Application of palladium complexes bearing acyclic amino(hydrazido)carbene ligands as catalysts for copper-free Sonogashira cross-coupling

Article abstract of DOI:10.1016/j.jcat.2015.06.001

Abstract Metal-mediated coupling of one isocyanide in cis-[PdCl₂(CNR¹)₂] (R¹ = C₆H₁₁ (Cy) 1, tBu 2, 2,6-Me₂C₆H₃ (Xyl) 3, 2-Cl-6-MeC₆H₃

Full text of DOI:10.1016/j.jcat.2015.06.001

Journal of Catalysis 329 (2015) 449–456
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Application of palladium complexes bearing acyclic
amino(hydrazido)carbene ligands as catalysts for copper-free
Sonogashira cross-coupling
Svetlana A. Timofeeva a, Mikhail A. Kinzhalov a, Elena A. Valishina b, Konstantin V. Luzyanin a,c,
,
⇑
Vadim P. Boyarskiy a, , Tatyana M. Buslaeva b, Matti Haukka d, Vadim Yu. Kukushkin a,e
⇑
a Institute of Chemistry, Saint Petersburg State University, Universitetsky pr. 26, 198504 Saint Petersburg, Russian Federation
b Moscow State University of Fine Chemical Technologies, Vernadsky Pr. 86, 119571 Moscow, Russia
c Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom
d Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014, Finland
e Institute of Macromolecular Compounds of Russian Academy of Sciences, Bolshoii Pr. 31, 199004 Saint Petersburg, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Metal-mediated coupling of one isocyanide in cis-[PdCl2(CNR1)2] (R1 = C6H11 (Cy) 1, tBu 2, 2,6-Me2C6H3
(Xyl) 3, 2-Cl-6-MeC6H3 4) and various carbohydrazides R2CONHNH2 [R2 = Ph 5, 4-ClC6H4 6, 3-NO2C6H4
7, 4-NO2C6H4 8, 4-CH3C6H4 9, 3,4-(MeO)2C6H3 10, naphth-1-yl 11, fur-2-yl 12, 4-NO2C6H4CH2 13, Cy
14, 1-(4-fluorophenyl)-5-oxopyrrolidine-3-yl 15, (pyrrolidin-1-yl)C(O) 16, 3-(3,5-di-tert-butyl-4-hydrox
yphenyl)propane-1-yl 17, EtNHC(O) 18] or sulfohydrazides R3SO2NHNH2 [R3 = Ph 19, 4-MeC6H4 20] led
to a series of (hydrazido)(amino)carbene complexes cis-[PdCl2{C(NHNHX)@N(H)R1}(CNR1)]; X = COR2,
SO2R3 (21–48, isolated yields 60–96%). All prepared species were characterized by elemental analyses
(C, H, N), HR ESI+-MS, IR, 1H and 13C{1H} NMR spectroscopy, and by a single-crystal X-ray diffraction
for 38. Complexes 21–48 demonstrated excellent activity as catalysts in copper-free Sonogashira cou-
pling of aryl iodides and a variety of aromatic terminal alkynes. Catalytic system runs in environmentally
benign EtOH ensuring product yields of up to 75–96% and TONs of up to 104. Mechanism of the
copper-free Sonogashira catalytic cycle involving 21–48 as catalysts was proposed upon identification
of key intermediates using HRESI-mass.
Article history:
Received 2 April 2015
Revised 30 May 2015
Accepted 1 June 2015
Keywords:
Acyclic diaminocarbenes
Palladium-(acyclic diminocarbene) catalysts
Copper-free Sonogashira coupling
Mechanism of catalytic action of Pd-ADCs
Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction
also known. Those include pioneering reports by Dhudshia and
Thadani [7] (catalyst loading: 1.5 mol%; product yields up to 91%;
In the last decade, complexes bearing acyclic diaminocarbene
(ADC) ligands [1,2] attracted attention as intriguing alternative to
complexes with N-heterocyclic species (NHCs) in the place of
catalysts in several cross-coupling processes [3,4]. Thus, palladium
complexes with ADC ligands were successfully employed in
Suzuki–Miyaura [5–7], Heck [3,4,7], Buchwald–Hartwig [3,4], and
Sonogashira reactions [3,4,7–11]. Among them, Sonogashira
coupling is typically performed in the presence of a copper
co-catalyst (that improves reaction efficiency but brings about
the generation of undesired by-products) [12–17], although a few
copper-free systems, that are free from above disadvantages, are
TONs up to 60), and by Luzyanin and Boyarskiy [10,11] (catalyst
loading: 0.1 mol%; product yields up to 99%; TONs up to 3.7 Â 104).
While application of palladium-ADCs as catalysts for
copper-free Sonogashira reaction is recognized, to the best of our
knowledge the mechanism of their action and the details of the
corresponding catalytic cycle remain unclear. Indeed, in one of
our recent works [11], we were able to establish for the first time
the identity of the real catalytic species, derived from
palladium-ADCs as precatalysts. However, we were unable to
establish the mechanism of the catalytic process employing
palladium-ADC catalysts, and no other data regarding the
mechanism of their action emerged up to date.
Recently, we reported that palladium complexes bearing
amino(hydrazido)carbene ligands (Fig. 1) act as excellent catalysts
of Suzuki–Miayura reaction performed in aqueous or organic
medium (yields up to 94%, TONs up to 9.4 Â 104) [6]. Motivated
⇑
Corresponding authors at: Institute of Chemistry, Saint Petersburg State
University, Universitetsky pr. 26, 198504 Saint Petersburg, Russian Federation
(K.V. Luzyanin and V.P. Boyarskiy).
