An Abnormal N-Heterocyclic Carbene-Based Palladium Dimer: Aqueous Oxidative Heck Coupling under Ambient Temperature

Article abstract of DOI:10.1002/adsc.201500220

Herein we report the synthesis of a new abnormal N-heterocyclic carbene (aNHC)-based ligand and its use in synthesizing an acetate bridged C-H activated palladium dimer (1). In complex 1, the carbene ligand displays an abnormal mode of binding with the palladium(II) ion. We used complex 1 as a catalyst for the oxidative Heck coupling reaction of arylboronic acids with electron-deficient as well as electron-rich olefins at ambient temperature in water to produce selectively the β-arylated products in good to excellent yields.

Full text of DOI:10.1002/adsc.201500220

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DOI: 10.1002/adsc.201500220
An Abnormal N-Heterocyclic Carbene-Based Palladium Dimer:
Aqueous Oxidative Heck Coupling Under Ambient Temperature
Pradip Kumar Hota,a Gonela Vijaykumar,a Anand Pariyar,a
Samaresh Chandra Sau,a Tamal K. Sen,a and Swadhin K. Mandala,*
a
Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur – 741246,
India
Fax: (+91)-33-4809-2033, phone: (+91)-990-367-6563; e-mail: swadhin.mandal@iiserkol.ac.in
Received: March 3, 2015; Revised: July 29, 2015; Published online: October 14, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500220.
Abstract: Herein we report the synthesis of a new tion of arylboronic acids with electron-deficient as
abnormal N-heterocyclic carbene (aNHC)-based well as electron-rich olefins at ambient temperature
ligand and its use in synthesizing an acetate bridged in water to produce selectively the b-arylated prod-
À
C H activated palladium dimer (1). In complex 1, ucts in good to excellent yields.
the carbene ligand displays an abnormal mode of
binding with the palladium(II) ion. We used complex Keywords: abnormal carbenes; b-arylated products;
1 as a catalyst for the oxidative Heck coupling reac- oxidative Heck coupling; palladium(II) complexes
Introduction
taining metal complexes have earlier demonstrated
excellent activity as catalysts,[6] resulting in the activa-
[6a]
[6b]
À
À
Carbenes featuring a neutral divalent carbon atom tion of inert bonds such as C H
and H H.
In
with the presence of two non-bonding electrons have some cases, the aNHC coordinated metal complexes
played a crucial role as transient intermediates in have outperformed the catalytic activity of their
a number of chemical transformatons. The isolation normal NHC counterparts (nNHC) as demonstrated
of stable N-heterocyclic carbenes (NHCs) by Arduen- by Nolan and co-workers in palladium-catalyzed
go et al. in 1991[1] is considered as the major break- Suzuki–Miyaura coupling and Heck coupling reac-
through in this field, which led to the development of tions.[7] Recently, we have established that this isolat-
numerous organometallic catalysts for a variety of or- ed aNHC derived from 1,3-bis(2,6-diisopropylphen-
ganic reactions.[2] This particular class of carbenes yl)-2,4-diphenylimidazolium salt is an outstanding
binds with metal ions through the carbon center ligand for designing useful organocatalysts[8] as well as
which is in between the two nitrogen atoms leading to organometallic catalysts[9] for a variety of organic
the normal NHC binding mode. However, in 2001, transformations. For example, we recently reported
À
Crabtree and co-workers first demonstrated that 2- the synthesis of halobridged C H bond activated pal-
pyridylmethylimidazolium salt on reaction with ladium complexes containing the same aNHC and
IrH5(PPh3)2, binds with the metal ion through a car- these palladium complexes were further used as cata-
bene center which has been generated between nitro- lysts for the Suzuki–Miyaura cross-coupling of chal-
gen and carbon atoms. This metal binding mode of lenging aryl chloride substrates.[9a] It delivered an ex-
NHC is known as the abnormal mode of binding.[3] cellent yield for the C Cl activation process at room
À
However, isolation of such an abnormal NHC in the temperature under
