New acyclic aminocarbene palladium(II) complexes as convenient catalysts for the Sonogashira and Suzuki cross-coupling

Article abstract of DOI:10.1007/s11172-013-0193-z

A reaction of isocyanide complex cis-[PdCl2(CNAr)2] with hydrazones H2N-N=CR1R2 proceeded at the carbon atom of one of the isocyanide groups and gave the corresponding diaminocarbene isocyanide palladium(II) complexes. These complexes showed high catalytic activity in the Sonogashira and Suzuki cross-coupling reactions.

Full text of DOI:10.1007/s11172-013-0193-z

Russian Chemical Bulletin, International Edition, Vol. 62, No. 6, pp. 1361—1365, June, 2013
1361
New acyclic aminocarbene palladium(II) complexes
as convenient catalysts for the Sonogashira and Suzuki crossꢀcoupling
E. A. Valishina,a T. M. Buslaeva,a and K. V. Luzyaninb,c
aM. V. Lomonosov Moscow State University of Fine Chemical Technologies,
86 prosp. Vernadskogo, 119571 Moscow, Russian Federation.
Fax: +7 (495) 936 8259. Eꢀmail: buslaevatm@mail.ru
bSt.ꢀPetersburg State University, Department of Chemistry,
26 Universitetskii prosp., 198504 St.ꢀPetersburg, Russian Federation
cUniversity of Liverpool, Department of Chemistry,
Crown Street, L69 7ZD Liverpool, United Kingdom,
Eꢀmail: konstantin.luzyanin@liv.ac.uk
A reaction of isocyanide complex cisꢀ[PdCl2(CNAr)2] with hydrazones H2N—N=CR1R2
proceeded at the carbon atom of one of the isocyanide groups and gave the corresponding
diaminocarbene isocyanide palladium(II) complexes. These complexes showed high catalytic
activity in the Sonogashira and Suzuki crossꢀcoupling reactions.
Key words: aminocarbene palladium complexes, metalꢀpromoted coupling, nucleophilic
addition, isocyanides, hydrazones, Sonogashira reaction, Suzuki reaction.
Metalꢀpromoted nucleophilic addition to coordinated
isocyanides is among the most promising methods for the
synthesis of complexes with acyclic aminocarbene ligands
[M]ADC, which are structural analogs of Nꢀheterocyclic
carbenes (Scheme 1).1—3 In the last years, complexes with
acyclic aminocarbenes were found to exhibit catalytic acꢀ
tivity in various processes of organic synthesis, which was
comparable and frequently even higher as compared to the
corresponding Nꢀheterocyclic carbenes.4,5 This allows one
to accomplish catalytic reactions, in particular, Suzuki
and Sonogashira crossꢀcoupling, under mild conditions
and using small amounts of the catalyst.4,5
examples of the addition of nucleophiles with the interꢀ
mediate sp2—sp3ꢀhybridization of the nitrogen atom,
namely, (Nꢀphenyl)benzamidine9 and benzophenone hyꢀ
drazone H2N—N=CPh2.10
In the latter case, the authors found that the coupling
products, aminocarbene complexes cisꢀ[PdCl2ꢀ
{C(NHN=CPh2)=NHR}(CNR)], exhibited high cataꢀ
lytic activity in the Suzuki crossꢀcoupling of aryl bromides
with phenylboronic acid.10
The hybridization of the nucleophilic center in hydrꢀ
azones is known to considerably depend on the nature of
substituents.11—13 Varying substituents, the hybridization
can be changed from sp3 to sp2, that, in turn, will affect
the donor properties of the carbene ligand obtained and,
therefore, the catalytic properties of the complex. Besides
the changes in the donor properties, the nature of substitꢀ
uents will also affect the steric properties of the ligand. It
is of interest to study specific features of these changes.
The purpose of this work is the synthesis of new repreꢀ
sentatives of palladium(II) aminocarbene complexes obꢀ
tained from hydrazones and Pdꢀcoordinated isocyanides
and evaluation of their catalytic properties in the Sonoꢀ
gashira and Suzuki crossꢀcouplings.
