Tandem Suzuki-Miyaura/transfer hydrogenation reaction catalyzed by a Pd-Ru complex bearing an anionic dicarbene

Article abstract of DOI:10.1016/j.jcat.2016.02.031

A hetero-bimetallic Pd-Ru N-Heterocyclic Dicarbene (NHDC) complex was synthesized from a Ag-Ru complex by transmetalation. The Pd-Ru system displays high catalytic activity in tandem Suzuki-Miyaura cross-coupling/transfer hydrogenation reaction of bromoaryl ketones with boronic acids in toluene and 2-propanol in the presence of KOH.

Full text of DOI:10.1016/j.jcat.2016.02.031

Journal of Catalysis 338 (2016) 222–226
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Tandem Suzuki–Miyaura/transfer hydrogenation reaction catalyzed
by a Pd–Ru complex bearing an anionic dicarbene
b,
Mario J. Bitzer a, Fritz E. Kühn a, , Walter Baratta
⇑
⇑
a Department of Chemistry & Catalysis Research Center, Molecular Catalysis, Technische Universität München, Lichtenbergstr. 4, D-85747 Garching bei München, Germany
b Dipartimento DI4A, Chimica, Università di Udine, Via Cotonificio 108, I-33100 Udine, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A hetero-bimetallic Pd–Ru N-Heterocyclic Dicarbene (NHDC) complex was synthesized from a Ag–Ru
complex by transmetalation. The Pd–Ru system displays high catalytic activity in tandem Suzuki–
Miyaura cross-coupling/transfer hydrogenation reaction of bromoaryl ketones with boronic acids in
toluene and 2-propanol in the presence of KOH.
Received 22 January 2016
Revised 19 February 2016
Accepted 28 February 2016
Ó 2016 Elsevier Inc. All rights reserved.
Keywords:
Anionic dicarbenes
Bimetallic complexes
Cross-coupling
Tandem catalysis
Transfer hydrogenation
1. Introduction
bene chemistry focus on the development of unusual coordination
geometries of the carbene ligand, and apart from abnormal carbe-
The combination of two catalytic transformations to a single
tandem process appears to be particularly appealing in organic
synthesis since this protocol reduces the number of synthetic steps
toward the target compound, with concomitant reduction of sol-
vent amounts and increase in overall yield [1,2]. Bimetallic systems
have become the subject of current research as they are expected
to induce cooperative effects among the involved metals [3–9].
For the Suzuki–Miyaura/transfer hydrogenation transformation
entailing C–C and C–H bond forming reactions, catalysts are mainly
based on heterogeneous systems [4,10] and enzymes [11–15]. By
contrast, homogeneous bimetallic catalysts for this tandem reac-
tion are less common [3,5,16,17], and some of them display higher
activity compared to the combination of the single monometallic
species [5,17]. Therefore, the design of novel heterobimetallic com-
plexes based on suitable ligands is crucial to achieve more efficient
catalysts for tandem reactions.
nes [27,28], imidazolyl-derived anionic dicarbenes (N-heterocyclic
dicarbenes, NHDCs) have recently attracted significant interest
[24,29–32]. These compounds inherently allow the synthesis of
bimetallic structures bearing two main-group elements
[29,30,32–36] or one main-group element and one transition metal
[37–41], and examples featuring two transition metal centers are
almost exclusively homobimetallic [42–48]. Although their ditopic
character renders these compounds these compounds highly
promising for homogeneous catalysis, only one example of a cat-
alytic application has been reported to date comprising an Au-B
system applied in the skeletal rearrangement of enynes [41]. We
describe here the isolation of
a rare example of a hetero-
bimetallic Pd–Ru NHDC complex that was found active as catalyst
in tandem Suzuki–Miyaura cross-coupling/transfer hydrogenation
reaction of bromoaryl ketones with boronic acids in toluene and
2-propanol.
In the last two decades, N-heterocyclic carbenes (NHC) have
been extensively employed as ligands for the preparation of effi-
cient homogeneous catalysts [18–23]. In particular, our groups
described several NHC Ru catalysts for the transfer hydrogenation
(TH) of ketones in 2-propanol [24–26]. Current advances in car-
2. Experimental
All reactions were performed under oxygen free, dry conditions
in an argon atmosphere using standard Schlenk and glovebox tech-
niques. The solvents used were purified, degassed, and dried
according to standard purification techniques [49] or obtained
from an MBraun solvent purification system. Complexes 1, 2 [24]
and [PdCl2(IMes)2] (IMes = 1,3-bis(mesityl)-imidazol-2-ylidene)
⇑
Corresponding authors.
