Radical C?H Bond Trifluoromethylation of Alkenes by High-Valent Copper(III) Trifluoromethyl Compounds

Article abstract of DOI:10.1002/adsc.201901105

A general and selective method is developed that allows direct vinylic C?H bond trifluoromethylation of 1,1-diarylalkenes by a high-valent copper(III) trifluoromethyl complex, producing biologically active trifluoromethylated alkenes (as well as trifluoro

Full text of DOI:10.1002/adsc.201901105

Title: Radical C-H Bond Trifluoromethylation of Alkenes by High-Valent
Copper(III) Trifluoromethyl Compounds
Authors: Hao-Ran Zhang, Chang Xiao, Song-Lin Zhang, and Xiaoming
Zhang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901105
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10.1002/adsc.201901105
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Radical C-H Bond Trifluoromethylation of Alkenes by High-
Valent Copper(III) Trifluoromethyl Compounds
Hao-Ran Zhanga, Chang Xiaoa, Song-Lin Zhanga* and Xiaoming Zhangb*
a
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122, China. Tel/Fax: 86-551-85917763; E-mail: slzhang@jiangnan.edu.cn
School of Food Science and Technology, Jiangnan University, Wuxi 214122, China. Tel/Fax: 86-551-85917763; E-
b
mail: xmzhang@jiangnan.edu.cn
Received:
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
H trifluoromethylation of simple aliphatic alkenes
allows direct vinylic C-H bond trifluoromethylation of 1,1- with gaseous CF3I.[6a] Qing et al. reported
Abstract. A general and selective method is developed that
diarylalkenes by a high-valent copper(III) trifluoromethyl
complex, producing biologically active trifluoromethylated
alkenes (as well as trifluoromethylated carbocyclic
compounds). This fundamental reactivity of Cu(III)-CF3
compounds has thus far been unknown. The presence of a
tertiary amine is crucial to this reaction, acting as both a
weak base and a single electron transfer (SET) promoter to
abstract the vinylic hydrogen. This method starts from bulk
olefins under cost-effective conditions (without the need
for external noble metal photocatalyst or stoichiometric
amounts of oxidant), and thus is valuable for practical and
sustainable applications.
stereoselective orthogonal access to both E- and Z-
trifluoromethylated styrenes exploiting a different
combination of photocatalyst and CF3 reagents
(Togni’s or Umemoto’s reagent) for the
trifluoromethylation of ortho-amino styrenes.[6b] Loh
et
al.
reported
a
copper-catalyzed
trifluoromethylation of styrenes with CF3SO2Na as
the CF3 reagent in the presence of excess peroxide as
the oxidant.[6c] Despite these elegant advances, either
expensive noble metal photocatalyst assisted by
photo-irradiation or excess external oxidants are
required wherein substrates containing redox
sensitive groups are not compatible, thus limiting the
generality and wide application of the methods.
Therefore, alternative methods are desirable for C-H
trifluoromethylation of simple alkenes.
Keywords: Trifluoromethylation; Cu(III)-CF3 compounds;
C-H activation; Trifluoromethylated alkenes; Tertiary
amine
Trifluoromethylated alkenes are an important
structural motif occurring in a range of biologically
active compounds. Therefore, great efforts have been
stimulated to develop efficient methods for
constructing the vinyl-CF3 bond. Despite the
impressive progress achieved for trifluoromethylation
of vinyl electrophiles[1] (e.g. halides, tosylates)
(Scheme 1a) and nucleophiles (e.g. vinyl boronic
acids,[2] carboxylic acids[3]) (Scheme 1b), and hydro-
trifluoromethylation of alkynes,[4] direct alkenes C-H
bond trifluoromethylation is much less developed.
Considering the ready availability and bulkness of
olefins, there is great demand for developing direct
alkene trifluoromethylation methods which are more
straightforward and sustainable without the need for
any prefunctionalization.
Very recently, activated alkenes functionalized
with strong electron-withdrawing or -donating groups,
typically acrylamides and enamides, have been
studied as substrates for C-H trifluoromethylation
(Scheme 1c).[5] However, general methods for simple
unactivated alkenes remain very rare.[6] Notably, Cho
et al. reported a photocatalyst induced E-selective C-
Scheme 1. Methods for vinyl-CF3 bond formation.
