Accordingly, great efforts have been made toward the efficient construction of vinyl−CF3 motif. Among them, some representative methods include cross-coupling of (pseudo)haloalkenes,3 trifluoromethylation of vinyl boron reagents,4 decarboxylative vinyl−CF3 bond formation,5 and direct trifluoromethylation of the C−H bond of alkenes (Scheme 1a−d).6,7 In addition to the de novo construction of the vinyl− CF3 bond, there are complementary methods involving manipulation of CF3-containing substrates such as functionalization of preformed Ar−CC−CF3,8 condensation of CF3CHPR3 ylide with carbonyl compounds, and alkene metathesis.9 However, these methods remain unsatisfactory due to limitations in the need for prefunctionalized substrates, atom- and step-economy, and regio- and stereoselectivity issues. Direct hydro-trifluoromethylation of alkynes is regarded as one of the most straightforward and step-economic methods for the construction of vinyl−CF3 bond from abundant alkynes without prefunctionalization. However, currently a major drawback of alkyne hydro-trifluoromethylation is the lack of stereoselectivity where a mixture of Z- and E-isomers is generally obtained.10 Selective synthesis of Z-isomeric vinyl− CF3 compounds is rather uncommon (Qing et al. reported a rare example of achieving both Z- and E-products controlled by the choice of the combination of CF3 source and © 2018 American Chemical Society

2. RESULTS AND DISCUSSION 2.1. Reaction Development and Substrate Scope. Our study began with reaction of p-chlorophenyl acetylene (2a) with (py)Cu(CF3)3 (1; py denotes pyridine) as the CF3 source and Ph3SiH, a common hydrogen precursor (Table 1).15 When Received: June 25, 2018 Published: July 31, 2018 10908

DOI: 10.1021/acs.joc.8b01586 J. Org. Chem. 2018, 83, 10908−10915

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The Journal of Organic Chemistry