E-mail addresses: fritz.kuehn@ch.tum.de (F.E. Kühn), walter.baratta@uniud.it
(W. Baratta).
http://dx.doi.org/10.1016/j.jcat.2016.02.031
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

M.J. Bitzer et al. / Journal of Catalysis 338 (2016) 222–226
223
[50] were obtained according to literature procedures. All further
chemicals were purchased from commercial sources and used as
received. NMR spectra were acquired on a Bruker Avance Ultra-
shield 400 MHz, a Bruker DPX 400 MHz spectrometer, and a Bruker
AC 200 spectrometer. All 1H and 13C chemical shifts are reported in
parts per million (ppm) relative to TMS, with the residual solvent
peak serving as internal reference [51]. 31P NMR spectra are refer-
enced to 85% H3PO4 as external standard. ESI mass spectra were
acquired on a Thermo Scientific LCG Fleet. GC analysis was done
with an Agilent 7890b GC system using an Agilent HP-5 column
mixture, 1 mL of a stock solution of catalyst 3 (1 mg/mL in toluene)
and 1 mL of 2-propanol were added (3: 0.5 mol%). The Schlenk tube
was sealed and heated to 100 °C. After 1 h, the reaction was termi-
nated by cooling with an ice bath and a 1 mL aliquot of the reaction
mixture was taken and filtered over a pad of silica. The sample was
diluted with toluene to a volume of 1 mL and characterized by gas
chromatography.
2.4. Tandem catalytic reactions using toluene followed by addition of
2-propanol
(30 m ꢀ 25 m ꢀ 0.32 mm) and a Varian CP3380 GC system using
l
a MEGA-DEX DETTBSBETA column (25.0 m ꢀ 25 m ꢀ 0.25 mm).
l
In a typical run, a Schlenk tube was charged with the ketone
(0.1 mmol), phenylboronic acid (0.175 mmol), anisole (25 mg)
and potassium hydroxide (0.3 mmol). This mixture was suspended
in the required amount of a toluene stock solution of the catalyst(s),
and the tube was sealed with a septum and heated to 100 °C for
15 min ([Pd]: 0.5 mol%, [Ru]: 1.0 mol%). 1 mL of 2-propanol was
added and after further 15 min at 100 °C the reaction was termi-
nated by cooling with an ice bath. A 1 mL aliquot of the reaction
mixture was taken and filtered over a pad of silica. The sample
was diluted with toluene to a volume of 1 mL and characterized
by gas chromatography.
Elemental analyses were carried out in the microanalytical labora-
tory at Technische Universität München.
2.1. Synthesis of complex 3
Complex
2
(109 mg, 0.05 mmol), [PdCl2(cod)] (15.5 mg,
0.055 mmol, 1.10 equiv.) and Ag2O (13.7 mg, 0.06 mmol) were sus-
pended in 10 mL of toluene and stirred at room temperature for
60 min under protection of light. The formed gray precipitate
was filtered off with a filter cannula and the volatiles of the filtrate
were removed under reduced pressure. The crude product was
washed with n-pentane (2 ꢀ 2 mL) and dried in vacuo. Complex 3
was obtained as brown-yellow powder in 80% yield (81.0 mg,
0.040 mmol). 1H NMR (400 MHz, CD2Cl2, RT): d = 7.91–7.87 (m,
4H, Ar–H), 7.46–7.31 (m, 16H, Ar–H), 7.16–7.09 (m, 14H, Ar–H),
6.98–6.92 (m, 6H, Ar–H), 6.85–6.82 (m, 6H, Ar–H), 6.64 (s, 2H,
3. Results and discussion
Recently, we described novel hetero-bimetallic NHDC com-
plexes based on Ag–Ru and Au–Ru metals [20], whose syntheses
entail the reaction of the abnormal carbene ruthenium complex
1 with Ag2O leading to formation of the Ag–Ru derivative 2
(Scheme 1).
