Photo-activation of Pd-catalyzed Sonogashira coupling using a Ru/bipyridine complex as energy transfer agent

Article abstract of DOI:10.1039/b618007h

The photo-activation of the Pd-catalyzed Sonogashira coupling reaction by using tris(bipyridine)ruthenium(II) complex (TB) as a co-catalyst was demonstrated. TB and its derivatives were used as co-catalysts in the photocatalytic systems composed of a phot

Full text of DOI:10.1039/b618007h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
Photo-activation of Pd-catalyzed Sonogashira coupling using a Ru/bipyridine
complex as energy transfer agent†
Masahisa Osawa,*a,b Hidetada Nagaib and Munetaka Akitab
Received 11th December 2006, Accepted 2nd January 2007
First published as an Advance Article on the web 15th January 2007
DOI: 10.1039/b618007h
The mixed catalyst system, Pd(CH3CN)2Cl2/P(t-Bu)3/[Ru(2,
2ꢀ-bipyridine)3]·2PF6, promotes the copper-free Sonogashira
coupling reaction of aryl bromides at room temperature
under irradiation of visible light.
Typical procedures for this Pd-catalyzed reaction involve the
use of CuI as a co-catalyst, an amine as a base and a phosphine
as the ligand for palladium, the key transition metal component.
Since the first report in 1975,5 a number of modifications and
improvements of the original protocol have been reported, such as
reaction in ionic liquids with the purpose of recycling the catalyst,6
the use of TBAF/Ag2O as effective co-catalysts,7 under aerobic
conditions for the practical convenience8 and recoverable dendritic
catalysts.9 In the conventional catalytic cycle, (i) formation of the
Pd(0) species via reductive elimination of the palladium diacetylide
intermediate and (ii) oxidative addition of aryl halide to the Pd(0)
species are involved as key steps. We envisaged that if TB(I)
or *TB(II) can be generated by irradiation of the mixed Pd–
TB catalyst system, the activated TB species may readily reduce
the Pd(II) precursor without forming the palladium diacetylide
intermediate to promote the catalytic reaction. In addition, the
activated TB species may affect the oxidative addition step.
The tris(bipyridine)ruthenium(II) complex (TB
=
[Ru(2,2ꢀ-
bipyridine)3]2+, Scheme 1) has been known as one of the ef-
fective groups which trap visible light efficiently because of its
outstanding photochemical, photophysical, and electrochemical
properties based on metal-to-ligand charge transfer (MLCT).1
The irradiation of TB(II) in solution yields *TB(II) (triplet excited
state), and if a sacrificial electron donor such as Et3N exists, TB(I)
(univalent state) is photochemically generated via *TB(II) (eqn (1)
and (2)).1
hv
∗
∗
TB(II) −−−→ TB (II) E0 ( TB (II) /TB (III)) = −0.84 V
(1)
(2)
hv
The palladium catalysts with bulky, electron-rich phosphine
ligands such as P(t-Bu)3 have been reported to exhibit unusu-
ally high reactivity for the Sonogashira coupling reaction of
aryl bromides at ambient temperature,10 as typically exempli-
fied by the Pd(PhCN)2Cl2/P(t-Bu)3/CuI/HN(i-Pr)2 (Buchwald
TB(II)
TB (I) E (TB (I) /TB (II)) = −1.28 V
−−−→
0
Et
N
3
3
and Fu),10a Pd2(dba)3/P(t-Bu)3/Et3N (Herrmann),10b and ((g -
allyl)PdCl)2/P(t-Bu)3/piperidine or DABCO systems (Soheili).10c
We have applied our TB strategy to the P(t-Bu)3 system, and opti-
mized the ratio of the palladium, phosphine, and TB components
for the standard reaction as shown in Table 1.11
Scheme 1 Structure of TB ([Ru(2,2ꢀ-bipyridine)3]2+).
