Pd(II) complexes with mixed benzothiazolin-2-ylidene and phosphine ligands and their catalytic activities toward C-C coupling reactions

Article abstract of DOI:10.1039/b713152f

Novel Pd(ii) mixed N,S-heterocyclic carbene (NSHC)-phosphine complexes of the general formula [PdBr₂(NSHC)(PR₃)] were obtained from bridge cleavage of dinuclear NSHC complexes of type [PdBr₂(NSHC)] ₂ [NSHC = 3-benzylbenzothiazolin-2-ylidene and 3- propylbenzothiazolin-2-ylidene] with triphenylphosphine, tricyclohexylphosphine and 2-diphenylphosphanyl-pyridine. All complexes have been fully characterized by ¹H and ¹³C NMR spectroscopy, ESI mass spectrometry and elemental analysis. The X-ray crystal structures of complexes 3-8 are reported. The complexes exhibit moderate to good catalytic activity in the Suzuki-Miyaura coupling reaction of aryl bromides and chlorides. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b713152f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Pd(II) complexes with mixed benzothiazolin-2-ylidene and phosphine ligands
and their catalytic activities toward C–C coupling reactions†
Swee Kuan Yen, Lip Lin Koh, Han Vinh Huynh* and T. S. Andy Hor*
Received 28th August 2007, Accepted 1st November 2007
First published as an Advance Article on the web 29th November 2007
DOI: 10.1039/b713152f
Novel Pd(II) mixed N,S-heterocyclic carbene (NSHC)-phosphine complexes of the general formula
[PdBr2(NSHC)(PR3)] were obtained from bridge cleavage of dinuclear NSHC complexes of type
[PdBr2(NSHC)]2 [NSHC = 3-benzylbenzothiazolin-2-ylidene and 3-propylbenzothiazolin-2-ylidene]
with triphenylphosphine, tricyclohexylphosphine and 2-diphenylphosphanyl-pyridine. All complexes
have been fully characterized by 1H and 13C NMR spectroscopy, ESI mass spectrometry and elemental
analysis. The X-ray crystal structures of complexes 3–8 are reported. The complexes exhibit moderate
to good catalytic activity in the Suzuki–Miyaura coupling reaction of aryl bromides and chlorides.
this paper. We also describe some selective catalytic data of the
Introduction
complexes with NSHC and phosphine ligands and demonstrate
The use of mixed-ligand complexes of palladium to promote
the rich potential of these complexes to support C–C coupling
catalysis is gathering momentum. This is driven by the expectation
reactions.
that the inherent hybrid character of such systems would be
electronically more sensitive and tunable to the needs of the
substrates. The “hetero-functionality” coordinating the metal
Results and discussion
center also makes it easier to impose chiral control. Among
the known dual-ligand systems, the pairing of N-heterocyclic
Solvent-free synthesis of benzothiazolium salt A
carbene (NHC) and phosphine is of special interest, because
of the established role of phosphines in catalysis as well as the
resemblance and synergy between these two classes of ligands.1 The
NHC-palladacycles of Nolan,2 NHC-phospha-palladacycles3 and
NHC-phosphine complexes of Herrmann,4 Cloke,5 and Welton,1c
diaminocarbene- and Fisher-carbene complexes with PPh3 of
Fu¨rstner6 and pyridylcarbene complexes of Lin7 are just a few
of the many successful examples. We are primarily focused on
N,S-heterocyclic carbenes based on benzothiazole,8 because the
additional heteroatom, in principle, could give a better balance to
moderate the r/p donor/acceptor character. Yet, they keep all the
benefits of NHCs such as easy electronic and steric modification
through the annulation with different ring systems, introduction
of different heteroatoms, and substituents at nitrogen, as well
as convenient synthesis, facile complexation and good chemical
stability etc.9 Our current understanding of NSHC complexes is
still relatively poor compared to the understanding of the classical
NHC complexes.10,11 The catalytic applications of NHC ligands
are hence well known,10 whereas those of NSHCs are of paucity.
The catalytic activities are accordingly poorly understood as well.
The complementary use of NSHC and phosphine in a dual ligand
system to support catalysis hence remains a largely unexplored
field, but yet with potentially rich returns. For this area to be
advanced, first and foremost we must have a general and easy
pathway to prepare such systems. This is the first objective of
3-Propylbenzothiazolium bromide (A) was prepared in 78% as
yellow powder from 1-bromopropane and benzothiazole at 120 ◦C
without solvent similarly to other derivatives8a (Scheme 1).
