Photoredox-Catalyzed Stereoselective Conversion of Alkynes into Tetrasubstituted Trifluoromethylated Alkenes

Article abstract of DOI:10.1002/anie.201505550

A regio- and stereoselective synthesis of trifluoromethylated alkenes bearing four different substituents has been developed. Stereocontrolled sulfonyloxytrifluoromethylation of unsymmetric internal alkynes with an electrophilic CF₃ reagent, namely the triflate salt of the Yagupol'skii-Umemoto reagent, in the presence of an Ir photoredox catalyst under visible-light irradiation afforded trifluoromethylalkenyl triflates with well-predictable stereochemistry resulting from anti addition of the trifluoromethyl and triflate groups. Subsequent palladium-catalyzed cross-couplings led to tetrasubstituted trifluoromethylated alkenes in a highly stereoselective manner. The present method is the first example of a facile one-pot synthesis of tetrasubstituted trifluoromethylated alkenes from simple alkynes.

Full text of DOI:10.1002/anie.201505550

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505550
German Edition: DOI: 10.1002/ange.201505550
Trifluoromethylation
Photoredox-Catalyzed Stereoselective Conversion of Alkynes into
Tetrasubstituted Trifluoromethylated Alkenes
Ren Tomita, Takashi Koike,* and Munetaka Akita*
Abstract: A regio- and stereoselective synthesis of trifluoro-
methylated alkenes bearing four different substituents has been
developed. Stereocontrolled sulfonyloxytrifluoromethylation
of unsymmetric internal alkynes with an electrophilic CF3
reagent, namely the triflate salt of the Yagupolꢀskii–Umemoto
reagent, in the presence of an Ir photoredox catalyst under
visible-light irradiation afforded trifluoromethylalkenyl tri-
flates with well-predictable stereochemistry resulting from
anti addition of the trifluoromethyl and triflate groups. Sub-
sequent palladium-catalyzed cross-couplings led to tetrasub-
stituted trifluoromethylated alkenes in a highly stereoselective
manner. The present method is the first example of a facile one-
pot synthesis of tetrasubstituted trifluoromethylated alkenes
from simple alkynes.
ical, agrochemical, and material sciences.[3] Step-economic
and versatile stereoselective synthetic methods towards
tetrasubstituted trifluoromethylated alkenes, however, have
been extremely limited thus far.[4] Hiyama and Shimizu
reported the synthesis of CF3-substituted triarylethenes
starting from CF3-substituted dichlorohydrin and 1,1-
dibromo-3,3,3-trifluoro-2-tosyloxypropene
(Scheme 1a).
Konno et al. developed methods based on the carbometala-
tion of CF3-substituted alkynes (Scheme 1b). However, these
excellent methods require CF3-substituted precursors, reduc-
ing their versatility and simplicity. In contrast, the direct
trifluoromethylation of alkynes is regarded as one of the most
straightforward strategies for the synthesis of CF3-substituted
alkenes. Nevertheless, highly stereoselective trifluoromethy-
lations of simple internal alkynes are still a great challenge in
spite of the recent development of various trifluoromethyla-
tion methods.[5,6] Herein, we describe a novel stereoselective
T
etrasubstituted alkenes represent a useful structural motif
that is found in biologically active natural products, drugs, and
functional materials.[1] Therefore, the development of new
and reliable methods for the stereoselective synthesis of
tetrasubstituted alkenes is in high demand.[2] Owing to the
unique properties of the trifluoromethyl (CF3) group, CF3-
substituted alkenes have great potential in the pharmaceut-
Table 1: Optimization of the photocatalytic trifluoromethyltriflation of
2a.