Scheme 1
Analysis of the literature data shows that cases of addiꢀ
tion of amines and hydrazines, classic sp3ꢀhybridized
Nꢀnucleophiles, to Pdꢀcoordinated isocyanides are most
often.1—3 Recently, there were described examples of adꢀ
dition of Nꢀnucleophiles (imines) with the sp2ꢀhybridizaꢀ
tion of the nucleophilic atom, such as 3ꢀiminoisoindolinꢀ
1ꢀone6,7 and 1,3ꢀdiiminoisoindoline,8 as well as the first
Results and Discussion
Synthesis of complexes. We studied the reactions beꢀ
tween the complex cisꢀ[PdCl2(CNAr)2] (1) and three
commercially available hydrazones H2N—N=CR1R2 2—4
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1361—1365, June, 2013.
1066ꢀ5285/13/6206ꢀ1361 © 2013 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 62, No. 6, June, 2013
Buslaeva et al.
(Scheme 2). A metalꢀpromoted coupling of this comꢀ
pounds led to the corresponding palladium(II) aminocarꢀ
bene complexes 5—7.
methanol) and insoluble in water. They were characterꢀ
ized by elemental analysis, IR spectroscopy, 1D and 2D
1H and 13C NMR spectroscopy, and ESI mass spectroꢀ
metry. The ESI+ mass spectra of the compounds obtained
exhibit peaks corresponding to the fragmentation prodꢀ
ucts with the elimination of one or two chloride ions,
namely, [M – 2Cl – H]+ (5 and 6) or [M – Cl]+ (7). The
signals have characteristic isotope clusters, which indiꢀ
cates the amount of palladium and chlorine atoms in the
corresponding ion.
Scheme 2
1
The proton signals in the H NMR spectrum and the
carbon signals in the 13C NMR spectrum were assigned
1
based on the H—1H COSY and APT experiments, reꢀ
1
spectively. In the H NMR spectra of compounds 5—7,
two broad peaks in the region  9.4—11.4 were observed
corresponding to two protons of the aminocarbene fragꢀ
1
ment Ccarbene—(NH)NHR. In the H NMR spectrum of
compound 7, an additional peak was found in the same
region corresponding to the proton of the HC=N group of
2ꢀhydroxybenzaldehyde hydrazone. The 13C NMR specꢀ
trum of hydrazones added to the coordinated isocyanide
exhibited a sharp shift of the signal for the quaternary
carbon atoms of the CN fragments to the lowꢀfield region
by approximately 50 ppm because of the conversion of the
latter to the carbene carbon atoms (165—178 ppm as comꢀ
pared to the starting 115 ppm).
Catalytic studies. The catalytic properties of aminoꢀ
carbene complexes 5—7 were studied in the Sonogashira
and Suzuki crossꢀcoupling reactions. Since effects of solꢀ
vents, bases, and temperature on the catalytic activity of
such aminocarbene complexes were studied earlier10 and
the scope of substrates was determined, in the present
work we limited our studies to the model substrates and
the optimal conditions found earlier.
Lately, the Sonogashira reaction became a widely used
method for the preparation of arylalkynes and conjugated
enynes.14—17 A classic version of the Sonogashira reaction
traditionally uses palladium phosphine complexes in the
presence of CuI as a cocatalyst. However, in the present
time the interest is growing to an alternative phosphineꢀ
free catalytic systems based, in particular, on palladaꢀ
cycles,14—17 nitrogenꢀcontaining heterocycles,14—17 heteroꢀ
cyclic and the corresponding acyclic diaminocarbenes.4
A high catalytic activity (TONs to 2000, TOFs to 280)
of acyclic aminocarbene complexes [PdCl{C(N(Ph)ꢀ
C(Ph)=NH)=NR}(CNR)] (R = But, Xyl) was reported9
in the coupling between 4ꢀO2NC6H4I and octꢀ1ꢀyne,
which was carried out under mild conditions (60 C) in
the nontoxic solvent (ethanol) in the presence of available
inorganic base (K2CO3) to obtain the product in up to
99% yield.
The reactions between compounds 1 and 2—4 were
found to proceed under conditions practically similar to
those described earlier,10 to be exact, upon reflux in chloꢀ
roform, however, the process required a longer time (9 h).
