Visible-Light-Promoted Arylation Reactions Photocatalyzed by Bismuth(III) Oxide

Article abstract of DOI:10.1002/ejoc.201701242

Bismuth(III) oxide has been successfully applied as a photocatalyst in the arylation of heteroarenes with diazonium salts. With a low catalyst loading (1 to 5 mol-%), this cheap and nontoxic semiconductor could efficiently promote the formation of the aryl radical under visible light irradiation. The arylated products are obtained in moderate to good yields, and the process admits straightforward scale-up (10 mmol; 1 mol-% Bi₂O₃). In two cases, the diazonium salt was generated in situ and used in the photocatalytic arylation in a tandem manner. Besides heteroarenes, Bi₂O₃ also catalyzed the arylation of differently substituted alkenes and alkynes, thus representing a viable and practical alternative to the more commonly used ruthenium complexes and organic dyes.

Full text of DOI:10.1002/ejoc.201701242

A Journal of
Accepted Article
Title: Visible Light-Promoted Arylation Reactions Photocatalyzed by
Bismuth(III) Oxide
Authors: Miquel A. Pericàs, Laura Buglioni, Paola Riente, and Emilio
Palomares
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201701242
Link to VoR: http://dx.doi.org/10.1002/ejoc.201701242
Supported by

European Journal of Organic Chemistry
10.1002/ejoc.201701242
COMMUNICATION
Visible Light-Promoted Arylation Reactions Photocatalyzed by
Bismuth(III) Oxide
[a]
[a]
[a],[b]
and Miquel A. Pericàs*[a],[c]
Laura Buglioni, Paola Riente, Emilio Palomares,*
processes initiated by the formation of phenyl radicals starting
from aryldiazonium salts 1.
Abstract: Bismuth(III) oxide has been successfully applied as a
photocatalyst in the arylation of heteroarenes with diazonium salts.
With a low catalyst loading (1 to 5 mol%), this cheap and non-toxic
semiconductor could efficiently promote the formation of the aryl
radical under visible light irradiation. The arylated products are
obtained in moderate to good yields, and the the process admits
X
N2BF4
X
catalyst or
R
+
R
photocatalyst
1
2
3
straightforward scale-up (10 mmol; 1 mol% Bi
diazonium salt was generated in situ and used in the photocatalytic
arylation in a tandem manner. Besides heteroarenes, Bi proved
2 3
O ). In two cases, the
catalyst = Cu(I) salt,[15] TiCl3,[17-19]
O
2 3
polyaniline,[20] hydrazines,[21]
also to catalyze the arylation of differently substituted alkenes and
alkynes, thus representing a viable and practical alternative to the
more commonly used ruthenium complexes and organic dyes.
photocatalyst = eosin Y,[22] porphirin,
[23]
[24]
Ru(II) complexes,[25]
TiO2 (mostly stoichiometric)[27]
methylacridine,
this work = Bi O3 (1 or 5 mol%) with visible light
2
Scheme 1. Arylation reaction with diazonium salts 1.
In the latest years, photocatalyzed transformations have been
receiving increasing attention.[1] Especially when promoted by
Particularly, the arylation of heteroarenes represents an
appealing reaction since it provides access to important
heterobiaryl structures which can find applications in several
scientific/technological fields for their biological[11] and optical[12]
properties. Due to the presence in its structure of a weak and
visible light-sources, photocatalysis represents an important pillar
in the development of greener synthetic strategies.[2] On the other
hand, the common use of metals like ruthenium and iridium as
catalysts in those processes constitutes a significant limitation,
because of their high cost and toxicity. For this reason, the quest
for cheaper, environmentally benign and non-toxic photocatalysts
labile C-N bond,[ diazonium salts have been widely exploited in
13]
arylation reactions.[
14]
The copper-catalyzed version of this
represents a fundamental challenge in synthetic chemistry.
