Metal-free, visible light-induced borylation of aryldiazonium salts: A simple and green synthetic route to arylboronates

Article abstract of DOI:10.1002/adsc.201200416

The eosin Y-catalyzed borylation of aryldiazonium salts under irradiation with visible light has been developed. This novel protocol provides an attractive route for the preparation of arylboronates, due to its operational simplicity and practicability as well as the mild reaction conditions.

Full text of DOI:10.1002/adsc.201200416

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DOI: 10.1002/adsc.201200416
Metal-Free, Visible Light-Induced Borylation of Aryldiazonium
Salts: A Simple and Green Synthetic Route to Arylboronates
Jian Yu,a Ling Zhang,a and Guobing Yana,*
a
Department of Chemistry, Lishui University, Lishui 323000, Peopleꢀs Republic of China
Fax : (+86)-578-227-1250; e-mail: gbyan@lsu.edu.cn
Received: May 10, 2012; Published online: October 2, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200416.
Abstract: The eosin Y-catalyzed borylation of aryl-
diazonium salts under irradiation with visible light
has been developed. This novel protocol provides
an attractive route for the preparation of arylboro-
nates, due to its operational simplicity and practica-
bility as well as the mild reaction conditions.
clear, their preliminary observations indicate that
a radical process can be involved.
As we know, aryldiazonium salts are an excellent
source of aryl radicals[9] and are often used as oxida-
tive quenchers in photoredox chemistry.[10] For exam-
ple, Sanford and co-workers in 2011 reported palladi-
um catalysis in combination with visible light photoca-
À
Keywords: arylboronates; aryldiazonium salts; bor-
ylation; photoredox catalysis; visible light
talysis for the arylation of aromatic C H bonds with
aryldiazonium salts.[11] Kçnig and co-workers devel-
À
oped the direct C H bond arylation of heteroarenes
with aryldiazonium salts catalyzed by eosin Y under
irradiation with visible light.[12] Compared with the
transition metal-catalyzed coupling reactions, these
Arylboronic acids or arylboronates represent versatile photoinduced redox processes with visible light repre-
building blocks in organic synthesis.[1] They have sent a unique strategy, because of their inherent
found wide applications in transition metal-catalyzed “green chemistry” features. Recently, photochemistry
cross-coupling reactions.[2] Due to their high stability has already attracted much attention from organic
and low toxicity, increasing attention has recently also chemists. The groups of MacMillan, Yoon and Ste-
been devoted to their applications in molecular recog- phenson have focused on the use of visible light pho-
nition and as pharmaceutical candidates.[3] Classical toredox catalysts such as ruthenium or iridium poly-
synthetic methods to prepare these boron coupounds pyridyl complexes to promote the enantioselective a-
are based on the reaction of trialkyl borates with alkylation and a-fluoroalkylation of aldehydes,[13] cy-
Grignard or lithium reagents.[4] This methodology, al- cloaddition reactions of enones[14] and reductive deha-
though still widely used in industry as well as in aca- logenations[15], respectively. In addition, the photoca-
À
demia, suffers some major drawbacks that include rig- talytic oxidation of C H bonds adjacent to nitrogen
orous anhydrous conditions and the limitation of sub- atoms of tertiary amines can afford highly reactive
strates. In the past decades, the transition metal-cata- iminium ion intermediates that subsequently react
À
lyzed borylation of aromatic halides or pseudohalides with various nucleophiles for the formation of C C,
[16]
À
À
has emerged as a general and powerful method for C N, and C P bonds. The development of efficient
the synthesis of arylboronates. Palladium[5] and and metal-free processes is highly desirable. The use
copper[6] catalysts have been shown to be effective for of organic dyes as photoredox catalysts,[12,16h] which
this transformation. More recently, significant prog- are environmentally friendly, inexpensive, and easy to
À
ress has also been made in selective borylation of C
handle, will be an attractive alternative to transition
H bonds of aromatic substrates by using rhodium and metal photocatalysts. Herein, we report a simple and
iridium complexes with B2pin2 or HBpin.[7] In addi- green borylation of aryldiazonium salts catalyzed by
tion, Wang and co-workers in 2010 developed a novel eosin Y under irradiation with visible light.
metal-free method for the synthesis of arylboronates
At the outset of this investigation, we used diazoni-
from arylamines with B2pin2 in the presence of tert- um salt 1a as model substrate to screen suitable reac-
butyl nitrite.[8] This cost-effective procedure serves as tion conditions and the results are summarized in
an important complementary route to arylboronates. Table 1. When 2 mol% eosin Y was used as the cata-
Although the mechanism of this reaction is not yet lyst, the borylation of 1a indeed took place in MeOH
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Jian Yu et al.
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Table 1. Optimization of the reaction conditions.[a]
Table 2. Substrate scope of visible-light photocatalysis cata-
lyzed borylation of aryldiazonium salts.[a]
Entry ArN2BF4 (1a–n)
Products (2a–n) Yield [%][b]
1
2
3
2a
2b
2c
76
70
80
Entry
Eosin Y
(mol%)
Solvent
Time
[h]
Yield
[%][b]
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
5
MeOH
MeCN
acetone
THF
H2O
MeCN/H2O (5/1)
MeCN
MeCN
MeCN
24
24
24
24
36
24
18
36
48
48
64
51
45
28
58
4
5
2d
2e
67
75
76
none
5
0[c]
40[d]
[a]
Unless otherwise noted, the reaction conditions are as
follows: 1a (1.5 equiv.), B2pin2 (0.3 mmol), eosin Y (2–
10 mol%), 25 W lamp, solvent (2 mL), at room tempera-
ture.
6
7
2f
2g
80
75
[b]
[c]
[d]
Isolated yield.
The reaction was carried out in the absence of eosin Y.
The reaction was carried out under ambient light.
8
9
2h
2i
78
60
at room temperature under irradiation with a 25 W
visible light lamp, albeit in low yield (entry 1). En-
couraged by this initial result, we proceeded to opti-
mize the proposed reaction. Several other solvents
were tested. It was found that acetonitrile (MeCN)
was the most suitable solvent to promote the reaction
with 64% yield (entry 2). Acetone and tetrahydrofur-
an (THF) afforded the borylation product in approxi-
mately the same yields (entries 3 and 4), whereas the
yield of the reaction slightly dropped in water
(entry 5). Reaction with a mixed solvent of MeCN
and water afforded 58% yield (entry 6). We then at-
tempted to increase the eosin Y catalyst loading, the
yield of the reaction was obviously improved and the
reaction time was shortened (entry 7). Finally, for
comparison the reaction was carried out in the ab-
sence of eosin Y catalyst, no product 2a could be de-
tected (entry 8). When the reaction was carried out
under ambient light, the reaction took longer with
dramatically decreased yield (entry 9). After exten-
sive screening of the reaction conditions, the opti-
mized reaction conditions were found to be: 5 mol%
eosin Y as catalyst and 25 W visible light lamp in
MeCN at room temperature.
10
11
2j
66
80
2k
12
13
2l
78
2m
2n
60
14
Trace
[a]
Unless otherwise noted, the reaction conditions are as
follows: 1a–n (1.5 equiv.), B2pin2 (1 mmol), eosin Y
(5 mol%), 25 W lamp, solvent (3 mL), at room tempera-
ture.
[b]
Isolated yield.
good yields. It was observed that the reaction was
Under the optimized conditions, the substrate marginally affected by electronic effects of the sub-
scope of this reaction was investigated. The results stituents of the aryldiazonium salts. The substituted
are summarized in Table 2. Aryldiazonium salts aryldiazonium salts bearing electron-deficient groups
having various electron-donating groups and electron- (entries 3, 6, 8, 11, 12) showed slightly higher reactivi-
withdrawing groups reacted smoothly and afforded ty than those bearing electron-donating groups (en-
the corresponding borylation products in moderate to tries 1 and 2). In addition, substantial steric hindrance
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Adv. Synth. Catal. 2012, 354, 2625 – 2628