a
very low catalyst loading
free state remained elusive until a decade later when (0.005 mol%) without any loss of appreciable catalytic
Bertrand and co-workers successfully strategized the activity over ten successive catalytic cycles.[9a] This
synthetic method of the first isolable abnormal NHC result clearly highlighted the usefulness of an abnor-
based
on
a
1,3-bis(2,6-diisopropylphenyl)-2,4- mal NHC in designing palladium-based catalysts for
diphenylimidazolium salt.[4] This particular report led C C coupling reactions. The present study reports the
to the anticipation that the access to this type of ab- synthesis of a new abnormal NHC-based ligand
normal NHC (aNHC) would fuel up the field of adopting a much simpler synthetic pathway and its
À
NHC-based organometallic catalysis.[5] aNHCs con- use in synthesizing an acetate bridged C H activated
À
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An Abnormal N-Heterocyclic Carbene-Based Palladium Dimer
palladium dimer. This palladium dimer was further
established as a useful catalyst for oxidative Heck
À
coupling reactions leading to new C C bond forma-
tion. After the first report by Heck[10] and further
modification by Uemura[11] and Mori,[12] the transition
metal-catalyzed coupling of organoboronic acids with
olefins, known as the oxidative Heck reaction, has
been highly investigated.[13] Despite many literature
reports on the oxidative Heck coupling reaction,[14]
the reaction suffers from a number of challenges in-
cluding unfriendly reaction conditions using solvents
such as DMF (dimethylformamide) or DMA (dime-
thylacetamide) as well as a higher loading of catalyst.
Scheme 1. Synthesis of ligand, L1HI.
The use of molecular oxygen as dedicated oxidant has and 2-bromoacetophenone. L1HI was characterized
been reported in palladium-catalyzed methodolo- by spectroscopic methods (HR-MS and NMR), ele-
gies.[15] However, the use of oxygen gas also led to un- mental analysis and a single crystal X-ray study. The
wanted side reactions. For example, arylzinc com- 1H NMR signal arising from the N-Me group of L1HI
pounds,[16] triarylbismuthines,[17] and arylboronic was assigned as a singlet at d=3.73 ppm. The suitable
acids[18] were reported to be dimerized in the presence crystals of L1HI were grown from dichloromethane at
of oxygen. However, Jungꢁs group reported the ambient temperature. Figure 1 shows the ORTEP dia-
oxygen-promoted[14a,15a] oxidative Heck coupling of or- gram of L1HI which reveals a cationic imidazolium
ganoboron compounds with olefins at 508C as well as backbone with iodide counter anion. The L1HI salt
at room temperature. In 2006, Jung and co-workers was further treated with palladium(II) acetate
also reported the first base-free oxidative Heck reac- [Pd(OAc)2] in 1,4-dioxane at 808C for 8 h resulting in
tion[15a] (DMA, 508C, 5 mol%). Xiao in 2008, devel- the formation of a dark yellow-coloured acetate-
oped the oxygen- and base-free olefination of arylbor- bridged palladium dimer 1 (60% isolated yield,
onic acids[19] (acetone, 708C, 20 h, 2 mol%). Previous Scheme 2).
literature studies showed that the oxidative Heck re-
Upon Pd(II) coordination, the N-methyl hydrogen
action, when attempted in water, it led to exclusive peak shifts from d=3.73 ppm to a more shielded
formation of the biaryl product via a homocoupling region (d=3.04 ppm). A new peak at d=1.24 ppm
1
reaction of the arylboronic acids.[18a–c] Recently, Mac- was observed in the H NMR spectrum which can be
gregor, Lee and co-workers reported that oxidative attributed to the methyl hydrogen nuclei from acetate
Heck coupling can be performed in water medium.[18d] groups.
Herein we report an oxidative Heck reaction using an
The abnormal carbon, C-5 upon Pd(II) coordina-
abnormal NHC-coordinated palladium(II) dimer as tion resonates at d=147.1 ppm as displayed by the
a catalyst. The catalytic reaction can be performed in
water using benzoquinone (BQ) as an inexpensive ox-
idant at ambient temperature under a very low cata-
lyst loading (1 mol%).