Organic dyes[3] and inorganic semiconductors[4,5] can efficiently
transformation (the Meerwein arylation) has been known since
the 1930s, but it suffers from some important drawbacks, such as
the usually low yields and the high catalyst loading required.[
Since then, many variations of the original procedure have been
reported (Scheme 1), involving both thermal[17-21] and
photochemical[22-27] reaction conditions. König and coworkers
substitute
metal
complexes
in
many
photocatalytic
15]
transformations. In 2014, our laboratory pioneered the use of
bismuth(III) oxide as an efficient photocatalyst. Bismuth(III) oxide
has the advantage to be not only very cheap, but also non-toxic.[6]
[16]
Moreover, in comparison with titanium(IV) oxide it possesses a
proved that this transformation can be photocatalyzed by eosin
smaller band gap (2.1 – 2.8 eV); so, it is able to absorb light in the
visible region of the electromagnetic spectrum.[7] Till now, this
Y,[
22a]
and that under visible light irradiation the phenyl radical is
formed from the diazonium salt 1. In the field of transition metal
semiconductor has been successfully used in the photocatalyzed
enantioselective -alkylation of aldehydes,[8] as well as in atom
photocatalysis, initial work dealing with the use of Ru(II)
[
25]
[9]
complexes as visible-light photocatalysts was followed by the
use of titanium(IV) oxide in spite of its larger band gap.[ In fact,
transfer radical addition (ATRA) reactions,
and in
27]
photopromoted radical thiol-ene reactions.[10] Looking for new
applications for this photocatalyst, we focused our attention on
it has been reported that TiO
which absorbs light in the visible spectrum.[27] Although
stoichiometric amounts of TiO were often necessary, the problem
was partially overcome by the immobilization of the
2
can react with 1 to form a diazoether,
2
[
a]
Dr. L. Buglioni, Dr. P. Riente, Prof. E. Palomares, Prof. M. A. Pericàsꢀ
Institute of Chemical Research of Catalonia (ICIQ)ꢀ
Avda.Països Catalans 16, 43007 Tarragona (Spain)
E-mail: epalomares@iciq.es; mapericas@iciq.es
semiconductor in the microchannels of
a continuous-flow
reactor.[28]
Herein, we report the successful application of inexpensive,
commercially available, and non-toxic bismuth(III) oxide to
promote the photocatalyzed -arylation of heteroarenes (Table 1).
To optimize conditions, the reaction of furan (2a) with
4-bromobenzenediazonium tetrafluoroborate (1a) was selected
as a benchmark. The light source used in the optimization process
was in all cases a household 23 W compact fluorescence lamp
http://www.iciq.org/research/research_group/prof-emilio-palomares/
http://www.iciq.org/research/research_group/prof-miquel-a-pericas/
Prof. E. Palomaresꢀ
[
[
b]
c]
Catalan Institution for Research and Advanced Studies (ICREA)
Avda. Lluis Companys 23, 08010 Barcelona (Spain)
Prof. M. A. Pericàsꢀ
Department de Química Orgànica, Universitat de Barcelona c/Martí i
CFL, see Supporting Information).[29]
(
Franqués 1-11, 08028 Barcelona (Spain
)
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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European Journal of Organic Chemistry
10.1002/ejoc.201701242
COMMUNICATION
Table 1. Optimization of reaction conditions.
(
2d) were tested in the reaction. The substitution pattern partially
affects the outcome of the transformation. In fact, when metha-
and ortho-substituted diazonium salts (1b and 1c, respectively)
were reacted with furan, the yields dropped down (compare 3aa,
3ba and 3ca), whereas the corresponding reactions with
thiophene were not affected (compare 3ab, 3bb, and 3cb).
O
N BF
2
Bi O (x mol%)
2 3
4
O
+
degassed solvent
CFL (23 W)
Br
Br
15 h
1a
2a
3aa
X
n
N2BF4
2 3
Bi O (5 mol%)
X
Catalyst
loading (mol%)
R
+
Entry[a]
Yield (%)[b]
R
2a (equiv.)