Entry
Deviations from the standard conditions[a]
3a[b] [%]
E/Z[c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
–
84
74[d]
84
62[e]
75
80
70
47
13
79
77
77
0
99:1
99:1
96:4
98:2
94:6
99:1
99:1
95:5
–
98:2
98:2
98:2
–
10 min
2 h
1a (1 equiv)
1b, 1 h
[Ru(bpy)3](PF6)2, 10 h
fac-[Ir(ppy)3], 2 h
CD3CN
[D6]acetone
2,6-lutidine
Na2CO3
Scheme 1. Stereoselective synthesis of tetrasubstituted trifluoromethy-
lated alkenes.
no base
no light
no photocatalyst
[*] R. Tomita, Dr. T. Koike, Prof. Dr. M. Akita
Chemical Resources Laboratory
0
–
Tokyo Institute of Technology
R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
E-mail: koike.t.ad@m.titech.ac.jp
[a] For detailed reaction conditions, see the Supporting Information.
[b] Yields were determined by 1H NMR spectroscopy using SiEt4 as an
internal standard. [c] The E/Z ratios were determined by 19F NMR
spectroscopy. [d] Unreacted 2a (13%) was observed. [e] Unreacted 2a
(23%) was observed. bpy=2,2’-bipyridine, dtbbpy=4,4’-di-tert-butyl-
2,2’-bipyridine, LED=light-emitting diode, ppy=2-pyridylphenyl.
makita@res.titech.ac.jp
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201505550.
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Table 2: Scope of the photocatalytic trifluoromethyltriflation of alkynes.[a,b,c]
these earlier projects prompted us to design
a method for the trifluoromethylating difunctional-
ization of alkynes through photoredox single elec-
tron transfer (SET) processes. Initially, we examined
the photocatalytic trifluoromethanesulfonyloxy-tri-
fluoromethylation (trifluoromethyltriflation) of 1-
phenyl-1-propyne (2a), an unsymmetric internal
alkyne, with the Yagupolꢀskii–Umemoto reagent
(1a). To our delight, the reaction of alkyne 2a with
1.8 equivalents of 1a in CD2Cl2 in the presence of an
Ir photoredox catalyst, namely [Ir(ppy)2(dtbbpy)]-
(PF6) (5 mol%), and 2,6-di-tert-butylpyridine under
visible-light irradiation (425 nm blue LEDs) for 0.5 h
afforded the desired product 3a in an NMR yield of
84% in a highly regio- and stereoselective manner
(E/Z = 99:1). Shorter (10 min) or longer reaction
times (2 h) did not lead to a significant deterioration
of the stereoselectivity (Table 1, entries 1–3), indi-
cating that an isomerization process is not involved
in the present reaction as a major reaction pathway.
Reducing the amount of 1a led to a lower conversion
of 2a (entry 4). The use of Umemotoꢀs reagent (1b)
lowered the yield and the efficiency owing to the
degradation of 1b under the reaction conditions
(entry 5). Other photocatalysts turned out to be
inefficient (entries 6 and 7). Other solvent systems,
such as CD3CN and [D6]acetone, gave complicated
mixtures of products (entries 8 and 9). The addition
of a bulky organic base, 2,6-di-tert-butylpyridine,
made the reaction cleaner, improving the yield of 3a
(entries 10–12).[10] Finally, irradiation with visible
light and the photoredox catalyst were shown to be
essential for the present reaction (entries 13 and 14).
The tetrasubstituted trifluoromethylalkenyl tri-
flates that were obtained through this photocatalytic
trifluoromethylation are shown in Table 2. The
reactions of 1-phenyl-1-propyne (2a) and various
derivatives (2b–2g) afforded the corresponding tri-
fluoromethylalkenyl triflates (3a–3g) in 49–86%
yield in a highly stereoselective manner (E/Z =
86:14–96:4). Remarkably, the present reaction toler-
ates a variety of functional groups on the arene ring,
such as halide (2c), ester (2d), primary amide (2e),
nitrile (2 f), and hydroxy (2g) groups. Furthermore,
3a:[d] 74%, E/Z=96:4
3d: 73%, E/Z=91:9
3g: 56%, E/Z=94:6
3b:[d] 86%, E/Z=96:4
3e: 49%, E/Z=86:14
3h: 76%, E/Z=97:3
3c: 64%, E/Z=92:8
3 f: 50%, E/Z=88:12
3i: 67%, E/Z=94:6
3j:[e] 52%, E/Z=83:17
3k:[f] 37%, E/Z=61:39
3l:[g] 57%, E/Z=85:15
3m:[g] 63%, E/Z=86:14
3n: 30%, E/Z=89:11
3o: 30%, E/Z=85:15
[a] For detailed reaction conditions, see the Supporting Information. [b] Yields of
isolated products. [c] The E/Z ratios were determined by 19F NMR spectroscopy of
the crude product mixtures. [d] 1a and base (1.8 equiv each). [e] 1a and base
(2.8 equiv each), 10 h. [f] 1a and base (3.0 equiv each), 12 h. [g] 1a and base
(2.2 equiv each).