Results and Discussion
We have designed and synthesized 3-methyl-1,2,4-tri-
phenyl-1H-imidazol-3-ium iodide salt (L1HI) with un-
symmetrical substituents on the two nitrogen atoms
(aromatic and aliphatic substituents, Scheme 1) by
adopting a simplified synthetic route. The newly de-
signed aNHC salt, L1HI represents a relatively less
hindered environment around the C-5 center than the
earlier reported 1,3-bis(2,6-diisopropylphenyl)-2,4-di-
phenylimidazolium salt.[4] L1HI was synthesized by
modifying the literature procedure.[20] The L1HI salt
was prepared by the reaction of 1,2,4-triphenylimida-
Figure 1. ORTEP diagram of L1HI with 50% probability el-
lipsoids. Hydrogen atoms have been omitted for the sake of
zole with methyl iodide in acetonitrile medium result-
ing in a 73% isolated yield. 1,2,4-Triphenyimidazole
was accomplished by the reaction between amidine clarity.
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Pradip Kumar Hota et al.
Scheme 2. Synthesis of complex 1.
13C NMR spectrum of 1. In addition to this observa-
tion, a new peak appeared at d=23.1 ppm, which can
be assigned as the methyl carbon resonance arising
from the bridging acetate group.[21] By analyzing the
spectroscopic data, we proposed the chemical struc-
ture of complex 1 as depicted in Scheme 2. Further-
more, the structure of 1 was established unambiguous-
ly by a single crystal X-ray study. The ORTEP dia-
gram of 1 is presented in Figure 2 which confirms an
acetate-bridged dimeric structure. Interestingly, the
Figure 2. ORTEP diagram of 1 with 50% probability ellip-
soids. Hydrogen atoms and solvent molecule (dichlorome-
thane) have been omitted for the sake of clarity. Selected
À
bond lengths (Š) and bond angles (deg): Pd(1A) C(5A),
À
À
1.976(5);
Pd(1A) C(7A),
1.983(6);
Pd(1A) O(1A),
À
À
2.143(4); Pd(1B) C(5B), 1.962(6); Pd(1B) C(7B), 1.972(6);
À
À
À
À
C(5A) Pd(1A) C(7 A), 80.60(2); C(5A) Pd(1A) O(1B),
À
À
À
173.45(18); C(7A) Pd(1A) O(1B), 93.52(19); C(7A)
À
À
À
Pd(1A) O(1A),
179.11(19);
O(1B) Pd(1A) O(1A),
À
À
87.25(15); C(5B) Pd(1B) C(7B), 80.70(2).
À
structure of 1 reveals that one of the ortho C H
À
bonds of the N-substituted phenyl group (C-7) has un- are measured as 1.983 Š (Pd1A C7A) and 1.972 Š
dergone activation via an orthometallation process. (Pd1B C7B). Furthermore, the DFT calculations re-
À
À
À
This type of C H bond activation during coordination vealed the nature of the Pd C bonds in which the
À
to palladium(II) with an NHC ligand is reminiscent to electron density contributions from Pd and C in Pd C
our earlier observation[9a] and was also reported by bonds are almost equal for both Pd C(aryl) and Pd
others.[21] The single crystals of 1 were grown by slow C(carbene) bonds with ~44% contribution from Pd
diffusion of hexane into a saturated solution of 1 in and ~56% from carbon. Figure 3 shows the HOMO
dichloromethane at room temperature. Figure 2 clear- and LUMO of 1. It can be seen clearly that the elec-
ly depicts the abnormal mode of binding of carbene
À
À
via C-5 center (C5A and C5B) with Pd(II) along with
À
ortho aryl C H bond activation at C-7 center (C7A
and C7B) forming Pd C(aryl) bonds. Each Pd(II) ion
À
in 1 adopts a square planar geometry on coordination
with two oxygen atoms from two bridging acetate
À
groups and two carbon atoms forming Pd C(aryl)
and Pd C(carbene) bonds. The distance between Pd1
À
and Pd2 is found to be 2.877 Š, which is relatively
À
larger to describe a formal Pd Pd bond distance
À
(2.72 Š) as observed in Pd wire containing Pd Pd co-
valent bonds.[22] The Pd–C(carbene) bond distances
À
are found to be 1.976 Š (Pd1A C5A) and 1.962 Š
(Pd1B C5B), which were comparable with our previ-
À
À
ously reported halo-bridged aryl C H bond activated
palladium dimers[9a]. The Pd C(aryl) bond distances Figure 3. HOMO and LUMO of 1.