2
Ar–H), 6.39–6.26 (m, 4H, Ar–H), 5.93 (t, JHH = 12.5 Hz, 2H, CHHP),
5.43 (s, 2H, Ar–H), 4.70 (m, 2H, CHHP), 4.05 (m, 2H, CHHP), 2.81 (d,
2JHH = 13.8 Hz, 2H, CHHP), 2.46 (s, 6H, CH3), 2.38 (s, 6H, CH3), 2.19
(s, 6H, CH3), 2.04 (s, 6H, CH3), 1.94 (s, 6H, CH3), 1.84 (s, 6H, CH3),
0.66 (s, 6H, CH3). 13C {1H} NMR (101 MHz, CD2Cl2, RT): d = 194.0
Treatment of the cationic Ag–Ru complex 2 with [PdCl2(cod)] in
toluene for 60 min affords the neutral Pd–Ru complex 3 by trans-
metalation (80% yield) [52,53]. The 1H NMR spectrum of 3 in CD2-
Cl2 shows six different signals for the mesityl methyl groups and
one signal for the acetate group. The 13C{1H} NMR of the normal
Ru carbene appears as a doublet of doublets at d = 194.0 ppm with
2
2
(dd, JCPcis = 11.2 Hz, JCPtrans = 101.0 Hz, NCN), 183.8 (s, OAc),
3
2
2
168.4 (d, JCP = 12.0 Hz, NC0N) 150.7 (dd, JCPcis = 9.8 Hz, JCP
-
cis = 19.4 Hz, NCCHN), 138.6, 138.0, 137.8, 137.7, 137.2, 136.7,
136.6, 136.3, 135.9, 135.2, 135.1, 134.9, 134.8, 134.7, 134.2,
134.1, 134.0, 131.7, 131.2, 130.6, 130.2, 129.3–128.0, 124.7,
123.8, 120.4 (aromatic carbon atoms), 53.4 (NCH2P), 48.1 (d,
1JCP = 39.1 Hz, NCH2P), 23.7 (CH3), 21.5 (CH3), 21.1 (CH3), 20.2
(CH3), 19.3 (CH3), 19.2 (CH3), 17.5 (CH3). 31P {1H} NMR (162 MHz,
a
2JCP = 101.0 and 11.2 Hz, due to the trans and cis P atoms;
whereas the abnormal Ru carbene signal is at d = 150.7 ppm
(2JCP = 9.8 and 19.4 Hz), in agreement with the data of 1 and 2
[24]. The formation of the Pd normal carbene is confirmed by the
2
3
CD2Cl2, RT): d = 81.6 (d, JPP = 25.0 Hz, major and minor isomers),
doublet at 168.4 ppm, displaying a JCP = 12.0 Hz. In addition, the
2
81.5 (d, 2JPP = 24.7 Hz, minor isomer), 57.5 (d, JPP = 24.7 Hz, minor
peaks at d = 53.4 and 48.1 ppm (1JCP = 39.1 Hz) are for the CH2P
moieties. Variable temperature 31P{1H} NMR studies in toluene-
d8 show that at 90 °C 3 displays two main doublets at d = 81.6
2
2
isomer), 57.0 (d, JPP = 25.0 Hz, major isomer), 56.5 (d, JPP = 24.7 -
Hz, minor isomer). Elemental analysis calcd (%) for C104H104Cl2N8-
O4P4PdRu2 ꢀ CH2Cl2: C, 59.53; H, 5.05 N, 5.29. Found: C, 59.27; H
5.20; N, 5.26. MS (ESI): m/z (%) = 1016.9 (30) [M2+], 998.6 (100)
[M2+ꢁCl)], 667.8 (55) [M3+ꢁCl].
2
and 57.0 ppm with JPP = 20.7 Hz. At 22 °C the complex exhibits a
pseudo-triplet at d = 81.8 ppm and four doublets in the range
2
58.3–56.8 ppm with JPP = 22–25 Hz, consistent with the presence
of four species, which is likely due to the different conformation
of the four five-membered P–NHC metallacycles of 3 [54]. The
Pd–Ru complex 3 is a novel example of a hetero-bimetallic NHDC
complex that can be easily prepared from the abnormal carbene
1, showing the potential of this route for the synthesis of multinu-
clear carbene complexes.
2.2. Transfer hydrogenation reactions
In a typical experiment, the reactor was charged with 2-
propanol (9.8 ml), the ketone (1 mmol), anisole (50 mg) and the
catalyst (0.05–0.1 mol%). The mixture was heated to 100 °C for
1 min and a 0.1 M solution of NaOiPr in 2-propanol (200
l
l,
Complex 3 was found active in the catalytic transfer hydrogena-
tion (TH) of ketones using 2-propanol as hydrogen source. Ace-
tophenone and 4-phenylacetophenone are easily reduced to the
corresponding alcohols with 3 as catalyst (0.05 mol%) in 120 min
at 100 °C in the presence of 5 mol% of NaOiPr (Table 1).
