Copper-catalyzed trifluoromethylation of aryl- and vinylboronic acids with generation of CF3-radicals

Article abstract of DOI:10.1039/c2cc36554e

The selective trifluoromethylation of aryl- and vinylboronic acids proceeds smoothly with CF₃SO₂Na (Langlois reagent) in the presence of copper catalysts and t-BuOOH. Therefore, the method relies both on transition metal catalysis an

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Copper-catalyzed Trifluoromethylation of Aryl- and Vinylboronic Acids
with Generation of CF3-radicals
Yang Li, Lipeng Wu, Helfried Neumann, and Matthias Beller*
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
Herein, we report copper-catalyzed trifluoromethylations of aryl
and vinyl boronic acids with in situ generated CF3-radicals from
50 the reaction of TBHP and CF3SO2Na at room temperature with
the mixture of water and DCM as solvents.
The selective trifluoromethylation of aryl- and vinylboronic
acids proceeds smoothly with CF3SO2Na (Langlois reagent) in
the presence of copper catalysts and t-BuOOH. Thereby, the
10 method relies both on transition metal catalysis and selective
radical reactions. Advantageously, the protocol can be
performed at room temperature under air atmosphere and
avoid the issue of poor regioselectivity
1. TM catalyzed or mediated trifluoromethylation:
[TM]
-
+
+
ArCF3
ArCF3
(Eq. 1)
(Eq. 2)
Ar
Ar
R
R
CF3
[TM]
+
CF3
R = H, X, B(OH)2 and its derivatives
2. Radical trifluoromethylation:
55
60
65
photoredox
catalysis
Trifluoromethylated aryl and heteroaryl compounds represent an
15 important structural motif in pharmaceuticals, agrochemicals, and
advanced organic materials.1 Due to the increasing interest on this
class of compounds, the development of new methodologies for
generation C-CF3 bonds has attracted significant attention in
recent years.2 More specifically, two major approaches for the
20 formation of these bonds have emerged in the last decade: On the
one hand, transition metal catalyzed or metal-mediated
trifluoromethylation reactions are applied using pre-
functionalized substrates such as aryl halides,3 arylboron
compounds,4 arenes substituted with directing groups,5 or
25 heteroarenes.6 Clearly, the major advantage of this approach is
the control of the specific regioselectivity. Unfortunately, these
+
CF3SO2R'
Ar
H
ArCF3
(Eq. 3)
or t-BuOOH
R' = Cl, Na
3. Cooperation of TM catalysis with radical trifluoromethylation:
photoredox
catalysis
(Eq. 4)
+
ArB(OH)2
CF3I
ArCF3
RCF3
cat. [Cu]
DMF, 60 °C
This work:
RB(OH)2
t-BuOOH
(Eq. 5)
CF3SO2Na
+
cat. [Cu]
DCM/H2O, rt
R = Ar, vinyl
Scheme 1 Strategies for the formation of C-CF3 bonds.
At the start of our investigations, 4-methoxyphenyl-boronic
acid (1a) was selected as
a
substrate for selective
trifluoromethylation. Obviously, there are two major problems in
this model reaction: The first issue is the competition of the direct
transformations
always
involve
relatively
expensive
trifluoromethylsilanes, such as Ruppert’s reagent (TMSCF3)3a-
70 radical reaction with the arene compared to the small amount of
in situ formed aryl metal species. The second issue is to avoid
oxidation of the arylboronic acid.10 To overcome these problems,
b,3g,3h,3j,3k,3m,4a-b,4f,5b,6
3c,3d
-
to generate a CF3 synthon
or TESCF3
30 (Eq 1), and S-(trifluoromethyl) thiophenium salts,3f,4d,4e,5 Togni’s
different metal catalysts including Pd,3d,3m,5,6a,11 and Cu
+
reagent4c,4i to generate a CF3 intermediate (Eq 2). A second
3e-3l,3n,4,6b
complexes3a-3c,
were tested. In addition, an extensive
synthetic approach takes advantage of highly reactive CF3
radicals.7 For examples, the groups of MacMillan7c and Baran7d
demonstrated elegantly the direct trifluoromethylation of arenes
35 and heterocycles. They used either visible light or t-BuOOH
(TBHP) to initiate CF3 radical formation from CF3SO2Cl or the
Langlois reagent (CF3SO2Na).7a,8 The latter reactions proceed
under mild conditions, albeit with poor regioselectivity (Eq 3).