À
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An Abnormal N-Heterocyclic Carbene-Based Palladium Dimer
Table 1. Optimization of the oxidative Heck coupling of p-
tron density on HOMO is exclusively localized on
both Pd atoms and along Pd C(carbene) and Pd
C(aryl) bonds.
tolylboronic acid and methyl acrylate at room temperatur-
À
À
e.[a]
Next, we investigated the catalytic activity of this
dimeric Pd(II) complex, 1, under the oxidative Heck
coupling reaction conditions at room temperature.
To optimize the reaction method, we investigated
the oxidative Heck reaction of p-tolylboronic acid
(1 mmol) with the benchmark electron-deficient
olefin, methyl acrylate (1.2 mmol). Initially, we found
that the use of simple Pd(OAc)2 as a catalyst
(1 mol%) in DMSO afforded the desired product in
37% yield (Table 1, entry 1), whereas under the iden-
tical conditions, catalyst 1 gave 70% yield (Table 1,
entry 3). The control run without catalyst did not pro-
ceed at all (Table 1, entry 4). In the absence of any
oxidant, the reaction does not proceed as revealed by
our control run in Table 1, entry 5. The choice of an
oxidant is essential, and BQ was the best among all
oxidants we examined (Table 1, entries 3 and 8–11).
Among the oxidants (BQ, TBHP, PhICl2, PhI(OAc)2
and K2S2O8) used in the present study, BQ resulted in
the best yield. Furthermore, we have tested the reac-
tion using different aprotic solvents as well as protic
solvents (Table 1, entries 12–23). To our delight, water
as solvent in presence of 1 mL trifluoroacetic acid
(TFA) showed the best catalytic result with an isolat-
ed yield of more than 90% at ambient temperature
using a 1 mol% loading of catalyst 1 (Table 1,
entry 23). We applied these conditions as standard
using water as solvent for further reactions. We ob-
served that the amount of TFA in the reaction
medium plays a crucial role. In the absence of TFA,
no conversion was observed (as shown in Table 1,
entry 6). Furthermore, we noticed from our optimiza-
tion experiments that the amount of TFA plays an im-
portant role. For example, when 0.3 mL or 0.6 mL of
TFA was used for the reaction, less conversion (~40%
with 0.3 mL TFA or ~84% with 0.6 mL TFA by
1H NMR spectroscopy, Table 1, entry 7) was observed
as compared to when 1 mL TFA was used (91% iso-
lated yield). A similar observation was also noted by
Yu and co-workers that excess TFA is required as
a promoter for better conversion in the ortho-trifluor-
omethylation of arenes.[23] However, the exact role of
Entry Catalyst
Oxidant
Solvent
Yield [%][b]
1
Pd(OAc)2 BQ
DMSO
H2O
DMSO
DMSO
H2O
H2O
H2O
DMSO
DMSO
37
48
70
0
2
3
4
Pd(OAc)2 BQ
catalyst 1 BQ
none BQ
5
6
7
8
catalyst 1 None
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 TBHP
catalyst 1 PhICl2
0
0[c]
40,[d] 84[e]
0
8
13
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
catalyst 1 PhI(OAc)2 DMSO
catalyst 1 K2S2O8
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
catalyst 1 BQ
DMSO
DMSO
DMA
DMF
1,4-dioxane
NMP
CH2Cl2
THF
EtOH
toluene
CH3CN
ethyl acetate 73
H2O 91
63[f]
25
53
50
34
86
37
3
88
5
[a]
All reactions were conducted with p-tolylboronic acid
(1 mmol), methyl acrylate (1.2 mmol), catalyst
1 (1 mol%), oxidant (1.2 equiv.), TFA (1 mL) and solvent
(2 mL) at room temperature for 12 h.
