Thioethers as directing group for the palladium-catalyzed direct arylation of arenes

Article abstract of DOI:10.1002/adsc.201200447

Thioethers have proven to be efficient directing groups for the arylation of arenes under palladium catalysis. The thioether group can be readily removed or converted to other functional groups. Kinetic isotopic effect studies reveal that the C-H cleavage of arenes might be the turnover-limiting step. Copyright

Full text of DOI:10.1002/adsc.201200447

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DOI: 10.1002/adsc.201200447
Thioethers as Directing Group for the Palladium-Catalyzed
Direct Arylation of Arenes
Jinzhong Yao,a Ming Yu,a and Yuhong Zhanga,b,*
a
Department of Chemistry, Zhejiang University, Hangzhou 310027, Peopleꢀs Republic of China
Fax : (+86)-571-87953244; e-mail: yhzhang@zju.edu.cn
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, Peopleꢀs Republic of China
b
Received: May 21, 2012; Revised: July 24, 2012; Published online: November 8, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200447.
Abstract: Thioethers have proven to be efficient di-
recting groups for the arylation of arenes under pal-
ladium catalysis. The thioether group can be readily
removed or converted to other functional groups.
Kinetic isotopic effect studies reveal that the C H
cleavage of arenes might be the turnover-limiting
step.
dination group with various metals in many useful
catalysts.[11] However, functionalization of thioether
À
derivatives using thioether as directing group by C H
activation has rarely been reported.[12] Recently, our
group explored the utilization of the thioether group
À
as directing group for the catalytic ortho-alkenylation
[13]
À
of arene C H bonds. It was established that thio-
ethers could functionalize as directing group for the
C H activation in palladium catalysis. Herein we
would like to report an efficient Pd-catalyzed aryla-
À
À
Keywords: arylation; C H bond activation; palladi-
um; potassium organotrifluoroborates; thioethers
tion of arenes with
a
variety of potassium
aryltrifluoroborates using thioethers as directing
AHCTUNGTRENNUNG
group. The thioether group could be easily removed
or transformed into other useful functionalities.
We envisaged that the arylation, one of the most
Biaryls are important building blocks of natural prod-
ucts, pharmaceuticals, functionalized materials, and important transformations in organic synthesis, could
asymmetric catalysts.[1] Consequently, the develop- also occur through the thioether-assisted activation of
À
ment of straightforward and environment-friendly C H bond in the presence of a palladium catalyst.
methods for the preparation of biaryls has attracted Accordingly, arylboronic acids,[14] which are widely
substantial interest. The famous cross-coupling reac- used coupling partners for aryl-carbon bonds, were
tions, i.e., Suzuki, Stille, Negishi, and Kumada reac- first tested. However, no desired product was detect-
tions, have become reliable tools for the construction ed and only homocoupling products of the arylboron-
of biaryls.[2] However, these methods usually require ic acid were obtained. Organotrifluoroborate salts
the prefunctionalized aromatic substrates (aryl halides have been well studied by Molander and others as an
or organometallic derivatives) as starting materials. In alternative to organoboronic acids in the Suzuki reac-
recent years, direct C H arylation has been intensive- tion and other coupling reactions.[15] These salts can
À
ly investigated owing to its atom- and step-economic be readily prepared through a variety of one-pot syn-
advantages.[3] In particular, chelation-assisted C H ar- thetic routes from readily available starting materi-
À
ylation has emerged as a powerful method for the ef- als.[16] By taking advantages of the attractive features
fective and regioselective construction of biaryls. Pre- of organotrifluoroborate salts, we examined the reac-
vious studies have demonstrated that ketones,[4] tion of potassium 4-methoxyphenyltrifluoroborate
amides,[5] imines,[6] pyridyl,[7] nitriles[8] and carboxylic (2a) with (3-methylbenzyl)
ACHTUNGTRENNUNG
acids[9] could serve as directing groups in the arylation presence of Pd
ACHTUNGTRENNUNG
of arenes. In these transformations, the directing (3 equiv.) at 1308C in DCE. To our delight, the de-
groups in some cases are irremovable and recalcitrant sired product was isolated in 40% yield (Table 1,
to undergo further synthetic elaboration, and hence entry 1). The homocoupling product from potassium
limited their wider applications.
phenyltrifluoroborate was not observed. Further opti-
The thioether is one of the most important func- mization revealed that the addition of 0.3 equiv. 1,4-
tional groups and exists in many bioactive compounds benzoquinone (BQ) resulted in a dramatic improve-
and natural products.[10] It has also been used as coor- ment of the reaction to afford a 73% yield (Table 1,
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Table 1. Optimization of reaction conditions.[a]
Table 2. Reaction of (3-methylbenzyl)ACTHUNGTRENNUNG
potassium aryltrifluoroborates.[a]
Entry Catalyst
Oxidant
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
Pd
Pd
PdCl2
N
Ag2CO3
Ag2CO3
Ag2CO3
DCE
DCE
DCE
DCE
DCE
DCE
40[c]
73
21
10
N.R.