Remarkably, with Pd–Ru complex 3 under these catalytic condi-
tions, 4-bromoacetophenone is poorly converted to 1-(4-
bromophenyl)ethanol (9% yield) and no dehalogenation products
(i.e. acetophenone or 1-phenylethanol) are observed in the reac-
tion mixture. Conversely, with Ru complex 1 (0.1 mol%) quantita-
tive reduction to 4-bromophenylethanol is attained in 20 min
(Table 1).
0.05 mmol, 5 mol%) was added to the stirred mixture. 1 mL ali-
quots of the reaction mixture were taken at the required reaction
times, cooled with an ice bath and filtered over a short pad of silica.
The sample was diluted with toluene to a volume of 1 mL and char-
acterized by gas chromatography.
2.3. Tandem reactions in toluene/2-propanol mixture
In
a typical run, a Schlenk tube was charged with 4-
bromoacetophenone (0.1 mmol), phenylboronic acid (0.125–
0.175 mmol), anisole (25 mg) and the base (0.2–0.3 mmol). To this

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M.J. Bitzer et al. / Journal of Catalysis 338 (2016) 222–226
Scheme 1. Synthesis of Pd–Ru NHDC complex 3 by transmetalation of 2.
Table 1
Table 2
Transfer hydrogenation of methylaryl ketones.
Effect of the base on the tandem Suzuki coupling/transfer hydrogenation reaction of
4-bromoacetophenone and phenylboronic acid with 3.
Base
%Aa
%Ba
%C
%D
NaOiPr 3.0 eq.
KOtBu 3.0 eq.
KOH 3.0 eq.
KOH 2.0 eq.
Cs2CO3 3.0 eq.
4
5
7
18
85
28
30
68
66
1
7
5
1
1
11
57
49
20
15
1
X
Catalyst
Yield (%)a
t (min)
H
3
3
3
1
95
92
9
120
120
120
20
Ph
Br
Br
Reaction conditions: 4-Bromoacetophenone (0.1 mmol), base, phenylboronic acid,
catalyst (0.005 mmol, 0.5 mol%), 2-propanol (1 mL), toluene (1 mL), 100 °C,
97
3
reaction time: 1 h.
Reaction conditions: Ketone (1 mmol), NaOiPr (0.05 mmol, 5 mol%), 3 (0.05 mol%)
or 1 (0.1 mol%), 2-propanol (10 mL), 100 °C.
a
Yields determined via GC analysis over two catalytic runs using anisole as
internal standard. The formation of Ph–Ph was below 10%. See SI for more detailed
results.
a
Yield determined via GC-FID analysis using anisole as internal standard.
Interestingly, the nature of the base and its concentration affect
the selectivity of the tandem reaction. While with the alkoxides
NaOiPr and KOtBu (3 equiv) the yield of B is in the range of 28–
30% with 49–57% of D, employment of KOH (2–3 equiv) increases
the amount of B up to 68%, with 1–20% of A, C and D as side prod-
ucts. Using Cs2CO3 in suspension, A is formed as the main product
(85%), with B, C and D in 1–11% yield (Table 2).
It is worth noting that the dehalogenation process leading to the
side products C and D requires high base concentrations and occurs
via hydridic Pd species forming in 2-propanol [55]. These findings
may suggest that in alcoholic media and at high alkoxide concen-
trations (NaOiPr, KOtBu), complex 3 mainly catalyzes dehalogena-
tion and further TH reactions, in accordance with other bimetallic
Pd–Ru systems [5,6,10,16,17]. Conversely, when the alkoxide
Complex 3 (0.5 mol%) catalyzes the tandem Suzuki–Miyaura/TH
reaction of 4-bromoacetophenone and phenylboronic acid (1.75
equivalents) to give 1-(4-biphenyl)ethanol (B) in 2-propanol/
toluene mixture (1/1 in volume) in the presence of 2–3 equivalents
of a base (Scheme 2), following the procedure described by Peris
et al. [17]. The analysis of the catalytic reactions also reveals the for-
mation of 4-phenylacetophenone (A), the product of the C–C cou-
pling reaction, as well as acetophenone (C) and 1-phenylethanol
(D) via dehalogenation (and TH) of 4-bromoacetophenone. In addi-
tion, no significant formation of 1-(4-bromophenyl)ethanol is
observed, indicating that under these catalytic conditions the
Suzuki–Miyaura reaction is faster than the reduction of 4-
bromoacetophenone via TH.