at −55.8 ppm was able to be isolated using column chromatography and determined to be Ar(Ph)CC(CF3)(H). This result indicates the competing hydro- and phenyltrifluoromethylation of alkynes. To attempt to avoid possible competing phenyl-trifluoromethylation, Et3SiH was then used as the hydrosilane in place of Ph3SiH. It was pleasing to see that the use of Et3SiH indeed gave selectively 4a in 66% NMR yield without appreciable other byproducts (entry 3). DMF solvent is crucial to the success; replacing it with other solvents such as DMA, NMP, and THF gave little or no 4a/4a′ but dominant 5a instead (entries 4−6). Notably, the prior heating of a DMF solution of NaOtBu and silane is required before the addition of DMF solution of Cu(III)−CF3 complex 1 and 2a for producing the desired 4a. Moreover, an excess amount of hydrosilane/NaOtBu is preferred for a high yield of the desired product 4a and to suppress the formation of byproducts (entry 7). 4a features a single 19F resonance at −57.6 ppm, a proton at 5.82 ppm with a doublet of quartet splitting (coupling constants of 12.6 and 9.0 Hz), and a proton at 6.90 ppm with doublet splitting (a coupling constant of 12.6 Hz). The NMR chemical shifts and particularly the coupling constants are indicative of a geminal position of CF3 and H,17 and vicinal cis-configuration of H and H on the double bond, which thus definitely determines the structure of 4a. The cis-configuration of 4a is further reflected by the presence of through-space 13 C−19F coupling between the fluorine nuclei of CF3 and ortho-carbon of the phenyl (relative to vinyl moiety) in 13C NMR of 4a, with a fine coupling constant of 2.4 Hz.18 This through-space spin−spin coupling is ascribed to the spatial proximity of the fluorines and ortho-carbon nuclei, rather than to the conventional through-bond transmission of spin−spin coupling.18 Such through-space 13C−19F coupling can serve as an additional, useful, and convenient tool in determining the Z/E configuration of trifluoromethylated alkenes. Next, a series of terminal arylalkynes were subjected to the optimal conditions (entry 3 in Table 1) to explore the synthetic compatibility of this method (Scheme 2). As can be seen, a broad range of functional groups can be tolerated on the aromatic ring, including alkyl, methoxy, chloro, bromo, fluoro, trifluoromethyl, nitrile, ester, ketone, and even unprotected aldehyde and NH2. Heteroaryl acetylene such as 3-pyridylacetylene gave the desired 4p in moderate 49% yield. In all cases, anti-hydro-trifluoromethylation occurs to selectively give 1,2-disubstituted Z-alkenes (except for 4q′with a pNO2 on the phenyl ring with dominant E-configuration), as reflected by the similar NMR features to those for 4a discussed above. Aliphatic terminal alkynes were also applicable for this reaction. For example, 4r and 4s were generated in 30% and 62% yields under the optimal conditions from the corresponding 3-aminoprop-1-yne and pentadec-1-yne (Scheme 2). However, internal alkynes, for example 1,2-diphenylacetylene, were completely inactive possibly due to the difficulty in trifluoromethylation by Cu(III)−CF3 complex 1. 2.2. Mechanistic Insights. To shed light on the model reaction in Table 1, 19F NMR monitoring of the reaction course under the optimized conditions showed that product 4a was produced in minutes with the detection of a minor amount of Ar−CC−CF3 (5a) intermediate (90%), followed by gradual consumption of 5a and the concurrent accumulation of the desired Nutrifluoromethylation (Nu: F or OAr) products.11 This kinetic difference suggests that this alkyne hydro-trifluoromethylation reaction might follow a mechanism that shows significant difference from that of the alkyne Nu-trifluoromethylation reactions. Control experiments show that silane is indispensable for the synthesis of product 4a; without silane, the reaction could not produce 4a but rather 5a as the dominant product (Scheme 3a).12c Notably, either strong basic NaOtBu or nonbasic KF could efficiently promote the formation of 5a (Scheme 3a). This somehow argues against the requirement of initial deprotonation for the generation of 5a. Furthermore, it was shown that disilane in place of hydrosilane could also be efficient for this reaction, generating the desired 4a in 83% yield (Scheme 3b). Radical trapping experiments showed that the presence of 2 equiv of TEMPO did not inhibit the reaction at all and even had a reverse effect of slightly improving the yield to 70% (Scheme 3c). The reaction using disilane showed some radical scavenger effect from TEMPO, and the yield was reduced to 56%, compared to 83% yield without TEMPO (Scheme 3b). 10910

DOI: 10.1021/acs.joc.8b01586 J. Org. Chem. 2018, 83, 10908−10915

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The Journal of Organic Chemistry Scheme 4. Proposed Mechanism

Scheme 5. Rationalization of the Generation of anti-Amino-trifluoromethylation Product 6

hydrosilane might be the most probable source of β-H. When Me3Si-SiMe3 is used, the in situ SiMe3 anion generated from reaction of NaOtBu with Me3Si-SiMe3 may decompose into a hydride and CH2SiMe2 and thus provides the source of β-H in 4a. On the basis of the above mechanistic results, a mechanism shown in Scheme 4 is proposed. The reaction is proposed to proceed through a crucial vinyl radical A which is formed from the radical reaction of Cu(III)−CF3 complex 1 with the alkyne (path a),19 or vinyl cation C generated from an alternative electrophilic trifluoromethylation of alkynes (path b) (note that vinyl cation C might also be generated by SET from vinyl radical A to the Cu(II) intermediate B) (Scheme 4).20 Subsequently, the intermediate 5a is formed by deprotonation of intermediate C with the release of a LCu(I)−CF 3 intermediate. Another possible way for the formation of 5a would involve initial deprotonation of the terminal alkyne by strong base, followed by outersphere nucleophilic attack of acetylide on the CF3 ligands of complex 1. Then LCu(I)−H species should be formed from reaction of the LCu(I)−CF3 with DMF promoted with silane, with the hydride coming from the DMF aldehyde moiety.21,22 Migratory insertion of 5a into LCu(I)−H would afford a vinyl−copper(I) intermediate

