A BODIPY-functionalized PdII photoredox catalyst for Sonogashira C-C cross-coupling reactions

Article abstract of DOI:10.1039/c9cc01365b

We report for the first time a BODIPY-functionalized dichloro(1,10-phenanthroline)palladium(ii) complex as an efficient photoredox catalyst for the Sonogashira C-C cross-coupling between phenylacetylene derivatives and iodobenzene derivatives with yields

Full text of DOI:10.1039/c9cc01365b

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DOI: 10.1039/C9CC01365B
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II
A BODIPY-functionalized Pd photoredox catalyst for Sonogashira
C-C cross-coupling reactions
a
a
a
a
Komadhie C. Dissanayake, Peters O. Ebukuyo, Yasser J. Dhahir, Kraig Wheeler and Hongshan
He,*
Received 00th January 20xx,
Accepted 00th January 20xx
a
DOI: 10.1039/x0xx00000x
We report for the first time a BODIPY-functionalized dichloro(1,10-
phenanthroline)palladium (II) complex as an efficient photoredox
catalyst for the Sonogashira C-C cross-coupling between
phenylacetylene derivatives and iodobenzene derivatives with
yields up to 92% under the visible light illumination at room
temperature.
[
Pd]
R1
+
X
R2
R1
R2
Pd(II)L2
F
B
N
F
F
N
N
B
F
N
A
R1
R2
R1X
Pd(0)L2
L
B
E
L
N
N
R1 Pd
L
X
Pd
Palladium-based catalysts have been widely used in organic
L
Pd
R2
1
Cl
Cl
1
-4
R1
chemistry for C-C cross-coupling reactions.
One typical
C
Cu
R2
R3NHX
example is the Sonogashira cross-coupling of aryl halides with
terminal alkynes at an elevated temperature. The reaction was
first reported in 1975 by Kenkichi Sonogashira, Yasuo Tohda,
D
L
R1 Pd
L
R2
N
N
N
N
F
CuX
Pd
B
R3N
5
H
R2
H
R2
Cl
Cl
F
and Nobue Hagihara. It was further developed through
CuX
2
3
extensive optimization and has become one of the most useful
6
reactions in organic synthesis. The catalytic system includes a
Scheme 1. Conventional Sonogashira C-C coupling reactions and structures
Pd(II) catalyst with a bulky, electron-rich phosphine ligand, of catalysts for this study.
copper iodide, and an amine. A typical combination is
In fact, photocatalysts have been used in organic synthesis.7-
However, there are only a few photocatalysts reported for
[
(Ph
accepted catalytic mechanism, in which Pd(II) is reduced to
Pd(0) by either Et N or PPh or both (process A). It then forms a
3 2 2 3 2
P) PdCl ]/PPh /CuI/Et NH. Scheme 1 shows a widely
1
9
1
6, 20-24
Sonogashira C-C cross-coupling reactions.
In 2007, Akita
et al reported that a mixture of tri(bipyridine)ruthenium
complex (8 mol%) with [Pd(CH CN) Cl ], and P(t-Bu) promoted
3
3
2
5
Pd-halide adduct through an oxidative addition (process B).
Next, a copper (I) acetylide from the copper cycle reacts with
Pd-halide compound through a transmetallation process to
form a Pd-acetylide intermediate (process C), which then
undergoes a trans-cis isomerization (process D) to produce
alkyne through a reductive elimination with simultaneous
3
2
2
3
the Sonogashira coupling reaction at room temperature under
the visible light illumination. It was found the yield of the
product was much higher than the conventional method. The
function of the ruthenium (II) complex in the catalysis was
unclear. In 2014, Mori et al found a Ru-Pd bimetallic complex
facilitated the Suzuki-Miyaura C-C coupling reactions between
bromobenzene and phenylboronic acid in the presence of PPh
CO in ethanol under Ar atmosphere. It was proposed that the
excited Ru(II)-polypyridyl moiety transferred electrons to the
palladium center reducing the Pd(II) to Pd(0), which ultimately
enhances the photocatalytic activity in the Suzuki–Miyaura
coupling reaction. In both studies, high dosage of ruthenium (II)
2
3
4
, 6
regeneration of Pd(0) catalyst. The reaction has been used
successfully in making numerous novel products, however,
significant efforts have been devoted to simplify or make the
reaction more efficient including developing Cu-free catalytic
systems. With a few exceptions, most reactions still require
3
,
K
2
3
4
,6
elevated temperature for high yields. In addition, high dosage
catalyst required (sometimes up to 5% Pd catalyst) also makes
the preparation not cost-effective. Therefore, it is of
importance to develop novel catalytic systems, such as
(
~8 mol%) was used, which could be problematic in regard to
the high cost and synthetic difficulty of these compounds.
