Sandmeyer-type reaction to pinacol arylboronates in water phase: A green borylation process

Article abstract of DOI:10.1055/s-0031-1290960

Copper(I)-catalyzed cross-coupling reactions of aryl diazonium salts with bis(pinacolato)diboron can proceed smoothly in the water phase at room temperature to give the corresponding arylboronate esters in good to high yields. The Sandmeyer-type borylation not only provides direct access to arylboronates bearing halo and acidic substituents, but also achieves a green borylation process. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290960

1394▌
LETTER
lSetaterndmeyer-Type Reaction to Pinacol Arylboronates in Water Phase:
A Green Borylation Process
Copper-Catalyzed Sandm
a
ey
,
e
b
r-Type Borylation in Water
a
a
b
Jie Zhang, Xiaolong Wang, Haitao Yu,* Jiahai Ye
a
School of Pharmacy, Nanjing University of Chiness Medicine, 138 Xianlin Rd., Nanjing 210046, P. R. of China
Fax +86(25)84767922; E-mail: njubilly@gmail.com
b
School of Chemical and Engineering, Nanjing University of Science and Technology, 200 Xiaolingwei, Nanjing 210094, P. R. of China
Received: 20.02.2012; Accepted after revision: 20.03.2012
in organic solvent, and yet are inherently inefficient to
acidic substituents, such as carboxyl, aminosulfonyl, etc.
Abstract: Copper(I)-catalyzed cross-coupling reactions of aryl di-
azonium salts with bis(pinacolato)diboron can proceed smoothly in
the water phase at room temperature to give the corresponding aryl- In this communication, we describe a facile and green
boronate esters in good to high yields. The Sandmeyer-type boryla-
tion not only provides direct access to arylboronates bearing halo
and acidic substituents, but also achieves a green borylation pro-
yldiazonium tetrafluoroborate salts with bis(pinacola-
cess.
method for the preparation of arylboronates bearing acidic
substituents by the classic Sandmeyer-type reaction of ar-
to)diboron. The reaction can be carried out in water under
Key words: aryldiazonium salts, pinacol arylboronate, borylation,
air at room temperature. To the best of our knowledge,
Sandmeyer reaction, 4-carboxylphenylboronate ester
this procedure has not been reported to date.
In light of a few reports of transformations to pinacol bo-
ronates that occur via aryldiazonium salts in the presence
Arylboronate esters and arylboronic acids may be one of
of a Pd catalyst, we surmised that it might be possible to
the most important intermediates that are used in a broad
1
2
3
achieve the transformation under traditional Sandmeyer
reaction conditions. Inspired by the report from Wang and
co-workers that Cu(I) and Cu(II) salts could afford pinac-
ol phenylboronate in 8% yield from phenyldiazonium ion
range of transition-metal-catalyzed C–C, C–N, C–O,
4
and C–P bond-forming reactions. Fundamental prepara-
tions of the boronate esters have been studied to a lesser
extent. The most commonly utilized methods to access ar-
ylboronate esters require harsh conditions or expensive
1
1
and B pin , we decided to explore the potential of the re-
2
2
reagents,5 such as the direct Pd-catalyzed attachment of
–8
action by extending the Sandmeyer-type reactions.
boron to aromatics to afford arylboronates using aryl ha- After some initial attempts, one can conclude that aryldi-
6
lides or triflates with a diboron pinacol.
azonium tetrafluoroborate salts were very suitable inter-
mediates for this transformation, which are easily
In recent ten years, investigations on the development of
synthetic methodologies for arylboronates have continu-
ously attracted the attention of organic chemists, and
some new representative synthetic alternatives to arylbo-
ronic acids and arylboronic esters are summarized as fol-
lows: (1) Strongin et al. afforded various arylboronic
esters via a palladium-catalyzed coupling of bis(pinacola-
to)diboron(B pin ) with arydiazonium tetrafluoro borate
salts under mild reaction conditions; (2) Andrus et al.
