Copper-catalyzed intramolecular carbotrifluoromethylation of alkynes for the construction of trifluoromethylated heterocycles

Article abstract of DOI:10.1002/chem.201404386

A mild and efficient copper-catalyzed intramolecular carbotrifluoromethylation of alkynes has been achieved in the presence of Togni reagent as trifluoromethylating reagent. The reaction tolerates a range of substrates to give a group of trifluoromethylated heterocycles with high selectivities. A plausible mechanism was proposed on the basis of experimental results. And the Togni award goes to Copper-catalyzed intramolecular carbotrifluoromethylation of alkynes is carried out with a Togni reagent as the trifluoromethylating reagent (see scheme). Various trifluoromethylated heterocycles are synthesized in moderate to good yields. Moreover, a range of common functional groups is tolerated under the reaction conditions.

Full text of DOI:10.1002/chem.201404386

DOI: 10.1002/chem.201404386
Communication
&
Heterocycles
Copper-Catalyzed Intramolecular Carbotrifluoromethylation of
Alkynes for the Construction of Trifluoromethylated Heterocycles
Yanan Wang, Min Jiang, and Jin-Tao Liu*[a]
Abstract: A mild and efficient copper-catalyzed intramo-
lecular carbotrifluoromethylation of alkynes has been ach-
ieved in the presence of Togni reagent as trifluorometh-
ylating reagent. The reaction tolerates a range of sub-
strates to give a group of trifluoromethylated heterocycles
with high selectivities. A plausible mechanism was pro-
posed on the basis of experimental results.
The introduction of trifluoromethyl groups into organic com-
pounds may profoundly change their properties.[1] Owing to
their high stabilities, hydrophobicities, metabolic activities, and
bioavailabilities, trifluoromethylated compounds have gained
more and more interest in a wide range of applications, espe-
cially in pharmaceuticals and agrochemicals.[2] Accordingly, the
development of efficient methods for the preparation of tri-
fluoromethylated compounds is of great importance. In the
past few years, various transition metal-mediated or -catalyzed
trifluoromethylation reactions have been developed by many
Scheme 1. a) Reported carbotrifluoromethylation of alkenes[7–9]; b) Carbotri-
fluoromethylation of alkynes (this work).
independent research groups to synthesize various CF3-con-
taining compounds.[3]
The synthesis of heterocyclic compounds catalyzed by tran-
sition metal complexes has also attracted significant attention.
Transition-metal-catalyzed reactions can directly construct
complicated molecules from readily accessible starting materi-
als under mild conditions. Recently, many groups have engag-
ed in the trifluoromethylation of alkenes coupled with intramo-
lecular reactions. For example, Buchwald and co-workers[4]
have reported the intramolecular oxytrifluoromethylation of al-
kenes. Sodeoka’s group[5] developed successfully a method of
intramolecular aminotrifluomethylation of alkenes. Liang and
co-workers[6] have reported the trifluoromethylation of alkenes
involving oximes. The groups of Sodeoka,[7] Zhu,[8] and Shi[9]
studied the carbotrifluoromethylation of alkenes independent-
ly (Scheme 1a). Our group was also interested in the trifluoro-
methylation of unsaturated alkanes and have developed an ef-
ficient copper-catalyzed oxytrifluoromethylation reaction of al-
lenes.[10] However, to our knowledge, studies of carbotrifluoro-
methylation of alkynes is quite rare.[11] We report herein the
copper-catalyzed intramolecular carbotrifluoromethylation of
alkynes for the construction of trifluoromethylated heterocy-
cles (Scheme 1b).
In our preliminary experiments, propargyl phenylether (1a)
was chosen as the substrate. The reaction was carried out with
1 equivalent of Togni reagent II (2a), 1.5 equivalents of 1a and
20 mol% of CuBr·Me2S in dichloroethane (DCE) at 608C. How-
ever, 19F NMR spectroscopy showed that the reaction was com-
plicated and no annular product was isolated (Scheme 2).
[a] Y. Wang, Dr. M. Jiang, Prof. J.-T. Liu
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of Organic Chemistry, Chinese Academy of Science
345 Lingling Road, Shanghai 200032 (P. R. China)
Fax: (+86)21-64166128
E-mail: jtliu@sioc.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404386.
Scheme 2. The reactions of 1a and 1a’ in the presence of Togni reagent II.
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Using 1a’ with a methyl group at the propargylic position as
substrate, a small amount of hydrotrifluoromethylation prod-
uct was formed, as shown by 19F NMR and GC-MS spectra of
the crude reaction mixture (see the Supporting Information).