The product yield under irradiation (entry 1) is noted to increase
dramatically compared with that in the dark or under irradiation
without TB (entries 7 and 8). The promoting effect of TB under ir-
radiation exceeds that of the conventional co-catalyst, CuI (entry 1
and 9).12 These observations indicate that photo-activated TB is
essential to the high performance. The Pd/P(t-Bu)3 is not crucial
to the catalytic activities (entries 1–2). On the other hand, the
amount of TB is critical. The reaction in the presence of 8% of
TB (entry 1) gave a result better than the results obtained in the
presence of a lesser amount (4%; entry 3) or an excess amount of
TB (16%; entry 4). (The Pd and P(t-Bu)3 concentrations were fixed
to 4%.) Furthermore, for the reactions with 6% of the Pd–P(t-Bu)3
reagent (entries 5 and 6), too, addition of an excess amount of
TB caused a decrease of the product yield. These results suggest
that addition of an excess amount of TB with the huge absorption
based on MLCT in the visible light region (e = 13 000, at 450 nm)1
causes a significant decrease of the light transmittance through the
reaction media. The results obtained so far reveal that the optimum
concentration of the TB component is 12.8 mM (55 mg in 5 mL
solvent) for the reaction carried out in a 13 mm-diameter Schlenk
tube. Having optimized the reaction conditions for the coupling
of bromobenzene and phenylacetylene we turned our attention
Therefore, TB and its derivatives are often used as co-catalysts
in the photocatalytic systems composed of a photsensitizer and
a catalyst, e.g., photodissociation of water2 and photoreduction
of CO2 to CO.3 Photo-activated *TB(II) and TB(I) states in
themselves are unable to catalyze a reaction, but they may be able
to play a crucial role to activate the precursor catalyst via electron
and/or energy transfer.4 To the best of our knowledge, there
is no precedent of a light-promoted, metal-catalyzed coupling
reaction and therefore it is challenging to study this new type
of multicomponent catalyst. As the first attempt, the Sonogashira
coupling reaction has been used and herein we describe the TB-
containing catalyst system that can undergo the Pd-catalyzed
Sonogashira coupling reaction under irradiation of visible light.
aRIKEN (The Institute of Physical and Chemical Research), Wako-shi,
Saitama, 351-0198, Japan. E-mail: osawa@riken.jp; Fax: +81 48 467 9389;
Tel: +81 48 462 1111
bChemical Resources Laboratory, Tokyo Institute of Technology, R1-27,
4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan; Fax: +81 45 924
5230; Tel: +81 45 924 5230
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b618007h
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 827–829 | 827
©

View Article Online
Table 1 Optimization of the ratio of palladium, phosphine and TB in the Pd-catalyzed Sonogashira coupling of phenylacetylene and bromobenzene
under irradiation of visible lighta
Entry
Pd(CH3CN)2Cl2 (%)
P(t-Bu)3 (%)
TB (%)
Yieldb (%)
Time/h
1
2
3
4
4
4
4
4
6
6
4
4
4
4
8
4
4
6
6
4
4
4
8
8
4
16
8
16
8
91
87
64
55
80
59
11
11
46
4
4
4
4
2
2
4
4
4
5
6
7c
8d
9e
—
—
a Reaction conditions: bromobenzene (0.8mmol), phenylacetylene (0.96 mmol). b GC yields. c In the dark. d Under irradiation without TB. e CuI (4%) was
added instead of TB.
to the function of TB in the catalytic cycle. The conversion rate
curves for the entries 1 and 7 in Table 1 (monitored by GC) are
shown as traces (a) and (b) in Fig. 1, respectively. The coupling
was completed within 5 h under irradiation to afford the product
in 95% yield (Fig. 1a), whereas the reaction in the dark gave the
product in 14% yield after 7 h (Fig. 1b). Furthermore, as can
be seen from trace (c), the catalytic reaction is promoted only
during the irradiated period. The role of TB in the catalytic cycle
Table 2 Pd-catalyzed Sonogashira coupling of various aryl bromides with aryl and alkyl acetylenes in the presence of TB
Entry
1
Aryl bromide
Acetylene
Reaction timea/h
2 (4)
Yield (%) irrad.b
99 (91)
Yield (%) darkc
30
2
3
6 (7)
99 (90)
85 (74)
36
14 (14)
Trace
4
5
6
7
8
9
1 (2)
5 (8)
99 (95)
95 (94)
97 (92)
95 (84)
99 (90)
80 (87)
21
14
34
18
34
33
5 (8)
6 (8)
10 (10)
18 (15)
a The values in parentheses are reaction time for the experiments to determine isolated yields. b GC yields and the values in parentheses are isolated yields
of the reaction products. c GC yields.