used a carbene–palladium catalyst to accomplish the bor-
ylation of aryldiazonium ions without added base; and
3) Wang et al. explored diazonium ions formed in situ us-
1
2
available from arylamines and prone to be isolated. Dur-
ing the course of our studies on the reaction, we prepared
pinacol phenylboronate catalyzed by CuBr with the use of
commercial acetone in 20% yield from phenyldiazonium
tetrafluoroborate and B pin . Considering that commer-
2
2
cial acetone contains a little water, we considered that –
compared with CuBr in pure organic solvent – Cu(I) ion-
ized in a small amount of water might give better yield. In
the studies of Wang and co-workers, Cu(I) and Cu(II) salt
catalysts could not be resolved in organic solvent, so the
yield was low. In several experiments, we adjusted the ra-
tio between water and acetone such that the clear solution
mixed with all materials including catalyst. We were
pleased to find that the reaction gave pinacolato phenylbo-
ronate in 70% yield within five hours at room tempera-
ture.
2
2
9
10
(
ing tert-butyl nitrite to give pinacol arylboronates in the
presence of benzoyl peroxide.11 It is worth noting that
these methods employ the cheap and abundant arylamines
as starting materials, so aryldiazonium salts easily ob-
tained from arylamines are highly attractive synthetic al-
ternatives to the corresponding halides and triflates. The Therefore we drew a conclusion that the borylation proce-
above procedures are highly reliable and tolerate a wide dure might tolerate the presence of free protons. In other
range of functional groups and allow easy access to a di- words, we might directly prepare pinacol arylboronates
verse set of arylboronate esters, but these reactions also substituted by groups containing active hydrogen, such as
require expensive palladium catalyst and ligand, proceed aminosulfonyl, carboxyl, etc. We selected 4-carboxylic
phenyldiazonium tetrafluoroborate (1a) as the model
compound to examine its behavior in the presence of dif-
ferent metal salts, which may accelerate the decomposi-
tion of the aryldiazonium ions (Table 1). Furthermore, the
SYNLETT 2012, 23, 1394–1396
Advanced online publication: 08.05.2012
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6
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5
2
1
4
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4
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2
0
9
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DOI: 10.1055/s-0031-1290960; Art ID: ST-2012-W0153-L
Georg Thieme Verlag Stuttgart · New York
©

LETTER
Copper-Catalyzed Sandmeyer-Type Borylation in Water
1395
a
Table 1 Reaction of 1a and B pin
CuBr to afford an isolated yield of 80% in a 2:1 MeCN–
water ratio (Table 1, entry 6).
2
2
N2BF4
Bpin
r.t.
Table 2 Synthesis of Pinacol Arylboronate
+
B2pin2
N2BF4
Bpin
COOH
COOH
2a
CuBr (5 mol%), r.t.
MeCN–H2O (2:1)
1a
R
+
B2pin2
R
1
b–n
2b–n
Entry
Solvent ratio 2:1 (v/v)Additive
mol%)
Time
(h)
Yield
(%)
b
(
Entry Productsa
Time (h) Yield (%)b
1
2
3
4
5
6
7
8
9
acetone–H O
Cu(Ac) (50)
6
6
35
21
15
12
2
2
O
O
acetone–H O
FeCl (10)
1
B
2b
2c
5
3
70
75
2
3
acetone–H O
Co(Ac) (10)
6
2
2
O
O
B
O
acetone–H O
CeCl (10)
6
2
3
H2N
S
2
2
O
70c
80c
65
71
68
10
acetone–H O
CuBr (5)
CuBr (5)
CuBr (20)
CuBr (5)
CuBr (5)
CuBr (5)
CuBr (5)
5
2
O
B
MeCN–H O
3
2
2d
3
78
O
MeCN–H O
3
HOOC
MeOOC
O
2
O
O
DME–H O
3
2
4
5
6
B
2e
2f
4
4
4
72
73
85
dioxane–H O
3
2
O
H Od
1
0
1
12
6
2
B
O
acetoned
1
25
O
a
Reaction conditions: 1a (1mmol), B pin (1 mmol), solvent (4.5
mL), r.t.