found Pd-decorated TiO2 nanoparticles
catalyzed the C-C cross-coupling effectively under 465 nm
photocatalytic systems, to address the aforementioned
2
0
concerns.1
Scaiano et al
illumination through direct excitation of Pd nanoparticles,
a. Department of Chemistry and Biochemistry, Eastern Illinois University, Charleston, whereas Huang et al found CuCl can also catalyze the C-C
26
IL 61920. Tel: 217-581-6231. Email: hhe@eiu.edu
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
cross-coupling under a blue LED irradiation. Herein, we report
our attempts to explore the feasibility of catalyzing the
Sonogashira C-C cross-coupling reaction using a BODIPY-
†
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COMMUNICATION
Journal Name
functionalized Pd(II) complex (1) under VIS illumination as
shown in Scheme 1. The catalyst is a dichloro(1,10- iodobenzene and phenylacetylene at ro
We found that complex 1 can catalyze the reactViioewnAbrtiecltewOenleinne
re The
D
o
O
m
I: 1
t
0
e
.
m
10
p
39
e
/
r
C
a
9
t
C
u
C
0
.
13
65B
phenanthroline)palladium (II) compound, in which two BODIPY reaction was carried out under nitrogen. In a typical reaction,
moieties are linked to the 1,10-phenanthroline through its 4,7 1.2 mmol phenylacetylene, 1.0 mmol of iodobenzene, 0.005
positions. BODIPY, an abbreviation of a class of dipyrromethene mmol PPh
3
, and catalyst 1 in 4.0 mL DMF were mixed with 1.0
compounds, has exceptional light absorption and fluorescence mL Et N in a Schlenk flask. A 13W Great Value LED lamp (1050
3
capability, and is easy to make. We envisioned that BODIPY lumens) was used as the light source. The distance between the
moieties in compound 1 will absorb the light and can transfer lamp and the flask was kept at 10 cm. The temperature of the
electrons to facilitate the reductive elimination. The use of flask was maintained at room temperature and the reaction
bidentate 1,10-phenanthroline in the compound 1 will mixture was drawn at different times for GC-MS analysis (see
eliminate trans-cis isomerization during the catalysis making ESI). Table 1 summarizes the results from the optimization.
the cross-coupling easier than the traditional coupling Upon illumination of the reaction for 24 h, 92% yield was
reactions. To the best of our knowledge, this is the first example obtained when 0.5% catalyst was used. In stark contrast, the
of a Pd(II) complex for photocatalytic Sonogashira C-C cross- reaction only gave 25% yield under a dark condition. Increasing
coupling reaction.
The complex 1 was prepared from a direct reaction between product was observed in the absence of catalyst, triethylamine
a ligand (BDP) and [Pd(CNPh) Cl ] in dichloromethane (DCM) at or triphenylphosphine. The reaction in other solvents such as
the catalyst to 5 mol% only increased the yield marginally. No
2
2
room temperature (Details can be found at ESI). The BDP was THF did not produce any product. In a comparative study, we
prepared from the reaction between 4,7-diformyl-1,10- performed the same reaction using the compound 2, in which
phenanthroline27 and 2,4-dimethylpyrrole in dry DCM under no BODIPY moiety was attached and compound exhibited weak
nitrogen, followed by reactions with 2,3-dichloro-5,6-dicyano- light absorption in the visible region. Only 17% yield was
2
8
1
,4-benzoquinone (DDQ), Et
3
N
and BF
3
•OEt
2
.
