Arylboration of alkenes by cooperative palladium/copper catalysis

Article abstract of DOI:10.1021/ja5029556

Arylboration of vinylarenes and methyl crotonate with aryl halides and bis(pinacolato)diboron by cooperative Pd/Cu catalysis has been developed, giving 2-boryl-1,1-diarylethanes and an α-aryl-β-boryl ester in a regioselective manner. The reaction is compatible with a variety of functionalities and amenable to be scaled-up to a gram scale with no detriment to the yield. A short synthesis of the biologically active compound CDP840 was performed using the present reaction as a key step.

Full text of DOI:10.1021/ja5029556

Communication
pubs.acs.org/JACS
Arylboration of Alkenes by Cooperative Palladium/Copper Catalysis
Kazuhiko Semba* and Yoshiaki Nakao*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: Arylboration of vinylarenes and methyl
crotonate with aryl halides and bis(pinacolato)diboron
by cooperative Pd/Cu catalysis has been developed, giving
2-boryl-1,1-diarylethanes and an α-aryl-β-boryl ester in a
regioselective manner. The reaction is compatible with a
variety of functionalities and amenable to be scaled-up to a
gram scale with no detriment to the yield. A short syn-
thesis of the biologically active compound CDP840 was
Figure 1. Medicines containing 1,1-diarylalkane structures.
performed using the present reaction as a key step.
Scheme 1. Working Hypothesis for Alkene Arylboration
lkylboranes are versatile synthetic intermediates in organic
A
synthesis,1 and some of them possess interesting biological
activity.2 They are prepared conventionally by hydroboration of
alkenes3 and by substitution reactions of boron electrophiles with
alkyllithium or Grignard reagents.4 In recent years, transition-
metal-catalyzed borylation reactions have been shown to be a
powerful synthetic method for the preparation of alkylboranes in
terms of functional group compatibility and unique reactivity.5
Carboboration6 of alkenes can be an effective method to pre-
pare highly functionalized alkylboranes starting from readily
available substrates.7 Whereas intramolecular carboboration of
alkenes using Pd or Cu catalysts has been studied extensively,7a−e
intermolecular carboboration of alkenes has rarely been achieved
and is limited to Cu-catalyzed benzylboration with highly elec-
trophilic benzyl chloride and bis(pinacolato)diboron (B2(pin)2)
(eq 1)7f and the uncatalyzed allylboration of highly reactive
and B2(pin)2 by Cu catalysis.12 Subsequently, cross-coupling of
4 with aryl halides in the presence of a Pd catalyst gives the
arylboration products.13 Highly reactive benzyl chloride can react
directly with 4 to achieve the benzylboration (eq 1).7f However,
it may be challenging to further expand the scope due to the low
reactivity of 4 toward the oxidative addition of other electrophiles
such as aryl halides. In our scenario, the aryl electrophiles could
be used as viable coupling partners by using Pd catalysts bearing
electron-rich phosphine ligands, which readily undergo oxidative
addition of aryl bromides and even aryl chlorides.14 To test the
working hypothesis, the cross-coupling reaction of β-borylalkyl-
copper 4a, which was prepared from styrene (1a), B2(pin)2, and
(IPr)Cu(OtBu),12 with phenyl bromide (2a) was performed
in the presence of 1.0 mol % Pd(OAc)2 and 2.0 mol %
dicyclohexyl(2-mesitylphenyl)phosphine (L; for the structure of
L, see Table 1) (eq 3). Gratifyingly, the desired coupling product
1-methylcyclopropene with triallylborane.7g Here we report
the first example of an intermolecular arylboration of various
vinylarenes and methyl crotonate 1 with aryl halides 2 and
B2(pin)2 by cooperative Pd/Cu catalysis,8 giving 2-boryl-1,1-
diarylethanes and an α-aryl-β-boryl ester 3 in a regioselective
manner (eq 2). The present transformation affords potentially
useful building blocks to access 1,1-diarylalkanes, which are widely
found in bioactive compounds and medicines such as Detrol,9
Zoloft,10 and Condylox11 (Figure 1).
