Direct Triflylation of Benzylic C—H Bonds with Pyridine as a Directing Group

Article abstract of DOI:10.1002/cjoc.201600826

The first example of benzylic C—H triflylation was accomplished with pyridine as a directing group. The reaction of various 2-benzylpyridines and (CF₃SO₂)₂O in the presence of NEt₃ in CH₂Cl₂ proceeded smoothly to afford the corresponding benzyl triflones in moderate to high yields.

Full text of DOI:10.1002/cjoc.201600826

COMMUNICATION
DOI: 10.1002/cjoc.201600826
Direct Triflylation of Benzylic C－H Bonds with Pyridine as a
Directing Group
Jun Yang,a Juanjuan Hu,a Yangen Huang,a Xiuhua Xu,*,b and Fengling Qing*,a,b
a College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620, China
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Shanghai 200032, China
The first example of benzylic C－H triflylation was accomplished with pyridine as a directing group. The reac-
tion of various 2-benzylpyridines and (CF3SO2)2O in the presence of NEt3 in CH2Cl2 proceeded smoothly to afford
the corresponding benzyl triflones in moderate to high yields.
Keywords triflylation, C－H functionalization, benzyl triflone, pyridine
Introduction
renes with NaSO2CF3.[9b] Herein, we disclose a direct
triflylation of benzylic C－H bonds for the preparation
of benzyl triflones (Scheme 1e).
Benzyl triflones constitute an important class of
compounds found in bioactive compounds[1] and cata-
lysts,[2] probably due to the strong electron-withdrawing
ability and high lipophilicity of the trifluoromethane-
sulfonyl (triflyl, SO2CF3, Tf) group.[3] In particular, they
are used as versatile synthetic intermediates in the prep-
aration of CF3SO2-containing compounds[4] and non-
fluorinated compounds.[5] Consequently, a number of
methods have been developed for the preparation of
benzyl triflones.[6] The representative methods include
nucleophilic triflylation of benzyl halides (Scheme 1a)[7]
and electrophilic triflylation of benzyl metal reagents
(Scheme 1b).[8] However, these methods suffer from the
need of prefunctionalization of benzylic substrates and/
or the low tolerance of functional groups. Alternatively,
benzyl triflones could also be prepared by addition reac-
tion of styrenes (Scheme 1c)[9] or oxidation of benzyl
trifluoromethyl sulfides (Scheme 1d).[10]
Scheme 1 Methods for the preparation of benzyl triflones
Previous work
- SO2CF3
R
(CF3SO2)2O
(b)
R
R
(a)
Ar
X
Ar
Ar
M
R
Ar
SO2CF3
(d)
(c)
oxidation
addition
SO2CF3
or
SCF3
Ar
Ar
This work
DG
DG
+ (CF3SO2)2O
(e)
Ar
H
Ar
SO2CF3
DG = pyridine
Recently, direct C－H functionalization reactions
have been recognized as atom- and step-economical
methods in organic synthesis.[11] By using these strate-
gies, various fluorinated compounds have been synthe-
sized through C－H fluorination,[12] C－H trifluoro-
methylation,[13] or C－H trifluoromethylthiolation.[14]
However, the analogous C－H triflylation reactions re-
main largely underdeveloped. Recently, Li and co-
workers reported a pioneering study on Rhodium-cata-
lyzed C－H triflylation of arenes with NaSO2CF3.[15]
This reaction proceeds by initial Rh-catalyzed C－H
hyperiodination followed by a chemoselective nucleo-
philic triflylation. Very recently, we developed an io-
dine-mediated regioselective C－H triflylation of sty-
Experimental
A glass tube equipped with a magnetic stirring bar
was charged with benzylpyridine (1.0 mmol),
Cu2(OH)2CO3•H2O (67.8 mg, 0.2 mmol), Et3N (303.6
mg, 3.0 mmol) in dry CH2Cl2 (10.0 mL), (CF3SO2)2O
(564.3 mg, 2.0 mmol) was added slowly under nitrogen
atmosphere at 0 ℃. The reaction mixture was then
slowly warmed to room temperature and stirred over-
night. After dilution with Et2O, the organic layer was
washed with H2O and brine, dried over anhydrous
Na2SO4 and concentrated under reduced pressure. The
residue was purified by column chromatography on sil-
ica gel to give the corresponding product.
*
E-mail: xuxiuhua@sioc.ac.cn, flq@mail.sioc.ac.cn; Tel.: 0086-021-54925187, Fax: 0086-021-64166128
Received November 23, 2016; accepted December 28, 2016; published online XXXX, 2017.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600826 or from the author.
In Memory of Professor Enze Min.
Chin. J. Chem. 2017, XX, 1—4
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1