In the course of the reaction, the solution gradually
changed its color from pale yellow to saturated gold.
The reaction progress was monitored using IR spectra,
in which the aminocarbene complexes 5—7 exhibited
only one strong absorption band at 2207 cm–1 responsible
for the vibration (CN), while the starting complex
cisꢀ[PdCl2(CNR)2] (1) had two such bands, and they were
found in the region of higher frequencies (2237 and
2212 cm–1). The presence of the only band in the IR specꢀ
tra of aminocarbene complexes indicates the presence of
only one isocyanide ligand. The formation of aminocarꢀ
bene complexes 5—7 was also confirmed by the emerꢀ
gence in the IR spectra of the strong band in the region
1510—1548 cm–1, which corresponded to the vibrations
(N—Ccarbene), that completely agreed with the data6—10
published earlier for the related aminocarbene complexes.
The isolated and purified reaction products 5—7, comꢀ
pounds cisꢀ[PdCl2{C(NHN=CR1R2)=NHAr}(CNAr)],
are yellow powders, stable in air, moderately soluble in
polar organic solvents (dichloromethane, chloroform,
As a model system for the Sonogashira reaction, we
used the system selected by us earlier between the stoichioꢀ
metric amounts of 4ꢀiodoanisole (0.10 mmol) and phenylꢀ
acetylene (0.10 mmol) in ethanol at 80 C and K2CO3

Palladium aminocarbene complexes
Russ.Chem.Bull., Int.Ed., Vol. 62, No. 6, June, 2013
1363
Table 1. Catalytic activity of complexes 5—7 in the Sonoꢀ
gashira and the Suzuki crossꢀcoupling reactions
(0.25 mmol) as a base (Scheme 3). An initial amount of
the catalyst was chosen to be 0.01 mol, the reaction time
4 h (Table 1).
Reaction
Product yield for the corresꢀ
ponding catalyst (%)
Scheme 3
5
6
7
Sonogashira coupling
Suzuki coupling
79 (75)
94
75 (50)
93
72 (56)
85
a In parentheses is given the yield after 1 h of using the larger
amount of phenylacetylene (0.15 mmol).
The reaction was carried out in ethanol at 80 C in the
presence of K2CO3 (0.15 mmol). The initial amount of
the catalyst was 1.00 nanomol, the reaction time 4 h. All
the compounds 5—7 were found to exhibit high catalytic
activity in the model Suzuki reaction: the yield of 4ꢀmethꢀ
oxybiphenyl reached 85—94% (see Table 1). The highest
yield of the product (94%) was obtained when catalyst 5
was used, like in the case of the Sonogashira reaction.
The maximum turnꢀover number of the catalyst (TON)
for compound 5 was 94000. No coupling of phenylborꢀ
onic acid with 4ꢀchloroanisole under such conditions
took place.
In conclusion, we have prepared earlier unknown
palladium(II) aminocarbene complexes 5—7 by the metꢀ
alꢀpromoted addition reaction of hydrazones to isocyaꢀ
nide. It should be emphasize that the most catalytic sysꢀ
tems described in the literature for the Sonogashira couꢀ
pling with high TON require either the use of toxic solꢀ
vents, and/or a copper cocatalyst, and/or a large amount
of the catalyst.14 The complexes obtained exhibit high catꢀ
alytic activity in the Sonogashira and Suzuki reactions.
Both processes can be carried out in nontoxic ethanol
using available inorganic base K2CO3. Besides, the Sonoꢀ
gashira reaction catalyzed by the aminocarbene complexꢀ
es under study does not require a cocatalyst. Complex 5
obtained by the addition of sterically less hindered benꢀ
zophenone hydrazone (2) to isocyanide displayed higher
catalytic activity as compared to complexes 6 and 7. Takꢀ
ing into account the fact that the hybridization of the
nucleophilic center in benzophenol hydrazone (2) is the
intermediate between the hybridizations in hydrazones of
compounds 3 and 4,11—13 a conclusion can be drawn that
for the crossꢀcoupling reaction to be successful, a catalyst
with a certain combination of donor and steric properties
is required.