Isolated yield based on p-tolylboronic acid. Benzoqui-
none is denoted by BQ, tert-butyl hydroperoxide is de-
noted by TBHP.
[b]
[c]
[d]
[e]
[f]
No TFA was used.
NMR conversion when 0.3 mL TFA was used.
NMR conversion when 0.6 mL TFA was used.
AcOH, 608C, 12 h.
TFA is not very clearly understood. When a reaction boronic acids with olefins and the results are summar-
between the catalyst and excess TFA was performed, ized in Scheme 3. Sterically hindered ortho-substitut-
the 19F NMR spectrum displayed a peak at d= ed phenylboronic acid was also tested displaying
À74.7 ppm (for free TFA, the fluorine nuclei resonate a moderate yield of 2g (50%). We have also examined
at d=À76.0 ppm) suggesting a new complex forma- a variety of arylboronic acid partners and olefins such
tion (see the Supporting Information). Based on an as methyl acrylate to ethyl acrylate and finally butyl
earlier study by Fujiwara and co-workers, it may be acrylate. For all these different types of coupling sub-
proposed that TFA converts the Pd-acetate bond into strates, we accomplished good to excellent yields. As
a Pd-trifluoroacetate bond.[24] However, all our at- can be seen from Scheme 3, the presence of an elec-
tempts to crystallize this compound failed. To expand tron-donating group in the arylboronic acid partner
the scope of the present reaction conditions with cata- enhances the yield of the coupling products (2b, 2f,
lyst 1, we examined the reaction of a variety of aryl- 2i, 2k, 2m). The oxidative Heck reaction using elec-
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Pradip Kumar Hota et al.
the a-arylated product.[19] Murugaiah et al. achieved
oxygen-promoted a-arylated products using electron-
rich olefins as the arylboronic acid partner.[26] Thus, it
is generally considered as a challenge to establish a b-
selective Heck reaction with an electron-rich olefin.
Awen et al. first demonstrated an oxidative b-selec-
tive Heck[27] coupling between arylboronic acid and
vinyl acetate at 608C in DMSO. Here we have taken
electron-rich vinyl acetate and vinyl pivalate sub-
strates for the oxidative Heck coupling at room tem-
perature in water (Scheme 4). It has been observed
Scheme 4. Oxidative Heck coupling with electron-rich ole-
fins.[a]
that instead of TFA, AcOH works successfully in the
case of electron-rich olefins (3a–3e). This might be
due to hydration of electron-rich olefins in strong
acidic medium such as TFA; an observation previous-
ly noted by Lei and co-workers.[27] We noticed that for
Scheme 3. Oxidative Heck coupling with electron-deficient
olefins.[a]
electron-rich olefins,
(20 mmol) is required as the reaction does not pro-
tron-deficient olefins and catalyst 1 leads to the for- ceed with 1 equivalent of olefin. Even the use of 5
mation of the trans b-arylated product exclusively. equivalents of electron-rich olefin results in only
a
large excess of olefin
Furthermore, we have tested the catalytic efficacy ~10% conversion. Earlier Lei and co-workers also
of catalyst 1 with electron-rich olefins. Generally, in used a large excess of electron-rich olefins (20 equiv.)
the case of electron-rich olefins, either mixtures of a- in the oxidative Heck coupling reaction.[27] By apply-
and b-arylated regio-isomeric products or exclusively ing our optimized conditions, we were able to obtain
a-arylated products are obtained.[19,25,26] Xiao et al. in only selectively b-arylated products in a good yields
2008 reported that the oxidative Heck coupling of ar- and no a-arylated product was found during the catal-
ylboronic acid with electron-rich olefins, resulting in ysis.
