Cobalt-Catalyzed Trifluoromethoxylation of Epoxides

Article abstract of DOI:10.1021/jacs.8b10298

A catalytic ring-opening reaction of epoxides by nucleophilic trifluoromethoxylation of trifluoromethyl arylsulfonate has been developed based on the use of a cobalt catalyst. This reaction provides an efficient, simple route for directly construction of a wide range of vicinal trifluoromethoxyhydrins under mild conditions. In addition, this method can convert terminal epoxides into target products with good chemo- and regioselectivity.

Full text of DOI:10.1021/jacs.8b10298

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Communication
Cobalt-Catalyzed Trifluoromethoxylation of Epoxides
Jie Liu, Yongliang Wei, and Pingping Tang
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Cobalt-Catalyzed Trifluoromethoxylation of Epoxides
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Jie Liu,§,† Yongliang Wei,§,† Pingping Tang*,†,‡
†State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin
300071, China
‡Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China
Supporting Information Placeholder
b) how to stabilize the trifluoromethoxy anion. To address the
first issue, we hypothesis that in the presence of catalytic
ABSTRACT: A catalytic ring-opening reaction of epoxides by
nucleophilic
trifluoromethoxylation
of
trifluoromethyl
SalenCoX (Figure 1c), axial anions X of SalenCoX could be used
arylsulfonate (TFMS) has been developed based on the use of a
cobalt catalyst. This reaction provides an efficient, simple route
–
to react with TFMS to generate OCF3 first, and ring-opening of
epoxide (1) with –OCF3 was achieved to generate the intermediate
A, which react with fluorophosgene generated in situ from –OCF3
to form intermediate B, and followed work-up to give the desired
vicinal trifluoromethoxyhydrins (3). The fluoride decomposed
from –OCF3 was further used to generate –OCF3 with TFMS
during the reaction (Figure 1b).
for directly construction of
a
wide range of vicinal
trifluoromethoxyhydrins under mild conditions. In addition, this
method can convert terminal epoxides into target products with
good chemo- and regioselectivity.
a) Known results
O
O
Nucleophilic ring-opening of epoxides represents an important
method to obtain, in single step, bifunctional organic molecules.1-3
O
S
OCF3
F
+
F
R
OCF3
Accordingly, various nucleophiles have been reported in the past
F
F
few years, such as amines4, thiols5, hydroxyl6-10, phenol11,
,
12
TFMS
b) Reaction design
carboxylic acids13-15, azide16, 17, cyanide18-20, halogen21-32 and so
on. In comparison, the trifluoromethoxy group (OCF3) is rarely
used as the nucleophile in the ring-opening reaction of epoxides,
even though OCF3-containing compounds are of great interest in
new drug and agro-chemical design due to its unique properties.33-
O
O
O
O
S
S
SalenCoX (cat)
OCF3
X
R
[Co]+OCF3
+
R
59
TFMS
It would be the poor nucleophilicity and unstability of
O
O
trifluoromethoxy anion limited its application in this area. To the
best of our knowledge, there has been only one report about the
construction of vicinal trifluoromethoxyhydrins through the ring-
opening of epoxides by now.60 In that case, the toxic
fluorophosgene with high pressure (4 MP) was used as the source
of the trifluoromethoxy anion (–OCF3) for nucleophilic attacking,
which apparently increased the risk and difficulty of operation.
Therefore, the nucleophilic ring-opening of epoxides by
trifluoromethoxy anion is extremely challenging. Herein, we
present a first example of cobalt-catalyzed ring-opening reaction
of epoxides with trifluoromethyl arylsulfonate (TFMS) under mild
conditions (Figure 1). And a bimetallic cooperative catalytic
process was determined by NMR and dynamic analysis as well.