E-mail addresses: konstantin.luzyanin@chem.spbu.ru (K.V. Luzyanin), vadim.
boyarskiy@chem.spbu.ru (V.P. Boyarskiy).
http://dx.doi.org/10.1016/j.jcat.2015.06.001
0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

450
S.A. Timofeeva et al. / Journal of Catalysis 329 (2015) 449–456
Table 1
R1
R1
Numbering scheme for palladium-ADC complexes derived from the addition of carbo
R2
R3
O
– (21–40) and sulfohydrazides (41–48) to isocyanides in cis-[PdCl2(CNR1)2] (1–4).
H
N
H
N
HN
Pd
HN
Pd
S
Cl
Cl
Cl
Cl
R1
R2
R2
N
N
H
O
O
H
Cy
Ph
4-ClC6H4
3-
21 fur-2-yl
22 Cy
23 4-(NO2)C6H4CH2
28
29
30
CNR1
CNR1
(NO2)C6H4
4-
(NO2)C6H4
4-MeC6H4
3,4-
(MeO)2C6H3
naphth-1-
yl
Fig. 1. Palladium complexes bearing acyclic amino(hydrazido)carbene ligands.
24 1-(4-fluorophenyl)-5-
oxopyrrolidine-3-yl
25 (pyrrolidin-1-yl)C(O)
26 3-(3,5-di-tert-butyl-4-
hydroxyphenyl)propane-1-yl
27 EtNHC(O)
31
32
33
by an outstanding catalytic efficiency of these compounds in
Suzuki–Miyaura cross-coupling, we decided to evaluate their
catalytic properties in related Sonogashira reaction. To our satisfac-
tion, application of these compounds allowed not only to perform
Sonogashira coupling under mild and copper-free conditions, but
also to get closer to the understanding of the mechanism of this
catalytic process.
34
tBu
4-ClC6H4
3-
(NO2)C6H4
3,4-
35 Cy
38
39
36 3-(3,5-di-tert-butyl-4-
hydroxyphenyl)propane-1-yl
37 EtNHC(O)
40
Our studies are partitioned in three parts. In the first, we
expand the series of palladium complexes with amino(hydrazido)-
carbene ligands as unexplored type of palladium-ADC species. In
the second, we evaluate catalytic properties of these complexes
in Sonogashira reaction, and finally, we attempt to identify cat-
alytic intermediates and to shed light on mechanism of the
Sonogashira process involving catalysts of this type. Our results
are disclosed below.
(MeO)2C6H3
Cy
tBu
Xyl
Ph
Ph
Ph
Ph
41 4-MeC6H4
42 4-MeC6H4
43 4-MeC6H4
44 4-MeC6H4
45
46
47
48
2-Cl-6-MeC6H3
(33, 34, 39, and 40) in good to excellent (86–96%) isolated yields.
Furthermore, the coupling of equimolar amounts of any of one
1–4 and 4-MeC6H4SO2NHNH2 (20) occurs under similar conditions
2. Results and discussion
and affords the aminocarbenes cis-[PdCl2{C(NHNHSO2R3)@N(H)R1}
In the previous study, a series of known [6] palladium-(hydrazi
(CNR1)] (45–48; 84–91%).
do)(amino)carbene complexes cis-[PdCl2{C(NHNHY)@N(H)R1}
(CNR1)]; Y = COR2, SO2R2 (21–32, 35–38, and 41–44; 60–96% iso-
lated yields) were prepared via the metal-mediated coupling of
one isocyanide ligand in cis-[PdCl2(CNR1)2] [R1 = C6H11 (Cy) 1, tBu
2, 2,6-Me2C6H3 (Xyl) 3, 2-Cl-6-MeC6H3 4] and various carbohy-
drazides R2CONHNH2 [R2 = Ph 5, 4-ClC6H4 6, 3-NO2C6H4 7,
4-NO2C6H4 8, 4-CH3C6H4 9, 3,4-(MeO)2C6H3 10, naphth-1-yl 11,
fur-2-yl 12, 4-NO2C6H4CH2 13; Cy 14, 1-(4-fluorophenyl)-5-oxopyr
rolidine-3-yl 15, (pyrrolidin-1-yl)C(O) 16)] or sulfohydrazide
PhSO2NHNH2 (19) (Scheme 1 and Table 1) [6]. We have now
extended this synthetic procedure by employing new carbohy-
drazides [R2 = 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propane-1-yl
17, EtNHC(O) 18] and one sulfohydrazide [R3 = 4-MeC6H4 20] as
reactants. Thus, we observed that the reaction between equimolar
amounts of 1 or 2 and carbohydrazides 17 and 18 proceeds in
refluxing CHCl3 for ca. 4 h. The subsequent workup provided the
Characterization of 21–48. The authenticity of known species
21–32, 35–38, and 41–44 was established upon comparison of
their 1H NMR spectra with those previously reported by some of
us [6], whereas new complexes 33, 34, 39, 40, and 45–48 were
characterized by elemental analyses (C, H, N), high resolution
ESI+-MS, IR, 1H and 13C{1H} NMR spectroscopies. In addition, the
structure of one species (38) was elucidated by a single-crystal
X-ray diffraction (Fig. 2).
The HR-ESI+ mass spectra of 33, 34, 39, 40, and 45–48 display an
ion due to [M+H]+ and/or a fragmentation corresponding to the
loss of HCls from the molecular ion, viz. [MÀCl]+ and
[MÀ2ClÀH]+, with the characteristic isotopic distribution. The IR
spectra exhibit from one to three weak to medium intensity broad
bands in the 3282–3042 cmÀ1 region that correspond to the NH
stretches. The medium/weak intensity bands in the range of
2983–2833 cmÀ1 are characteristic of
ms(CAH) and mas(CAH) vibra-
tions. The examination of the IR spectra revealed the presence of
one strong stretching vibration in the range between 2229 and
2224 cmÀ1 (R1 = Cy, t-Bu) or 2210 and 2204 cmÀ1 (R1 = Xyl,
aminocarbene species cis-[PdCl2{C(NHNHC(O)R2)@N(H)R1}(CNR1)]
O
R2
2-Cl-6-MeC6H3) from
m(C„N) of unreacted isocyanide ligands.