Scheme 2. Tandem Suzuki–Miyaura coupling/transfer hydrogenation vs. dehalogenation/transfer hydrogenation reaction catalyzed by 3.

M.J. Bitzer et al. / Journal of Catalysis 338 (2016) 222–226
225
Fig. 1. Time course of tandem Suzuki–Miyaura/TH reaction catalyzed by 3 using toluene and 2-propanol.
Table 3
Tandem Suzuki coupling/transfer hydrogenation of several substrates catalyzed by 3 using toluene and 2-propanol.
Catalyst
Ketone
Boronic acid
Product
Yield (%)
3
95a
3
94b
3
3
3
98b
95b
96b
3
93b
3
97b
0a
-
1 + [PdCl2(IMes)2]
76a,c
Reaction conditions: Ketone (0.1 mmol), KOH (0.3 mmol), boronic acid (0.175 mmol), catalyst 3 (0.005 mmol, 0.5 mol%), toluene (1 mL), after 15 min: addition of 2-propanol
(1 mL), 100 °C, reaction time: 30 min. The formation of Ph–Ph was below 10%.
a
Yields determined via GC-FID analysis over two catalytic runs using anisole as internal standard.
Yields determined via NMR analysis over two catalytic runs using anisole as external standard.
1: 0.01 mmol, 1.0 mol%, [PdCl2(IMes)2]: 0.005 mmol, 0.5 mol%. Under these conditions, significant amounts of an unidentified side product were obtained. See SI for
b
c
further details.
concentration is low, 3 promotes the Suzuki–Miyaura reaction.
Therefore, the formation of the dehalogenated product should be
hindered if alkoxides and alcohols are avoided during the C–C cou-
pling reaction, which is the first step of the desired reaction
(Scheme 2). Accordingly, the reaction protocol was modified by
running the tandem reactions under alcohol-free conditions until
the coupling of the bromoarene with the boronic acid was com-
pleted, followed by addition of 2-propanol to initiate the TH
reaction.
15 min, with quantitative conversion to 4-acetylbiphenyl (Fig. 1).
Subsequent addition of 2-propanol at 100 °C leads to completion
of the reaction after further 15 min (overall time 30 min), affording
1-(4-biphenyl)ethanol in 95% yield.
This catalytic procedure has also been applied to 3-
bromoacetophenone and to several boronic acids affording the cor-
responding biphenyl alcohols in excellent yields (93–98%) at
0.5 mol% of 3 in 30 min (Table 3).
The reaction tolerates a variety of functional groups, including
OMe, F and CF3 in varying substitution patterns, enabling a benign
and efficient two-step synthesis of several organic compounds
Thus, 4-bromoacetophenone, phenylboronic acid, KOH and 3
(0.5 mol%) were suspended in toluene and heated to 100 °C for

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[8] A. Zanardi, J.A. Mata, E. Peris, Chem. Eur. J. 16 (2010) 10502.
[9] A. Zanardi, J.A. Mata, E. Peris, Chem. Eur. J. 16 (2010) 13109.
[10] D. Zhang, T. Cheng, G. Liu, Appl. Catal. B 174–175 (2015) 344.
[11] S. Borchert, E. Burda, J. Schatz, W. Hummel, H. Gröger, J. Mol. Catal. B Enzym.
84 (2012) 89.
[12] V. Gauchot, W. Kroutil, A.R. Schmitzer, Chem. Eur. J. 16 (2010) 6748.
[13] A. Prastaro, P. Ceci, E. Chiancone, A. Boffi, R. Cirilli, M. Colone, G. Fabrizi, A.
Stringaro, S. Cacchi, Green Chem. 11 (2009) 1929.
[14] E. Burda, W. Hummel, H. Gröger, Angew. Chem. Int. Ed. 47 (2008) 9551.
[15] R. Kourist, J. González-Sabín, R. Liz, F. Rebolledo, Adv. Synth. Catal. 347 (2005)
695.
[16] S. Gonell, M. Poyatos, J.A. Mata, E. Peris, Organometallics 31 (2012) 5606.
[17] A. Zanardi, J.A. Mata, E. Peris, J. Am. Chem. Soc. 131 (2009) 14531.