sulfonyloxytrifluoromethylation of internal unsymmetric
alkynes mediated by visible-light photoredox catalysis.[7]
a substituent in the ortho position of the arene ring (2h) did
not induce a deterioration of yield and stereoselectivity (3h:
76%, E/Z = 97:3).
A
shelf-stable and easy-to-use electrophilic CF3 reagent,
S-(trifluoromethyl)diphenylsulfonium trifluoromethanesulfo-
nate (1a; Yagupolꢀskii–Umemoto reagent),[8] was found to be
a key compound for the difunctionalization of alkynes. The
obtained trifluoromethylalkenyl triflates were readily con-
verted into tetrasubstituted trifluoromethylated alkenes in
a stereocontrolled manner by well-established Pd-catalyzed
coupling reactions. This method thus enables the facile one-
pot synthesis of CF3-substitued alkenes bearing four different
substituents (Scheme 1c).
À ꢀ À
Aryl alkyl acetylenes (Ar C C R), such as 2i, 2j, and
2k, can also be trifluoromethylated under these reaction
conditions. Substrates with alkyl groups bulkier than a methyl
group, such as ethyl and n-butyl groups, still afforded the
E stereoisomer selectively (3i: 67%, E/Z = 94:6; 3j: 52%,
E/Z = 83:17). An even bulkier substituent, namely a cyclo-
hexyl group, had a significant harmful effect on the efficiency
and stereoselectivity (3k: 37%, E/Z = 61:39). The reactions
of methyl 3-phenyl-2-propynoate (2l) and 4-phenyl-3-butyn-
2-one (2m) also proceeded in a stereoselective manner to give
useful a-CF3-substituted enones (3l: 57%, E/Z = 85:15; 3m:
63%, E/Z = 86:14).[6o] Furthermore, diaryl acetylenes (2n and
Previously, we had developed a regiospecific trifluorome-
thylating difunctionalization reaction of alkenes mediated by
photoredox catalysis.[9] The experience accumulated during
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Chemie
2o) afforded the corresponding diaryl trifluoromethylalkenyl
triflates in moderate yields and stereoselectivities (3n: 30%,
E/Z = 89:11; 3o: 30%, E/Z = 85:15). A simple aliphatic
alkyne, 6-dodecyne, did not afford the corresponding product.
These results show that the present photocatalytic reaction
can be used for the regio- and highly stereoselective
difunctionalization of aryl acetylenes with trifluoromethyl
and triflate groups.
While investigating the scope, we found that the reaction
products of 1-aryl-1-propynes bearing an electron-donating
group (EDG) on the aryl ring could not be isolated
presumably owing to their susceptibility to hydrolysis. There-
fore, the trifluoromethylating sulfonyloxylation of alkynes
with other external nucleophiles was examined. The reaction
with a tetraalkylammonium tosylate (1.1 equiv) yielded the
corresponding trifluoromethylalkenyl tosylates 4 as isolable
products. The tosylate system is amenable to the synthesis of
anisyl and N-protected aminophenyl trifluoromethylalkenyl
tosylates (4p: 67%, E/Z = 94:6; 4q: 57%, E/Z = 80:20; 4r:
58%, E/Z = 70:30). The tosylate system also exhibited a high
stereoselectivity as the trifluoromethyl and tosylate groups
were added to the alkyne in an anti fashion (Scheme 2).
Scheme 3. One-pot stereocontrolled synthesis of tetrasubstituted tri-
fluoromethylated alkenes from alkynes. Yields of 5 are based on 2a.
to afford 5b (E/Z = 8:92) in 49% yield (see the Supporting
Information).