D that is hydrogenated to give the desired products 4.22 Nevertheless, it should be emphasized that other mechanistic alternatives cannot be completely excluded at the current stage. This mechanistic proposal is generally consistent with various experimental observations shown in Scheme 3. Particularly, it can rationalize the unusual excellent Zstereoselectivity and the DMF participation to provide the αH atom. Furthermore, the TEMPO effect for the model reaction and how anti-amino-trifluoromethylation product 6 is formed can also be explained by this mechanism. As shown in Scheme 5, in the presence of TEMPO, vinyl radical A might be driven to undergo hydrogen atom abstraction by TEMPO to produce 5a with the release of TEMPO-H. TEMPO-H can react with the LCu(I)−silyl intermediate to produce LCu(I)− amino species F via a N−O bond oxidative cleavage/reductive elimination sequence.22 Finally, migratory insertion of 5a into LCu(I)−amino species F and subsequent hydrodemetalation produces 6.

3. CONCLUSION Thus, a hydro-trifluoromethylation reaction across the triple bond of terminal alkynes is developed exploiting the novel reactivity of Cu(III)−CF3 complex, which gives regio- and 10911

DOI: 10.1021/acs.joc.8b01586 J. Org. Chem. 2018, 83, 10908−10915

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The Journal of Organic Chemistry