[18] K. Riener, S. Haslinger, A. Raba, M.P. Högerl, M. Cokoja, W.A. Herrmann, F.E.
Kühn, Chem. Rev. 114 (2014) 5215.
[19] M.N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 510 (2014) 485.
[20] T. Dröge, F. Glorius, Angew. Chem. Int. Ed. 49 (2010) 6940.
[21] S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612.
[22] F.E. Hahn, M.C. Jahnke, Angew. Chem. Int. Ed. 47 (2008) 3122.
[23] W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290.
[24] M.J. Bitzer, A. Pöthig, C. Jandl, F.E. Kühn, W. Baratta, Dalton Trans. 44 (2015)
11686.
[25] J. Witt, A. Pöthig, F.E. Kühn, W. Baratta, Organometallics 32 (2013) 4042.
[26] W. Baratta, J. Schütz, E. Herdtweck, W.A. Herrmann, P. Rigo, J. Organomet.
Chem. 690 (2005) 5570.
without isolation of intermediate products. By contrast, in the
absence of 3 no conversion of 4-bromoacetophenone to the desired
product is observed. To establish the advantage of the hetero-
bimetallic system 3 with respect to single Pd and Ru complexes,
the compounds [PdCl2(IMes)2] and 1, which show stereoelectronic
properties similar to those of 3, were used in the tandem catalysis.
By performing the reaction of 4-bromoacetophenone with phenyl-
boronic acid using [PdCl2(IMes)2] and 1 in 1:2 molar ratio, only 76%
of the desired product is obtained, with a significant amount of an
unidentified side product (Table 3), indicating a higher selectivity
of 3 compared to the single complexes. Furthermore, addition of
Hg does not affect the catalytic reactions, suggesting that the tan-
dem catalysis with 3 occurs under homogeneous conditions [56].
The low catalyst loading of 3 and short reaction time associated
with the use of inexpensive KOH base make this catalytic system
highly attractive for the Suzuki–Miyaura coupling/transfer hydro-
genation process, demonstrating the potential of hetero-
bimetallic NHDC catalysts for application in tandem reactions.
[27] R.H. Crabtree, Coord. Chem. Rev. 257 (2013) 755.
4. Conclusions
[28] P.L. Arnold, S. Pearson, Coord. Chem. Rev. 251 (2007) 596.
[29] Y. Wang, Y. Xie, P. Wei, H.F. Schaefer, G.H. Robinson, Dalton Trans. (2016),
http://dx.doi.org/10.1039/C5DT03894D.
[30] A.J. Martínez-Martínez, M.Á. Fuentes, A. Hernán-Gómez, E. Hevia, A.R.
Kennedy, R.E. Mulvey, C.T. O’Hara, Angew. Chem. 127 (2015) 14281.
[31] J.B. Waters, J.M. Goicoechea, Coord. Chem. Rev. 293–294 (2015) 80.
[32] T. Simler, P. Braunstein, A.A. Danopoulos, Chem. Commun. 52 (2016) 2717.
[33] A. El-Hellani, V. Lavallo, Angew. Chem. Int. Ed. 53 (2014) 4489.
[34] Y. Wang, M.Y. Abraham, R.J. Gilliard, P. Wei, J.C. Smith, G.H. Robinson,
Organometallics 31 (2012) 791.
[35] Y. Wang, Y. Xie, M.Y. Abraham, P. Wei, H.F. Schaefer III, P.v.R. Schleyer, G.H.
Robinson, Organometallics 30 (2011) 1303.
[36] Y. Wang, Y. Xie, M.Y. Abraham, P. Wei, H.F. Schaefer III, P.v.R. Schleyer, G.H.
Robinson, J. Am. Chem. Soc. 132 (2010) 14370.
[37] R.A. Musgrave, R.S.P. Turbervill, M. Irwin, R. Herchel, J.M. Goicoechea, Dalton
Trans. 43 (2014) 4335.
[38] C. Pranckevicius, D.W. Stephan, Chem. Eur. J. 20 (2014) 6597.
[39] J.B. Waters, R.S.P. Turbervill, J.M. Goicoechea, Organometallics 32 (2013) 5190.
[40] R.A. Musgrave, R.S.P. Turbervill, M. Irwin, J.M. Goicoechea, Angew. Chem. Int.
Ed. 51 (2012) 10832.