Scheme 1 Synthesis of benzothiazolium salt A.
1
The H NMR spectrum of A in CDCl3 shows a pseudo-sextet
at 2.17 ppm and two sets of triplets at 5.12 and 1.08 ppm
characteristic of the propyl substituent. The downfield resonance
at 12.09 ppm for the SCHN proton indicates the formation
of an benzothiazolium salt. This is further supported by a 13C
signal at 164.9 ppm for the SCN carbon. The positive mode ESI
mass spectrum shows a base peak at m/z = 178 corresponding
to the [M − Br]+ cation. X-Ray single-crystal diffraction of
A confirmed the benzothiazolium structure with a N-propyl
substituent (Fig. 1). The unit cell contains two independent
molecules, which show disorder of the propyl-substituents.
Synthesis of dinuclear palladium(II) complexes
We have previously described the synthesis of dimeric complex 1
bearing the 3-benzylbenzothiazolin-2-ylidene ligand.8a Using the
same general method, we prepared the dinuclear Pd(II) carbene
complex 2 with 3-propylbenzothiazolin-2-ylidene ligands from the
reaction of two equivalents of azolium salt A with one equivalent
Department of Chemistry, National University of Singapore, 3 Science Drive
3, 117543, Singapore. E-mail: andyhor@nus.edu.sg; chmhhv@nus.edu.sg;
Fax: +65 67791691; Tel: +65 65162658
† CCDC reference numbers 658815–658821. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b713152f
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The geminal coupling of the benzylic protons indicates that there
is no or only a slow and hindered rotation around the Pd–Ccarbene
bond due to the bulky PPh3 ligand. Complex 4 behaves similarly.
Its propyl substituent also exhibits an AX system with two sets
of triplets for CH2CH2CH3 at 4.81 and 4.05 ppm and two sets
of multiplets for CH2CH2CH3 at 2.41 and 1.79 ppm (Table 1).
The terminal methyl group of the propyl substituent remains
unaffected, but shifts slightly downfield by ∼0.4 ppm compared
to salt A. Apparently, the terminal methyl group is further away
from the Pd–Ccarbene bond and thus has greater flexibility. This
phenomenon once again indicates no free rotation of the Pd–
Ccarbene bond due to the neighboring bulky phosphine ligand.
The 13C NMR signals for the carbenoid carbons of 3 and 4
appear at 208.4 and 206.3 ppm, respectively (Table 1). They are
downfield shifted compared to those of the mononuclear solvate
complexes reported earlier (191.0 ppm for trans-[PdBr2(NSHC)-
(CH3CN)] and 191.9 ppm for trans-[PdBr2(NSHC)(DMF)]).8a
This observation is in line with a lower Lewis-acidity of the Pd(II)
centers in 3 and 4, which is caused by an additional strong r-
donating phosphine ligand.
Fig. 1 ORTEP view of the molecule C10H12BrNS (A) with 50% thermal
ellipsoids and labelling scheme; hydrogen atoms and water molecules are
◦
˚
omitted for clarity. Selected bond lengths (A) and angles ( ): S1–C1 1.68(4),
N1–C1 1.32(4), N1–C8 1.48(5), S2–C11 1.68(3), N2–C11 1.33(4), N2–C18
1.47(4), N1–C1–S1 114.60(3) C1–N1–C8 123.20(3), N2–C11–S2 114.60(2),
C11–N2–C18 122.80(3).
of Pd(OAc)2 in 76% yield. The absence of the downfield signal in
the 1H NMR spectrum of 2 for the SCHN proton characteristic for
salt A indicates a successful complexation. The propyl resonances
are similar to those of A, but the aromatic resonances are slightly
shifted upfield by ∼0.3 ppm. Although the carbenoid carbon
could not be detected in the 13C NMR spectrum, the FAB
mass spectrometric data agrees with a dinuclear skeletal structure
(m/z = 807) corresponding to [2 − Br]+.