= 5.9 Hz), 136.7 (s), 129.1 (q, J = 2.5 Hz), 126.1 (s), 123.0 (q, J = 271.1 Hz), 117.1 (q, J = 34.9 Hz), 112.8 (s), 21.3 (s). The spectral data are in accordance with the literature report.10e (Z)-1-Ethyl-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4c; 9 mg, 45%). Yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.37 (d, J = 8.1 Hz, 2H), 7.23 (d, J = 8.1 Hz, 2H), 6.91 (d, J = 12.6 Hz, 1H), 5.73 (dq, J = 12.6, 9.2 Hz, 1H), 2.69 (q, J = 7.6 Hz, 2H), 1.27 (t, J = 7.6 Hz, 3H). 19 F NMR (376 MHz, CDCl3) δ −57.55 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 145.5 (s), 139.6 (q, J = 5.9 Hz), 131.0 (s), 129.2 (q, J = 2.5 Hz), 127.9 (s), 122.9 (q, J = 271.2 Hz), 117.0 (q, J = 35.0 Hz), 28.7 (s), 15.4 (s). Anal. Calcd for C11H11F3: C, 65.99; H, 5.54. Found: C, 66.28; H, 5.73. (Z)-1-(tert-Butyl)-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4d; 10 mg, 44%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.47− 7.35 (m, 4H), 6.90 (d, J = 12.7 Hz, 1H), 5.73 (dq, J = 12.6, 9.3 Hz, 1H), 1.35 (s, 9H). 19F NMR (376 MHz, CDCl3) δ −57.56 (s, 3F). 13 C NMR (101 MHz, CDCl3) δ 152.5 (s), 139.5 (q, J = 5.9 Hz), 129.0 (q, J = 2.6 Hz), 125.9 (s), 125.3 (s), 123.0 (q, J = 271.1 Hz), 117.0 (q, J = 35.1 Hz), 34.7 (s), 31.2 (s). The spectral data are in accordance with the literature report.23,24 (Z)-1-Methoxy-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4e; 15 mg, 74%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.42 (d, J = 8.7 Hz, 2H), 6.93 (d, J = 8.8 Hz, 2H), 6.85 (d, J = 12.7 Hz, 1H), 5.67 (dq, J = 12.6, 9.3 Hz, 1H), 3.86 (s, 3H). 19F NMR (376 MHz, CDCl3) δ −57.53 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 160.3 (s), 139.2 (q, J = 6.0 Hz), 130.9 (q, J = 2.7 Hz), 126.1 (s), 123.1 (q, J = 270.9 Hz), 115.7 (q, J = 35.1 Hz), 113.8 (s), 55.3 (s). The spectral data are in accordance with the literature report.10e (Z)-1-Bromo-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4f; 16 mg, 64%). White solid; 1H NMR (400 MHz, CDCl3) δ 7.53 (d, J = 8.5 Hz, 2H), 7.32−7.25 (m, 2H), 6.88 (d, J = 12.6 Hz, 1H), 5.83 (dq, J = 12.6, 9.0 Hz, 1H). 19F NMR (376 MHz, CDCl3) δ −57.63 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 138.4 (q, J = 5.8 Hz), 132.5 (s), 131.6 (s), 130.5 (q, J = 2.5 Hz), 123.4 (s), 122.6 (q, J = 271.4 Hz), 118.8 (q, J = 35.0 Hz). The spectral data are in accordance with the literature report.23 (Z)-1-Fluoro-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4g; 6 mg, 32%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.47−7.32 (m, 2H), 7.13−7.05 (m, 2H), 6.90 (d, J = 12.6 Hz, 1H), 5.78 (dq, J = 12.6, 9.0 Hz, 1H). 