In this work, we reported the isolation of a rare bimetallic N-
heterocyclic dicarbene (NHDC) complex bearing Pd and Ru, start-
ing from a Ag–Ru NHDC species via transmetalation. This complex
was found highly active in tandem Suzuki–Miyaura/transfer
hydrogenation reaction of bromoacetophenones and boronic acids
with formation of biphenyl alcohols with high selectivity (up to
98%), using 0.5 mol% of the catalyst and in a short time. It was
established that the bimetallic catalyst is superior to the combina-
tion of related monometallic species, leading to higher selectivity
in the tandem reaction. The nature of the base and the reaction
media strongly affect the selectivity of the tandem reaction. This
protocol entails the use of KOH as base in toluene, followed by
addition of 2-propanol, avoiding dehalogenation reactions. Current
investigations are underway to extend the number of bimetallic
catalysts based on NHDC ligands and to apply them in tandem
transformations, including asymmetric reactions.
[41] S. Kronig, E. Theuergarten, C.G. Daniliuc, P.G. Jones, M. Tamm, Angew. Chem.
Int. Ed. 51 (2012) 3240.
[42] D.R. Armstrong, S.E. Baillie, V.L. Blair, N.G. Chabloz, J. Diez, J. Garcia-Alvarez, A.
R. Kennedy, S.D. Robertson, E. Hevia, Chem. Sci. 4 (2013) 4259.
[43] A. Krüger, E. Kluser, H. Müller-Bunz, A. Neels, M. Albrecht, Eur. J. Inorg. Chem.
(2012) 1394.
Acknowledgments
[44] Y. Wang, Y. Xie, M.Y. Abraham, R.J. Gilliard, P. Wei, C.F. Campana, H.F. Schaefer,
P.v.R. Schleyer, G.H. Robinson, Angew. Chem. Int. Ed. 51 (2012) 10173.
[45] U.J. Scheele, S. Dechert, F. Meyer, Chem. Eur. J. 14 (2008) 5112.
[46] M.R. Crittall, C.E. Ellul, M.F. Mahon, O. Saker, M.K. Whittlesey, Dalton Trans.
(2008) 4209.
We thank the Erasmus STA program and the TUM Graduate
School for financial support.
[47] A.A. Danopoulos, D. Pugh, J.A. Wright, Angew. Chem. Int. Ed. 47 (2008) 9765.
[48] C.E. Ellul, M.F. Mahon, O. Saker, M.K. Whittlesey, Angew. Chem. Int. Ed. 46
(2007) 6343.
[49] W.L.F. Armarego, C.L.L. Chai, Purification of Laboratory Chemicals,
Butterworth-Heinemann, Oxford, United Kingdom, 2003.
[50] S. Zhu, R. Liang, H. Jiang, Tetrahedron 68 (2012) 7949.
[51] G.R. Fulmer, A.J.M. Miller, N.H. Sherden, H.E. Gottlieb, A. Nudelman, B.M. Stoltz,
J.E. Bercaw, K.I. Goldberg, Organometallics 29 (2010) 2176.
[52] To increase the yield of 3, the reaction of 2 with [PdCl2(cod)] was carried out in
the presence of Ag2O.
[53] Mes = 2,4,6-trimethylphenyl; cod = cyclooctadiene.
[54] See Supporting Information for further details.
[55] O. Navarro, N. Marion, Y. Oonishi, R.A. Kelly, S.P. Nolan, J. Org. Chem. 71 (2006)
685.
[56] R.H. Crabtree, Chem. Rev. 112 (2012) 1536.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2016.02.031.
References
[1] J.-C. Wasilke, S.J. Obrey, R.T. Baker, G.C. Bazan, Chem. Rev. 105 (2005) 1001.
[2] A. Ajamian, J.L. Gleason, Angew. Chem. Int. Ed. 43 (2004) 3754.
[3] J.A. Mata, F.E. Hahn, E. Peris, Chem. Sci. 5 (2014) 1723.
[4] D. Zhang, J. Xu, Q. Zhao, T. Cheng, G. Liu, ChemCatChem 6 (2014) 2998.
[5] N. Dehury, S.K. Tripathy, A. Sahoo, N. Maity, S. Patra, Dalton Trans. 43 (2014)
16597.
[6] S. Sabater, J.A. Mata, E. Peris, Nat. Commun. 4 (2013) 2553.
[7] S. Sabater, J.A. Mata, E. Peris, Organometallics 31 (2012) 6450.