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An Abnormal N-Heterocyclic Carbene-Based Palladium Dimer
Scheme 5. Chemoselectivity study with catalyst 1.
In addition, to check the chemoselectivity among CH3CN, DMSO and CH2Cl2 were dried over CaH2 and dis-
tilled before use. All chemicals were purchased from Sigma-
Aldrich or Merck or Spectrochem. All the reagents were
used directly as received. Analytical thin layer chromatogra-
phy (TLC) was performed on a Merck 60 F254 silica gel
plate (0.25 mm thickness). Column chromatography was per-
formed on Merck 60 silica gel (100–200 mesh). 1H and
13C NMR spectra were recorded on a JEOL ECS 400 MHz
spectrometer and on a Bruker Avance III 500 MHz spec-
trometer. All chemical shifts are reported in ppm using tet-
ramethylsilane as a reference. Chemical shifts (d) downfield
from the reference standard were assigned positive values.
The HR-MS data were obtained using a Finnigan MAT
8230 instrument. Elemental analyses were carried out using
a Perkin–Elmer, series 2, 2400 CHN analyzer and samples
were prepared by keeping them under reduced pressure
(10À2 mbar) for overnight. The melting point was measured
in a sealed glass tube on a Büchi B-540 melting point appa-
ratus and was uncorrected.
various competitive coupling possibilities, we studied
intermolecular competition reaction using equimolar
amounts of 4-iodotoluene, 4-tert-butylphenylboronic
acid, and methyl acrylate (Scheme 5). Under the reac-
tion condition, it afforded exclusively the oxidative
Heck coupled product without formation of any Heck
or Suzuki coupled products.
Conclusions
In conclusion, we have stabilized a palladium acetate
unit in the form of a dimer using a newly designed
aNHC ligand. In this acetate-bridged palladium dimer
(1), the carbene is attached to the palladium via an
abnormal mode of binding. We have applied this
dimer as a catalyst for oxidative Heck coupling reac-
tions of arylboronic acids with electron-deficient ole-
fins under a low catalyst loading (1 mol%) at ambient
temperature in water for the first time. This catalyst
Synthesis of L1HI
The synthesis of 1,2,4-triphenylimidazole was accomplished
was also efficiently used for electron-rich olefins. In by reaction between amidine[20c] and 2-bromoacetophenone
where H2O-THF (1:10) mixture was used as solvent. Then
the corresponding salt was obtained using methyl iodide in
acetonitrile medium at reflux condition; isolated yield: 73%;
mp 192–1948C. 1H NMR (400 MHz, CDCl3, 258C, TMS):
d=7.91 (d, J=7.6 Hz, 2H), 7.88–7.86 (m, 2H), 7.58 (d, J=
6.8 Hz, 2H), 7.51–7.49 (m, 4H), 7.44–7.35 (m, 6H), 3.73 (s,
3H); 13C NMR (100 MHz, CDCl3, 258C, TMS): d=144.9,
135.8, 134.9, 132.1, 131.8, 130.7, 130.5, 130.2, 129.7, 129.2,
129.1, 126.4, 125, 121.5, 120.5, 35.1; ESI-HR-MS: m/z (%)=
311.16 [M+]; elemental analysis: anal. calcd. for C22H19IN2:
C 60.29, H 4.37, N 6.39; found: C 60.21, H 4.39, N 6.35.
the case of electron-rich olefins, selectively the b-ary-
lated product was obtained in good isolated yield.
Formation of any a-arylated product or mixture of a-
and b-arylated product was not observed during this
catalytic reaction. Thus, in this study, we have been
able to perform the oxidative Heck coupling in
a greener way, i,e., at room temperature in water
under a low catalyst loading with a catalyst that dis-
played superior catalytic activity over the well studied
palladium acetate catalyst.