Considering the Thorpe–Ingold effect,[12] which demon-
strates that increasing the size of two substituents on a tetrahe-
dral center leads to enhanced reactions between parts of the
other two substituents in the cyclization reaction, we prepared
alkyne 1b with two methyl substituents at the propargyl posi-
tion and continued to study its carbotrifluoromethylation. Grat-
ifyingly, the corresponding five-membered cyclic products 3b
were obtained as a mixture of two isomers in 61% overall
yield with a ratio of 11:1 under the same conditions (Table 1,
total yield of products E- and Z-3b was slightly lowered. Ume-
moto’s reagent (2c) failed to give any cyclic product with full
recovery of the starting materials (Table 1, entries 2 and 3).
Next, we focused on the catalyst and solvent effects. However,
various copper salt catalysts, such as CuCl, CuI, [(CH3CN)4Cu]-
[PF6], CuBr, Cu(OTf)2, and CuBr2, could not improve the yield of
3b in DCE (Table 1, entries 4–9). Using dichloromethane (DCM)
as solvent, the CuI-catalyzed reaction afforded 3b in 63% over-
all yield with high selectivity, whereas the total yield was only
41% with CuBr·Me2S (Table 1, entries 10 and 11). Therefore, CuI
was chosen as the catalyst to further study the solvent effect.
Much lower yields were obtained with N-methyl-2-pyrrolidone
(NMP), acetonitrile, dioxane or dimethyl sulfoxide (DMSO) as
solvent (Table 1, entries 12–15). Ethyl acetate and tert-butyl al-
cohol were also less effective for the reaction (Table 1, en-
tries 16 and 17). Therefore, DCM was identified as the best sol-
vent for this reaction among the solvents tested. Further inves-
tigation showed that the catalyst loading could be lowered to
10 mol% without any loss in yield or stereoselectivity (Table 1,
entry 18). Carrying out the reaction in DCM under reflux and
increasing the amount of 1b to 2.0 equivalents gave isomers
E- and Z-3b in 67% combined yield (Table 1, entry 19). At-
tempts to use PPh3 or PCy3 as an additive did not further im-
prove the yield (Table 1, entries 20 and 21). Surprisingly, the
total yield of E- and Z-3b was improved to 74% in the pres-
ence of B2Pin2 (Table 1, entry 22). Control experiments indicat-
ed that the copper salt was important for this reaction
(Table 1, entries 23 and 24). Thus, the optimal conditions were
set to 10 mol% of CuI, 2.0 equivalents of 1b, and 10 mol% of
B2Pin2 in DCM under reflux.
Table 1. Optimization of the reaction conditions.[a]
Entry
[Cu]
Additive
Solvent
E-3b/Z-3b
Yield [%][b]
1
CuBr·Me2S
CuBr·Me2S
CuBr·Me2S
CuCl
—
—
—
DCE
DCE
DCE
56:5
53:5
NR
2[c]
3[d]
4
—
DCE
41:4
47:5
47:5
54:5
38:4
44:4
37:4
57:6
13:<5
14:<5
19:<5
37:4
49:5
50:5
58:6
61:6
24:<5
52:5
67:7
NR
5
6
7
8
9
10
11
12
13
14
15
16
CuI
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
DCE
DCE
DCE
DCE
[(CH3CN)4Cu][PF6]
CuBr
Cu(OTf)2
CuBr2
CuBr·Me2S
CuI
With the optimized reaction conditions in hand, we studied
the scope of alkyne substrates for this copper-catalyzed carbo-
trifluoromethylation reaction. As shown in Table 2, a range of
terminal alkynes 1 with different substituents in the benzene
ring could react with 2a and give the expected carbotrifluoro-
methylation products in moderate to good yields. Both elec-
tron-rich and electron-deficient alkynes are compatible with
DCE
DCM
DCM
NMP
CH3CN
dioxane
DMSO
EA
tBuOH
DCM
DCM
DCM
DCM
DCM
DCM
DCM
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
—
17
18[e]
19[e,f,g]
20[e,f,g]
21[e,f,g]
22[e,f,g]
23[g]
24[g]
PPh3
PCy3
B2Pin2
B2Pin2
—
—
NR
[a] Reaction conditions: 1b (0.15 mmol), 2a (0.1 mmol), copper catalyst
(0.02 mmol), solvent (1 mL), 608C, 4 h, under N2; [b] determined by
19F NMR spectroscopy using hexafluorobenzene as internal standard;
[c] Togni reagent I was used as trifluoromethylating reagent; [d] Umemo-
to reagent was used as trifluoromethylating reagent; [e] 0.1 equivalents
of CuI were used; [f] 0.2 mmol of 1b was used; [g] the reaction was car-
ried out in DCM at reflux.
entry 1). The stereochemistry of the newly formed double
bond in the major product was established as E by NOE experi-
ments. Inspired by the result, we screened different conditions
to find a suitable protocol for the selective formation of cycli-
zation products. Using Togni reagent I (2b) instead of 2a, the
Figure 1. ORTEP representation of major isomer 3k.