S
O
HO
R1
OCF3
R2
OCF3
SalenCoX (cat)
R
+
R1
R2
3
TFMS
1
TFMS
O
[Co]+
OCF3
+
F
work-up
F
F
O
F
[Co]O
R1
OCF3
R2
O
OCF3
R2
R1
A
B
I
II
c) Structure of catalyst and
Recently, our group reported a series of applications of
trifluoromethyl
arylsulfonate
(TFMS)
as
the
trifluoromethoxylation reagent for the introduction of
trifuoromethoxyl group into the target molecules.61-65 And its
trifluoromethoxy anion –OCF3 could be generated in situ by AgF,
CsF (Figure 1a). We wondered whether it could be possible to
achieve the nucleophilic ring-opening of epoxides by
trifluoromethoxy anion from TFMS. However, we realized that
two issues have to be considered: a) fluoride salts is not suitable
to activate the TFMS to generate the –OCF3 since the nucleophilic
ring-opening of epoxides by fluoride anion could happen;27-32 and
N
O
N
O
I:
Catalyst
(X =DNP)
Co
X
rac - 1,2-Diaminocyclohexane
tBu
tBu
II:
Catalyst
(X =DNP)
tBu
tBu
R, R - 1,2-Diaminocyclohexane
SalenCoX
Figure 1. Cobalt-catalyzed trifluoromethoxylation of epoxides
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Our
investigation
started
using
2,3-epoxy-1,2,3,4-
from 45% to 95%. Acyclic substrates 1e and 1f were also
tetrahydronaphthalene (1a) as the model substrate to optimize the
reaction conditions. Different axial anions X of SalenCoX were
evaluated as shown in Table 1. Compared with other Lewis acids
(see Supporting Information for more details), 17% desired
product 3a was found through work-up of intermediate B when
10 % mol SalenCoOTs was applied as the catalyst. Encouraged by
this result, the electron-withdrawing axial anion X such as
PhCO2 , CF3CO2 and DNP (Table 1, entries 9 to 11) exhibited
the best catalytic activity, and DNP gave the highest yield 73%
(Table 1, entry 11). In contrast, anions with poor nucleophilicity,
smoothly transformed into the desired products (3e, 3f) with
excellent yields. Moreover, the transformation was compatible
with the presence of some functionalities (ester, ether and amide)
in the substrates. It is worth mentioning that less than 5%
fluorination byproduct through ring opening of epoxides by
fluoride was observed.
Table 2. Substrate scope for ring-opening reaction of meso
-epoxides.a
–
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
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O
O
Catalyst I (10 mol%)
nBu4N+DNP- (10 mol%)
S
O
–
–
OCF3
HO
R
OCF3
R
such as BF4 and SbF6 , extremely suppressed the product
formation. And TFMS was left without any decomposition in
those cases. The role of axial anion DNP is not only easy to
dissociate from central metal for the activation of epoxide, but
+
R
R
MeCN, r.t. 24 h
1
TFMS (2)
3
–
OH
also can react with TFMS to generate OCF3. In 2010, Langlois
OH
OH
O
O
O
and coworkers reported nBu4N+ could stabilize the –OCF3.66 Based
on this result, nBu4N+DNP– was applied in this reaction. The
highest yield 88% was achieved when 10% mol nBu4N+DNP– was
used (Table 1, entry 12), while further increasing the amount of
ammonium salt had a negative effect on the reaction (Table 1,
entries 12 to 14). The yield was further up to 97% when 3.0 equiv
of TFMS was used (Table 1, entry 15). The control reactions in
the absence of cobalt catalyst showed none of the
trifluoromethoxylation product (Table 1, entry 16 and 17). After
extensive optimization of reaction conditions, 97% product 3a
could be obtained as the highest yield when the reaction of 1a was
conducted with 10% mol catalyst I, 10% mol nBu4N+DNP-, 3
equiv TFMS (2) in MeCN under an N2 atmosphere at room
temperature.
Ph
OCF3
O
OCF3
OCF3
3b, 83%c
3c, 90%b,d
3a, 95%
OH
O
O
OCF3
OCF3
HO
HO
Ph
3f, 85%c,d
OBn
3e, 95%c,e
Ph
BnO
OCF3
3d, 45%b,d
OH
OH
OH
O
O
O
N
BnO
BnO
EtO
OCF3
3g, 75%b,d
OCF3
3h, 63%b,e
OCF3
3i, 87%b,e
OH
OCF3
OH
OH
Table 1. Screening of reaction conditions.a
BnO
OCF3
OCF3
O
O
3j, 63%b,d
3k, 49%f (95%g)
3l, 67%b,d (98%g)
S
OH
SalenCoX (10 % mol)
OCF3

+
nBu4N+DNP-
MeCN, r.t., 24h
OCF3
a) Reaction conditions: 1 (1.0 equiv), TFMS (2, 3.0 equiv),
Catalyst I (0.1 equiv), nBu4N+DNP– (0.1 equiv), MeCN, N2, 25
°C. Yields of isolated products are given. b) 0.15 equiv of
Catalyst I and nBu4N+DNP– were used. c) 0.2 equiv of catalyst I
and nBu4N+DNP– were used. d) 4 equiv of TFMS (2) was used. e)
5 equiv of TFMS (2) was used. f) no nBu4N+DNP– was used. g)
Yields were determined by integration of the 19F NMR spectrum
using benzotrifluoride as an internal standard.