A possible reaction mechanism for the photocatalytic
sulfonyloxytrifluoromethylation is illustrated in Scheme 4.
Up to the regiospecific generation of the a-styryl-type
trifluoromethylalkenyl radical 6’, which is stabilized by
delocalization to the p orbitals of the phenyl ring,[11a,c] the
reaction appears to proceed through a mechanism similar to
that of the previously reported photoredox-catalyzed trifluor-
omethylation of olefins.[9,12] Key intermediate 6’ can undergo
1e oxidation by the highly oxidizable Ir photocatalyst
(IrIV)[11b,12] to give trifluoromethylalkenyl cation 6+. The
highly stereoselective formation of trifluoromethylalkenyl
triflate 3 can be interpreted in terms of the strongly electro-
negative properties of the CF3 group:[4a,13] Nucleophilic attack
of the triflate anion favors anti addition with respect to the
CF3 group owing to electrostatic repulsion, as supported by
the electrostatic potential map of 6a+ (see the Supporting
Information). We also suggest that another possible mecha-
nism proceeds through direct addition of the triflate anion to
6’ followed by 1e oxidation (see the Supporting Information).
We cannot determine the exact reaction pathway at present.
In conclusion, we have developed novel trifluoromethyl-
triflation and -tosylation reactions of alkynes mediated by
photoredox catalysis. The present photocatalytic system
Scheme 2. Photocatalytic trifluoromethyltosylation of alkynes bearing
an electron-donating group (EDG). Reaction conditions: [Ir(ppy)2-
(dtbbpy)](PF6) (5 mol%), 2,6-di-tert-butylpyridine (1.8 equiv). [a] 1a
and base (1.5 equiv each) were used. [b] [NMe4][OTs] was used.
EDG=electron-donating group.
Next, we examined the stereocontrolled synthesis of
trifluoromethylalkenes bearing four different substituents
through Pd-catalyzed coupling reactions of the trifluorome-
thylalkenyl triflates. As shown in Scheme 3, CF3-substituted
alkenyl triflate 3a, generated in situ by the photoredox-
catalyzed reaction, was directly subjected to various Pd-
catalyzed reactions without purification and after solvent
exchange. As a result, the tetrasubstituted CF3-substituted
alkene 5a was obtained in 60% yield in a highly stereoselec-
tive manner ((1E,3Z)/(1E,3E) = 92:8) in a one-pot process.
The present, highly programmable strategy enabled the
stereocontrolled synthesis of a variety of tetrasubstituted
CF3-substituted alkenes 5a–5d in facile one-pot reactions. It
turned out that the stereochemistry of 3a was almost retained
during the Pd-catalyzed reactions under mild conditions (RT–
408C). Furthermore, trifluoromethylalkenyl tosylate 4p also
underwent a Pd-catalyzed coupling with phenylboronic acid
Scheme 4. Possible reaction mechanism.
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20, 16806 – 16829; b) T. Besset, T. Poisson, X. Pannecoucke,
Chem. Eur. J. 2014, 20, 16830 – 16845; c) T. Besset, T. Poisson, X.
Pannecoucke, Eur. J. Org. Chem. 2015, 2765 – 2789; d) C.
Alonso, E. M. de Marigorta, G. Rubiales, F. Palacios, Chem.
Rev. 2015, 115, 1847 – 1935; e) P. Gao, X.-R. Song, X.-Y. Liu, Y.-
M. Liang, Chem. Eur. J. 2015, 21, 7648 – 7661.
turned out to be powerful for the regio- and highly
stereocontrolled synthesis of trifluoromethylalkenes from
simple alkynes bearing a wide variety of functional groups.
Furthermore, by combining this process with well-established
palladium catalysis, various tetrasubstituted CF3-substituted
alkenes were obtained in a highly stereoselective manner in
a facile one-pot process. The synthesis of new and functional
multisubstituted trifluoromethylated alkenes is currently
underway in our laboratory.
[6] a) P. G. Janson, I. Ghoneim, N. O. Ilchenko, K. J. Szabó, Org.
Lett. 2012, 14, 2882 – 2885; b) H. Egami, R. Shimizu, M.