3a was obtained in 82% yield. In contrast, no trace amount of 3a
was observed in the absence of Pd(OAc)2 or Pd(OAc)2/L.15
To achieve a cooperative catalytic reaction, we optimized
the reaction conditions employing 1a, 2a, and B2(pin)2 as model
substrates (Table 1). After a variety of reaction parameters were
screened, the arylboration of 1a gave 3a regioselectively in high
yield using the standard conditions: Pd(OAc)2 (1.0 mol %), L
Our working hypothesis to realize the arylboration of alkenes
is shown in Scheme 1. The first step involves the catalytic
generation of β-borylalkylcopper 4 via the reaction of an alkene
Received: March 24, 2014
Published: May 8, 2014
© 2014 American Chemical Society
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Communication
formation of 3a (entry 17). These results clearly indicate the
cooperative action of the Pd and Cu catalysts in the arylbora-
tion of styrene. When the arylboration of 1a was performed
in the presence of a catalytic (5.0 mol %) or stoichiometric
(0.25 mmol) amount of TEMPO under the standard conditions,
3a was obtained in 94% or 45% yield, respectively. These results
show that the present reaction does not proceed through a
radical mechanism mediated by alkoxide bases.18
Table 1. Optimization of the Arylboration Reaction
The reaction on a 0.25 mmol scale under the standard con-
ditions gave 3a in 77% yield after purification by medium-
pressure column chromatography on silica gel (Table 2, entry 1).
The scope of substrates was surveyed employing various styrene
derivatives (1a−e) and aryl bromides (2a−m) (Table 2). All of
the reactions proceeded with excellent regioselectivity, and various
functional groups such as amino, siloxy, acyl, alkoxycarbonyl,
aminocarbonyl, cyano, chloro, fluoro, and trifluoromethyl groups
were tolerated under the standard conditions. Neither electron-
donating (entries 2−5) nor electron-withdrawing (entries 6−10)
groups at the para position of 2a affected the yield, and the
corresponding 1,1-diarylethylboronates were obtained in good to
high yields. The procedure could be scaled up to a gram scale
(entry 3). An ortho substituent was also compatible (entry 11).
Heteroaromatic bromides 2l and 2m reacted efficiently, giving 3l
and 3m in high yields (entries 12 and 13). Electron-rich and
electron-deficient styrene derivatives (1b−d) gave the corre-
sponding products in good yields (entries 14−16). 2-Vinyl-
naphthalene (1e) afforded the desired product 3p in 71% yield
with (ClIPr)CuCl as the Cu catalyst (entry 17). In addition to
vinylarenes, methyl crotonate (1f) also participated in the present
arylboration to give β-boryl ester 3q in 66% yield as a mixture of
diastereomers when (IPr*)CuCl was used (see Table 1 for the
structure of IPr*) instead of (IPr)CuCl (entry 18), whereas
aliphatic alkenes such as 1-octene and cyclohexene reacted sluggishly
under the standard reaction conditions. The arylboration of
diphenylacetylene (7) also proceeded efficiently under the standard
conditions, affording alkenylboronate 8 in 70% yield (eq 4).
variation from the standard yield of 3a + 3a′ yield of 5 yield of 6
ab
,
c
ab
,
ad
,
entry
conditions
(%) (3a/3a′)
(%)
(%)
1
2
3
4
5
6
7
8
9
none
>95 (>99:1)
58 (98:2)
87 (85:15)
21 (94:6)
88 (>99:1)
91 (>99:1)
20 (90:10)
45 (96:4)
67 (>99:1)
95 (>99:1)
71 (94:6)
0
5
8
PPh3 instead of L
PCy3 instead of L
PtBu3 instead of L
S-phos instead of L
X-phos instead of L
dppe instead of L
dppp instead of L
ClIPr instead of IPr
26
7
28
11
8
29
10
7
10
3
29
28
3
15
19
11
9
10 MeIPr instead of IPr
4
11 IMes instead of IPr
8
30
8
12 LiOMe instead of NaOMe
13 LiOtBu instead of NaOMe
14 Cs2CO3 instead of NaOMe
15 CsF instead of NaOMe
16 without Pd(OAc)2/L
17 without (IPr)CuCl
0
>95 (>99:1)
1
0
9
3
18
3
0
0
0
22
0
0
0
36
a
Determined by GC analysis with tridecane as an internal standard.