R1
One strong band between 1590 and 1565 cmÀ1 was attributed to
H2N NH
R2
CNR1
CNR1
Cl
Cl
H
N
HN
Pd
m
(CcarbeneAN) in the newly formed ADC-moiety.
5–18
The 1H NMR spectra of 33, 34, 39, 40, and 45–48 display a peak
Pd
Cl
Cl
N
H
R1 = Cy, tBu
in the range of d 7.6–10.3 assigned to the PdACcarbene@N(H)R1 pro-
O
ton. In addition, 1H NMR spectra exhibit two broad peaks from
CNR1
CHCl3, reflux, 4 h
PdACcarbene@N(H)N(H)Y protons in the range of
d 8.8–11.0.
1–4
21−40
Addition of 5–21 to 1–4 in all possible combinations is accompa-
nied by a pronounced downfield d 13C shift of one of the isocyanide
quaternary C atom to the range specific for ADC PdACcarbene
(d 160–224 ppm) [5,6,18–25]. The Ccarbene@NH 13C signals in 34–
35, 40–41, and 46–49 were found to resonate at ca. d 180 ppm, that
is ca. 65 ppm downfield shifted vs. 1 (e.g. 115 ppm for C„N in
cis-[PdCl2(CyNC)2]). These are the typical patterns observed for
the related complexes with amino(hydrazido)carbene ligands [6].
The crystallographic data and processing parameters for 38 are
listed in Table S1 (see Supporting Information), the plot for the
O
S
R1
O
R3
R3
O
H2N NH
H
N
HN
S
19−20
Cl
Cl
N
H
O
Pd
R1 = Cy, tBu, Xyl, 2-Cl-6-MeC6H3
CNR1
CHCl3, reflux, 4 h
41−48
Scheme 1. Synthesis of the amino(hydrazido)carbene-palladium complexes.

S.A. Timofeeva et al. / Journal of Catalysis 329 (2015) 449–456
451
of a base. Under these conditions, we observed that the conversion
of 4-iodoanisole is essentially complete after ca. 1 h. No visible
catalyst decomposition was observed.
Comparison of the catalytic activity of complexes 21–48 in the
model Sonogashira reaction shows that all of them exhibit elevated
and comparable efficiencies (Table 2 contains results for the most
active catalysts; for the full data set see Table 2S of Supporting
Information). The discrepancy in observed activity between 21
and 48 is lower than a medium value of 89 5% with an exception
of precatalysts 21, 27, 30, and 39. Slightly lower activity of these
species can be at least partially explained by their lower solubility
in ethanol even at 80 °C.
Indeed, with the majority of 21–48, the coupling product was
obtained in 75–96% yields (catalyst loading of 0.01 mol%).
Complexes 22 (entry 1, Table 2, product yield 94%), 28 (entry 3,
92%), 38 (entry 7, 93%), and 44 (entry 7, 92%) as the most active
were tested at lower catalyst loading (0.001 mol% instead of
0.01 mol%). Moderate catalytic activity was observed, and the pro-
duct yield was 26–34% (entries 2, 4, 6 and 8, Table 2), with the
maximum yield of 33% for complex 38. No product formation
was observed upon further decreasing the catalyst loading to
10À10 mol (0.0001 mol%).
As the next step, we examined the scope of our catalytic system
using representative catalysts 28, 38, and 44. Aryl iodides and
aromatic terminal alkynes containing both electron-donating
and electron-withdrawing substituents can be successfully
transformed into the target products in good to excellent yields
Fig. 2. View of 38 with the atomic numbering schemes. Thermal ellipsoids are
drawn with the 50% probability level. Hydrogen labels are omitted for simplicity.
Selected bond lengths (Å) and angles (°): Pd1AC13 1.942(4); Pd1AC1 1.978(3);
Pd1ACl2 2.3234(8); Pd1ACl1 2.3520(8); O1AC6 1.209(4); N1AC1 1.313(4);
N1AC(2) 1.503(4); N2AC1 1.340(4); N2AN3 1.389(4); N3AC6 1.353(4); N4AC13
1.140(4); N4AC14 1.464(4); C6AC7 1.510(5); C13APd1AC1 88.6813;
C13APd1ACl2 177.04(10); C1APd1ACl2 88.49(9); Cl2APd1ACl1 92.10(3);
C1AN1AC(2) 129.3(3); C1AN2AN3 122.0(2); C6AN3AN2 120.6(3); C13AN4AC14
177.3(4); N1AC1AN2 117.6(3); N1AC1APd1 129.3(2); N2AC1APd1 113.1(2);
O1AC6AN3 122.2(3); O1AC6AC7 124.9(3); N3AC6AC7 112.8(3).
(Table
3 and 3S). No formation of undesired homocoupling
products were detected in any of these trials. Attempts to employ
aryl bromides as substrates, e.g. 4-bromoanisol and
1-bromo-4-nitrobenzene under studied conditions were unsuc-
cessful, however, we were able to transform those species under
more drastic conditions (see Supporting Information for more
details). Thus, moderate to good yields of the corresponding prod-
ucts were obtained starting from 4-bromoanisol (58%) and
1-bromo-4-nitrobenzene (95%) at 130 °C for 24 h of reaction using
1 mol% of precatalyst.
All prepared palladium complexes 21–48 were evaluated as
catalysts for Sonogashira coupling of aryl iodides and aromatic
terminal alkynes containing both electron-donating and electron-
withdrawing substituents showing excellent activity (products
yields up to 75–96%, TONs up to 104).
structure can be found in Fig. 2, and bond lengths and angles are
given in the figure legend.