828 | Dalton Trans., 2007, 827–829
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Notes and references
1 A. Juris, V. Balzani, F. Barigelletti, S. Compagna, P. Belser and A. Von
Zelewsky, Coord. Chem. Rev., 1988, 84, 85–277, and references therein.
2 (a) V. Balzani, L. Moggi, M. F. Manfrin, F. Bolletta and M. Gleria,
Science, 1975, 189, 852–856; (b) C. Crentz and N. Sutin, Proc. Natl.
Acad. USA, 1975, 72, 2858–2862; (c) J. R. Bolton, Science, 1978, 202,
705–710; (d) E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca and M.
Gra¨etzel, J. Am. Chem. Soc., 1981, 103, 6324–6329.
3 (a) J.-M. Lehn and R. Ziessel, Proc. Natl. Acad. Sci. USA, 1982, 79,
701–704; (b) R. Maidan and I. Willner, J. Am. Chem. Soc., 1986, 108,
8100–8101.
4 For recent reports on visible light sensitization of chemical net
reactions, see: (a) V. W.-W. Yam, V. W.-M. Lee and K.-K. Cheung,
Organometallics, 1997, 16, 2833–2841; (b) M. Osawa, M. Hoshino and
Y. Wakatsuki, Angew. Chem., Int. Ed., 2001, 40, 3472–3474; (c) A.
Inagaki, S. Edure, S. Yatsuda and M. Akita, Chem. Commun., 2005,
5468–5470.
Fig. 1 Conversion rates for the Sonogashira coupling in the presence of
TB; (a) with irradiation of 150 W-Xe lamp (420 < k < 800 nm), (b) in
the dark and (c) irradiated on and off. Solid and dotted curves are for the
periods with irradiation or in the dark, respectively. aGC yield.
5 K. Sonogashira, Y. Toda and N. Hagiwara, Tetrahedron Lett., 1975, 50,
4467–4470.
6 (a) T. Fukuyama, M. Shinmen, S. Nishitani, M. Sato and I. Ryu, Org.
Lett., 2002, 4, 1691–1694; (b) S. B. Park and H. Alper, Chem. Commun.,
2004, 1306–1307.
7 A. Mori, J. Kawashima, T. Shimada, M. Sugro, K. Hirabayashi and Y.
Nishihara, Org. Lett., 2000, 2, 2935–2937.
8 (a) B. Liang, M. Dai, J. Chen and Z. Yang, J. Org. Chem., 2005, 70,
391–393; (b) J.-H. Li, Y. Liang and Y.-X. Xie, J. Org. Chem., 2005, 70,
4393–4396.
9 (a) K. Heuze´, D. Me´ry, D. Gauss and D. Astruc, Chem. Commun.,
2003, 2274–2275; (b) K. Heuze´, D. Me´ry, D. Gauss, J.-C. Blais and D.
Astruc, Chem.–Eur. J., 2004, 10, 3936–3944.
10 (a) T. Hundertmark, A. F. Littke, S. L. Buchwald and G. C. Fu, Org.