Yang et al.
Table 1 Optimization of reaction conditionsa
COMMUNICATION
Results and Discussion
In 2015, Kuninobu and Kanai reported a benzylic
C－H perfluoroalkylation using N-oxide/BF2Rf com-
plexes (Rf＝CF3, A, Scheme 2) as substrates.[13f] The
reaction mechanism involves the deprotonation to form
enamine-type intermediate B and nucleophilic attack of
CF3 group of the enamine moiety. Inspired by this ele-
gant work and on the basis of our experience in the mi-
gration reactions of triflyl group,[16] we speculated that
the reaction of pyridines with triflylating reagents gave
the corresponding complexes E,[17] which might under-
go an intramolecular rearrangement to form the trifly-
lated products F.
Entry Catalyst
Base
─
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Yieldb/%
0
1
2
3
4
5
6
7
8
9
10
－
－
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
35
FeCl3
46
ZnCl2
45
Scheme 2 C—H trifluoromethylation and triflylation
MnCl2
CuSO4
Cu2O
41
50
49
CuSO4•5H2O
Cu(OAc)2
57
44
Cu2(OH)2CO3•H2O NEt3
75
11c Cu2(OH)2CO3•H2O NEt3
12d Cu2(OH)2CO3•H2O NEt3
63
54
13
14
15
16
17
Cu2(OH)2CO3•H2O DBU
Trace
4
Cu2(OH)2CO3•H2O NaHCO3 CH2Cl2
Cu2(OH)2CO3•H2O NEt3
Cu2(OH)2CO3•H2O NEt3
Cu2(OH)2CO3•H2O NEt3
Toluene
CH3CN
48
30
CH2ClCH2Cl 42
18e Cu2(OH)2CO3•H2O NEt3
CH2Cl2
0
a Reaction conditions: 1a (0.20 mmol), (CF3SO2)2O (0.40 mmol),
catalyst (0.04 mmol), base (0.60 mmol), solvent (2.0 mL), under
N2, r.t., overnight. b Yields determined by 19F NMR spectroscopy
using trifluoromethylbenzene as an internal standard. c Cu2(OH)2-
CO3•H2O (0.02 mmol). d Cu2(OH)2CO3•H2O (0.20 mmol). e CF3-
SO2Cl (0.40 mmol) was used as the SO2CF3 source.
To test our hypothesis, we chose 2-benzylpyridine
(1a) as a model substrate to investigate the C－H trifly-
lation (Table 1). The reaction of 1a and (CF3SO2)2O in
CH2Cl2 gave a new product, probably the pyridinium
salt (Entry 1).[17] To our delight, the desired product 2a
was observed in 35% yield when NEt3 was added to the
reaction mixture (Entry 2). Inspired by our previous
work on C－H triflylation,[15] we then explored the ef-
fect of catalytic metal salt on this reaction. The addition
of different metal salts, including FeCl3, ZnCl2, MnCl2,
and a series of copper salts resulted in higher yields
(Entries 3－10), although the exact role of the metal salt
in this reaction remains unclear. The highest yield of 2a
was achieved in the presence of catalytic Cu2(OH)2CO3•
H2O (Entry 10). Neither increasing nor decreasing the
amount of Cu2(OH)2CO3•H2O could improve the yield
(Entries 11 and 12). Further screening of the bases
(DBU and NaHCO3) and solvents (toluene, CH3CN, and
CH2ClCH2Cl) showed that NEt3 was the best choice of
base and CH2Cl2 was the best choice of solvent (Entries
13－17). It was noteworthy that none of the desired
product was detected when CF3SO2Cl was used as the
SO2CF3 source (Entry 18). Probably the weak nucleo-
philicity of the chloride anion hindered this transfor-
mation. Moreover, the analogous reactions of 1a with
(CF3CO)2O or Bz2O did not proceed.
When pyridine was switched to other analogues, in-
cluding pyridine derivatives with substituents (Me,
OMe, F, and Br) at different positions, pyrimidine, ben-
zo[d]oxazole, benzo[d]thiazole, and imine, the C—H
triflylation took place in low yields (Scheme 3). These
results indicated that both of the steric and electronic
effects were crucial to this reaction. Thus, we focused
on the triflylation of benzylic C－H bonds with pyridine
as a directing group. As shown in Table 2, a variety of
2-benzylpyridines 1 with substituents on the benzyl ring
were investigated. Substrates 1b－1d bearing ortho-
methyl, meta-methyl, or para-methyl afforded the cor-
responding products 2b－2d in 40%, 54%, and 72%
yields, respectively. These results once again showed
the effect of steric hindrance on the yield. Both of elec-
tron-donating and -withdrawing groups, such as alkyl
(1e), aryl (1f), ester (1j), and nitrile (1k), were well tol-
erated in the reaction. It was noteworthy that substrates
1g－1i bearing aryl halides could be transformed to the
corresponding products 2g－2i in moderate yields, thus
providing opportunities for additional transformations.
2
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Chin. J. Chem. 2017, XX, 1—4