Solvent
degassed DMF
CFL 23 W
n
n = 1 or 2
15 h
1
a-h
2a-d
3
1
5.0
5.0
0
27
27
27
27
27
27
27
27
10
10
27
27
DMSO
DMSO
DMSO
64
20
25
15
–
2[c]
With furan (2a):
O
O
O
3[d]
4
Br
3ca, 45%
O
5.0
5.0
5.0
10[e]
5.0[f]
5.0
10
CH
3
CN
Br
Br
3
aa, 86%
3ba, 45%
5
EtOH
DMF
DMF
DMF
DMF
DMF
DMF
DMF
O
O
6
86
50
54
36
36
70
74
7
O2N
F3C
3
da, 97%
3ea, 55%
3fa, 51%
8
[a]
66%
O
O
9
10
MeOC
3ga, 67%
NC
1
1[g]
2[h]
1.0
1.0
3ha, 48%
1
With thiophene (2b):
S
S
S
[
a] 1a (0.25 mmol), 2a (27 equiv.), Bi
light (CFL, 23 W), 15 h. [b] After column chromatography. [c] The reaction was
performed in the dark. [d] The reaction was carried out for 65 h. [e] BiVO (10
mol%) was used instead of Bi . [f] Bi (5.0 mol%) was used instead of Bi
g] The reaction was carried out on 1 mmol scale (0.27 g of 1a). [h] The reaction
2 3
O (5 mol%), degassed DMF (1 ml), visible
4
Br
Br
3cb, 32%
Br
O
2 3
2
S
3
2 3
O .
3ab, 47%
3bb, 42%
[
2%[a]
4
was carried out on 10 mmol scale (2.7 g of 1a).
S
S
S
To our delight, in degassed DMSO and with 5 mol% of catalyst,
the corresponding product 3a was obtained in 64% yield (Table 1,
entry 1). When performing the reaction either in the dark (entry 2)
or in the absence of the catalyst (entry 3), 3a was isolated in low
yields, even after prolonged reaction time. From the screening of
different solvents (entries 4 – 6), DMF emerged as the best option.
O2N
MeOC
NC
3
db, 45%
3gb, 49%
3hb, 59%
With N-Boc pyrrole (2c):[b]
Boc
N
Boc
Boc
N
N
In ethanol, in turn, only degradation products could be observed
by H NMR in the reaction crude (entry 5).[30] BiVO
1
and Bi
4
2 3
S
proved not to be as efficient as Bi
the excess of furan used in the reaction (from 27 to 10
equivalents), led to a substantial drop in yield (entry 9). On the
2
O
3
(entries 7 and 8). Decreasing
O2N
MeOC
3gc, 56%
NC
3dc, 47%
3hc, 48%
2 3
other hand, increasing the loading of Bi O did not led to any yield
With pyridine (2d):[c]
improvement (entry 10). Interestingly, only 1 mol% of catalyst is
sufficient to catalyze the arylation, as it was proved when scaling-
up the reaction to 1 mmol or even up to 10 mmol (entries 11 and
N
N
1
2). In this way, it was possible to isolate 1.65 g of product 3aa in
Br
O2N
a single preparative experiment not requiring any extension in
reaction time. With the optimized conditions in hand, we started
to explore the substrate scope (Scheme 2). The reactions were
performed on 0.25 mmol scale and, for practical reasons, a 5
3ad, 0%
3dd, 50%
Scheme 2. Substrate scope. [a] The reaction was promoted by sunlight for 8 h
in Tarragona (Spain; 41.11º N) in the second week of May. [b] With 12 equiv. of
2
c. [c] With 1.2 equiv. of DIPEA.
2 3
O
was used.[31] Not only furan (2a), but also thiophene
mol% of Bi
2b), N-Boc-pyrrole (2c, with only 12 equivalents), and pyridine
(
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European Journal of Organic Chemistry
10.1002/ejoc.201701242
COMMUNICATION
The presence of electron-withdrawing groups in the structure of
the diazonium salt seems to have a positive influence on the
reaction outcome. However, even the electron-neutral diazonium
salt 1e delivered the corresponding product 3ea in moderate yield
Ph
O
OH
Me
Me
N2BF4
(a)
A)
Me
Me
+
O2N
Ph
O2N
(
55%). 4-Nitrophenyl derivatives 3da, 3db, 3dc, 3dd were
1
b, 4.0 equiv.
5, 1.0 equiv.
6, 75%
8, 90%
isolated in moderate (45-50%) to excellent (97%) yields.