Lett., 2000, 2, 1729–1793; (b) V. P. W. Bo¨hm and W. A. Herrmann,
Eur. J. Org. Chem., 2000, 3679–3681; (c) A. Soheili, J. Albaneze-Walker,
P. G. Dormer and D. L. Hughes, Org. Lett., 2003, 5, 4191–4194.
11 Typical Procedure (Table 1, entry 2). 4-Bromobenzene (0.126 g,
0.80 mmol), Pd(CH3CN)2Cl2 (0.0084 g, 0.032 mmol), and TB (0.055 g,
0.064 mmol) were added to a dried Schlenk tube (13 mm in diameter)
with a TeflonTM stop cock. The vessel was degassed then backfilled
with nitrogen followed by addition of DMF (4.0 mL). Then, P(t-
Bu)3 (97.5 lL of a 10 wt% solution in n-hexane, 0.032 mmol),
phenylacetylene (105 mL, 0.96 mmol), mesitylene (internal standard:
50 lL, 0.362 mmol) and Et3N (1.0 mL, 7.18 mmol) were added via a
syringe to the stirred reaction mixture under an atmosphere of nitrogen.
The irradiation of visible light was carried out by a 150-W Xenon lamp
(Ushio SX-UI 150XQ) through a Toshiba Y-42 glass filter and a Sigma-
koki CLDF-50S colored filter (420 < k < 800 nm). The diameter of the
effective irradiation field was 50 mm. For the dark reactions, the Schlenk
tube wrapped with an Al-foil was irradiated to make the reaction
temperature similar. The yields of products and the consumption of
aryl bromides were determinedby GC.
is unknown, but we guess that the TB(I) state is the key acceleration
factor because the degradation products of triethylamine (the
sacrificial electron donor; eqn (2)) are detected by GC-MS.13,14
We screened the reaction of various aryl bromides; the results are
summarized in Table 2.15 Activated aryl bromides react with aryl-,
trimethylsilyl-, and alkyl-acetylenes in excellent yields within a
short reaction time (entries 1–4). The less reactive bromobenzene,
4-bromoanisole, and 4-bromotolene also smoothly couple at
ambient temperature (entries 5–8). Even sterically hindered 2-
bromo-1,3-dimethylbenzene coupled with phenylacetylene high
efficiently (entry 9). For every case (entry 1–9), the remarkable
promotion effects of irradiation of visible light are evident as
compared with the reactions in the dark.
It should be noted that under the same conditions, the reaction
of 4-chloroacetophenone16 and phenylacetylene (eqn (3)) afforded
4-acetylphenyl phenyl acetylene in 36% GC yield.17 This result
suggests that the activated TB species affect the oxidative addition
step in the catalytic cycle.
(3)
In summary, we have demonstrated the photo-activation of the
Pd-catalyzed Sonogashira coupling reaction by using TB as a
co-catalyst in place of CuI. Attempts to apply this strategy to
other transition-metal catalyzed coupling reactions are currently
underway in our laboratory.
12 Conditions (Table 1, entry 9). 4-Bromobenzene (0.126 g, 0.80 mmol),
phenylacetylene (105 lL, 0.96 mmol), Pd(CH3CN)2Cl2 (0.0084 g,
0.032 mmol), CuI (6.10 mg, 0.032 mmol), Et3N (1.0 mL, 7.18 mmol),
DMF (4.0 mL).
13 P. J. DeLaive, B. P. Sullivan, T. J. Meyer and D. G. Whitten, J. Am.
Chem. Soc., 1979, 101, 407–408.
14 HN(Et)2 (m/z = 73, 58) and CH3CHO (m/z = 44, 29) were detected
by GC-MS. These, two compounds were derived from hydrolysis of
Et2N+ CHCH3.
15 See ESI† for experimental details.
=
Acknowledgements
16 Reports on copper-free Sonogashira coupling of aryl chlorides at room
temperature are still few, see: D. Me´ry, K. Heuze´ and D. Astruc, Chem.
Commun., 2003, 1934–1935.
17 We observed extensive dimerization of phenylacetylene giving 1,4-
diphenylbutadiyne before the consumption of 4-chloroacetophenone.
M. A. is grateful to the Ministry of Education, Culture, Sports,
Science and Technology of the Japanese Government (Grant-in-
Aid for Scientific Research on Priority Areas, No. 18065009) for
financial support of this research.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 827–829 | 829
©