19F NMR (376 MHz, CDCl3) δ −57.64 (s, 3F), −111.63 (s, 1F). The spectral data are in accordance with the literature report.10e (Z)-1-(Trifluoromethyl)-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4h; 13 mg, 54%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.66 (d, J = 8.2 Hz, 2H), 7.51 (d, J = 8.1 Hz, 2H), 7.00 (d, J = 12.6 Hz, 1H), 5.92 (dq, J = 12.6, 8.8 Hz, 1H). 19F NMR (376 MHz, CDCl3) δ −57.69 (s, 3F), −62.87 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 138.1 (q, J = 5.8 Hz), 137.2 (s), 130.9 (q, J = 32.7 Hz), 129.0 (q, J = 2.4 Hz), 125.3 (q, J = 3.7 Hz), 122.4 (q, J = 271.5 Hz), 121.2 (q, J = 272.2 Hz), 120.3 (q, J = 35.0 Hz). The spectral data are in accordance with the literature report.9a (Z)-4-(3,3,3-Trifluoroprop-1-en-1-yl)benzonitrile (4i; 7 mg, 36%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.69 (d, J = 8.3 Hz, 2H), 7.49 (d, J = 8.2 Hz, 2H), 6.98 (d, J = 12.6 Hz, 1H), 5.95 (dq, J = 12.6, 8.7 Hz, 1H). 19F NMR (376 MHz, CDCl3) δ −57.72 (s, 3F). 13 C NMR (101 MHz, CDCl3) δ 138.2 (s), 137.6 (q, J = 5.7 Hz), 132.1 (s), 129.3 (q, J = 2.5 Hz), 122.3 (q, J = 271.7 Hz), 121.0 (q, J = 35.0 Hz), 118.3 (s), 112.7 (s). The spectral data are in accordance with the literature report.9a,25 (Z)-1-(4-(3,3,3-Trifluoroprop-1-en-1-yl)phenyl)ethan-1-one (4j; 11 mg, 51%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.98 (d, J = 8.4 Hz, 2H), 7.49 (d, J = 8.2 Hz, 2H), 7.00 (d, J = 12.6 Hz, 1H), 5.91 (dq, J = 12.6, 8.8 Hz, 1H), 2.64 (s, 3H). 19F NMR (376 MHz, CDCl3) δ −57.64 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 197.4 (s), 138.5 (q, J = 5.8 Hz), 138.3 (s), 137.1 (s), 129.0 (q, J = 2.5 Hz), 128.3 (s), 122.5 (q, J = 271.5 Hz), 120.0 (q, J = 35.0 Hz), 26.6 (s). The spectral data are in accordance with the literature report.3f Methyl (Z)-4-(3,3,3-Trifluoroprop-1-en-1-yl)benzoate (4k; 13 mg, 57%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 8.06 (d, J = 8.4 Hz, 2H), 7.47 (d, J = 8.2 Hz, 2H), 7.00 (d, J = 12.6 Hz, 1H), 5.90

stereoselectively (Z)-β-CF3 styrenes. A range of functional groups can be tolerated on the aromatic ring of arylalkynes. This method provides a complementary approach to the previously developed various vinyl−CF3 bond-forming reactions wherein E-isomeric products or a mixture of E/Z isomers were often obtained. Intensive mechanistic studies including various control experiments, NMR monitoring of the reaction course, radical trapping experiments, and deuterium labeling experiments support a reaction mechanism involving the initial formation of Ar−CC−CF3 intermediate and the in situ generation of LCu(I)−H species. These two compounds undergo migratory insertion to give a LCu(I)−vinyl intermediate that finally produces the desired (Z)-products after hydrogenation. Further efforts are still needed to fully understand this reaction and explore its synthetic applications.