The obtained after 24 h illumination. This indicates clearly the key
1
compositions of the BDP and complex 1 were ascertained by H role of BODIPY moieties in the catalysis.
NMR, MS and elemental analysis. The structure of the complex
Table 1. Optimization of reaction conditions.a
1
was ascertained by single crystal X-ray diffraction analysis with
the ORTEP diagram of 1 shown in Fig. 1. The central Pd is
coordinated by two N and two Cl atoms forming a square planar
geometry. Two BODIPY moieties are almost perpendicularly
attached to the 4,7 positions of 1,10-phenanthroline with
torsion angles ~ 99.63° and 105.50°, respectively. The complex
has a strong absorption in the visible region with a peak position
at 509 nm as shown in Fig. 2. The complex showed very weak
fluorescence, which is a stark contrast to the BDP with an
extremely strong emission at 525 nm.
I
o
DMF, 25 C
+
(1)
1
or 2, Et3N, PPh3
Entry
Catalytic system
Yield (%)b
4
h
17h
ND
ND
14
3
24h
ND
ND
18
5
1
2
3
4
5
6
7
1(0.5 mol%)
Et N + PPh
1 (0.5 mol%)+ Et N
IL
IL
IL
IL
IL
D
IL
ND
ND
6
3
3
3
1 (0.5 mol%)+ PPh
3
0.06
23
5
1 (0.5 mol%)+ Et
1 (0.5 mol%)+ Et
2 (0.5 mol%)+ Et
3
N + PPh
3
N + PPh
3
N + PPh
3
3
3
86
22
13
92
25
17
3
aThe typical reaction conditions were used. b: GC-MS yield. ND, not detected.
Table 2 summaries the outcomes of reactions between
iodobenzene and phenylacetylene derivatives in the presence
of 1 under optimized conditions. When phenylacetylene
reacted with iodobenzene derivatives having a substituent in
the para position, yields varied from 1% to 92%. Only a trace
2 2
amount of coupling product was observed when R = NH . On
the contrary, the variation of the yields was relatively small
when iodobenzene reacted with phenylacetylene derivatives.
The yields vary from 64% to 98%. The yields are slightly lower
Fig. 1. ORTEP diagrams of 1 in two different views (50% thermal ellipsoid
probability).
when a substituent (OCH
position of phenylacetylene. It was found that a combination of
and 3 in the reaction also produced a decent yield for the
3 3 2
, CH or NO ) was introduced to para
2
reaction between iodobenzene and phenylacetylene as shown
in Table 3. Results clearly demonstrate the compound 1 acted
as a photocatalyst for C-C coupling reactions. It is worth pointing
out that no homocoupling product was observed in all cases.
Several reaction mechanisms have been proposed for
1, 29
copper-free reactions
and a widely accepted one is a
Fig. 2. Absorption spectra (a) and emission spectra (b) of BDP and 1 in
2
dichloromethane at room temperature.
deprotonation mechanism, in which a  -acetylene-palladium
intermediate is formed.
3
0
Experimental and theoretical
2
| J. Name., 2012, 00, 1-3
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studies31, 32 indicate the high energy barrier for the formation of Table 3. Yields of different C-C coupling reactions using 2 and 3
a
View Article Online
DOI: 10.1039/C9CC01365B
this species. By the observation that substituents on the
phenylacetylene exerted minimal impact on the reaction yield
R1
I
+
o
DMF, 25 C
R1
R2
(3)
2
, 3, Et3N, PPh3
(entries 11-14 in Table 2), the reaction proceeds most likely
R2
through a transmetallation mechanism proposed recently by
Entry
Substituents
Yield (%)
b
Kosmrlj et al.3
2, 33
The catalytic cycle of our reactions started
R
R
4h
15
3
17h
84
20
9
24h
87
21
9
1
2
II
0
II
from the reduction of Pd to Pd . The Pd should be reduced to
1
2
3
4
H
H
H
H
H
0
OCH3
CH
NO
Pd by Et
3
4
N and or PPh
3
as observed in traditional Sonogashira
reactions. This is also supported by the low yield of product in
the absence of Et N or PPh (entries 3 and 4 in Table 1) as well
3
1
2
0.6
3
3
3
3
as very similar yields of reactions between one with 1 in the dark aThe reaction conditions: 2 (0.5 mol%) 3 (1 mol%), phenylacetylene (1.2 mmol),
(0.005 mmol), and Et N (1.0 mL).