1
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10.1002/adsc.201901105
Advanced Synthesis & Catalysis
In continuation of our recent interest in developing
diazabicyclo[2.2.2]octane), pyridine, DBN (1,5-
Cu(III)-CF3 chemistry,[7-9] we report herein the direct
C-H bond trifluoromethylation of 1,1-diarylalkenes
diazabicyclo[4.3.0]non-5-ene)
and
dimethylaminopyridine (DMAP) (entries 5-9). This
led to the finding that DBU is a suitable organic base
compromising the efficiency and the selectivity. The
organic base is crucial to this reaction; without it, the
reaction could not give the desired 3a or 4a in
significant amounts (entry 10).
with
a
Cu(III)-CF3 compound under simple
conditions (Scheme 1d), without the need for either
photocatalyst/photoirradiation or external oxidant.
The Cu(III)-CF3 compound offers CF3 radical to
selectively add to the alkene double bond, giving a
crucial carbon-centred tertiary radical that is
sufficiently stabilized with two aryl and an alkyl
groups (Scheme 1d). The presence of DBU facilitates
single electron transfer (SET) and hydrogen-
abstraction events to occur with the tertiary radical to
give the desired trifluoromethylated alkenes. Notably,
such trifluoromethylated alkenes with two vicinal
aryl groups are structural motif with important
biological activity (e.g., anti-breast cancer drug
panomifene), but are very challenging to prepare.[8c,10]
Table 1. Optimization of reaction conditions.a)
a)
Reaction conditions: 1 (0.2 mmol), 2a (0.2 mmol), base
(0.6 mmol), 4,4’-difluorobiphenyl (0.2 mmol, internal
standard) and solvent (2 mL) stirred under N2. b) 19F NMR
yield.
Scheme
2.
Substrate
scope
of
this
C-H
trifluoromethylation reaction. Both 19F NMR and isolated
yields (in parentheses) are provided if available. The E/Z
ratios were determined by 19F NMR analysis of the crude
reaction mixture.
Our study began by reaction of (phen)CuIII(CF3)3
(1) (phen denotes phenanthroline)[7b] and 1,1-
diphenylethylene (2a) under N2 condition. With CsF
or NaOtBu as the additive, the conversion of 2a is
low to give a mixture containing majorly the desired
3a and byproduct 4a arising from alkene
hydrotrifluoromethylation (Table 1, entries 1-
3).[6f,11,12] It was delighting to see that when an
organic base Et3N was used, the conversion of 2a was
greatly improved to almost quantitative, albeit the
selectivity was low to give a mixture of 3a and 4a in
a 1:1 ratio (entry 4). To improve the selectivity, other
organic bases were then examined including DBU
(1,8-diazabicyclo[5.4.0]undec-7-ene), DABCO (1,4-
With the optimized conditions in hand (Table 1,
entry 5), a range of 1,1-diarylalkenes were studied to
show the synthetic compatibility (Scheme 2).[13]
Functional groups such as alkyl, phenyl, methoxy,
amino, fluoro, chloro, nitro and cyano are tolerated
on the phenyl ring. Heteroarenes, such as pyridine
and thiophene are also compatible to produce 3o and
3p. It is interesting to see that 2q with the two phenyl
rings tethered by an ethylene bridge, was able to
2
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10.1002/adsc.201901105
Advanced Synthesis & Catalysis
produce the desired 3q in a 21% isolated yield.
Generally, symmetric 1,1-diarylalkenes give a single
dominant product (3a-3f) while unsymmetrical 1,1-
diarylalkenes give a mixture of E/Z isomers in a ratio
ranging from 1:1 to 6:1. The 2r with a phenyl and a
methyl at the alkenes gave a complex mixture that
was inseparable containing the desired 3r. This result
also highlights the importance of two aryl/heteroaryl
groups for stabilizing the potential tertiary radical
(Scheme 1d) and thus achieving high efficiency and
selectivity.