Sodeoka, Tetrahedron Lett. 2012, 53, 5503 – 5506; c) S. Mizuta,
S. Verhoog, K. M. Engle, T. Khotavivattana, M. OꢀDuill, K.
Wheelhouse, G. Rassias, M. MØdebielle, V. Gouverneur, J. Am.
Chem. Soc. 2013, 135, 2505 – 2508; d) N. Iqbal, J. Jung, S. Park,
E. J. Cho, Angew. Chem. Int. Ed. 2014, 53, 539 – 542; Angew.
Chem. 2014, 126, 549 – 552; e) T. Xu, C. W. Cheung, X. Hu,
Angew. Chem. Int. Ed. 2014, 53, 4910 – 4914; Angew. Chem.
2014, 126, 5010 – 5014; f) G.-C. Ge, X.-J. Huang, C.-H. Ding, S.-
L. Wang, L.-X. Dai, X.-L. Hou, Chem. Commun. 2014, 50, 3048 –
3051; g) J. Xu, Y.-L. Wang, T.-J. Gong, B. Xiao, Y. Fu, Chem.
Commun. 2014, 50, 12915 – 12918; h) P. Gao, Y.-W. Shen, R.
Fang, X.-H. Hao, Z.-H. Qiu, F. Yang, X.-B. Yan, Q. Wang, X.-J.
Gong, X.-Y. Liu, Y.-M. Liang, Angew. Chem. Int. Ed. 2014, 53,
7629 – 7633; Angew. Chem. 2014, 126, 7759 – 7763; i) H. Egami,
T. Ide, M. Fujita, T. Tojo, Y. Hamashima, M. Sodeoka, Chem.
Eur. J. 2014, 20, 12061 – 12065; j) Y. Wang, M. Jiang, J.-T. Liu,
Chem. Eur. J. 2014, 20, 15315 – 15319; k) Y.-L. Ji, J.-H. Lin, J.-C.
Xiao, Y.-C. Gu, Org. Chem. Front. 2014, 1, 1280 – 1284; l) Z.
Hang, Z. Li, Z.-Q. Liu, Org. Lett. 2014, 16, 3648 – 3651; m) Y. Li,
Y. Lu, G. Qiu, Q. Ding, Org. Lett. 2014, 16, 4240 – 4243; n) A.
Maji, A. Hazra, D. Maiti, Org. Lett. 2014, 16, 4524 – 4527; o) Y.-P.
Xiong, M.-Y. Wu, X.-Y. Zhang, C.-L. Ma, L. Huang, L.-J. Zhao,
B. Tan, X.-Y. Liu, Org. Lett. 2014, 16, 1000 – 1003; p) Y.-L. Ji, J.-
H. Lin, J.-C. Xiao, Y.-C. Gu, Eur. J. Org. Chem. 2014, 7948 –
7954; q) H.-L. Hua, Y.-T. He, Y.-F. Qiu, Y.-X. Li, B. Song, P. Gao,
X.-R. Song, D.-H. Guo, X.-Y. Liu, Y.-M. Liang, Chem. Eur. J.
2015, 21, 1468 – 1473; r) Y. Wang, M. Jiang, J.-T. Liu, Org. Chem.
Front. 2015, 2, 542 – 547.
[7] For selected reviews on photoredox catalysis, see: a) T. P. Yoon,
M. A. Ischay, J. Du, Nat. Chem. 2010, 2, 527 – 532; b) J. M. R.
Narayanam, C. R. J. Stephenson, Chem. Soc. Rev. 2011, 40, 102 –
113; c) J. Xuan, W.-J. Xiao, Angew. Chem. Int. Ed. 2012, 51,
6828 – 6838; Angew. Chem. 2012, 124, 6934 – 6944; d) C. K. Prier,
D. A. Rankic, D. W. C. MacMillan, Chem. Rev. 2013, 113, 5322 –
5363; e) D. P. Hari, B. Kçnig, Angew. Chem. Int. Ed. 2013, 52,
4734 – 4743; Angew. Chem. 2013, 125, 4832 – 4842; f) M. Reck-
enthäler, A. G. Griesbeck, Adv. Synth. Catal. 2013, 355, 2727 –
2744; g) M. N. Hopkinson, B. Sahoo, J.-L. Li, F. Glorius, Chem.