For the catalysts 5—7 and the reaction time of 4 h, the
yields of the crossꢀcoupling products were 70—80%. Our
attempts to shorten the reaction time to 1 h by the use of
50% excess of phenylacetylene were successful in the case
of catalyst 5, whereas for catalysts 6 and 7 they resulted in
the decrease in the yield of the final product to 50%. Thus,
catalyst 5 turned out to be more active than its analogs 6
and 7. The maximum turnꢀover number of the catalyst
(TON) for compound 5 was 7900. It is important to note
that the reaction did not require the presence of a copper
cocatalyst. Our result almost 1000 times exceeded that
described earlier in the literature18 under similar condiꢀ
tions (EtOH, 80 C, K2CO3, 6 h, 96% yield, ncat = 10–5 mol,
n
start.comp. = 10–3 mol, TON = 9.6).
Our attempts to carry out the Sonogashira reaction
with cheaper and more available aryl bromides (4ꢀbromoꢀ
anisole) were unsuccessful: under the selected conditions
even when the reaction time was prolonged to 8 h, the
yield of the product did not exceed 5%. Carrying out the
Sonogashira reaction with 4ꢀbromoanisole in its classic
version, i.e., in the presence of the copper(I) iodide cocatꢀ
alyst and with an excess of PPh3, did not result in the
considerable increase of the coupling product yield, either.
The Suzuki coupling19—22 was chosen as the second
catalytic reaction, namely, the coupling reaction of
4ꢀbromoanisole (0.10 mmol) and phenylboronic acid
(0.12 mmol) (Scheme 4).
Scheme 4
Experimental
Reactants and solvents used in the work were purchased
from Alfa Aesar. The complex [PdCl2(MeCN)2]23 and isocyanꢀ
ide 2ꢀ(4ꢀMeOC6H4CO2)C6H4NC24 were synthesized accordꢀ
ing to the procedures published earlier. NMR spectra were reꢀ

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Russ.Chem.Bull., Int.Ed., Vol. 62, No. 6, June, 2013
Buslaeva et al.
corded on Bruker Avance II (UltraShieldTM Magnet) and Brukꢀ
er Avance DRX (UltraShieldTM Magnet) spectrometers at 20 C
(400.130/500.130 MHz (1H), 100.613/125.758 MHz (13C), solꢀ
vent CDCl3). Chemical shifts (in ppm) were determined relative
to the signal of the solvent. The signals in the 1H and 13C NMR
spectra were assigned based on the 1H—1HꢀCOSY and APT
experiments, respectively. Elemental analysis of compounds was
carried out on a HewlettꢀPackard 185 B CHNꢀanalyzer. IR specꢀ
tra were obtained on a JASCO FT/IRꢀ4000 spectrometer equippꢀ
ed with a PIKE MIRacle—Single Reflection ATR module in the
range of frequencies 4000—400 cm–1 (resolution of the specꢀ
trometer 5 cm–1). Mass spectra of the samples dissolved in
methanol (in some cases, in acetonitrile) were obtained on
a VARIAN 500ꢀMS LC mass spectrometer (electrospray ionizaꢀ
tion, the ion flow of +5 kV, capillary voltage 30 V, loading RF
100%) (ESMS+).
Palladium(II) dichloro[2ꢀ(4ꢀmethoxybenzoyloxy)phenylisoꢀ
cyanide]{[2ꢀ(4ꢀmethoxybenzoyloxy)phenylamino][2ꢀ(9Hꢀfluoꢀ
renꢀ9ꢀylidene)hydrazino]carbene} (6). Found (%): C, 58.95;
H, 3.66; N, 6.21. C43H33N4O6Cl2Pd. Calculated (%): C, 58.75;
H, 3.78; N, 6.37. MS (ESI+, 30 V, MeOH) m/z: 805 [M – 2 Cl –
– H]+. IR, /cm–1: 2965—2839 (C—H); 2206 (CN); 1605
(N=C); 1541 (N—Ccarbene); 760 (C—Harom). 1H NMR (CDCl3),
: 10.57 (s, 1 H); 10.08 (s, 1 H, Ccarbene—(NH)NH); 8.16—7.95,
7.67—7.29, 7.21—6.69 (m, 24 H, Ar); 3.86, 3.57 (both s, 6 H,
OCH3). 13C{1H} NMR (CDCl3), : 178.2 (Ccarbene—NH); 164.3
(C=N); 152.6 (C=O); 147.2 (C—O); 140.8 (C—N); 133.9—120.2
(Ar); 114.1 (CN); 55.6, 55.3 (OCH3).