55
49
31
21
22
Entry Substrate 2
Product 3
Yield [%][b]
73
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Ag2CO3
AgOAc
AgOTFA DCE
1
Ag2O
DCE
toluene
xylene
1,4-dioxane 12
i-PrOH
DMF
9
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
10
11
12
13
14
15
2
3
69
72
N.R.
N.R.
N.R.
29[d]
DMSO
DCE
[a]
Reaction conditions: (3-methylbenzyl)ACTHNUTRGNEUNG(p-tolyl)sulfane 1a
(0.2 mmol), potassium aryltrifluoroborate 2a (0.6 mmol),
palladium catalyst (0.02 mmol), BQ (0.06 mmol), oxidant
(0.6 mmol), solvent (2 mL) , 1308C , 24 h.
Isolated yield.
In the absence of BQ.
The reaction was conducted at 1108C.
4
54
[b]
[c]
[d]
5
6
7
8
9
70
entry 2).[17] Other palladium catalysts, including PdCl2
and Pd(PPh3)2Cl2, showed relatively lower efficiency
(Table 1, entries 3 and 4), and Pd(PPh3)4 was com-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
pletely inactive (Table 1, entry 5). Among the oxi-
dants examined, Ag2CO3 gave the highest yield
(Table 1, entry 2). A comparable reaction efficiency
was presented by AgOAc, while AgOTFA and Ag2O
showed poor reactivity (Table 1, entries 6–8). The sol-
vent played an important role in this transformation.
Dichloroethane was the best solvent for this reaction.
Other solvents, such as toluene, xylene, and dioxide,
led to poor yields (Table 1, entries 9–11). The reaction
did not occur in i-PrOH, DMF and DMSO (Table 1,
entries 12–14). The temperature was found to influ-
ence the reactivity. For example, lowering the temper-
ature to 1108C caused the yield to decrease to 29%
(Table 1, entry 15).
59
N,R
35
51
Next, we turned our attention to investigate the
scope of the potassium aryltrifluoroborates as shown
in Table 2. In most cases, good results were obtained
for the potassium trifluoroarylborates with an elec-
tron-donating group in the aromatic ring (Table 2, en-
tries 1–6). Potassium aryltrifluroborates with ortho
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Thioethers as Directing Group for the Palladium-Catalyzed Direct Arylation of Arenes
Table 3. Reaction of thioethers with potassium 4-methoxy-
Table 2. (Continued)
Entry Substrate 2
phenyltrifluoroborate.[a]
Product 3
Yield [%][b]
10
41
11
53
Entry Substrate 1
Product 3
Yield
[%][b]
[a]
Reaction conditions: thioether arene 1a (0.2 mmol), po-
tassium aryltrifluoroborate (0.6 mmol), Pd(OAc)2
2
ACHTUNGTRENNUNG
(4.5 mg, 0.02 mmol), BQ (6.5 mg, 0.06 mmol), Ag2CO3
(164.4 mg, 0.6 mmol), DCE (2 mL), 1308C, 24 h.
Isolated yield.
1
63
57
[b]
2
3
substitutents could not participate the reaction
(Table 2, entry 7), illustrating that the steric hindrance
played a role in the reaction. The presence of elec-
tron-withdrawing groups such as Cl, F, and CN in the
aromatic ring of aryltrifluoroborate salts led to lower
efficiency (Table 2, entries 8–10). Potassium 2-naph-
thalenyltrifluoroborate underwent the reaction
smoothly and a 53% yield was afforded (Table 2,
entry 11). The super reactivity of potassium aryltri-
fluoroborates might be rationalized by the greater nu-
cleophilicity of potassium trifluoroborates over the
corresponding boronic acids.[9c,15c]
The substituent groups on the thioether arenes in-
fluenced the direct arylation reactions as summarized
in Table 3. Arenes containing electron-withdrawing
substituents exhibited lower yields in the arylation
than their electron-rich counterparts. For example,
the yields of Cl- and F-substituted arenes were con-
siderably depressed in contrast to those of Me- and
MeO-substituted arenes (Table 3, entries 1–7). In the
61
4
5
58
31
6
7
34
36
case of benzylACHTUNGTRENNUNG(p-tolyl)sulfane 1i, the diarylated prod-
uct was isolated in 12% yield, which led to a lower
yield of 3s. It should be noted that various thioethers
were well tolerated and showed the good reactivity to
afford the corresponding arylated products (Table 3,
entries 9 and 10).