3a
1a
TFMS (2)
nBu4N+DNP-
(mol%)
Entry
X
Yield (%)d
OTs
Cl
1
2
-
-
17
46
0
BF4
3
-
SbF6
SCN
OTf
4
-
0
5
-
8
Besides meso-epoxides, rac-epoxides were also explored and
the results were listed in Table 3. In general, these terminal epox-
ides were easier to be converted into the vicinal trifluoromethoxy-
hydrins with yield range from 62% to 93% (3m to 3r). Ether,
nitrile and halogen were all tolerated. Notably, the catalytic
system displayed excellent regioselectivities in the ring-opening
of substrates 1m to 1p. There only secondary alcohols 3m to 3p
were formed. The high regioselectivities were found with
substrates 1q and 1r, and less than 2% primary alcohol byproduct
was found in the reaction. Inspired by these results, we are aware
that this catalytic system could retain the stereochemistry at the
methine carbon during the ring-opening process. Therefore, chiral
pure substrate (S)-1m was used and gave the product 3m in 88%
yield with > 99% ee (Supporting Information). This indicated that
catalyst I predominately induced the ring opening at the
methylene carbon of (S)-1m and the stereochemistry at the
methine carbon was kept. Unfortunately, the lower
regioselectivity was observed when styrene epoxide 1s was used
due to conflicting steric and electronic biases to nucleophilic
attack.16, 69 The conversion of asymmetric inner epoxy-alkane 1t
shown the steric factor had a big influence on the ring-opening
position. The less hinder carbon of epoxy group was more favored
6
-
21
47
27
70
63
73
88
72
62
97
0
NO3
OAc
PhCO2
CF3CO2
DNP
DNP
DNP
DNP
DNP
-
7
-
8
-
9
-
10
11
12
13
14
15b
16c
17c
-
-
10
20
40
10
-
-
10
0
a) General conditions: 1a (1.0 equiv), 2 (2.0 equiv), cobalt (10
mol%), MeCN, 25 °C, N2. b) 3.0 equiv of 2 was used. c) no cobalt
catalyst was used. d) Yields were determined by 19F NMR with
benzotrifluoride as a standard. DNP = 2, 4-dinitrophenoxy
With the optimized conditions in hand, we then investigated
the scope of the ring-opening reaction of meso-epoxides with
TFMS first. As shown in Table 2, Five-, six-, seven-membered
ring substrates (1a to 1d, 1g to 1l) were successfully converted
into the desired vicinal trifluoromethoxyhydrins with yield range
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attack of –OCF3 to form the product 3t as the main product.
Information).72, 73 These observations provide solid evidence that a
bimetallic cooperation exists in the ring-opening reaction of
epoxides with TFMS. Jacobsen further proposed two limiting
geometries for the enantioseletivity-determining transition state:
“head to head” and “head to tail”. And only the “head to tail”
geometry could induce the asymmetric ring opening of epoxides,
which was confirmed by a dimeric salen chromium complex
(similar with catalyst III in Figure 3) who can simulate the “head
to tail” geometry through its intramolecular bimetallic pathway.74,
75 To our surprise, when chiral catalyst III was applied in the ring
opening of 1a with TFMS, except trifluoromethoxylated product
3a (47%, 19F NMR yield), there was also fluorinated product 3aa
Except regioselectivity, catalyst I also shown its excellent
chemoselectivity in the ring-opening of substrate 1u with two
different epoxy groups, and the ring-opening only occurred at the
terminal epoxy group due to more steric hinder structure of left
epoxy group prevent the catalyst I to activate it.
Table 3. Substrate scope for ring-opening reaction of rac-
epoxides with TFMS.a
9
O
O
Catalyst I (10 mol%)
nBu4N+DNP- (10 mol%)
S
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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O
OCF3
HO
R1
OCF3
R2
+
R2
R1
MeCN, r.t. 24 h
(40%,
19F
NMR
yield).
And
in
contrast
with
1
TFMS (2)
3
trifluoromethoxylated product 3a still with low level of
stereocontrol (ee = 12%), fluorinated product 3aa gave a
remarkable result (87% ee) (Figure 3). This reveals that
fluorinated product 3aa was produced through the intramolecular
bimetallic pathway (“head to tail”) just like the previous report,31,
OH
OH
OH
O
OCF3
OCF3
O
OCF3
BnO
OCF3
NC
b
3m
3n
, 93%
3o
3r
, 93%
, 85%
74, 75
but trifluoromethoxylated product 3a was not. Hence,
OH
OH
OH
obviously one cobalt center of catalyst II could hardly induce the
nucleophile –OCF3 to approach to an epoxide activated by another
cobalt center through the “head to tail” geometry. The barrier may
be caused by the steric repulsion between CF3 and the activated
epoxide or salen ligand. And this may explain the loss of stereo
control of catalyst II in the ring opening of epoxides with TFMS
even if the reaction proceeds with a bimetallic cooperative
catalysis.