The 31P NMR signals of the coordinated phosphine is also
slightly downfield-shifted (27.3 ppm for 3 and 27.7 ppm for 4,
Table 1) as compared to the 1,3-diisopropylbenzimidazolin-2-
ylidene analogue cis-[PdBr2(NHC)PPh3] (26.6 ppm).1d
Cleavage of 1 and 2 with tricyclohexylphosphine (PCy3)
Cleavage of 1 and 2 with triphenylphosphine (PPh3)
Bridge cleavage also occurs readily with the bulky and more basic
phosphine PCy3 to give 5 and 6 again as cis-isomers. An AX system
is also observed in the 1H NMR spectra (CDCl3) for the geminal
methylene protons of the N-substituents of complexes 5 and 6
(Table 1). The 13C carbenoid carbon signals occur at 208.7 ppm
and 206.0 ppm for 5 and 6, respectively. Upon coordination, the
31P signal of the basic phosphine ligand shifts downfield (44.7 ppm
and 43.7 ppm for complexes 5 and 6, respectively) as compared
to the free ligand (11.8 ppm). ESI-MS analysis also supports
the formation of 5 and 6 with isotopic envelopes centered at
m/z = 972 and 644, respectively, corresponding to the [M − Br]+
fragments.
Similar bromo-bridged complexes are known to be cleaved by
coordinating solvents such as CH3CN and DMF, giving the
mononuclear solvate complexes.8a Dinuclear Pd(II) complexes 1
and 2 are thus readily cleaved by various ligands to form a
range of mononuclear monocarbene complexes. Thus, complexes
1 and 2 react with PPh3 to give the mixed complexes 3 and 4,
respectively, which have been isolated as cis-isomers (scheme 2).
The 1H NMR spectrum (CDCl3) of 3 exhibits an AX system with
two sets of doublets for the geminal methylene protons of the
benzyl substituent at 6.53 and 5.00 ppm (Dd(AX) = 1.35 ppm)
with a geminal coupling constant of 2J(HH) = 15.10 Hz (Table 1).
Cleavage of 1 and 2 with 2-diphenylphosphanylpyridine (PPh2Py)
As a means to introduce an additional ligand function to this
series, we have used a pyridylphosphine PPh2Py as an incoming
co-ligand. The expected carbene–phosphine complexes 7 and 8
have been similarly isolated. Similar AX systems were observed
1
in the H NMR spectra for complexes 7 and 8. Their 13C NMR
spectra show the carbenoid carbon signals at 207.7 and 205.4 ppm
for complexes 7 and 8, respectively. Furthermore, sharp singlets
at 24.5 and 24.6 ppm were found in the 31P NMR spectra of
complexes 7 and 8, respectively. Compared to those of complexes
3 and 4, these resonances are shifted highfield by 2.8 and 3.1 ppm,
respectively, maybe indicating a less electron rich phosphine donor.
1
There is no evidence of pyridyl coordination from the H NMR
spectra. The positive mode ESI mass spectra are dominated by an
isotopic pattern centered at m/z = 675 and 627 corresponding to
a monocation [M − Br]+ formed upon bromide dissociation from
7 and 8, respectively.
Scheme 2 Synthesis of Pd(II) mixed NSHC-phosphine complexes 1–8.
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Table 1 Selected 1H, 31P and 13C NMR and crystallographic data for complexes 3–8
3
4
5
6
7
8
d(1H): NCHH, NCHH (ppm)
Dd(HH)
6.53, 5.00
1.53
15.10, 15.10
—
27.3
4.81, 4.05
0.76
5.65, 5.05
12.60, 12.44
27.7
6.69, 6.14
0.55
15.75, 15.75
—
44.7
5.39, 4.46
0.93
5.25, 4.00
12.45, 12.45
43.7
6.78, 5.26
1.52
15.10, 15.80
—
24.5
5.04, 4.28
0.76
5.03, 4.83
12.63, 12.60
24.6
2J(HH)/Hz
3J(HH)/Hz
d(31P): PR3 (ppm)
d(13C): Ccarbene (ppm)
208.4
206.3
208.7
206.0
207.7
205.4
˚
Bond Lengths/A
Pd1–P1
Pd1–C1
Pd1–Br1
Pd1–Br2
S1–C1
N1–C1
N1–C8
2.27(2)
1.98(6)
2.46(8)
2.47(8)
1.72(6)
1.33(7)
1.47(7)
2.27(1), 2.28(1)
1.99(4), 1.96(4)
2.47(6), 2.47(6)
2.48(6), 2.48(6)
1.70(4), 1.71(4)
1.33(5), 1.32(5)
1.48(5), 1.48(5)
2.30(2)
1.96(7)
2.47(1)
2.48(1)
1.71(7)
1.34(8)
1.48(8)