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Communication
protected amine substrate was used, the reaction could
also occur to give the corresponding cyclic product, but
the yield was kind of lower (3t, 34%), and 6-endo carbo-
cyclization product was formed as the major product
(see the Supporting Information). It is worth mentioning
that the reaction of the substrate with a methylene link-
age took place successfully at 808C to give the expect-
ed products 3u in 43% yield, along with the hydrotri-
fluoromethylation product in moderate yield (see the
Supporting Information). Furthermore, the X-ray crystal
structure of major isomer 3k was determined (Figure 1;
CIF data are presented in the Supporting Informa-
tion).[13]
Table 2. Scope of the intramolecular carbotrifluoromethylation of al-
kynes.[a]
Interestingly, when a phenyl group was introduced at
the terminal position of the alkyne, six-membered cyclic
products were obtained under similar conditions. Using
DCM as solvent, the carbotrifluoromethylation reaction
of O-tethered alkyne 4a without two methyl groups at
the propargyl position took place smoothly in a sealed
tube at 808C to give the unexpected 2H-chromene de-
rivative 5a, isolated in 58% yield, and the 5-exo carbo-
trifluoromethylation product was not detected. Similar
transformation could be achieved with N-tethered
alkyne 4b, affording the corresponding 1,2-dihydroqui-
noline derivative 5b.
However, if the substituent at the terminal position of
alkyne was methyl, 2H-chromene 5c was obtained in
30% yield and less than 10% yield of five-membered
cyclic product 5c’ was also detected (Scheme 3).
To gain insight into the reaction mechanism, we per-
formed the reaction in the presence of 1.5 equivalents
of 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO).
19F NMR spectroscopy showed that the reaction was in-
hibited and the radical trapping product TEMPO-CF3
was obtained in 91% yield (Scheme 4). However, when
another radical scavenger, 2,6-di-tert-butyl-4-methylphe-
nol (BHT, 2.0 equiv), was added, isomers E- and Z-3a
were obtained in 67% total yield. We hypothesized that
TEMPO might destroy Togni’s reagent under the reac-
tion conditions.[14]
[a] Reaction conditions:
1 (0.2 mmol), 2a (0.1 mmol), CuI (0.01 mmol), B2Pin2
(0.01 mmol), DCM (1 mL), reflux, 4 h, under N2, yield of isolated product (yield de-
termined by 19F NMR spectroscopy using hexafluorobenzene as internal standard is
given in parentheses); [b] reaction time was 7 h; [c] reaction time was 8 h; [d] reac-
tion time was 11 h; [e] reaction time was 17 h; [f] the reaction was conducted in
a sealed tube at 808C for 12 h.
Based on the experimental results, a plausible mecha-
nism is proposed (Scheme 5). Initially, the interaction of
the reaction conditions. The substrates bearing methoxy or
aryl substituents gave the cyclic products in moderate yields.
Halide-substituted substrates also reacted well to give the
cyclic products in good yields. In the case of an iodo-substitut-
ed substrate, the iodo substituent remained intact under the
reaction conditions, and the corresponding products were ob-
tained in 70% overall yield with an E/Z ratio of 10:1. The sub-
strates containing an electron-deficient substituent such as
para-nitro and para-cyano afforded the corresponding cyclic
products in higher yields. Moderate to good yields were also
achieved when the two methyl substituents at the propargylic
position were replaced by two ethyl (3l), cyclopentyl (3m), or
cyclohexyl (3n) groups. Furthermore, many functional groups,
such as hydroxy, ketone, ester, amide, and aldehyde, were tol-
erated in this carbotrifluoromethylation reaction. When an un-
Scheme 3. The reactions of alkynes 4 with Togni reagent II (2a).
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In conclusion, the copper-cat-
alyzed intramolecular carbotri-
fluoromethylation of alkynes was
achieved under mild conditions.
The reaction tolerates a wide
range of functional groups, pro-
viding a practical method for the
synthesis of various trifluorome-
thylated heterocycles. Further
studies regarding the scope,
mechanism, and synthetic appli-
cation of this reaction are in
progress in our laboratory.
Scheme 4. Reaction in the presence of TEMPO or BHT
Experimental Section
General procedure for copper-cata-
lyzed intramolecular carbotrifluoro-
methylation of alkynes: To a reactor charged with Togni reagent II
(0.1 mmol), CuI (0.01 mmol), B2Pin2 (0.01 mmol), and dichlorome-
thane (1 mL) was added alkyne (0.2 mmol) under N2 atmosphere.
The mixture was stirred under reflux and monitored by TLC. After
the completion of the reaction, the solvent was removed under re-
duced pressure and the residue was purified by column chroma-
tography to give the product (petroleum ether).
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (No. 21172243) is gratefully acknowledged.
Keywords: alkynes
· carbotrifluoromethylation · copper ·
heterocycles · homogeneous catalysis
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Published online on October 13, 2014
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