Metal-Free, Visible Light-Induced Borylation of Aryldiazonium Salts
Schlenk round-bottomed flask under a nitrogen atmosphere.
Then 3 mL MeCN were added by syringe. The resulting re-
action solution was stirred at room temperature under irra-
diation with a 25 W visible light lamp. The reaction progress
was monitored by GC-MS. After the completion of the reac-
tion, the solution was filtered though a short column of
silica gel and washed with EtOAc. The filtrate was concen-
trated under reduced pressure to leave a crude product,
which was purified by flash column chromatography on
silica gel to afford the final products.
Scheme 1. Proposed mechanism for photocatalytic boryla-
tion of aryldiazonium salts.
Acknowledgements
We thank the Natural Science Foundation of Zhejiang Prov-
ince (No. Y4110324, LY12B02006) for financial support.
of the aryldiazonium salts was obvious. For example,
the reaction with para-chloro- or para-bromo-substi-
tuted aryldiazonium salts afforded higher yields than
those of ortho-chloro- or ortho-bromo-substituted ar-
yldiazonium salts (entries 3/4 and 8/9). It is particular-
ly noteworthy that chloro, bromo and fluoro groups
are tolerated in the reaction, which is advantageous
for further transformations (entries 3–10). The reac-
tion tolerated a wide range of functional groups, such
as alkoxy, alkyl, chloro, bromo, fluoro, nitro, and
acetyl groups. However, the application of this pro-
cess is problematic for the borylation of heteroaro-
matic diazonium salts (entry 14). The reason for the
failure of the reaction here is less clear.
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