Palladium(II) dichloro[2ꢀ(4ꢀmethoxybenzoyloxy)phenylisoꢀ
cyanide]{[2ꢀ(4ꢀmethoxybenzoyloxy)phenylamino]ꢀ{[2ꢀ(2ꢀhydrꢀ
oxyphenyl)hydrazino]carbene} (7). Found (%): C, 54.39; H, 3.72;
N, 6.11. C37H31N4Cl2O7Pd. Calculated (%): C, 54.13; H, 3.81;
N, 6.82. MS (ESI+, 30 V, MeOH) m/z: 786 [M – Cl]+. IR,
/cm–1: 2967—2934 (C—H); 2208 (CN); 1605 (N=C); 1510
(N—Ccarbene); 759 (C—Harom). 1H NMR (CDCl3), : 11.39
(s, 1 H); 10.61 (s, 1 H, Ccarbene—(NH)NH); 10.08 (s, 1 H, HC=N);
8.38—7.29, 7.13—6.72 (both m, 20 H, Ar); 5.29 (s, 1 H, OH);
3.87, 3.83, 3.73 (3s, 6 H, OCH3). 13C{1H} NMR (CDCl3),
: 164.7, 159.8 (Ccarbene—NH, C=N); 153.9 (C=O); 144.25
(C—N, C—O, C—OH); 133.5—122.2 (Ar); 114.3 (CN); 55.6,
55.4 (OCH3).
Palladium(II) dichlorobis[2ꢀ(4ꢀmethoxybenzoyloxy)phenylꢀ
isocyanide] cisꢀ[2ꢀ(4ꢀMeOC6H4CO2)C6H4NC]2PdCl2 (1). The
source of palladium [PdCl2(MeCN)2] (0.50 g, 2 mmol) was susꢀ
pended in approximately 50 mL of chloroform, followed by a slow
dropwise addition of isocyanide 2ꢀ(4ꢀMeOC6H4CO2)C6H4NC
(1.0 g, 4 mmol) over 30 min. The mixture was refluxed for 4 h in
an oil bath. After cooling to 20 C, the reaction mixture was
filtered off, the filtrate was concentrated dry in vacuo. The resiꢀ
due was thrice washed with diethyl ether (2×10 mL) and dried
in vacuo to the constant weight. Found (%): C, 52.95; H, 3.56;
N, 3.98. C30H22Cl2N2O6Pd. Calculated (%): C, 52.69; H, 3.24;
N, 4.10. MS (ESI+, 30 V, MeOH) m/z (Irel (%)): 646 [M – Cl]+.
IR (KBr), /cm–1: 2965—2839 (C—H); 2236, 2212 (CN); 1740
Suzuki crossꢀcoupling. The base K2CO3 (20.7 mg, 0.15 mmol),
4ꢀbromoanisole (18.7 mg, 0.1 mmol) and phenylboronic acid
(12.2 mg, 0.1 mmol) were mixed in a 10ꢀmL tube, followed by
the addition of a 1•10–9 M solution of catalyst 5—7 in ethanol
(1 mL). The tube was tightly capped, placed in an oil bath preꢀ
liminary heated to 80 C, and allowed to stand for a certain time
with stirring. After cooling of the reaction mixture to 20 C, it
was dried in vacuo to the constant weight, followed by the addiꢀ
tion of a calculated amount of the standard 1,2ꢀdimethoxyethane
(9.0 mg, 0.1 mmol). The reaction mixture was extracted with
CDCl3 (3×0.25 mL) and all the fractions were combined. Analyꢀ
1
(C=O); 1511 (C—O); 1233 (C—N); 754 (C—Harom). H NMR
(CDCl3), : 8.26—7.29, 7.00—6.84 (m, 16 H, Ar); 3.87, 3.82
(both s, 6 H, OCH3). 13C{1H} NMR (CDCl3), : 169.3 (C=O);
168.7 (C—O); 146.5 (C—N); 137.5—146.4 (Ar); 114.3 (CN);
55.5 (OCH3).