Synthesis of Complex 1
Experimental Section
Under an argon atmosphere, a 50-mL Schlenk flask was
charged with Pd(OAc)2 (224 mg, 1 mmol), L1HI (438 mg,
1 mmol), and 1,4-dioxane (15 mL). The reaction mixture
was heated to 808C for 8 h. After cooling to room tempera-
ture, all volatiles were removed under reduced pressure
from the reaction mixture. Complex 1 was obtained by re-
crystallization from CH2Cl2/hexane as yellow crystals; yield:
571 mg (0.6 mmol, 60%); mp 298–3038C. 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=7.67 (t, J=7.6 Hz, 1H),
Materials and Instrumentation
Unless stated otherwise, all reactions were performed in
flame-dried glassware under oxygen-free atmosphere
(argon) using standard Schlenk techniques or inside an
MBraun Glovebox maintained below 0.1 ppm levels of O2
and H2O. THF, toluene, pentane, hexane and benzene were
dried over a sodium/benzophenone mixture prior to use.
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Pradip Kumar Hota et al.
7.57 (d, J=6.8 Hz, 3H), 7.50–7.37 (m, 6H), 7.29 (d, J=
7.6 Hz, 1H), 7.04 (d, J=7.6 Hz, 1H), 6.79 (t, J=7.6 Hz,
1H), 6.60 (t, J=7.6 Hz, 1H), 5.86 (d, J=8.4 Hz, 1H), 3.04
(s, 3H), 1.24 (s, 3H); 13C NMR (125 MHz, CDCl3, 258C,
TMS): d=179.3, 147.1, 146.7, 136.7, 136.2, 134.9, 131.5,
131.4, 130.1, 129.8, 129.7, 129.6, 129.2, 127.9, 125.4, 123.1,
121.6, 111.9, 32.0, 23.0; ESI-HR-MS: m/z (%)=969.41 [M+
NH4]+; elemental analysis: anal. calcd. for C48H40N4O4Pd2: C
60.71, H 4.25, N 5.90; found: C 60.65, H 4.27, N 5.88.
silica gel using a mixture of hexane and ethyl acetate
(100:2) as eluent.
General Procedure for the Oxidative Heck Reaction
of Arylboronic Acids with Electron-Rich Olefins
A 10-mL round-bottom flask containing a stirrer bar was
charged with arylboronic acid (1 mmol), catalyst
1 (0.01 mmol), BQ (1.2 equiv.), acetic acid (AcOH) (1 mL)
and water (2 mL). Then the olefin (20 equiv.) was added to
the reaction mixture. The reaction was monitored by TLC
using ethyl acetate and hexane as the mobile phase. The re-
action mixture was diluted with water and excess aqueous
NaHCO3 solution was added to neutralize the acetic acid.
After stirring the reaction mixture for 10 min, it was extract-
ed by diethyl ether (15 mL) for 3 times. The organic parts
were combined together and dried over Na2SO4. The pure
product was obtained by column chromatography on silica
gel using a mixture of hexane and ethyl acetate (100:2) as
eluent.
X-Ray Crystallographic Details for Compounds L1HI
and 1
The single-crystal X-ray diffraction data of the crystals were
collected on a SuperNova, Dual, Mo at zero, Eos diffrac-
tometer at 295 K for L1HI and 100 K for complex 1 using
graphite-monochromatic Mo Ka radiation (l=0.71073 Š).
Atomic coordinates, isotropic and anisotropic displacement
parameters of all the non-hydrogen atoms of the two com-
pounds were refined using Olex2,[28] and the structure was
solved with the Superflip[29] structure solution program using
charge flipping and refined with the ShelXL[30] refinement
package using least squares minimization. Structure graphics
shown in the figures were created using the diamond soft-
ware package version 2.1. A summary of the crystal data is
given below.
General Procedure for the Chemoselectivity Study
A 10-mL round-bottom flask containing a stirrer bar was
charged with 4-tert-butylphenylboronic acid (1 mmol), 4-io-
dotoluene (1 mmol), catalyst 1 (0.01 mmol), BQ (1.2 equiv.),
trifluoroacetic acid (TFA) (1 mL) and water (2 mL). Then
methyl acrylate (5 equiv.) was added to the reaction mixture.