Particularly, in order to obtain the pyridine derivative 3dd, it was
necessary to modify the standard conditions by addition of 1.2
equivalents of diisopropylethylamine (DIPEA). This base is
probably necessary to neutralize the tetrafluoroboric acid formed
during the reaction progress, which likely protonates the pyridine
moiety, leading to deactivation in the absence of a base. On the
other hand, 3ad was not obtained even in the presence of added
base. Electron-withdrawing in the para position of the
arenediazonium salt (i.e.; 4-Trifluoromethyl-, 4-acetyl-, and 4-
ciano, in 1f, 1g, and 1h, respectively) were all well tolerated, and
the reactions with these substrates furnished the corresponding
products in moderate yields, irrespectively of the heteroarene
partner (see products 3fa–3ha, and 3gb–3hc). From a practical
perspective, it was possible to improve the yield of the
transformation by simply exposing the reaction vessel to solar
irradiation for 8 hour, instead of using a 23 W CLF. This was
proved with products 3fa and 3cb, where yields increased by 15%
and 10%, respectively. Interestingly, it is also possible to generate
N2BF4
Ph
(
b)
B)
C)
+
Ph
OAc
O
O2N
O N
2
1b, 1.0 equiv.
7, 10 equiv.
Me
N2BF4
Br
O
+
(c)
Me
O
N
Br
N
Me
Me
1b, 3.0 equiv.
9, 1.0 equiv.
10, 46%
Scheme 4. Other applications of Bi
A) Bi (2.5 mol%), Ph PAuCl (2.5 mol%), degassed MeOH, CFL (23 W), 20
h; B) Bi (5.0 mol%), degassed DMF, CFL (23 W), 20 h; C) Bi (5.0 mol%),
degassed DMSO, CFL (23 W), 20 h.
2 3
O -photocatalyzed arylations. Conditions:
2
O
3
3
2
O
3
2 3
O
In accordance with literature precedents,[22,27] we tentatively
propose for the arylation reaction a mechanism involving first the
2 3
photoinduced excitation of Bi O . (Scheme 5). In this way, one
electron gets promoted from the valence band (VB) to the
conduction band (CB), generating positive holes (h ) in the
the starting material in situ, as demonstrated by Ranu and
coworkers (Scheme 3).[22b] Starting from the corresponding
+
semiconductor. This is followed by electron transfer to the
diazonium salt 1, this leading to its transformation into the phenyl
radical A, with evolution of nitrogen gas and tetrafluoroborate
aniline 4, tert-butyl nitrite works as a diazotizing agent to form 1.
In the presented instance, the heterobiaryl compounds 3ha and
3hb were obtained in yields comparable to the ones of the
standard procedure. This indicates the non-interference of tert-
anion. The so-formed species
A would then attack the
butyl nitrite with the reaction pathway triggered by Bi
2
O
3
.
heteroarene 2, affording the radical B. From this point, two
possible pathways can occur: i) a radical crossover, where the
catalyst is re-generated, by abstraction of an electron from the
intermediate radical B, furnishing the cation species C; and ii) a
situation where B directly reacts with the diazonium salt 1 yielding
the radical A, which starts the cycle again, following a radical
chain propagation mechanism. In any case, C is deprotonated by
the tetrafluoroborate, giving rise to the target arylated product 3.
Bi O (5 mol%)
BuONO (1.2 equiv.)
X
2
3
NH2
X
t
+
NC
degassed DMF
CFL 23 W
NC
4
X = O , 2a
2b
3ha, 59%
3hb, 52%
15 h
S
,
Scheme 3. In situ generation of diazonium salt and concomitant photocatalytic
arylation mediated by Bi
O .
2 3
N2BF4
R
In order to prove the efficiency of bismuth(III) oxide in related
photocatalytic arylations, we also studied the addition of phenyl
radicals to non-aromatic, yet unsaturated systems. (Scheme 4).