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EXPERIMENTAL SECTION

General Experimental Details. All chemicals were purchased commercially except complex (py)Cu(CF3)3 (1) (py denotes pyridine) which was prepared according to the procedure we developed recently.12c DMF and CH2Cl2 solvents were simply dried over Na2SO4 before use to extrude adventitious water. Other reactants were used as received without further purification. All the reactions were performed in a Schlenk tube under N2 atmosphere which was realized through evacuation/backfill techniques after three times. Reaction progress was monitored by TLC analysis with stains visualized under UV irradiation and 19F NMR analysis of the crude mixture. NMR yields were determined using 19F NMR analysis of the crude mixture with 4,4-difluorobiphenyl as the internal standard (117.0 ppm). Column chromatography on silica gel was used to obtain purified products that are suitable for NMR spectroscopic characterization. NMR spectra were recorded on a 400 MHz spectrometer and processed with MestReNova program. Chemical shifts are reported in ppm and referenced to residual solvent peaks or TMS. NMR signals are reported as follows to delineate possible splitting: s, singlet; bs, broad singlet; d, doublet; t, triplet; q, quartet; dd, doublet of doublet; dq, doublet of quartet; m, multiplet. Coupling constants are reported in hertz where present. All 13C and 19F NMR spectra were obtained with proton decoupling. General Procedure for the Hydro-trifluoromethylation Reactions. To an oven-dried 25 mL Schlenk tube equipped with a stir bar were added silane (3; 0.4 mmol) and NaOtBu (0.4 mmol). The Schlenk tube was evacuated and refilled with dry nitrogen. Anhydrous DMF (2 mL) was then added via syringe. The contents of the tube were stirred at 100 °C for 1 h (heated in an oil bath). Then a DMF solution (1 mL) of (py)Cu(CF3)3 (1; 0.1 mmol) and alkynes (2; 0.1 mmol) was slowly injected using a syringe. Stirring was continued for 12 h until the reaction was complete as indicated by TLC analysis. The reaction mixture was then cooled to room temperature and extracted with dichloromethane. The combined organic layers were washed three times with copious amounts of water and once with brine and then dried over magnesium sulfate. The organic phase was then condensed by evaporation in vacuum. The resulting residue was purified by column chromatography on silica gel (eluent: n-hexane/dichloromethane = 1:0−5:1 (v/v)) to give the final product 4. (Z)-1-Chloro-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4a; 10 mg, 50%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.42−7.32 (m, 4H), 6.90 (d, J = 12.6 Hz, 1H), 5.82 (dq, J = 12.6, 9.0 Hz, 1H). 19 F NMR (376 MHz, CDCl3) δ −57.63 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 138.4 (q, J = 6.1 Hz), 135.1 (s), 132.1 (s), 130.3 (q, J = 2.4 Hz), 128.6 (s), 122.6 (q, J = 271.3 Hz), 118.7 (q, J = 34.9 Hz). The spectral data are in accordance with the literature report.9a (Z)-1-Methyl-4-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4b; 12 mg, 65%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.35 (d, J = 8.1 Hz, 2H), 7.21 (d, J = 8.0 Hz, 2H), 6.91 (d, J = 12.6 Hz, 1H), 5.74 (dq, J = 12.6, 9.2 Hz, 1H), 2.40 (s, 3H). 19F NMR (376 MHz, CDCl3) δ −57.53 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 139.6 (q, J 10912