Eur. J. 2014, 20, 3874 – 3886; h) T. Koike, M. Akita, Inorg. Chem.
Front. 2014, 1, 562 – 576; i) T. Koike, M. Akita, Top. Catal. 2014,
57, 967 – 974.
Acknowledgements
This work was supported by the JSPS (KAKENHI Grants
26288045, 15K13689, and 15J12072) and the Naito Founda-
tion.
Keywords: alkenes · alkynes · fluorine · photocatalysis ·
trifluoromethylation
How to cite: Angew. Chem. Int. Ed. 2015, 54, 12923–12927
Angew. Chem. 2015, 127, 13115–13119
[1] a) O. Reiser, Angew. Chem. Int. Ed. 2006, 45, 2838 – 2840;
Angew. Chem. 2006, 118, 2904 – 2906; b) A. B. Flynn, W. W.
Ogilvie, Chem. Rev. 2007, 107, 4698 – 4745; c) E.-i. Negishi, Z.
Huang, G. Wang, S. Mohan, C. Wang, H. Hattori, Acc. Chem.
Res. 2008, 41, 1474 – 1485; d) M. Irie, T. Fukaminato, K.
Matsuda, S. Kobatake, Chem. Rev. 2014, 114, 12174 – 12277.
[2] For selected reports of the synthesis of tetrasubstituted alkenes,
see: a) E.-i. Negishi, Y. Zhang, F. E. Cederbaum, M. B. Webb, J.
Org. Chem. 1986, 51, 4082 – 4083; b) M. Hojo, Y. Murakami, H.
Aihara, R. Sakuragi, Y. Baba, A. Hosomi, Angew. Chem. Int. Ed.
2001, 40, 621 – 623; Angew. Chem. 2001, 113, 641 – 643; c) K.
Itami, T. Kamei, J.-i. Yoshida, J. Am. Chem. Soc. 2003, 125,
14670 – 14671; d) C. Zhou, D. E. Emrich, R. C. Larock, Org. Lett.
2003, 5, 1579 – 1582; e) X. Zhang, R. C. Larock, Org. Lett. 2003,
5, 2993 – 2996; f) K. Shibata, T. Satoh, M. Miura, Org. Lett. 2005,
7, 1781 – 1783; g) M. G. Suero, E. D. Bayle, B. S. L. Collins, M. J.
Gaunt, J. Am. Chem. Soc. 2013, 135, 5332 – 5335; h) F. Xue, J.
Zhao, T. S. A. Hor, T. Hayashi, J. Am. Chem. Soc. 2015, 137,
3189 – 3192.
[3] a) M. Shimizu, T. Hiyama, Angew. Chem. Int. Ed. 2005, 44, 214 –
231; Angew. Chem. 2005, 117, 218 – 234; b) A. Rivkin, T.-C.
Chou, S. J. Danishefsky, Angew. Chem. Int. Ed. 2005, 44, 2838 –
2850; Angew. Chem. 2005, 117, 2898 – 2910; c) M. Shimizu, Y.
Takeda, M. Higashi, T. Hiyama, Angew. Chem. Int. Ed. 2009, 48,
3653 – 3656; Angew. Chem. 2009, 121, 3707 – 3710; d) Z. Shi, J.
Davies, S.-H. Jang, W. Kaminsky, A. K.-Y. Jen, Chem. Commun.
2012, 48, 7880 – 7882; e) P. Innocenti, K.-M. J. Cheung, S.
Solanki, C. Mas-Droux, F. Rowan, S. Yeoh, K. Boxall, M.
Westlake, L. Pickard, T. Hardy, J. E. Baxter, G. W. Aherne, R.
Bayliss, A. M. Fry, S. Hoelder, J. Med. Chem. 2012, 55, 3228 –
3241.