HPLC yield with commercial 2a as external standard.
Yields after isolation by column chromatography.
Single solvent.
2
O N
B
2
2
2g
O
b
O
c
B
d
7
8
2h
2i
5
6
54
24
O
O2N
optimization of the reaction conditions, including solvent
and reaction time, were then investigated.
O
O
B
NO2
Me
As the catalyst, CuBr was superior to other inorganic salts
Table 1, entries 1–5). We also tested other Cu(I) salts,
(
like CuI and CuCl. Although CuI gave a comparable
yield, the reaction solution showed a deeper color and pu-
rification of the product was problematic. Additionally,
CuCl yielded only 50% of the desired compound. Increas-
ing the amount of CuBr led to a sharp decrease, with 0.2
equivalents of additive giving the desired compound in
only 65% yield (Table 1, entry 7). As it is suggested that
the Sandmeyer reaction proceeds via a radical mecha-
O
O
B
9
2j
4
70
O N
2
O
1
1
1
0
1
2
3
Br
B
B
2k
2l
5
3
3
6
40
77
68
42
O
O
13
nism, a lot of halogen anions can take a competing reac-
tion with B pin as radical receptor. It was indeed
NC
F3C
Me
2
2
O
observed that only a trace of the desired compound could
be obtained with 4-carboxylic phenyldiazonium bromide
as starting material under the same conditions. Finally, we
found that the reaction was marginally affected by sol-
vents (Table 1, entries 5–11). It was observed that pure ac-
etone or pure water as solvents afforded 1b in only 25%
and 10% yields, respectively (Table 1, entries 10 and 11).
The optimized reaction conditions used only 5 mol% of
O
B
2m
2n
O
O
1
B
O
a
1
All products were confirmed by comparison of H NMR and mass
spectra.
Isolated yields.
b
©
Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1394–1396

1
396
J. Zhang et al.
LETTER
(5) For reviews, see: (a) Ishiyama, T.; Miyaura, N. J. Synth.
With these results in hand, we examined the scope of the
reaction and obtained a variety of the expected pinacol–
substituted phenylboronates in moderate to good yields
Org. Chem. Jpn. 1999, 57, 503. (b) Ishiyama, T.; Miyaura,
N. J. Organomet. Chem. 2000, 611, 392. (c) Miyaura, N. In
Catalytic Heterofunctionalization; Togni, A.; Grützmacher,
H., Eds.; Wiley-VCH: Chichester, 2001, Chap. 3.
1
4
under the optimized reaction conditions (Table 2).
All of the reactions were completed within six hours and
afforded the desired pinacol arylboronates. Moreover
present reaction rules of the steric and electronic effects
were in agreement with results reported which using aryl-
diazonium salts as borylation material.9 The meta- and
para-substituted aryldiazonium salts favored the reaction
while ortho-substituted aryldiazonium salts gave only
lower yields (Table 2, entry 8). Substrates with electron-
withdrawing groups showed better reactivity than elec-
tron-donating groups did. But the electronic effect might
play a dominant role in the case of pinacol 3-nitro-5-meth-
ylphenyl boronate (Table 2, entry 9). Finally, it was worth
noting that substrates bearing bromo and aminosulfonyl
substituents could also be employed in this reaction af-
fording moderate yields (Table 2, entries 2 and 10).
(6) (a) Murata, M.; Watanabe, S.; Masuda, Y. J. Org. Chem.
1
997, 62, 6458. (b) Murata, M.; Oyama, T.; Watanbe, S.;
Masuda, Y. J. Org. Chem. 2000, 65, 164. (c) Baudoin, O.;
Guénard, D.; Guéritte, F. J. Org. Chem. 2000, 65, 9268.
(
d) Broutin, P. E.; Cerna, I.; Campaniello, M.; Leroux, F.;
–11
Colobert, F. Org. Lett. 2004, 6, 4419. (e) Murata, M.;
Sambommatsu, T.; Watanabe, S.; Masuda, Y. Synlett 2006,
1867.