b
c
d
Based on 1a. Determined by GC analysis. Based on 2a.
(2.0 mol %), (IPr)CuCl (5.0 mol %), and NaOMe (0.15 mmol)
in toluene at 80 °C for 2 h (entry 1). Notably, the formation
of the expected byproducts 5 and 6, which could be derived
from Cu-catalyzed hydroboration16 of 1a and Pd- or Cu-catalyzed
borylation17 of 2a, respectively, were suppressed under the stan-
dard conditions. The effects of the phosphine ligands on Pd are
summarized in entries 2−8. When PPh3 or PtBu3 was employed,
the yield of 3a was low and large amounts of byproducts 5 and
6 were observed (entries 2 and 4). In the case of PCy3, the
regioselectivity was modest (entry 3). Dicyclohexylbiarylphos-
phines such as S-phos and X-phos gave good results in terms of
the observed activity and selectivity (entries 5 and 6). Bidentate
phosphine ligands were not effective for this reaction (entries 7
and 8). Other N-heterocyclic carbene (NHC) ligands on Cu
were also investigated (entries 9−11). Although less electron-
donating ClIPr resulted in moderate activity, the use of more
electron-donating MeIPr showed higher productivity (entries 9
and 10). With sterically less demanding IMes, poorer yield
and regioselectivity were observed, with the formation of a large
amount of 6 (entry 11). The effect of various bases on the
reaction was also tested (entries 12−15). LiOtBu was also
effective in promoting the reaction (entry 13), whereas other
bases such as LiOMe, Cs2CO3, and CsF did not afford the desired
product (entries 12, 14, and 15). In the absence of Pd(OAc)2/L,
3a was not formed, as was the case with the use of a stoichio-
metric amount of 4a (eq 3), and only 5 was obtained in 22% yield
(entry 16). In addition, when the reaction was performed with-
out (IPr)CuCl, a large amount of 6 was observed without the
Next, we examined the arylboration of 1a with inexpensive and
readily available aryl chlorides (9a−c) (eq 5). Activated aryl
chloride 9a underwent the arylboration under conditions similar
to the standard ones (conditions A). When X-phos and LiOtBu
instead of L and NaOMe (conditions B) were used, the products
were obtained in good yields also from nonactivated aryl chlorides
such as 9b and 9c as well as aryl triflate 10.
A plausible reaction mechanism is depicted in Scheme 2. The
catalytic cycle consists of two individual cycles involving (A) Pd
and (B) Cu catalysis. Cycle A is initiated by the oxidative addition
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Table 2. Arylboration of 1 with 2 and B2(pin)2
Scheme 2. Plausible Reaction Mechanism
investigated (eq 6). Although 3p was obtained in 74% yield via
arylboration of 1e with 2a and B2(pin)2, 16 was not converted to
styrene arylboration product 3a. Another possible reaction
pathway would involve an initial Pd-catalyzed Mizoroki−Heck
(MH) reaction of the vinylarene to give the 1,1-diarylethene and
subsequent Cu-catalyzed hydroboration,16 but this is unlikely
because the MH reaction typically gives the 1,2-diarylethene.21
The facts that 4a givesthearylboration productingoodyieldinthe
presence of Pd catalyst (eq 3) and that the reaction requires both
Pd and Cu catalysts (Table 1), in addition to the above discussion,
support the proposed cooperative Pd/Cu catalysis.