and 2 under illumination (entries 6 and 7 in Table 1). The iodobenzene (1.0 mmol), DMF (4.0 mL), PPh
illumination may enhance the reduction of Pd as observed in
3
3
II
Light source: Great Value LED bulb, 13 W, 1050 lumens. The distance between
lamp and the reaction flask was about 10 cm. b: GC-MS yield.
several other heterogeneous photocatalytic C-C cross-coupling
2
0, 34
reactions
(
due to the unique photo induced electron transfer
3
5
PET) process between BODIPY moiety and its orthogonally
oriented aromatic group in its meso position; however,
electrochemistry studies showed quite similar redox potential
profiles of BODIPY moieties and Pd center in complex 1 in the
dark and under illumination as shown in Fig. 3 (a). Therefore,
II
PET may have minimal effect on the reduction of Pd . After the
II
0
Pd was reduced to Pd , PPh
with the fourth site being coordinated possibly by solvent (s).
We monitored the absorption spectral change of 1 + Et N + PPh
3
coordinated to the metal center
3
3
3
Fig. 3. CV profiles of 1 in CH CN in the dark and under illumination (a) and
in DMF under nitrogen. As shown in Fig. 3 (b), a new broad peak absorption spectral changes of 1 + Et N + PPh in DMF upon illumination (b)
3
3
at room temperature. All measurements were carried out under nitrogen.
around 540 nm appeared under light illumination. This
observation indicated the formation of active species, such as
[
3
Pd(BDP)(PPh )(S)]. This is consistent with the isolated three-
II
36, 37
coordinate Pd complex by Hartwig and co-workers.
As
shown in Scheme 2, the formed active species will go oxidative
addition with iodobenzene to form intermediate II, which will
transform to III through the transmetallation with the
intermediate IV. The reductive elimination will produce the
product and release the initial active species I. Due to the
bidentate nature of phenanthroline, no trans-cis isomerization
will be involved.
Table 2. Yields of different C-C coupling reactions using photocatalyst 1.a
R1
I
+
o
DMF, 25 C
R1
R2
(2)
1
, Et3N, PPh3
R2
Entry
Substituents
Yield (%)b
17h
18
ND
74
86
43
20
38
R
1
R
2
4h
7
24h
36
Very low
83
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
OCH
NH
CH
H
3
2
ND
27
23
15
11
12
2
3
92
50
Cl
COOCH
COOEt
CHO
3
23
40
Scheme 2. Proposed photocatalytic C-C cross-coupling reactions between
iodobenzene and phenylacetylene. S: solvent; Ar and Ar ; aromatic groups.
1 2
8
15
9
33
18
98
87
64
88
We envision the function of BODIPY moieties in the catalysis
CN
8
2
6
is to facilitate the reductive elimination. Hwang et al reported
a photo-induced Sonogashira reaction catalyzed by CuCl under
a blue LED at room temperature. It was proposed that excitation
of copper phenylacetylide facilitate the coupling between
acetylene and led to the formation of electron-deficient
acetylene moiety via ligand to metal charge transfer, making
1
1
1
1
1
0
1
2
3
4
H
CH
N(CH
NO
CN
NO
H
H
H
H
2
ND
17
44
40
66
8
91
82
63
3
3
)
2
2
86
a
b
The typical reaction conditions were used. GC-MS yield.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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acetylene positive for electron-rich group-substituted phenyl 8. J. Xuan and W.-J. Xiao, Angew. Chem. Int. Ed., 2012, 51, 6828-
View Article Online
DOI: 10.1039/C9CC01365B
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6838.
3
8
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