Direct Triflylation of Benzylic C－H Bonds with Pyridine as a Directing Group
Scheme 3 Screening of other directing groups
product was observed. The triflylation of substrates
bearing another substituent at the benzylic position
failed to give the corresponding products. Finally, the
structure of product 2 was confirmed by X-ray crystal-
lographic analysis of compound 2e (see the supporting
information).
Although the detailed mechanism of this reaction as
well as the precise role of Cu2(OH)2CO3•H2O remains
unclear. We proposed a plausible mechanism based on
the previous work (Scheme 4).[13f,16,17] The reaction of
2-benzylpyridine (1) with (CF3SO2)2O gave the corre-
sponding triflylpyridinium triflate G, which then un-
derwent deprotonation in the presence of a base to gen-
erate enamine intermediate H. Finally, the copper-as-
sisted migration of the triflyl group in intermediate H
afforded the desired product 2.
Scheme 4 Proposed reaction mechanism
Table 2 Triflylation of benzylic C－H bonds with pyridine as a
directing groupa
Me Py
Py
Py
Me
SO2CF3
SO2CF3
SO2CF3
2a, 62%
Py
2b, 40%
2c, 54%
Py
Py
SO2CF3
Ph
SO2CF3
SO2CF3
t-Bu
Me
2d, 72%
Conclusions
2e, 46%
Py
2f, 80%
Py
Py
In conclusion, we have developed a novel C－H tri-
flylation of 2-benylpyridines with (CF3SO2)2O for the
preparation of triflylated 2-benylpyridines. A triflylpyri-
dinium triflate was proposed as the intermediate in-
volved in this reaction. Further investigation of the
mechanism and potential applications of this protocol,
as well as the development of other C－H triflylation
reactions, are in progress.
SO2CF3
SO2CF3
SO2CF3
Cl
Br
F
2h, 58%
2g, 55%
2i
, 50%
Py
Py
R
SO2CF3
SO2CF3
2j (R = CO2Me), 60%;
2k (R = CN), 48%
2l, 50%
Acknowledgement
Py
Py
Py
We are grateful for the financial support from Na-
tional Natural Science Foundation of China (Nos.
21502215, 21421002, 21332010, 21272036), Strategic
Priority Research Program of the Chinese Academy of
Sciences (No. XDB20020000), Youth Innovation Pro-
motion Association CAS (No. 2016234), and Shanghai
Pujiang Program (No. 15PJ1410300).
Me
Cl
Cl
SO2CF3
2o, trace
SO2CF3
2n, 49%
SO2CF3
N
2m, 47%
Me
a
Reaction conditions: 1 (1.0 mmol), (CF3SO2)2O (2.0 mmol),
Cu2(OH)2CO3•H2O (0.20 mmol), NEt3 (3.0 mmol), CH2Cl2 (10.0
mL), under N2, r.t., overnight, isolated yields.
2-(Naphthalen-2-ylmethyl)pyridine (1l) also delivered
the triflylated product 2l in 50% yield. The disubstituted
benylpyridines 1m and 1n were also suitable substrates
applicable in the current reaction. However, when
di(pyridin-2-yl)methane (1o) was subjected to the
standard reaction conditions, only trace of the desired
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