Synthesis of aminocarbene complexes 5—7 (general proceꢀ
dure). A solution of hydrazone H2N—N=CR1R2 [R1, R2 = Ph,
benzophenone hydrazone (2); R1 + R2 = 9Hꢀfluorenylidene,
9Hꢀfluorenꢀ9ꢀone hydrazone (3); R1 = 2ꢀHOC6H4, R2 = H,
2ꢀhydroxybenzaldehyde hydrazone (4)] (0.196 g, 0.194 g, 0.136 g,
respectively, 1 mmol) in chloroform (2 mL) was added to
a solution or a suspension of complex 1 (0.683 g, 1 mol). The
reaction mixture was refluxed for 9 h, during which the mixture
gradually changed its color from light yellow to bright yellow.
Then, the mixture was concentrated at 20—25 C, the product
was extracted with chloroform (2×5 mL). The bright yellow soꢀ
lution was filtered from the insoluble precipitate, and the filtrate
was concentrated dry at room temperature in vacuo. The dry
residue was washed with diethyl ether (2×2 mL) and dried
in vacuo at room temperature to the constant weight.
Palladium(II) dichloro[2ꢀ(4ꢀmethoxybenzoyloxy)phenylisoꢀ
cyanide]{[2ꢀ(4ꢀmethoxybenzoyloxy)phenylamino]ꢀ2ꢀdiphenylꢀ
methylidenehydrazino)carbene} (5). Found (%): C, 57.75; H, 3.90;
N, 6.20. C43H35N4O6Cl2Pd. Calculated (%): C, 58.62; H, 4.00;
N, 6.36. MS (ESI +, 30 V, MeOH) m/z: 808 [M – 2 Cl – H]+.
IR, /cm–1: 2933—2839 (C—H); 2206 (CN); 1605 (N=CPh2);
1542 (N—Ccarbene); 760 (C—Harom). 1H NMR (CDCl3), : 9.87
(s, 1 H); 9.43 (s, 1 H, Ccarbene—(NH)NH); 8.60—7.28, 7.14—6.86
(m, 26 H, Ar); 3.88, 3.76 (both s, 6 H, OCH3). 13C{1H} NMR
(CDCl3), : 177.3, 164.5 (Ccarbene—NH, C=N); 158.3 (C=O);
147.1 (C—O); 143.7 (C—N); 135.6—119.9 (Ar); 114.2 (CN);
55.7, 55.5 (OCH3).
1
sis was carried out using H NMR spectroscopy. The peaks of
the products were correlated with the data published earlier. The
yield of the product was determined by the integration of selectꢀ
ed peaks with the peaks of the standard.
Sonogashira crossꢀcoupling. The base K2CO3 (34.5 mg,
0.25 mmol), 4ꢀiodoanisole (23.4 mg, 0.1 mmol), and phenylꢀ
acetylene (10.2 mg, 0.1 mmol) were mixed in a 10ꢀmL tube,
followed by the addition of a 1•10–9 M solution of catalyst 5—7
in ethanol (1 mL). Further, the reaction was carried out as deꢀ
scribed in the preceding procedure.
This work was financially supported by the Federal
Target Program "Scientific and ScientificꢀPedagogical
Specialists of the Innovative Russia" (State Contract
P–676, 20.05.2010), St.ꢀPetersburg State University (Reꢀ
search Grant for the years 2011—2013), the Russian Founꢀ
dation for Basic Research (Project No. 11–03–00048ꢀa),
and the Council on Grants at the President of the Rusꢀ
sian Federation for the study overseas in 2011/2012
academic year (No. 2057), as well as the Portugal Fund of
Science and Technology Fundação para a Ciência e
a Tecnologia — FCT, Portugal, scientific projects
PTDC/QUIꢀQUI/098760/2008 and PTDC/QUIꢀQUI/
109846/2009).

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12. Y. P. Kitaev, B. I. Buzykin, Usp. Khim. (Russ. Chem. Rev.),
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Received March 15, 2013 ;
in revised form May 17, 2013