Palladium-catalyzed borylation of aryl arenesulfonates with dialkoxyboranes

Article abstract of DOI:10.1246/cl.2011.962

The cross-coupling of aryl arenesulfonates with dialkoxyboranes proceeded in the presence of Bu₄NI and a catalytic amount of [Pd(dba) ₂]/1,1'-bis(di-tert-butylphosphano)ferrocene, giving good yields of the corresponding arylboronates. The protocols tolerate a wide range of functional groups, including ester, nitrile, and ketone.

Full text of DOI:10.1246/cl.2011.962

doi:10.1246/cl.2011.962
962
Published on the web September 5, 2011
Palladium-catalyzed Borylation of Aryl Arenesulfonates with Dialkoxyboranes
Miki Murata,* Takeshi Oda, Yosuke Sogabe, Hirofumi Tone, Takeshi Namikoshi, and Shinji Watanabe
Department of Materials Science and Engineering, Kitami Institute of Technology,
165 Koencho, Kitami, Hokkaido 090-8507
(Received May 25, 2011; CL-110440; E-mail: muratamk@mail.kitami-it.ac.jp)
[Pd(dba)2] / D-t-BPF
The cross-coupling of aryl arenesulfonates with dialkoxy-
Et3N
Bu4NI
boranes proceeded in the presence of Bu4NI and a catalytic
amount of [Pd(dba)2]/1,1¤-bis(di-tert-butylphosphano)ferrocene,
giving good yields of the corresponding arylboronates. The
protocols tolerate a wide range of functional groups, including
ester, nitrile, and ketone.
+
ArSO3
(RO)2B
H
dioxane
100 °C
FG
1
2
(RO)2B
H
+
FG
FG
Arylboronic acids and their esters are crucial intermediates
for versatile transition-metal-catalyzed C-C bond-forming re-
actions such as the palladium-catalyzed Suzuki-Miyaura cross-
coupling1 and the rhodium-catalyzed Miyaura conjugate addi-
tion.2 Not surprisingly, the versatility of these methodologies has
led to considerable development of C-B bond formation. From
environmental and economic points of view, the transition-
metal-catalyzed borylation of ubiquitous C-H bonds of arenes
is an ultimate goal, however, this method has been a little
problematic with regard to the control of regioselectivity to
date.3 In contrast, the borylation of aryl electrophiles has proven
to be a regiospecific and powerful method for C-B bond
formation.4 Although these techniques involve the use of
tetraalkoxydiborons or dialkoxyboranes 1, 1 is more cost-
effective and atom-economical boron sources.
In 1997, we demonstrated the first examples of the
borylation of aryl halides and triflates with pinacolborane (1a)
by using a suitable palladium catalyst and tertiary amine.5
Thereafter, numerous catalyst systems have been reported to
affect the borylation of aryl halides with 1.6-9 Despite several
advantages of employing aryl sulfonates, most studies have not
focused on the borylation of aryl sulfonates. Recently, Percec
has reported the use of aryl mesylates and tosylates as effective
electrophiles in the nickel-catalyzed borylation using neopen-
tylglycolborane, although no example of substrates containing
ketones has been disclosed.10 Since these sulfonates are cheaper
and more stable than the corresponding triflates, they should be
more attractive substrates.
Nonetheless, we would suggest that a drawback of Percec’s
procedure was that neopentylglycolborane should be prepared in
situ, presumably due to its lower stability. In contrast, 1a and
hexyleneglycolborane (1b) can be isolated, and a further
important point is that these dialkoxyboranes exhibit good
reactivity for the palladium-catalyzed borylation of aryl hal-
ides.5-7,11 The purpose of our studies is to explore a substrate
scope of the borylation using such dialkoxyboranes 1. In this
paper, we wish to report an efficient protocol for the palladium-
catalyzed borylation of aryl arenesulfonates 2 (Scheme 1).
As a test for the optimization of reaction parameters, ethyl
4-(phenylsulfonyloxy)benzoate (2a) was used as a substrate for
the palladium-catalyzed borylation using 1a. The results are
summarized in Table 1. The borylation of 2a using [Pd(dba)2]
and 1,1¤-bis(di-tert-butylphosphano)ferrocene (D-t-BPF) as a
3
4
Me
Me
Me
Me
O
O
O
O
1
1a
BH ( ) ,
1b
BH (
:
)
Me
Me
Me
2 : Ar = 4-MeC6H5, Ph, 4-CF3C6H5
Scheme 1.
Table 1. Palladium-catalyzed borylation of 2a with 1aa
Yield/%b
Entry
Ligand
Additive
3a
4a
1
2
3
4
5
6
D-t-BPF
D-t-BPF
D-t-BPF
D-t-BPF
none
8
90
37
19
19
12
39
7
10
22
28
0
Bu4NI
Bu4NBr
Bu4NCl
Bu4NI
Bu4NI
D-i-PPFc
t-Bu-DPEphosd
aReaction conditions: 1a (1.0 mmol), 2a (0.50 mmol),
[Pd(dba)2] (0.025 mmol), ligand (0.025 mmol), Et3N (1.5
mmol), additive (0.55 mmol), in dioxane (2 mL) at 100 °C,
24 h. GC yields are based on 2a. 1,1¤-Bis(diisopropylphos-
phano)ferrocene. dBis[2-(di-tert-butylphosphano)phenyl] ether.
b
c
catalyst gave only 8% yield of the borylated product 3a due to a
strong tendency to produce the reduced arene 4a (Entry 1).
Recently, we reported that addition of an iodide anion source
improved the product selectivity on the borylation of aryl
chlorides, although we still have no definitive explanation for
the role of halide ion.6b,12 We then examined a treatment with
additional tetrabutylammonium halides, and found that the
borylation proceeded selectively in the presence of Bu4NI
(Entry 2). Bu4NBr or Bu4NCl was less effective as expected
(Entries 3 and 4). As for the catalyst system, the combination
of [Pd(dba)2] and D-t-BPF was found to be efficient for this
reaction, whereas the use of other ligands such as D-i-PPF and
t-Bu-DPEphos gave lower yields (Entries 5 and 6). Et3N as a base
and 1,4-dioxane as a solvent, which were extremely effective for
the reaction of aryl halides, could also be used with 2a.5-7
After optimization of the reaction conditions, we inves-
tigated the scope of the borylation of the aryl arenesulfonates 2
Chem. Lett. 2011, 40, 962-963
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