The reaction was monitored by TLC using ethyl acetate and
hexane as the mobile phase. The reaction mixture was dilut-
ed with water and excess aqueous NaHCO3 solution was
added to neutralize the TFA. After stirring of reaction mix-
ture for 10 min, it was extracted by diethyl ether (15 mL)
for 3 times. The organic parts were combined together and
dried over Na2SO4. The pure product was obtained by
column chromatography on silica gel using a mixture of
hexane and ethyl acetate (100:2) as eluent as eluent.
Crystal data for L1HI: C22H19IN2, MW=438.29, 0.06”
0.10”0.25 mm3, orthorhombic, space group Pna21, a=
10.4487(5) Š, b=13.3244(7) Š, c=14.5118(10) Š, a=b=
g=90.00, V=2020.4(2) Š3, Z=4, calculated density=
1.441 gcmÀ3, m(MoKa)=1.591 mmÀ1, T=295 K, q range for
data collection=2.47–24.90, 3170 reflections measured, 1619
independent (Rint =0.0273), R1=0.0392 [I>2s(I)], wR2=
0.0834 (all data).
Crystal data for 1: C51H46Cl6N4O4Pd2, Mr=1204.42, 0.12”
0.30”0.61 mm3, monoclinic, space group P21/c, a=
12.4488(3) Š, b=13.9100(4) Š, c=28.8105(7) Š, a=g=
90.00, b=96.64, V=4955.4(2) Š3, Z=4, calculated density=
1.618 gcmÀ3, m(MoKa)=1.099 mmÀ1, T=100 K, q range for
data collection=2.93–26.37, 10132 reflections measured,
10688 independent (Rint =0.0355), R1=0.0611 [I>2s(I)],
wR2=0.1721 (all data). CCDC 1046972 (1) and CCDC
1044268 (L1HI) contain the supplementary crystallographic
data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Computational Details
All calculations were performed using the Gaussian 09[31]
program. Density functional theory (DFT) calculations em-
ployed the B3LYP functional using all electron basis sets.
The geometry was fully optimized without symmetry con-
straints. All the atoms are treated with double-z quality
(DZVP) basis sets of LANDLZ quality.[32] Optimized geo-
metries and orbital diagrams were plotted in the Chemcraft
visualization software and calculations were performed in
an octa-core, 64 bit HP Z420 workstation.
General Procedure for the Oxidative Heck Reaction
of Arylboronic Acids with Electron-Deficient Olefins
Supporting Information
A 10-mL round-bottom flask containing a stirrer bar was
charged with arylboronic acid (1 mmol), catalyst
1 (0.01 mmol), BQ (1.2 equiv.) trifluoroacetic acid (TFA)
(1 mL) and water (2 mL). Then the acrylate (1.2 equiv.) was
added to the reaction mixture. The reaction was monitored
by TLC using ethyl acetate and hexane as the mobile phase.
The reaction mixture was diluted with water and excess
aqueous NaHCO3 solution was added to neutralize the
TFA. After stirring the reaction mixture for 10 min, it was
extracted by diethyl ether (15 mL) for 3 times. The organic
parts were combined together and dried over Na2SO4. The
pure product was obtained by column chromatography on
1
X-ray crystallography details, H and 13C NMR data along
with spectra are available in the Supporting Information.
Acknowledgements
We thank SERB (DST), India (Grant No. SR/S1/IC-25/2012)
for financial support. PKH is thankful to IISER-Kolkata for
a research fellowship. GVK and SCS are thankful to UGC,
New Delhi for research fellowships. AP thanks SERB (DST)
3168
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Adv. Synth. Catal. 2015, 357, 3162 – 3170

FULL PAPERS
An Abnormal N-Heterocyclic Carbene-Based Palladium Dimer
for a research fellowship. TKS is thankful to CSIR, New
Delhi for a research fellowship. PKH thanks Dr. Sudipta
Raha Roy for his constructive suggestiosn during the work.
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Adv. Synth. Catal. 2015, 357, 3162 – 3170