In 38, the chlorides are mutually in the cis position
[Cl2APd1ACl1 92.10(3)°] and a slightly distorted square planar
environments around the metal center are completed with one
unreacted isocyanide ligand and one carbene ligand, viz.
C(NHNHCOC6H11)@NHR1. The PdACcarbene bond lengths [Pd1AC1
1.978(3) Å] are nearly similar to those in related PdANHC com-
plexes (typically 1.99–2.00 Å) [26] suggesting comparable binding
strength of the ligand to the metal. The PdACcarbene bond lengths
are comparable to those reported for the related palladium
aminocarbene complexes cis-[PdCl2{C(N(H)N@CPh2)@N(H)Xyl}
(CNXyl)] (1.924(3) Å) [5], cis-[PdCl{C(N@C(C6R2R3R4R5CON))@
N(H)Cy}(CNCy)]
(2.001(3) Å)
[27],
and
cis-[PdCl{C(N@
C(C6H4CNHN))@N(H)Xyl}(CNXyl)] (2.015(9) Å) [28]. In 38, the car-
bene moiety is roughly planar and the angles around the carbene
C1 atoms range from 113.1° to 129.3° therefore sustaining the
sp2 hybridization of this atom. The two CcarbeneAN bond lengths
of the carbene fragment are slightly different [N1AC1 1.313(4),
Table 2
Comparison of the catalytic activity of 21–48.
I
precatalyst
EtOH, K2CO3,
N2AC1 1.340(4) Å]. This observation indicates substantial p –p
p
p
MeO
80 o
C
interactions within the N1AC1AN2 fragments, as detected in the
abovementioned palladium-aminocarbene complexes [5,27,28].
In the isocyanide ligand, the C„N triple bond (N4AC13
1.140(4) Å) has a normal value for the typical triple CN bond
(1.136–1.144 Å) and is coherent with that observed in the related
isocyanide palladium complexes, e.g. cis-[PdCl2(CNR)2] [R = Cy
(1.128–1.142 Å), tBu (1.108–1.149 Å), Xyl (1.145–1.156 Å)] [29–31].
Application of 21–48 as catalysts for Sonogashira cross-coupling.
Taking into consideration our previous reports on application of
the palladium-ADCs in a copper-free Sonogashira reaction [11],
we evaluated catalytic properties of 21–48 in this process. The
reaction of 4-iodoanisole with phenylacetylene accomplishing
1-methoxy-4-(phenylethynyl)benzene has been chosen as a model
system for the copper-free Sonogashira coupling. We employed
EtOH as an environmentally benign solvent and K2CO3 in the place
OMe
Entry
Cat
Yield, % (TON)
1–2
3–4
5–6
7–8
22
28
38
44
94 (9.4 Â 103); 34 (3.4 Â 104)a
92 (9.3 Â 103); 26 (2.6 Â 104)a
93 (9.3 Â 103); 33 (3.3 Â 104)a
92 (9.2 Â 103); 31 (3.3 Â 104)a
K2CO3 (2.5 Â 10À4 mol, 2.5 equiv), 4-iodoanisole (1.0 Â 10À4 mol, 1.0 equiv),
phenylacetylene (1.5 Â 10À4 mol, 1.5 equiv); selected catalyst (1 Â 10À8 mol); EtOH
(1 mL), 80 °C, 1 h.
a
Yields at catalyst loading 0.001 mol% (reaction time increased to 5.5 h).

452
S.A. Timofeeva et al. / Journal of Catalysis 329 (2015) 449–456
Table 3
Product scope for the copper-free Sonogashira system employing 21–48.
R5
I
catalyst
R5
R4
EtOH, K2CO3, 80 o
C
R4
Products and yields, % with the selected (catalyst)
86 (44)
89 (38)
97 (28)
92 (44)
74 (28)
75 (38)
86 (38)
93 (38)
79 (44)
83 (38)
98 (38, 44)
99 (38, 44)
H(O)C
H(O)C
H(O)C
F
Me
OMe
Me
OMe
OMe
99O(M2e8, 38)
8O6M(e44)
7O4M(e38)
98 (28)
Base K2CO3 (2.5 Â 10À4 mol, 2.5 equiv), aryl iodide (1.0 Â 10À4 mol, 1 equiv), alkyne (1.5 Â 10À4 mol, 1.5 equiv); selected catalyst (1 Â 10À8mol); EtOH (1 mL), 80 °C, 1 h.
Comparison of our results with those previously reported by
Dhudshia and Thadani [7] indicates that Pd-ADC catalysts from this
study are significantly more efficient than former, although they
are only slightly better than the system based on the related palla-
dium complexes with amino(hydrazido)carbene ligands reported
previously by some of us [11].
Approaches to identification of catalytic species. Despite the fact
that Sonogashira coupling is one of the most extensively explored
cross-coupling reactions, its mechanism remains unclear even in
case of an apparently more transparent copper-free process [32].
Furthermore, the replacement of the catalyst with one of a
different type alters dramatically and unpredictably the nature
of the catalytic intermediates. While application of popular
palladium-NHC species in Sonogashira reaction is well docu-
mented [32–34], only limited evidence on the nature of the
catalytically active species and mechanism were obtained, and
cycle (e.g. products of the oxidative addition or alkyne binding to
the palladium center) and to confirm the proposed catalytic cycle.