963
Table 2. Palladium-catalyzed borylation of representative 2
however, these studies were limited to aryl halides and
triflates.11 The second portion of this work involved the
application of this protocol to the borylation of 2 with 1b.
Several aryl sulfonates having suitable leaving groups were
borylated using 1b without any difficulty (Entries 14-16). Both
of the dialkoxyboranes, 1a and 1b, were tolerant of a variety
of common functional groups. As a result, the presence of
functional groups, such as esters (Entries 3, 13, and 14), ketones
(Entries 4, 12, and 16), and nitriles (Entries 1, 2, and 15), in the
starting 2 did not interfere with the outcome of the present
borylation.
In conclusion, we have developed a general method for the
borylation of aryl arenesulfonates 2 using dialkoxyboranes 1.
The combination of [Pd(dba)2] and D-t-BPF was a versatile
catalyst system, and the treatment with additional TBAI was
critical for the selective borylation. Investigations for mecha-
nistic studies and the extension to other organic electrophiles are
currently underway in our laboratory.
Entry
1
1
2
3
Yield/%a
PhSO3
CN
77
1a
3b
(2b)
PhSO3
PhSO3
2
3
4
71
71
87
3c
3d
3e
NC
(2c)
CO2Me
O
(2d)
PhSO3
PhSO3
Ph
(2e)
(2f)
CF3
5
6
85
83
24
12
35
80
74
3f
TsO
CF3
3f
(2g)
(2h)
(2i)
PhSO3
TsO
7
3g
3g
3g
3g
3h
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
8
N SO3
9
N
(2j)
References and Notes
ArSO3
10
11
1
2
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ArSO3
OMe
(2l)b
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13
76
3i
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(2m)b
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(2n)b
CO2Me
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14
15
16
62
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1b
2a
2b
2m
3k
3l
3m
b
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96 h.
8
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using 1 (Table 2).15 In the first part of this study, 1a was used as
the borylation reagent. Under our standard conditions, electron-
deficient aryl benzenesulfonates were efficiently converted to
the corresponding products 3 (Entries 1-5). Also, tosylate was
a facile leaving group, and 3f was obtained in good yield
(Entry 6). In contrast to the electron-deficient substrates, an
electron-neutral aryl sulfonate, phenyl benzenesulfonate (2h),
did not have enough reactivity and as a consequence 2h was
recovered (Entry 7). Attempts to alleviate this problem through
the use of 2i or 2j13 as a substrate also resulted in low conversion
to product (Entries 8 and 9). We found however, phenyl
4-trifluoromethylbenzenesulfonate (2k) participated in the pres-
ent palladium-catalyzed borylation to give the desired product
3g in high yield (Entry 10). Furthermore, the borylation of
electron-rich (Entry 11), -deficient (Entry 12), and sterically
hindered aryl 4-trifluoromethylbenzenesulfonates (Entry 13)
could also be accomplished.14
9
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14 The reaction of benzenesulfonate counterparts of 2l-2n gave the
corresponding products 3h-3j in µ20% yields.
15 Supporting Information is available electronically on the CSJ-
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Recently, it was shown that 1b was a cost-effective
borylation reagent for the palladium-catalyzed borylation;
Chem. Lett. 2011, 40, 962-963
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/