To gain insight into the reaction mechanism, an in-
termolecular isotope kinetic experiment was carried
out. BenzylACHTUNGTRENNUNG(p-tolyl)sulfane 1i and its deuterated ana-
8
9
41[c]
logue 1i-d5 were equivalently subjected to the reac-
tion (Scheme 1). A significant kinetic isotope effect
À
(KIE) of 2.7 was observed, indicated that the C H
cleavage might be involved in the rate-determining
65
step in this new catalytic system.[20]
In order to demonstrate the potential synthetic ap-
plication of the new reaction, we explored transfor-
mations of the arylated products with the model prod-
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Jinzhong Yao et al.
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arenes, which is consistent with the electrophilic palla-
dation mechanism.[18] According to our previous stud-
ies[13] and the experimental results, a plausible reac-
tion mechanism is proposed for this new arylation
process as shown in Scheme 3. The electrophilic palla-
dation and deprotonation occurs at the ortho-position
of thioether arene to give the intermediate A, which
reacts with the potassium aryltrifluoroborate to afford
Table 3. (Continued)
Entry Substrate 1
Product 3
Yield
[%][b]
10
51
the (aryl)ACTHNUGRTNEUNG(aryl)palladium(II) species B. The subse-
[a]
Reaction conditions: (3-methylbenzyl)ACTHUNGTRENNUNG
quent reductive elimination liberates the biaryl prod-
uct with the formation of Pd(0), which is oxidized by
Ag2CO3 to regenerate the Pd(II) for the next catalytic
cycle. Further research is required to elucidate the de-
tailed reaction mechanism.
1
(0.2 mmol),
potassium
aryltrifluoroborate
(0.6 mmol), Pd
ACHTUNGTRENNUNG
0.06 mmol), Ag2CO3 (164.4 mg, 0.6 mmol), DCE (2 mL),
1308C, 24 h.
Isolated yield.
[b]
[c]
12% of diarylation product was also isolated.
In summary, we have developed an efficient proto-
col for regioselective arylation of arenes via the thio-
À
ether-assisted C H bond activation by the palladium
uct 3c (Scheme 2). The cleavage of the phenylthio catalysis. Potassium aryltrifluoroborate were proven
group was achieved under reductive conditions using
Raney Ni in the presence of hydrazine hydrate to
obtain the corresponding arylation product 5c in 73%
yield. By regulating the dosage of the oxidant m-
CPBA and reaction time, the thioether could be oxi-
dized to afford the sulfone 6c or sulfoxide 7c in good
yields.
In principle, two different reaction pathways can be
envisioned for the initial arylation: (i) the electrophil-
ic palladation and subsequent arylation;[18] (ii) direct-
2
[19]
À
ed CACHTUNGTRENNUNG(sp ) H activation and subsequent arylation.
In our experiment, the electron-rich thioether arenes
showed higher reactivity than the electron-difficient
Scheme 1. Kinetic isotope experiment.
Scheme 3. Proposed mechanism.
Scheme 2. Removal and transformation of thioethers.
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Thioethers as Directing Group for the Palladium-Catalyzed Direct Arylation of Arenes
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to be the suitable coupling partners. This transforma-
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cient synthesis of a variety of biaryls and sulfur-con-
taining compounds. Further development of other C
H functionalizations by the use of thioether as direct-
ing group is underway in our laboratories.
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(27.4 mg, 0.2 mmol), potassium 4-methoxyphenyltrifluorobo-
rate 2a (128.4 mg, 0.6 mmol), Pd(OAc)2 (4.5 mg, 0.02 mmol),
ACHTUNGTREN(NUNG p-tolyl)sulfane 1a
AHCTUNGTRENNUNG
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a Teflon-lined cap, and the reaction mixture was stirred at
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Acknowledgements
Funding from National Basic Research Program of China
(No. 2011CB936003) and Natural Science Foundation of
China (No. 21072169) is acknowledged.
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