In pursuit of this study, we resumed our previous attempts of
identification of catalytic species that are formed throughout the
catalytic cycle. In the first set of experiment, we performed several
catalytic runs of the model system in the presence of metallic Hg
(the mercury drop test [35]) using five representative precatalysts
22, 28, 38, 42, and 44. Similar reaction rates for the corresponding
processes both in the absence and in the presence of metallic Hg
for any combination of substrates were detected. Furthermore,
using representative precatalysts time-dependent reaction profiles
for a model reaction system were acquired (see Supporting
Information). On basis of those, no induction period in the system
was detected, and the accumulation of the product in the initial
period followed nearly linear time-dependence. Both observations
suggest that no formation of palladium nanoparticles occurs
during the reaction and that the catalytic system operates under
typically homogeneous conditions.
the key step of the process involves the
p-coordination of an
alkyne to a unsaturated palladium-NHC center followed by proton
removal from alkyne upon action of base that occurs directly at the
metal. In case of palladium-ADC species, in our previous work we
got some first evidence on the nature of the catalytic species
formed from the palladium-ADC precatalysts. Indeed, we estab-
lished that the [M(carbene)] fragment is responsible for the cat-
alytic action and that the mechanism is homogeneous. We were,
however, unable to identify other intermediates of the catalytic
In the second set of experiments, HRESI-MS monitoring (see
Figs. 1S–4S, Supporting Information) of the reaction mixture
for the model catalytic system [data with representative precata-
lyst 42, viz. cis-[PdCl2{C(NHNHS(O)2Ph)@N(H)tBu}(CNtBu)];
C16H26N4Cl2O2PdS, M = 514.0183 g/mol] allowed the identification
of intermediates formed in the course of our palladium-
ADC-catalyzed process (Scheme 2).

S.A. Timofeeva et al. / Journal of Catalysis 329 (2015) 449–456
453
π
Scheme 2. Proposed mechanism for the copper-free Sonogashira coupling catalyzed by 21–48 based upon the identification of catalytic intermediates.
Indeed, we initiated our reaction by addition of K2CO3 to the
model reaction mixture at RT. Immediately after the addition,
the signal corresponding to the [M(carbene)(CNR)]2+ species [in
the MS-spectra: [MÀ2ClÀH]+, m/z: 443.0764 (found), 443.0728
(calcd.)] was detected in the spectrum. This species is formed upon
removal of the chlorides from the starting 42 (Fig. 1S), and contains
palladium(II) center; its reduction to palladium(0) derivatives does
not proceed at RT.
Upon heating of the mixture several new peaks emerged in the
HRESI-MS spectra. Indeed, the first group of signals with m/z of
701.0287 [MÀ2Cl+MeOC6H4I+Na]+ (m/z: 701.0245, calcd.) matches
to the product of oxidative addition of MeOC6H4I to the pre-formed
[M(carbene)(CNR)]0 (Fig. 2S; Oxidative addition step from
Scheme 2). Unfortunately, the latter species was not evident in
the spectra at any stage of the catalytic process, hence, we believe
that [M(carbene)(CNR)]0, being coordinatively unsaturated, reacts
promptly with the MeOC6H4I substrate. The signal due to the
oxidative addition remains present until full disappearance of the
substrates and we believe in this case, it represents the catalyst
resting state.
Second set of the signals with m/z of 675.1680 detected in the
model catalytic reaction corresponds to the product of the alkyne
binding to the metal center with subsequent coordination of the
isocyanide [MÀ2Cl+MeOC6H4+CCPh+Na]+ (m/z: 675.1592, calcd.)
(Proton removal/Alkyne binding step, Fig. 3S).
To bring additional evidence on the mechanism of the process,
step-by-step formation of the catalytic intermediates, was
explored. Thus, the HRESI-MS spectra of the reaction mixture in
the absence of phenyl acetylene (addition of MeOC6H4I and
K2CO3 to the solution of 42 in EtOH followed by heating of the
resulting mixture) brought about the emergence of similar signal
due to oxidative addition in HRESI-MS (m/z of 701.0287
[MÀ2Cl+MeOC6H4I+Na]+; m/z: 701.0245, calcd.). Subsequent addi-
tion of phenylacetylene to this solution and heating led to appear-
ance of the signal with m/z of 675.1680 (product of the alkyne
binding to the metal center with subsequent coordination of the
isocyanide [MÀ2Cl+MeOC6H4+CCPh+Na]+; m/z: 675.1592, calcd.).
Summarizing our data with the previously reported mechanistic
studies on Sonogashira catalysis [32,36,37], we propose the full
catalytic cycle employing palladium-ADC precatalysts (Scheme 2).
Throughout the catalytic run, we detected the gradual deceler-
ation of the process. After 1 h (typical time for one catalytic exper-
iment), the addition of the fresh substrates and the base to the
system does not lead to generation of a new portion of product.
In the HRESI-MS spectra taken at this time (Fig. 4S), we detected
the disappearance of the signals from [M(carbene)(CNR)]2+ and
oxidation addition product [MÀ2Cl+MeOC6H4I+Na]+ along with
the accumulation of the palladium clusters containing several
palladium atoms. Despite the previous reports on catalytic activity
of small palladium clusters (see Ref. [38] and references therein), in
our case these compounds are apparently not catalytically
competent. Indeed, we consider their formation as one of the
plausible catalyst deactivation pathways. It was previously
reported [39] that the generation of the catalytically active clusters
from the corresponding complexes is inhibited in the presence of
the carbene ligands presumably due to their strong binding to
the metal core.
3. Conclusions and final remarks
In pursuit of the studies, we prepared several new amino
(hydrazido)carbene-palladium complexes via the metal-mediated

454
S.A. Timofeeva et al. / Journal of Catalysis 329 (2015) 449–456
addition of carbo- and sulfohydrazides to one of the coordinated
isocyanides in cis-[PdCl2(CNR1)2]. Evaluation of the catalytic
properties of prepared palladium-ADC species in Sonogashira
coupling indicated that all of them exhibit high catalytic activity.
Indeed, with catalyst loading as low as 0.01 mol%, catalytic
system that runs in environmental benign EtOH under
copper-free conditions afforded the target internal alkynes in
75–96% yields and with TONs up to 104. We were also able to
propose the full catalytic cycle for this copper-free Sonogashira
protocols upon identification of key intermediates, including cat-
alytically active [M(carbene)(CNR)] species, and intermediates
formed upon oxidative addition of aryl halide and alkyne binding.