Finally, to demonstrate the synthetic utility of the alkylboro-
nates obtained by the present method, we examined their use in
the Suzuki−Miyaura cross-coupling reaction (eq 7). 3b (Table 2,
entry 2) was reacted with 4-iodopyridine in the presence of
PdCl2(dppf) (20 mol %) and KOH(aq) to give CDP840, a
phosphodiesterase IV inhibitor, in 62% yield.22
In conclusion, we have developed an arylboration reaction of various
vinylarenes and methyl crotonate with aryl halides and B2(pin)2 by
cooperative Pd/Cu catalysis, giving 2-boryl-1,1-diarylethanes and an
α-aryl-β-boryl ester in highly chemo- and regioselective manners.
The synthetic utility of the alkylboron compounds obtained by the
present method was also demonstrated by a short synthesis of the
phosphodiesterase IV inhibitor CDP840. This novel cooperative
catalytic system has great potential in the development of
carboboration and other carbon−carbon bond-forming reactions.
a
b
Isolated yields of 3. Run for 5 h using 1a (7.5 mmol), 2c (11 mmol),
c
B2(pin)2 (11 mmol), and NaOMe (11 mmol). Pd(OAc)2 (2.0 mol %)
and L (4.0 mol %) were used. Run at 50 °C for 4 h using 1a
d
e
(0.38 mmol) and 2a (0.25 mmol). Run at 50 °C for 4 h using
f
g
(ClIPr)CuCl (5.0 mol %). Run at 50 °C for 4 h. Run for 0.5 h.
h
i
(ClIPr)CuCl (5.0 mol %) was used. (IPr*)CuCl (5.0 mol %), 1f
j
(0.38 mmol), and 2c (0.25 mmol) were used. 3q was isolated as an
80:20 mixture of diastereomers.
of aryl halide 2 to LPd(0) 11 (step a in cycle A). On the basis of
the results shown in eq 3, transmetalation between LPdAr(X) 12
and β-borylalkylcopper 4 (step b in cycle A), which is generated
via addition of borylcopper 13 across 1 (step e in cycle B),12 is
followed by reductive elimination to give 3 and regenerate 11
(step c in cycle A). In cycle B, borylcopper 13 is produced by the
reaction of copper alkoxide 14 with B2(pin)2 (step d in cycle B).19
After transmetalation, LCu-X 15 reacts with the metal alkoxide base
to regenerate 14 (step f in cycle B).
ASSOCIATED CONTENT
* Supporting Information
Procedures, analytical data, and NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
One could argue that the reaction proceeds through dibora-
tion of the vinylarene to give the 1-aryl-1,2-diborylethane followed
by cross-coupling with the aryl halide. To check this possibility, the
reaction of 1,2-diboryl-1-phenylethane 16 (prepared by NaOtBu-
catalyzed diboration20 of 1a) with 2a in the presence of 1e was
AUTHOR INFORMATION
Corresponding Authors
semba.kazuhiko.5n@kyoto-u.ac.jp
nakao.yoshiaki.8n@kyoto-u.ac.jp
■
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Communication
(9) Srinivas, K.; Srinivasan, N.; Reddy, K. S.; Ramakrishna, M.; Reddy,
C. R.; Arunagiri, M.; Kumari, R. L.; Venkataraman, S.; Mathad, V. T. Org.
Process Res. Dev. 2005, 9, 314 and references therein.
Notes
The authors declare no competing financial interest.
(10) Vukics, K.; Fodor, T.; Fischer, J.; Fellegvar
Process Res. Dev. 2002, 6, 82 and references therein.
́
(11) Abad, A.; Lopez-Perez, J. L.; del Olmo, E.; García-Fernandez, L.
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́ ́
i, I.; Levai, S. Org.
ACKNOWLEDGMENTS
■
́
́
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” (22105003) from MEXT.
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