The three studied reactions had been previously reported with
ruthenium(II) complexes as photocatalysts. In the first case
1
C
e-
+ e-
BF4
N2 + BF4
X
X
n
n
CB
VB
H
R
R
h+
Scheme 4-A),[32] Bi
3
+ BF4
HBF4
- e-
(
2
O
3
works as a co-catalyst together with a
HBF4
-
Bi2O3
R
gold(I) complex: whereas the semiconductor likely promotes the
formation of the phenyl radical, the gold activates the propargyl
alcohol 5, allowing the formation of the desired product 6 in high
yield. The photocatalytically-generated aryl radical can also
efficiently attack enol acetate 7, affording 8 in 90% yield, a result
X
n
R
X
A
B
n
2
that favorably compares with the one reported with [Ru(bpy)
(
3 2
]Cl
1
Scheme 4-B) and the same substrate.[33] Moreover, 3,3-
radical chain propagation
disubstituted oxindole 10 could be isolated in 46% yield starting
from N-arylacrylamide 9 and 1b (Scheme 4-C).[34] Although the
yield achieved in this case is somewhat lower than with
ruthenium(II), the avoidance of expensive transition metal clearly
showcases the interest of the present approach.
2 3
Scheme 5. Proposed mechanism of the transformation photocatalyzed by Bi O
+
(
VB = valence band, CB = conductive band, h = hole).
In summary, we have found a new use for Bi
2 3
O as a
photocatalyst in Meerwein-type arylation reactions. This cheap
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European Journal of Organic Chemistry
10.1002/ejoc.201701242
COMMUNICATION
and non-toxic semiconductor can catalyze the formation of aryl
radicals starting from easily available diazonium salts. Only 5
mol% of catalyst is necessary to prepare a wide range of
differently-substituted -arylated heteroarenes, and the catalyst
loading can be reduced down to 1 mol% for experiments in larger
scales. In this way, it is possible to perform the reaction at 10
mmol scale, without any significant yield loss. The process
tolerates in situ generation of the diazonium salts from the
corresponding anilines. Yields vary from moderate to excellent,
the combination of electron-rich heteroarenes with electron-poor
diazonium salts usually leading to optimal yields. Interestingly, the
use of sunlight to promote the reaction can shorten reaction times
and improve the outcome of the transformation, probably as the
result of more homogeneous illumination. As an extension of this
chemistry, bismuth(III) oxide has also been successfully applied
to promote the aryl addition to different unsaturated systems.
2
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solved in DMF (1.5 ml) under argon. The solution was degassed with an
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4
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Acknowledgements
This work was funded by MINECO (grant CTQ2015-69136-R,
MINECO/FEDER), DEC (grant 2014SGR827) and the CERCA
Programme/Generalitat de Catalunya. We also thank the Severo Ochoa
Excellence
Accreditation
2014-2018
(SEV-2013-0319).
L.B.
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acknowledges the COFUND-Marie Curie action of the European Union’s
FP7 (291787-ICIQ-IPMP) for a postdoctoral fellowship.
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Keywords: Bismuth oxide • photocatalysis • arylation • visible
light • diazonium salts
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European Journal of Organic Chemistry
10.1002/ejoc.201701242
COMMUNICATION
[
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[30] This fact is remarkable, since Rueping et al. selected ethanol as the
2
solvent for the same transformation catalyzed by TiO , whereas no
product was observed in other solvents. Ethanol was necessary in their
case in order to form a diazoether, which was the species absorbing the
visible light.[27] Thanks to its smaller band gap, Bi
2 3
O does not require the
formation of the diazoether species (see UV-visible spectra in the
Supporting Information), so the reaction proceeds well without alcoholic
solvents.
[
31] This involves weighing 6 mg of Bi
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reproducible manner. At the same scale, working with 1 mol% of Bi
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European Journal of Organic Chemistry
10.1002/ejoc.201701242
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Bi
2
O
3
is a cheap, non-toxic material, which is
L. Buglioni, P. Riente, E. Palomares*, A.
M. Pericàs*
able to absorb light in the visible region. The
formation of aryl radical can be catalysed by
Page No. – Page No.
Bi
contrast with semiconductors like TiO
of Bi enables low catalyst loadings (1 to 5
mol%), even in a 10 mmol scale. Arylations
mediated by Bi are chemoselective, allow
2
O
3
,
when irradiated by visible light. In
2
, the use
Visible
Light-Promoted
Arylation
by
2 3
O
Reactions
Photocatalyzed
Bismuth(III) Oxide
2 3
O
the in situ preparation of the starting diazonium
salt and deliver the arylation products in
moderate to good yields.
Key Topic: Photocatalytic arylation
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