Catalyst deactivation in this system is accompanied by the
generation of small Pd clusters that are not catalytically
competent.
33. Yield 86%. Anal. Calcd for C31H50N4Cl2O2Pd: C, 54.11; H, 7.32; N,
8.14. Found: C, 54.14; H, 7.31; N, 8.13. HR-MS (ESI+, 70 V, MeOH),
m/z: calc 687.2418 for C31H51N4Cl2O2Pd+, found 687.2421 [M+H]+,
calc 651.2652 for C31H50N4ClO2Pd+ found 651.2658 [MÀCl]+. IR
(KBr, selected bands, cmÀ1):
m(OAH) 3642 (s),
m
(NAH) 3240 (m),
(C@O) 1708 (s),
m
m
(CAH) 2962–2870 (m), m(C„N) 2227 (s),
m
(CcarbeneAN) 1565 (s). 1H NMR (CDCl3, d): 1.14–3.00 (m, 42H,
CH2, CH3), 3.84–4.00 (m, 1H, CHNC), 4.38–4.57 (m, 1H, NHCH),
7.04 (s, 2H, aryls), 7.69 (s, broad, 1H, CcarbeneANHACy), 8.88 (s,
4. Experimental section
broad,
1H,
C
carbeneANHANH),
9.36
(s,
broad,
1H,
C
carbeneANHANH). 13C{1H} NMR (CDCl3, d): 23.2 (2C, CH2), 24.6
Materials and instrumentation
(CH2), 24.8 (2C, CH2), 24.9 (CH2), 30.4 (6C, CH3), 31.0 (CH2), 31.7
(2C, CH2), 34.3 (2C, C(CH3)), 36.1 (CH2), 42.6 (CH), 55.7 (CH), 59.1
(CH), 118.6 (C„N), 125.2 (CH, aryls), 131.1 (C, aryls), 135.5
(C, aryls), 151.9 (C, aryls), 169.7 (CO), 177.9 (Ccarbene).
Solvents, PdCl2, and all organic compounds were obtained from
commercial sources and used as received, apart from chloroform,
which was purified by the conventional distillation over calcium
chloride. The complexes cis-[PdCl2(CNR1)2] (1–4) [15,16] and
22–33, 36–39, and 42–45 [6] were prepared by the reported proce-
dures. C, H, and N elemental analyses were carried out on a Euro EA
3028 HT CHNSO analyzer. Mass spectra were acquired on Bruker
micrOTOF spectrometer equipped with electrospray ionization
(ESI) source, MeOH was used as the solvent. The instrument was
operated at positive ion modes using m/z range of 50–3000. The
capillary voltage of the ion source was set at À4500 V (ESI+) and
the capillary exit at +(70–150) V. The nebulizer gas pressure was
0.4 bar and drying gas flow was 4.0 L/min. The most intensive peak
in the isotopic pattern is reported. Infrared spectra were recorded
on a Shimadzu FTIR 8400S instrument (4000–400 cmÀ1) in KBr
pellets. 1H, 13C{1H}, and DEPT NMR spectra were acquired on a
Bruker 400 MHz Avance spectrometer at ambient temperature.
X-ray Structure Determination. The crystal of 38 was immersed
in cryo-oil, mounted in a MiTeGen loop, and measured at a tem-
perature of 123 K. The X-ray diffraction data were collected on an
34. Yield 91%. Anal Calcd for C18H31N5Cl2O2Pd: C, 41.04; H, 5.93; N,
13.29. Found: C, 41.18; H, 5.92; N, 13.25. HR-MS (ESI+, 70 V, MeOH),
m/z: calc 490.1196 for C18H31N5ClO2Pd+, found 599.2344 [MÀCl]+,
calc 454.1429 for C18H30N5O2Pd+, found 454.1435 [MÀ2ClÀH]+. IR
(KBr, selected bands, cmÀ1):
m(NAH) 3245–3063 (m),
m
(CAH)
2962–2853 (m),
m
(C„N) 2229 (s),
m
(C@O) 1680 (s),
m(CcarbeneAN)
1580 (s). 1H NMR (DMSO-d6, d): 0.96–2.20 (m, 23H, CH2 + CH3),
3.06–3.27 (m, 2H, CH2), 4.08–4.20 (m, 1H, CHNC), 4.28–4.45 (m,
3
1H, NHCH), 7.80 (d, JH,H = 8.9 Hz, 1H, CcarbeneANHACy), 8.73 (t,
3JH,H = 5.9 Hz, 1H, NHEt), 10.26 (s, broad, 1H, CcarbeneANHANH),
10.77 (s, broad, 1H, CcarbeneANHANH). 13C{1H} NMR (DMSO-d6, d):
14.8 (CH3), 22.3 (3C, CH2), 24.8 (CH2), 25.4 (4C, CH2), 31.5 (2C,
CH2), 34.1 (2C, CH2), 54.6 (CH), 59.8 (CH), 159.0 (CO), 159.3 (CO),
176.4 (Ccarbene), C„N were not detected.