Crucial electronic effects of aryl/heteroaryl groups
have been observed in two aspects. First, electron-
donating groups improve the reaction efficiency with
higher yields while substrates with electron-
withdrawing groups react sluggishly. This is obvious
by the reactivity order for the preparation of 3b-d >
3a > 3e-f, and the better reactivity toward 3p with an
electron-rich thiophene than to 3o with an electron-
poor pyridine. Second, the electronic property of the
aryl/heteroaryl groups has a significant influence on
the diastereoselectivity for unsymmetrical 1,1-
diarylalkenes. The isomeric products with the CF3
trans to the more electron-rich aryl group are more
favored than the isomers with cis-CF3. The extent of
this diastereo bias largely depends on the electronic
difference between the two geminal aryl groups, and
becomes more predominant for substrates with two
electronically distinct aryl groups, such as 3m, 3o and
3p, as reflected by the higher ratios of
diastereoisomers.
(2,2,6,6-tetramethylpiperidyloxy) (Scheme 4a). This
led to a low NMR yield of only 9% for 3a, implying
that radical species might be involved in the reaction
course. To prove the intermediacy of radical species,
radical clock experiment using α-cyclopropyl styrene
2t as the substrate was conducted under the optimized
conditions with the expectation of the occurrence of
ring opening of the cyclopropyl group. Indeed, a
trifluoromethylated dihydronaphthalene
5
was
obtained in 24% NMR yield as yellow oil (Scheme
4b). The formation of 5 is rationalized by the ring
opening of cyclopropyl group in the initial
intermediate III, giving a homoallylic radical IV
which then proceeds via aromatic radical substitution
to give 5. This result of radical clock experiment, the
radical trapping study (Scheme 4a), the formation of
indene 3s using 1,3-dienes in Scheme 3, and the
observation of byproduct of type 4a in most of our
studies, provide compelling evidence supporting the
involvement of radical trifluoromethylation of the
alkene double bond with the Cu(III)-CF3 compound.
Scheme 4. Radical scavenger and radical clock
experiments.
Scheme 3. Application to the synthesis of
trifluoromethylated indene.
As a further demonstration of the synthetic
potential, reaction of 1,3-diene 2s with Cu(III)-CF3
compound 1’ can efficiently give trifluoromethylated
indene 3s in a high yield of 84% from a formal aryl-
trifluoromethylation (Scheme 3). A radical species I
is proposed, generated from trifluoromethylation of
the right double bond, to participate in aromatic
radical attack to produce radical species II.
Aromatizing hydrogen abstraction from II would
give favorably indene 3s. 2,2,2-Trifluoroethylated
indenes are potentially an important motif for
electric/optic materials design.
Radical trapping study was conducted in the
presence of two equivalents of additional TEMPO
Scheme 5. Plausible mechanism.
3
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10.1002/adsc.201901105
Advanced Synthesis & Catalysis
As a result, a plausible mechanism involves the
Ikeda, S. Nomura, A. Tarui, K. Sato and A. Ando,
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H. S. Krishnan, P. V. Jog, A. P. Iyer and G. A. Olah,
Org. lett. 2012, 14, 1146-1149; e) J.-j. Ma, W.-b. Yi,
G.-p. Lu and C. Cai, Adv. Synth. Catal. 2015, 357,
3447-3452; f) P. V. Ramachandran and W.
Mitsuhashi, Org. Lett. 2015, 17, 1252-1255; g) P.
Huang, Y. Li, X. Fu, R. Zhang, K. Jin, W. Wang and
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P. Midya, J. Rana, T. Abraham, B. Aswin and E.
Balaraman, Chem. Commun. 2017, 53, 6760-6763.
initial addition of a CF3 radical to the double bond to
form a key tertiary carbon radical A (Scheme 5).[14]
The fate of A depends largely on the reaction
conditions. In the presence of DBU, this intermediate
tends to undergo SET to give tertiary carbocation
B,[8,15] which is then deprotonated by DBU to produce
the desired product 3.[16] The released ammonium and
-
the phenCu(I)(CF3)2 anion generated from SET
process are proposed to participate in further
exothermic decomposition, for example, pathways
involving the generation of nitrogen ylide[17] as well
as R3N•HF,[8e] which provides strong thermodynamic
driving force for the formation of product 3.