DOI: 10.1021/acs.joc.8b01586 J. Org. Chem. 2018, 83, 10908−10915

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(dq, J = 12.6, 8.8 Hz, 1H), 3.95 (s, 3H). 19F NMR (376 MHz, CDCl3) δ −57.66 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 166.6 (s), 138.6 (q, J = 5.8 Hz), 138.1 (s), 130.4 (s), 129.5 (s), 128.8 (q, J = 2.5 Hz), 122.5 (q, J = 271.5 Hz), 119.9 (q, J = 34.9 Hz), 52.2 (s). The spectral data are in accordance with the literature report.23,25 (Z)-3-(3,3,3-Trifluoroprop-1-en-1-yl)aniline (4m; 8 mg; 43%). Yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.19 (t, J = 7.8 Hz, 1H), 6.88 (d, J = 12.7 Hz, 1H), 6.83 (d, J = 7.7 Hz, 1H), 6.76 (m, 1H), 6.70 (dd, J = 7.9, 2.0 Hz, 1H), 5.75 (dq, J = 12.6, 9.1 Hz, 1H), 3.61 (bs, 2H). 19F NMR (376 MHz, CDCl3) δ −57.30 (s, 3F). Anal. Calcd for C9H8F3N: C, 57.76; H, 4.31; N, 7.48. Found: C, 58.05; H, 4.11; N, 7.26. (Z)-1-Methyl-3-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4n; 8 mg; 43%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.41−7.17 (m, 4H), 6.93 (d, J = 12.6 Hz, 1H), 5.77 (dq, J = 12.6, 9.1 Hz, 1H), 2.39 (s, 3H). 19F NMR (376 MHz, CDCl3) δ −57.49 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 139.8 (q, J = 5.9 Hz), 137.9 (s), 129.8 (s), 129.6 (q, J = 2.4 Hz), 128.2 (s), 126.0 (q, J = 2.6 Hz), 122.8 (q, J = 271.3 Hz), 117.8 (q, J = 34.9 Hz), 113.6 (s), 21.3 (s). The spectral data are in accordance with the literature report.26 (Z)-1-Chloro-2-(3,3,3-trifluoroprop-1-en-1-yl)benzene (4o; 5 mg, 22%). Yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.49−7.40 (m, 2H), 7.36−7.28 (m, 2H), 7.11 (d, J = 12.4 Hz, 1H), 5.93 (dq, J = 12.3, 8.5 Hz, 1H). 19F NMR (376 MHz, CDCl3) δ −57.93 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 136.6 (q, J = 5.7 Hz), 133.2 (s), 130.3 (q, J = 3.3 Hz), 130.1 (s), 129.2 (s), 126.6 (s), 122.6 (q, J = 271.7 Hz), 120.0 (q, J = 34.3 Hz), 116.5 (s). The spectral data are in accordance with the literature report.9a (Z)-3-(3,3,3-Trifluoroprop-1-en-1-yl)pyridine (4p; 4 mg, 23%). Yellow oil; 1H NMR (400 MHz, CDCl3) δ 9.03 (m, 2H), 7.83 (d, J = 7.7 Hz, 1H), 7.56 (d, J = 8.8 Hz, 1H), 6.97 (d, J = 12.6 Hz, 1H), 5.97 (dq, J = 12.6, 8.8 Hz, 1H). 19F NMR (376 MHz, CDCl3) δ −57.89 (s, 3F). The spectral data are in accordance with the literature report.24 Phenyl-trifluoromethylation Product Using Ph3SiH as the Silane. Yellow oil, 12 mg, 42% yield. 1H NMR (400 MHz, CDCl3) δ 7.38 (m, 5H), 7.27 (dd, J = 8.1, 1.4 Hz, 2H), 7.22 (d, J = 8.4 Hz, 2H), 6.17 (q, J = 8.2 Hz, 1H). 19F NMR (376 MHz, CDCl3) δ −55.57 (s). 13 C NMR (101 MHz, CDCl3) δ 151.3 (q, J = 5.6 Hz), 139.7 (s), 135.7 (s), 134.7 (s), 130.5 (d, J = 1.8 Hz), 129.7 (s), 128.6 (s), 128.4 (s), 127.9 (s), 123.0 (q, J = 270.7 Hz), 116.0 (q, J = 34.0 Hz). The spectral data are in accordance with the literature report.7d (E)-1-(1-(4-chlorophenyl)-3,3,3-trifluoroprop-1-enyl)-2,2,6,6-tetramethylpiperidine (6; 9 mg, 26%). Colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.67−7.30 (m, 4H), 6.01 (q, J = 8.7 Hz, 1H), 1.87− 1.48 (m, 6H), 1.19 (s, 12H). 19F NMR (376 MHz, CDCl3) δ −51.80 (s, 3F). 13C NMR (101 MHz, CDCl3) δ 165.6 (q, J = 5.6 Hz), 135.8 (s), 131.7 (s), 129.7 (q, J = 1.7 Hz), 128.4 (s), 125.6 (q, J = 266.5 Hz), 94.8 (q, J = 35.6 Hz), 61.0 (s), 39.6 (s), 32.1 (s), 20.7 (s), 16.9 (s). Anal. Calcd for C18H23ClF3N: C, 62.51; H, 6.70; N, 4.05. Found: C, 62.77; H, 6.59; N, 4.11.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (no. 21472068). Financial support from MOE & SAFEA for the 111 Project (B13025), is also gratefully acknowledged.