Agilent Technologies Supernova diffractometer using Cu K
a radi-
ation (k = 1.54184 Å). The CrysAlisPro [40] program package was
used for cell refinements and data reductions. The structure was
solved by charge flipping method using the SUPERFLIP [41] pro-
gram. An analytical absorption correction (CrysAlisPro) [40,42]
was applied to the data. Structural refinement was carried out
using SHELXL-97 [43] with the Olex2 [44] and SHELXLE [45] graph-
ical user interfaces. Solvents of crystallization (CHCl3, iPr2O,
CH2Cl2) were severely disordered and partially lost. Only one
CHCl3 molecule could be located from the difference Fourier
map. However, even that was partially lost and therefore refined
with occupancy of 0.25. Other solvent molecules could not be
located. The contribution of solvent to the calculated structure
factors was taken into account by using a SQUEEZE routine of
PLATON [46]. The crystal under investigation was weakly diffract-
ing and the cyclohexyl and tert-butyl groups were slightly disor-
dered. However, no disorder model was applied in the final
refinement. Therefore, the Ueq for one of the cyclohexyl carbon
(C11) remained high. Also, the poor diffraction power was the
reason for missing of some reflections. The hydrogen atoms were
positioned geometrically and constrained to ride on their parent
atoms, with CAH = 0.98–1.00 Å, NAH = 0.88 Å, and Uiso = 1.2–1.5
39. Yield 96%. Anal. Calcd for C27H46N4Cl2O2Pd: C, 50.99; H, 7.29; N,
8.81. Found: C, 51.07; H, 7.28; N, 8.77. HR-MS (ESI+, 70 V, MeOH),
m/z: calc 599.2339 for C27H46N4ClO2Pd+, found 599.2344 [MÀCl]+,
calc 563.2572 for C27H45N4O2Pd+, found 563.2579 [MÀ2ClÀH]+. IR
(KBr, selected bands, cmÀ1):
m
(OAH) 3641 (s),
(C„N) 2227 (s),
(CcarbeneAN) 1566 (s). 1H NMR (CDCl3, d): 1.41 (s, 18H, CH3), 1.54
m
(NAH) 3242 (m),
m(CAH) 2957–2871 (m),
m
m(C@O) 1707 (s),
m
U
eq(parent atom). The crystallographic details are summarized
(s, 9H, CH3), 1.68 (s, 9H, CH3), 2.34–3.10 (m, 4H, CH2), 7.10 (s, 2H,
aryls), 7.77 (s, broad, 1H, CcarbeneANHAtBu), 8.88 (s, broad, 1H,
in Table S2.

S.A. Timofeeva et al. / Journal of Catalysis 329 (2015) 449–456
455
C
carbeneANHANH), 9.55 (s, broad, 1H, CcarbeneANHANH). 13C{1H}
d(CAH from Ar) 730 (m). 1H NMR (DMSO-d6, d): 1.48 (s, 9H, CH3),
1.56 (s, 9H, CH3), 2.44 (s, 3H, CH3), 7.47 (d, 3JH,H = 8.0 Hz, 2H, aryls),
NMR (CDCl3, d): 29.8 (CH3), 30.4 (CH3), 31.3 (CH2), 31.4 (CH3),
34.3 (C), 36.3 (CH2), 55.2 (C), 59.5 (C), 118.3 (C„N), 125.4 (CH,
aryls), 131.4 (C, aryls), 135.4 (C, aryls), 151.9 (C, aryls), 174.2 (CO),
177.4 (Ccarbene).
3
7.69 (s, 1H, CcarbeneANHAtBu), 7.76 (d, JH,H = 8.0 Hz, 2H, aryls),
10.13 (s, 1H, NH), 10.69 (s, 1H, NH). 13C{1H} NMR (DMSO-d6, d):
21.5 (CH3), 29.7 (3C, CH3), 31.0 (3C, CH3), 54.7 (C), 61.0 (C), 128.5
(2C, CH, aryls), 130.1 (2C, CH, aryls), 135.0 (C, aryls), 144.6 (C, aryls),
178.9 (Ccarbene), C„N were not detected.
40. Yield 90%. Anal. Calcd for C14H27N5Cl2O2Pd: C, 35.42; H, 5.73; N,
14.75. Found: C, 35.47; H, 5.78; N, 14.68. HR-MS (ESI+, 70 V, MeOH),
m/z: calc 438.0883 for C14H27N5ClO2Pd+, found 438.0890 [MÀCl]+.
IR (KBr, selected bands, cmÀ1):
m(NAH) 3241–3042 (m),
m(CAH)
47. Yield 84%. Anal. Calcd for C25H28N4Cl2O2PdS: C, 47.97; H, 4.51;
N, 8.95. Found: C, 48.06; H, 4.53; N, 8.91. HR-MS (ESI+, 70 V, a mix-
ture of MeOH and DMSO), m/z: calc 589.0651 for C25H28N4ClO2PdS+,
2983–2833 (m),
m
(C„N) 2224 (s),
m
(C@O) 1680 (s),
m(CcarbeneAN)
3
1576 (s). 1H NMR (DMSO-d6, d): 1.08 (t, JH,H = 7.2 Hz, 3H, CH3),
found 589.0655 [MÀCl]+. IR (KBr, selected bands, cmÀ1):
m
(NAH)
1.49 (s, 9H, CH3), 1.60 (s, 9H, CH3), 3.09–3.27 (m, 2H, CH2), 7.66
3
3270–3154 (m), m(C„N) 2210 (s), m(CcarbeneAN) 1565 (s), mas(SO2)
(s, 1H, CcarbeneANHAtBu), 8.74 (t, JH,H = 5.9 Hz, 1H, NHEt), 10.40
1334 (s),
m
s(SO2) 1170 (s). 1H NMR (DMSO-d6, d): 2.23 (s, 6H,
(s, broad, 1H,
C
CcarbeneANHANH), 10.72 (s, broad, 1H,
carbeneANHANH). 13C{1H} NMR (CDCl3, d): 14.8 (CH3), 29.7 (CH3),
CH3), 2.33 (s, 6H, CH3), 2.40 (s, 3H, CH3), 7.06–7.44 (m, 8H, aryls),
3
7.74 (d, JH,H = 8.0 Hz, 2H, aryls), 10.20 (s, 1H, NH), 10.38 (s, 1H,
31.3 (CH3), 34.1 (CH2), 55.0 (C), 59.3 (C), 159.0 (CO), 159.5 (CO),
176.0 (Ccarbene), C„N were not detected.