Inspired by these results, we envisioned to bring together the
40 advantages of both strategies by combining transition metal
catalysis with the high reactivity of CF3 radicals. Probably, in
such an approach the regioselectivity might be controlled by
transition metal catalysis, while the CF3 radical is generated
under mild reaction conditions from less expensive reagents.
45 Notably, during our work Sanford and co-workers reported an
approach towards this direction using the CuOAc-catalyzed
trifluoromethylation with CF3I using visible light at 60°C (Eq 4).9
75 variation of reaction parameters, e.g. ligands, solvents,
temperature, additives, etc. were performed. To our delight, the
desired product 4a was obtained in decent yield (53%) using
simple Cu(OAc)2 as catalyst in a mixture of DCM and water as
solvent at rt (Table 1, entry 1). Noteworthy, without Cu(OAc)2
80 less than 5% of product was detected (Table 1, entry 2).
Cu(I)OAc and other copper catalysts exhibited lower efficiency
than Cu(OAc)2 (Table 1, entries 3-8). Under slightly acidic
reaction conditions the chemoselectivity of this transformation is
increased. Hence, using 2.5 equiv of NH4Cl promoted a higher
85 yield of 4a (Table 1, entries 1, 9 and 10). Notably, the function of
the ligand (2,4,6–collidine) to the metal is important to this
reaction, too (Table 1, entries 11-15). In the absence of 2,4,6–
collidine, the reaction yield was decreased sharply to only 22%
(Table 1, entry
11). Increasing the amount of the
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Table 2 Substrate scope of the Cu-catalyzed trifluoromethylation
of arylboronic acidsa†
trifluoromethylation reagent (NaSO2CF3) improved the yield to
64% (Table 1, entries 1, 16 to 17) and raising the ratio of TBHP
to NaSO2CF3 led to a slightly higher efficiency (Table 1, entry
18). Finally, addition of 0.24 equiv of imidazole as an additive
gave the desired product in a 74% yield (Table 1, entry 19).
5
Table
1
Copper-catalyzed trifluoromethylation of 4-
methoxyphenylboronic acid: Variation of reaction conditionsa
20 mol% [Cu]
2,4,6-collidine
+
TBHP
MeO
B(OH)2
+
CF3SO2Na
MeO
CF3
DCM, H2O
rt, air, 6 h
1a
2
3
4a
CF3SO2Na TBHPb
NH4Cl
(equiv)
2,4,6-collidine
(equiv)
Entry [Cu]
Yield(%)c
(equiv)
(equiv)
10
15
20
Cu(OAc)2
1
2
3
4
5
6
5.0
5.0
5.0
10.0
10.0
10.0
2.5
2.5
2.5
2.0
2.0
2.0
53
<5
40
CuOAc
Cu(CF3CO ) •H O
5.0
5.0
5.0
10.0
10.0
10.0
2.5
2.5
2.5
2.0
2.0
2.0
44
40
38
2
2
2
CuBr
CuI
7
8
9
Cu(CH3CN)4BF4 5.0
10.0
10.0
10.0
2.5
2.5
3.0
2.0
2.0
2.0
2.0
36
44
36
Cu(CH3CN)4OTf
Cu(OAc)2
5.0
5.0
10
11
Cu(OAc)2
Cu(OAc)2
5.0
5.0
10.0
10.0
2.0
2.5
39
22
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
5.0
5.0
5.0
10.0
10.0
10.0
2.5
2.5
2.5
1.0
1.5
2.5
46
49
49
12
13
14
15
16
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
5.0
6.0
10.0
12.0
2.5
2.5
3.0
2.0
28
55
Compared with the trifluoromethylation of arylboronic acids, the
60 vinylboronic acids are less sensitive towards the influence of
substituents (Table 3, 6a-6g). Even with strong electron-
withdrawing substituents (6f, 6g), the trifluoromethylation
proceeded smoothly with good yields. It should be noted that the
trifluoromethylation occurred highly selective and no (Z) isomer
17
18
19
7.0
7.0
7.0
14.0
16.1
16.1
2.5
2.5
2.5
2.0
2.0
2.0
64
66
74d
a All the reactions were performed on 0.25 mmol scale in DCM (4 mL) and H2O (1.5 mL).
b TBHP (tert-butyl hydroperoxide) was used as 70% solution in water. c GC yield is shown
and dodecane was used as internal standard. d 0.24 equiv of imidazole was added.