(7) (a) Brown, H. C.; Srebnik, M.; Cole, T. E. Organometallics
1
986, 5, 2300. (b) Brown, H. C.; Cole, T. E.
Organometallics 1983, 2, 1316.
(
8) (a) Fürstner, A.; Seidel, G. Org. Lett. 2002, 4, 541. (b) Zhu,
L.; Duquette, J.; Zhang, M. J. Org. Chem. 2003, 68, 3729.
(c) Giroux, A. Tetrahedron Lett. 2003, 44, 233.
(
d) Billingsley, K.; Barder, T. E.; Buchwald, S. L. Angew.
Chem. Int. Ed. 2007, 46, 5359.
(9) Willis, M. D.; Strongin, M. R. Tetrahedron Lett. 2000, 41,
683.
8
In conclusion, we have developed a new procedure, which
is compatible with green chemistry principles, for the syn-
thesis of arylboronates. Putative advantages over the
known methodology include: 1) providing direct access to
arylboronates bearing halo and acidic substituents; 2)
mild conditions and using water as solvent; and 3) a cata-
lytic amount of Cu(I) salt additive reduces the metal con-
tamination. Though the negative effect from steric
hindrance can reduce the scope of the method, we believe
that this protocol is a useful addition to known procedures
for arylboronate synthesis.
(
10) Ma, Y. D.; Song, C.; Jiang, W.; Xue, G. P.; Cannon, F. J.;
Wang, X. M.; Andrus, B. M. Org. Lett. 2003, 5, 4635.
(11) Mo, F. Y.; Jiang, Y. B.; Qiu, D.; Zhang, Y.; Wang, J. B.
Angew. Chem. Int. Ed. 2010, 49, 1846.
(
12) General Procedure for the Synthesis of 1a–n
A solution of aromatic amines (20 mmol) in HCl (6 mL) in
a flask was cooled to 0 °C and diazotized with 1.42 g (22
mmol) of NaNO . The temperature must be kept below 5 °C.
2
After 15 min, the ice-cold solution of NaBF was rapidly
4
poured into the flask, which had been cooled below 0 °C.
The temperature should remain below 5 °C. Powerful
stirring was required to agitate the thick magma at this stage.
After 30 min stirring, the solid was filtered with Büchner
funnels. The crystal of aryldiazonium salts was washed with
iced H O, with MeOH, and with commercial Et O.
Acknowledgment
2
2
(
(
13) Kochi, J. K. J. Am. Chem. Soc. 1957, 79, 2942.
14) General Procedure for the Synthesis of 2a–n
Financial support was received from the Natural Science Foundati-
on of Jiangshu Province (BK2011813).
Aryldiazonium tetrafluoroborate salts (2 mmol) and B pin2
2
(
2 mmol) were resolved in MeCN (6 mL) and H O (3 mL).
2
After the solution was clear, CuBr (0.1 mmol) was added.
The resulting reaction mixture was allowed to stir for 5–6 h
at r.t. The solution was then concentrated under reduced
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1) For reviews, see: (a) Suzuki, A. Acc. Chem. Res. 1982, 15,
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1
pressure. The residue was diluted with brine. The H
was extracted with EtOAc several times. The combined
organic layer was dried over anhyd Na SO and concen-
2
O layer
(c) Miyaura, N. Top. Curr. Chem. 2002, 219, 11.
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2
4
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(
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purified on silica gel by flash column chromatography with
EtOAc and hexane.
1
Compound 2a: H NMR (400 MHz, CDCl
): δ = 1.39 (s, 12
3
(3) Xu, J. M.; Wang, X. Y.; Shao, C. W.; Su, D. Y.; Cheng,
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2 H, HAr). MS (EI): m/z (%) = 248 (2.8) [M ], 233 (57.3),
3
+
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Synlett 2012, 23, 1394–1396
© Georg Thieme Verlag Stuttgart · New York