NH), 10.89 (s, 1H, NH). 13C{1H} NMR (DMSO-d6, d): 18.5 (2C, CH3),
19.3 (2C, CH3), 21.6 (C, CH3), 125.6 (C„N), 128.2 (CH, aryls), 128.5
(CH, aryls), 128.6 (CH, aryls), 128.8 (CH, aryls), 129.9 (CH, aryls),
131.0 (CH, aryls), 135.3 (C, aryls), 135.9 (C, aryls), 136.0 (C, aryls),
136.7 (C, aryls), 137.9 (C, aryls), 144.3 (C, aryls), 182.9 (Ccarbene).
45. Yield 86%. Anal. Calcd for C21H32N4Cl2O2PdS: C, 43.35; H, 5.54;
N, 9.63. Found: C, 43.38; H, 5.57; N, 9.61. HR-MS (ESI+, 70 V,
MeOH), m/z: calc 581.0731 for C21H33N4Cl2O2SPd+ found 581.0736
[M+H]+, calc 545.0964 for C21H32N4ClO2PdS+, found 545.0970
48. Yield 88%. Anal. Calcd for C23H22N4Cl4O2PdS: C, 41.43; H, 3.33;
N, 8.40. Found: C, 43.46; H, 3.31; N, 8.37. HR-MS (ESI+, 70 V,
MeOH), m/z: calc 628.9558 for C23H22N4Cl3O2PdS+, found
[MÀCl]+. IR (KBr, selected bands, cmÀ1):
m(NAH) 3272–3124 (m),
m
m
(CAH) 2933–2856 (m), m(C„N) 2225 (s), m(CcarbeneAN) 1590 (s),
as(SO2) 1345 (s),
628.9562 [MÀCl]+. IR (KBr, selected bands, cmÀ1):
m(NAH) 3240–
m
s(SO2) 1169 (s). 1H NMR (DMSO-d6, d): 1.00–
3064 (m),
(s),
m(C„N) 2204 (s), m(CcarbeneAN) 1565 (s), mas(SO2) 1340
1.97 (m, 20H, CH2), 2.43 (s, 3H, CH3), 4.17–4.42 (m, 2H, CH), 7.45
m
s(SO2) 1170 (s), d(CAH from Ar) 775 (m). 1H NMR (DMSO-d6,
3
3
(d, JH,H = 8.0 Hz, 2H, aryls), 7.74 (d, JH,H = 8.0 Hz, 2H, aryls), 8.36
(d, 3JH,H = 9.4 Hz, 1H,
carbeneANHACy), 10.03 (s, 1H,
carbeneANHANH), 10.42 (s, 1H, CcarbeneANHANH). 13C{1H} NMR
d): 2.36 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.40 (s, 3H, CH3), 7.26–7.36
C
3
(m, 5H, aryls), 7.46–7.59 (m, 3H, aryls), 7.81 (d, JH,H = 8.0 Hz, 2H,
C
aryls),
C
10.32
(s
1H,
C
carbeneANHAAr),
10.36(s,
1H,
(DMSO-d6, d): 21.5 (C, CH3), 22.1 (3C, CH2), 24.8 (CH2), 25.2 (3C,
CH2), 31.5 (3C, CH2), 54.7 (CH), 59.8 (CH), 128.5 (2C, CH, aryls),
130.0 (2C, CH, aryls), 135.2 (C, aryls), 144.3 (C, aryls), 178.8
(Ccarbene), C„N were not detected.
carbeneANHANH), 11.05 (s, 1H, CcarbeneANHANH). 13C{1H} NMR
(DMSO-d6, d): 18.9 (2C, CH3), 19.8 (2C, CH3), 21.6 (C, CH3), 124.5
(C„N), 127.5 (CH, aryls), 128.2 (CH, aryls), 128.8 (CH, aryls), 129.7
(CH, aryls), 130.0 (CH, aryls), 130.1 (CH, aryls), 132.2 (C, aryls),
132.6 (C, aryls), 135.1 (C, aryls), 136.4 (C, aryls), 137.9 (C, aryls),
139.0 (C, aryls), 140.7 (C, aryls), 144.4 (C, aryls), 183.7 (Ccarbene).
General procedure for the catalytic Sonogashira cross-coupling.
K2CO3 (2.5 Â 10À4 mol, 2.5 equiv), aryl iodide (1.0 Â 10À4 mol, 1.0
equiv), and alkyne (1.5 Â 10À4 mol, 1.5 equiv) were mixed in a
10-mL vial, followed by addition of a solution of the selected
catalyst (1 Â 10À8 mol) in EtOH (1 mL). The vial was placed in a
preheated oil bath at 80 °C and stirred for 1 h. After cooling to
20–25 °C, the reaction mixture was evaporated to dryness under
a stream of dinitrogen followed by addition of 1.0 equiv of
1,2-dimethoxyethane as NMR internal standard, and extraction of
the reaction mixture with three 0.20-mL portions of CDCl3. All frac-
tions were joined and analyzed by 1H NMR spectroscopy. The pro-
duct peak assignments were based on the authentic samples or on
published data [47–53], whereas quantifications were performed
46. Yield 91%. Anal. Calcd for C17H28N4Cl2O2PdS: C, 38.54; H, 5.33; N,
10.57. Found: C, 38.62; H, 5.37; N, 10.55. HR-MS (ESI+, 70 V, a mix-
ture of MeOH and DMSO), m/z: calc 493.0651 for
C
17H28N4ClO2PdS+, found 493.0651 [MÀCl]+. IR (KBr, selected bands,
cmÀ1):
2227 (s),
m
(NAH) 3282–3128 (m),
m
(CAH) 2950–2830 (m),
m(C„N)
m(CcarbeneAN) 1580 (s), mas(SO2) 1345 (s), ms(SO2) 1168 (s),

456
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Acknowledgments
Financial support of Russian Science Foundation (Grant
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