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REFERENCES

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The Journal of Organic Chemistry (15) Hydrosilane in conjunction with NaOtBu or KOtBu has been proposed to generate silyl or hydrogen radicals. See: (a) Liu, W.-B.; Schuman, D. P.; Yang, Y.-F.; Toutov, A. A.; Liang, Y.; Klare, H. F. T.; Nesnas, N.; Oestreich, M.; Blackmond, D. G.; Virgil, S. C.; Banerjee, S.; Zare, R. N.; Grubbs, R. H.; Houk, K. N.; Stoltz, B. M. Potassium tert-Butoxide-Catalyzed Dehydrogenative C−H Silylation of Heteroaromatics: A Combined Experimental and Computational Mechanistic Study. J. Am. Chem. Soc. 2017, 139, 6867−6879. (b) Smith, A. J.; Young, A.; Rohrbach, S.; O’Connor, E. F.; Allison, M.; Wang, H.S.; Poole, D. L.; Tuttle, T.; Murphy, J. A. Electron-Transfer and Hydride-Transfer Pathways in the Stoltz-Grubbs Reducing System (KOtBu/Et3SiH). Angew. Chem., Int. Ed. 2017, 56, 13747−13751. (16) The products 4a, 4a′, and 5a feature characteristic 19F signals at ca. −57, −63, and −50 ppm. (17) Vicinal H−CF3 coupling constant is typically less than 2 Hz on the double bond. Refer to: (a) Bégué, J.-P.; Bonnet-Delpon, D.; Mesureur, D.; Ourevitch, M. Determination of Z and E Configurations in Trifluoromethylated Vinyl Compounds: 3J(CF) Coupling Constants as a Criterion for Configurational Assignments. Magn. Reson. Chem. 1991, 29, 675−678. (b) Matsubara, K.; Oba, A.; Usui, Y. Configurational assignments in trifluoromethylvinyl compounds using through-space carbon−fluorine coupling constants. Magn. Reson. Chem. 1998, 36, 761−765. (18) For discussion on through-space coupling, see: (a) Hierso, J.-C. Indirect Nonbonded Nuclear Spin−Spin Coupling: A Guide for the Recognition and Understanding of “Through-Space” NMR J Constants in Small Organic, Organometallic, and Coordination Compounds. Chem. Rev. 2014, 114, 4838−4867. (b) Reference 17b. (c) Domański, S.; Staszewska-Krajewska, O.; Chaładaj, W. PdCatalyzed Carbonylative Carboperfluoroalkylation of Alkynes. Through-Space 13C−19F Coupling as a Probe for Configuration Assignment of Fluoroalkyl-Substituted Olefins. J. Org. Chem. 2017, 82, 7998−8007. (19) The generation of vinyl radical A and Cu(II) intermediate B may occur through a direct outersphere attack of electron-rich Cβ atom of alkyne to the electrophilic CF3 of Cu(III) complex 1. The homolytic Cu−CF3 bond dissociation and subsequent addition of · CF3 with alkyne seems unlikely because no detectable of TEMPOCF3 was observed in the presence of TEMPO. (20) For the vinyl cation proposal, see: (a) Walkinshaw, A. J.; Xu, W.; Suero, M. G.; Gaunt, M. J. Copper-Catalyzed Carboarylation of Alkynes via Vinyl Cations. J. Am. Chem. Soc. 2013, 135, 12532− 12535. For the proposal of SET from vinyl radical to generate vinyl cation, see refs 11 and 14a,14b. (21) Panne, P.; Naumann, D.; Hoge, B. Cyanide initiated perfluoroorganylations with perfluoroorganosilicon compounds. J. Fluorine Chem. 2001, 112, 283−286. (22) For selected recent examples of Cu(I)-hydride in catalysis, see: (a) Zhu, S.; Niljianskul, N.; Buchwald, S. L. Enantio- and Regioselective CuH-Catalyzed Hydroamination of Alkenes. J. Am. Chem. Soc. 2013, 135, 15746−15749. (b) Zhu, S.; Buchwald, S. L. Enantioselective CuH-Catalyzed Anti-Markovnikov Hydroamination of 1,1-Disubstituted Alkenes. J. Am. Chem. Soc. 2014, 136, 15913− 15916. (c) Yang, Y.; Shi, S.-L.; Niu, D.; Liu, P.; Buchwald, S. L. Catalytic asymmetric hydroamination of unactivated internal olefins to aliphatic amines. Science 2015, 349, 62−66. (d) Pirnot, M. T.; Wang, Y.-M.; Buchwald, S. L. Copper Hydride Catalyzed Hydroamination of Alkenes and Alkynes. Angew. Chem., Int. Ed. 2016, 55, 48−57. (e) Miki, Y.; Hirano, K.; Satoh, T.; Miura, M. CopperCatalyzed Intermolecular Regioselective Hydroamination of Styrenes with Polymethylhydrosiloxane and Hydroxylamines. Angew. Chem., Int. Ed. 2013, 52, 10830−10834. (f) Nishikawa, D.; Hirano, K.; Miura, M. Asymmetric Synthesis of α-Aminoboronic Acid Derivatives by Copper-Catalyzed Enantioselective Hydroamination. J. Am. Chem. Soc. 2015, 137, 15620−15623. (g) Xi, Y.; Butcher, T. W.; Zhang, J.; Hartwig, J. F. Regioselective, Asymmetric Formal Hydroamination of Unactivated Internal Alkenes. Angew. Chem., Int. Ed. 2016, 55, 776− 780.

(23) Hafner, A.; Fischer, T. S.; Bräse, S. Synthesis of CF3-Substituted Olefins by Julia-Kocienski Olefination Using 2-[(2,2,2Trifluoroethyl)sulfonyl]benzo[d]thiazole as Trifluoromethylation Agent. Eur. J. Org. Chem. 2013, 2013, 7996−8003. (24) Straathof, N. J.; Cramer, S. E.; Hessel, V.; Noel, T. Practical Photocatalytic Trifluoromethylation and Hydrotrifluoromethylation of Styrenes in Batch and Flow. Angew. Chem., Int. Ed. 2016, 55, 15549−15553. (25) Ayeni, D. O.; Mandal, S. K.; Zajc, B. Julia−Kocienski approach to trifluoromethyl-substituted alkenes. Tetrahedron Lett. 2013, 54, 6008−6011. (26) Lishchynskyi, A.; Mazloomi, Z.; Grushin, V. V. Trifluoromethylation and Pentafluoroethylation of Vinylic Halides with Low-Cost RfH-Derived CuRf (Rf = CF3, C2F5). Synlett 2014, 26, 45−50.

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