With the optimized reaction conditions in hand, the substrate
65 was observed during this protocol.4c,4d,13e,13f,14
. Next, (Z)-
25 scope was investigated. Arylboronic acids with electron-donating
substituents (methoxy, methyl, 3,4-methylenedioxy) underwent
trifluoromethylation in good yields (Table 2, 4a, 74% yield, 4b,
66% yield, 4c, 61%). Classic hydroxyl protecting groups (Bn and
TBS) are well-tolerated in this reaction (Table 2, 4d, 73% yield,
30 4e, 69% yield). Notably, methylthio and N,N-dimethylamino
substituents survive under the mild reaction conditions, albeit in
modest yields (Table 2, 4f, 47% yield, 4g, 39% yield). We
observed that steric hindrance in ortho position to the boryl group
decreased the reaction efficiency somewhat (Table 2, 4h, 4i).
potassium styryltrifluoroborate was selected as a representative of
Z-alkenylboronic acid derivatives for stereoselectivity studies.
Contrary to the (E)-vinylboronic acids, the (Z)-isomer gave a
mixture of regioisomers with a ratio of E:Z = 1 : 0.7. The
70 formation of this mixture is attributed to higher stability of the
(E)–isomer.
Table 3 Trifluoromethylation of vinylboronic acidsa
35
Decreasing of substrates’ electron density induced some
lower reaction efficiency (Table 2, 4j-4o). Halide-substituted
starting materials represent especially interesting substrates
because of further functionalization of trifluoromethylated arenes.
Hence, we were pleased to find that the reactions of 4-
40 iodophenylboronic acid as well as the corresponding bromides
and chlorides worked reasonably well when increasing the
amount of Cu(OAc)2 (Table 2, 4m-4o). Some typical examples of
heteroaromatic substrates were also investigated. In fact, the
arylboronate derived from benzofuran underwent the reaction
45 with good yield (Table 2, 4p) and the corresponding boronic acid
derived from an indole derivative also gave an acceptable yield
(Table 2, 4q). Then, 6-bromohexylboronic acid was selected as
an example to investigate the activity of the alkylboronic acids.
However, no desired product was observed in this case.
75
Based on our observations, we propose the reaction
mechanism as shown in Scheme 2. On the one hand,
transmetallation of the arylboronic acid takes place with the
active Cu(II) species (7) to give the aryl copper(II) complex (8).
On the other hand, the CF3 radical is generated from the reaction
80 of TBHP with NaSO2CF3. Reaction of both active species should
afford the arylcopper(III)CF3 intermediate (9) (path A).15
The desired product is obtained by reductive elimination of
intermediate 9 which also releases the Cu(I) complex (10).
Finally, 10 is re-oxidized to the active Cu(II) catalyst (7) to close
85
50
Conjugated aromatic systems with trifluoromethyl
substituents such as β-trifluoromethylstyrene derivatives have
found wide use in Organic Light Emitting Diodes (OLEDs) and
they are of general interest for other applications as advanced
materials.12 Hence, we were pleased to find that our novel
55 protocol can be also applied to vinylboronic acids.13
2
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-
CF3SO2
•
trace metal
t-BuO•
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CF3
+ SO2
+
t-BuO-
t-BuOOH
OH-
R-
Cu(II)Ln
7
65
70
75
80
85
90
ArB(OH)2
R•
•
CF3
B(OH)2Ln
5
Cu(III)CF3Ln
11
Cu(I)Ln
ArCu(II)Ln
Path A
10
8
Path B
ArB(OH)2
4
For recent selected samples, see: (a) L. Chu, F.-L. Qing, Org. Lett.
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B(OH)2Ln
•
ArCF3
CF3
CF3Cu(III)LnAr
10
9
R = CF3 or t-BuO
Scheme 2 Proposed reaction mechanism
the catalytic cycle. Another mechanistic pathway which we can
not exclude at this point is the reaction of the Cu(II) complex 7
15 with CF3 radicals to generate the aryl copper(III) complex 11.6b
Subsequent transmetallation with the aryl- or vinylboronic acid
will lead again to intermediate 9 (path B).
5
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In summary, this communication reports a convenient Cu-
catalyzed trifluoromethylation of aryl- and vinylboronic acids
20 using less expensive and stable CF3SO2Na as CF3 source.
Although a large quantity of TBHP was used, synthetic
applications are not limited because of the very low cost. The
protocol is robust and the reactions work in water and DCM
under air atmosphere at room temperature. Notably, the presented
25 methodology makes use of cooperative transition metal catalysis
and CF3 radical formation. Furthermore, it suggests a new
orientation in organic transformations.9,16
6
7
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Leibniz-Institut für Katalyse e.V. an der Universität Rostock; Albert
:
100
30 Einstein
Str.
Matthias.Beller@catalysis.de
29a,18059
Rostock,
Germany;
E-mail
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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