OCF3
O
OCF3
OH
Cl
3q
, 62%
3p
, 81%
, 85%
OH
OCF3
OCF3
3s 3s'
67% combined yield
/
= 1 : 2.9
+
3s
3s'
OCF3
Me
HO
BnO
OH
Me
F3CO
BnO
3t 3t'
/
= 10 : 1
+
88% combined yieldb,d
3t'
3t
OH
O
c
O
OCF3
3u
, 53%
Me
Me
Me
a) Reaction conditions: 1 (1.0 equiv), TFMS (2, 4.0 equiv),
Catalyst I (0.1 equiv), nBu4N+DNP- (0.1 equiv), MeCN, N2, 25
°C. Yields of isolated products are given. b) 3 equiv of TFMS (2)
was used. c) 5 equiv of TFMS (2) was used. d) 0.15 equiv of
Catalyst I and nBu4N+DNP- were used. r.r. ratio was determined
by integration of the 1H NMR or 19F NMR spectrum.
Figure 2. 19F NMR spectra of a mixture of catalyst II and
TFMS in CH3CN
Enantioselective
ring
opening
of
epoxides
by
trifluoromethoxylation anion was also investigated. Initially, the
reaction was conducted with the chiral pure catalyst II.
Unfortunately, the desymmetrization of meso-epoxide (1a) only
gave the desired product (3a) in 95% yield with 6% ee, and the
kinetic resolution of rac-epoxide (1m) afforded the desired
product (3m) in 80% yield with 13% ee. In order to explain this
phenomenon, we performed some preliminary studies which help
us gain more insight into the reaction mechanism. According to
Jacobsen’s reports, the metallic salen complex behave as a Lewis
acid to activate epoxide and also as a nucleophile delivery agent
through a bimetallic cooperation, which is the key point to realize
the high enantioselective ring- opening of epoxides.67-69 Hence, a
mixture of catalyst II and TFMS in MeCN was monitored by 19F
NMR first to indicate the existence of SalenCoOCF3 (around –
18.8 ppm, Figure 2).61, 70, 71 Moreover, kinetic study about catalyst
II revealed a linear correlation between rate constants (k) vs
[catalyst II]2, reflecting a second-order dependence on catalyst.
And the same conclusion was also obtained by analyzing the data
with the variable time normalization analysis graphical methods
developed by Burés (see more detail in the Supporting
N
O
N
O
O
5
N
O
N
Co
Co
tBu
III
Catalyst
O
O
O
O
tBu
DNP
DNP
tBu
tBu
tBu
tBu
O
O
S
OH
F
OH
+
OCF3
III
Catalyst
OCF3
O
+
MeCN, 24 h
3a
1a
TFMS (2)
3aa
Yield: 40%
ee: 87%
Yield: 47%
ee: 12%
Figure 3. Ring-opening reaction of substrate 1a with TFMS
catalyzed by catalyst III
In conclusion, we have reported the first ring-opening reaction
of epoxides with trifluoromethyl arylsulfonate (TFMS) to
construct vicinal trifluoromethoxyhydrins under mild conditions.
This method enables the transformation of various epoxides into
the corresponding trifluoromethoxylation products and a range of
functional groups are compatible. Especially terminal epoxides
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an efficient method for the synthesis of optically active 1,2-diol
can be converted into the products with good chemo- and
regioselectivity. Mechanism studies reveal the reaction may be
achieved through a bimetallic cooperation. Further efforts will
focus on designing new chiral catalyst to achieve the vicinal
trifluoromethoxyhydrins with high enantioselectivity.
monoethers. Angew. Chem. Int. Ed. 1998, 37, 2223-2226.
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ASSOCIATED CONTENT
Supporting Information
9
Experimental procedures and characterization of all new
compounds including 1H, 13C and 19F NMR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
ptang@nankai.edu.cn
Author Contributions
§J. L. and Y. W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We gratefully acknowledge the State Key Laboratory of
Elemento-organic Chemistry for generous start-up financial
support. This work was supported by the National Key Research
and Development Program of China (2016YFA0602900), NFSC
(21522205, 21672110) and the Fundamental Research Funds for
the Central Universities.
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O
O
Cobalt
catalyzed
S
O
HO
R1
OCF3
R2
OCF3
9
+
R1
R2
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Me
21 examples
up to 95% yields
TFMS
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