[8] a) V. V. Lyalin, V. V. Orda, L. A. Alekseeva, L. M. Yagupolꢀskii,
Zh. Org. Khim. 1984, 20, 115 – 118; b) T. Umemoto, S. Ishihara,
J. Am. Chem. Soc. 1993, 115, 2156 – 2164; c) J.-J. Yang, R. L.
Kirchmeier, J. M. Shreeve, J. Org. Chem. 1998, 63, 2656 – 2660.
[9] a) Y. Yasu, T. Koike, M. Akita, Angew. Chem. Int. Ed. 2012, 51,
9567 – 9571; Angew. Chem. 2012, 124, 9705 – 9709; b) Y. Yasu, T.
Koike, M. Akita, Chem. Commun. 2013, 49, 2037 – 2039; c) Y.
Yasu, T. Koike, M. Akita, Org. Lett. 2013, 15, 2136 – 2139; d) T.
Koike, M. Akita, Synlett 2013, 24, 2492 – 2505; e) Y. Yasu, Y.
Arai, R. Tomita, T. Koike, M. Akita, Org. Lett. 2014, 16, 780 –
783; f) R. Tomita, Y. Yasu, T. Koike, M. Akita, Angew. Chem.
Int. Ed. 2014, 53, 7144 – 7148; Angew. Chem. 2014, 126, 7272 –
7276; g) R. Tomita, Y. Yasu, T. Koike, M. Akita, Beilstein J. Org.
Chem. 2014, 10, 1099 – 1106.
[4] a) G. NØmeth, R. Kapiller-Dezsçfi, G. Lax, G. Simig, Tetrahe-
dron 1996, 52, 12821 – 12830; b) X. Liu, M. Shimizu, T. Hiyama,
Angew. Chem. Int. Ed. 2004, 43, 879 – 882; Angew. Chem. 2004,
116, 897 – 900; c) Y. Takeda, M. Shimizu, T. Hiyama, Angew.
Chem. Int. Ed. 2007, 46, 8659 – 8661; Angew. Chem. 2007, 119,
8813 – 8815; d) T. Konno, Synlett 2014, 1350 – 1370; e) K.
Aikawa, N. Shimizu, K. Honda, Y. Hioki, K. Mikami, Chem.
Sci. 2014, 5, 410 – 415.
[10] The present reaction gave a small amount of 1-phenylprop-1-en-
1-yl trifluoromethanesulfonate, a product of the addition of
TfOH to the alkyne, as a side product in the absence of a base.
[5] For recent reviews on the trifluoromethylation of alkynes, see:
a) S. Barata-Vallejo, B. LantaÇo, A. Postigo, Chem. Eur. J. 2014,
12926 www.angewandte.org
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The addition of a base effectively prevents formation of this side
product.
[11] a) C. Galli, A. Guarnieri, H. Koch, P. Mencarelli, Z. Rappoport,
J. Org. Chem. 1997, 62, 4072 – 4077; b) P. C. Montevecchi, M. L.
Navacchia, Tetrahedron 2000, 56, 9339 – 9342; c) U. Wille, Chem.
Rev. 2013, 113, 813 – 853.
+ 0.80 V vs. [Cp2Fe] compared to MnIII oxidants; see: M. S.
Lowry, J. I. Goldsmith, J. D. Slinker, R. Rohl, R. A. Pascal, Jr.,
G. G. Malliaras, S. Bernhard, Chem. Mater. 2005, 17, 5712 – 5719.
[13] a) T. Katagiri, K. Uneyama, J. Fluorine Chem. 2000, 105, 285 –
293; b) T. Katagiri, S. Yamaji, M. Handa, M. Irie, K. Uneyama,
Chem. Commun. 2001, 2054 – 2055.
[12] The cyclic voltammogram for 1a contained an irreversible
reduction wave at À1.11 V vs. [Cp2Fe], indicating that 1a is
reduced by the photoexcited [IrIII(ppy)2(dtbbpy)]+ (À1.37 V vs.
[Cp2Fe]). Furthermore, the highly oxidizable Ir species
[IrIV(ppy)2(dtbbpy)]2+ exhibits a high oxidation potential at
Received: June 16, 2015
Revised: August 8, 2015
Published online: September 11, 2015
Angew. Chem. Int. Ed. 2015, 54, 12923 –12927
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