Alternatively, without the efficient assistance of a
tertiary amine, intermediate A might also take a
hydrogen from the environment (e.g. the solvent) to
produce the hydrotrifluoromethylation byproduct 4a.
In the cases of using 2s and 2t as the substrates, the
initial radical A from radical trifluoromethylation (i.e.,
intermediate I and III) can cyclize with neighbouring
aryl ring (Scheme 3) or rearrange to another radical
species driven by subsequent cyclization to a stable
cyclic compounds (Scheme 4b).
[2] Vinyl-Boron, see: a) T. Liu and Q. Shen, Org. Lett.
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In conclusion, this study describes a direct C-H
bond trifluoromethylation of simple alkenes by
Cu(III)-CF3 compound in synergy of a tertiary amine.
This reaction occurs under simple and cost-effective
conditions, without the need for external oxidant or
photocatalyst. It is attractive for preparing 1,1-diaryl
trifluoromethylated alkenes and trifluoromethylated
carbocycles from simple alkenes. This study
consolidates one fundamental reactivity property of
crucial Cu(III)-CF3 compounds that is currently
unknown.
[4] Hydro-trifluoromethylation of alkynes, see: a) S.
Mizuta, S. Verhoog, K. M. Engle, T. Khotavivattana,
M. O’Duill, K. Wheelhouse, G. Rassias, M.
Médebielle and V. Gouverneur, J. Am. Chem. Soc.
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Experimental Section
In an oven-dried 25-mL Schlenk tube equipped with a
magnetic stir bar were charged with phenCu(CF3)3 (1) (90
mg, 0.2 mmol) and 1,1-disubstituted alkene (2) (0.2 mmol).
The Schlenk tube was evacuated and refilled with dry
nitrogen. A DCE (2 mL) solution of DBU (0.6 mmol) was
then added by syringe. The contents in the tube were
vigorously stirred for 18 hours at 90℃ (seated in an oil
bath). The reaction mixture was then allowed to cool to
room temperature, diluted with CH2Cl2 and then filtrated.
The filtrate was concentrated by vacuum evaporation. The
resulting residual was purified by preparative TLC
(petroleum ether/ethyl acetate = 30:1 (v/v)) to provide
trifluoromethylated alkene products 3.
[5] For activated alkenes, see: a) T. Besset, D. Cahard
and X. Pannecoucke, J. Org. Chem. 2013, 79, 413-
418; b) Z. Fang, Y. Ning, P. Mi, P. Liao and X. Bi,
Org. Lett.,2014, 16, 1522-1525; c) C. Feng and T.-P.
Loh, Chem. Sci. 2012, 3, 3458-3462; d) C. Feng and
T.-P. Loh, Angew. Chem., Int. Ed. 2013, 52, 12414-
12417; e) N. O. Ilchenko, P. G. Janson and K. J.
Szabó, Chem. Commun. 2013, 49, 6614-6616; f) H.
Jiang, W. Huang, Y. Yu, S. Yi, J. Li and W. Wu,
Chem. Commun. 2017, 53, 7473-7476; g) L. Li, J.-Y.
Guo, X.-G. Liu, S. Chen, Y. Wang, B. Tan and X.-Y.
Liu, Org. Lett. 2014, 16, 6032-6035; h) X. Zhang, P.
Huang, Y. Li and C. Duan, Org. Biomol. Chem. 2015,
13, 10917-10922; i) L.-H. Wu, J.-K. Cheng, L. Shen,
Z.-L. Shen and T.-P. Loh, Adv. Synth. Catal. 2018,
360, 3894-3899.
Acknowledgements
This study was supported by the National Natural Science
Foundation of China (No. 21472068).
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4
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10.1002/adsc.201901105
Advanced Synthesis & Catalysis
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5
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Advanced Synthesis & Catalysis
COMMUNICATION
Radical C-H Bond Trifluoromethylation of Alkenes
by High-Valent Copper(III) Trifluoromethyl
Compounds
Adv. Synth. Catal. 2019, Volume, Page – Page
H.-R. Zhang, C. Xiao, S.-L. Zhang* and X. Zhang*
6
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