2.29(2)
1.97(6)
2.47(9)
2.48(8)
1.72(6)
1.35(7)
1.46(7)
2.26(8)
1.97(3)
2.47(4)
2.47(4)
1.70(3)
1.33(4)
1.47(4)
2.27(8)
1.97(3)
2.48(4)
2.46(4)
1.71(3)
1.33(4)
1.47(4)
Angles/◦
C1–Pd1–P1
C1–Pd1–Br1
P1–Pd1–Br1
C1–Pd1–Br2
P1–Pd1–Br2
Br1–Pd1–Br2
N1–C1–S1
N1–C1–Pd1
S1–C1–Pd1
C1–N1–C8
93.31(2)
175.16(2)
87.48(5)
87.76(12)
177.71(5)
91.29(3)
110.80(4)
131.70(4)
117.50(3)
123.10(5)
93.62(1), 87.83(1)
178.11(1), 178.85(1)
88.26(3), 91.40(3)
94.40(2)
175.8(2)
89.31(6)
86.20(2)
175.65(6)
89.96(4)
110.70(5)
130.70(5)
118.50(4)
122.10(6)
94.10(2)
174.39(2)
91.40(5)
84.27(15)
176.90(5)
90.28(3)
110.40(4)
126.30(4)
123.00(3)
123.20(5)
91.35(9)
178.61(8)
88.03(2)
87.37(9)
175.93(2)
93.34(1)
111.40(2)
126.90(2)
121.61(2)
122.70(2)
91.27(9)
178.02(9)
89.85(2)
88.45(8)
177.35(3)
90.51(1)
111.40(2)
128.7(2)
119.89(2)
123.80(3)
86.34(1), 87.55(1)
179.88(4), 171.93(3)
91.7798(2), 93.11(2)
110.70(3), 111.10(3)
127.80(3), 129.10(3)
121.30(2), 119.60(2)
123.00(4), 123.00(4)
Molecular structures
Suzuki–Miyaura coupling reactions
Single crystals suitable for X-ray diffraction of complexes 3–8 were
obtained from diffusion of Et2O into saturated CH2Cl2 solutions.
Their solid-state molecular structures are depicted in Fig. 2 and
selected bond parameters are summarized in Table 1. As an
exception, the unit cell of complex 4 contains two independent
but very similar molecules. For clarity, only one is depicted. As
expected all complexes are mononuclear and essentially square-
planar in geometry. The carbene and the phosphine ligands are
found in the cis-position to each other. Furthermore, the carbene
ring planes of all complexes are twisted out of the {PdCPBr2}
coordination planes with dihedral angles ranging from 74.4◦–
88.7◦ in order to avoid intraligand repulsion. It is noteworthy,
that the greater angles were found for the tricyclohexylphosphine
derivatives 5 and 6 due to the greater steric demand of this
phosphine ligand. The Pd–Ccarbene and the Pd–P bonds amounting
With the isolation and characterization of this series of mixed
carbene–phosphine complexes, we are able to study and di-
rectly compare the catalytic activities with their cis-bis(carbene)
analogues8a or standard phosphine based catalysts such as
[PdBr2(PPh3)2]. The activities of 3–8 toward the Suzuki–Miyaura
coupling reactions of various aryl bromides and chlorides with
phenyl boronic acid in the presence of Cs2CO3 as base in toluene
yielding the corresponding biphenyl products are given in Table 2.
All the complexes are air-stable and hence there is no necessity for
glove-box handling. Toluene is the solvent of choice as complexes
3–8 do not dissociate in this solvent. For example, the 31P NMR
spectrum for complex 3 in deuterated toluene (23.18 ppm) remains
unchanged for up to one week.
All the mixed carbene–phosphine complexes give rise to
highly active catalysts for electron poor substrates such as 4-
bromoacetophenone giving quantitative yields in most cases (Ta-
ble 2, runs 1–6). For the activated substrate 4-bromobenzaldehyde,
complexes 3, 5 and 7 with the 3-benzylbenzothiazolin-2-ylidene
ligand give quantitative yields, which compare better to the per-
formance of complexes 4, 6 and 8 with the 3-propylbenzothiazolin-
2-ylidene ligand (runs 9–14). The former are comparable to
[PdBr2(PPh3)2] (Table 2, runs 8 and 16). The bis(carbene) com-
plex cis-[PdBr2(NSHC)2]8a bearing the 3-benzylbenzothiazolin-2-
ylidene ligand on the other hand gives yields of only 71–88%
(runs 7, 15).
˚
˚
to 1.96(7)–1.99(4) A and 2.26(8)–2.30(2) A, respectively, are
unexceptional and in the similar range found for a benzimidazolin-
2-ylidene analogue.1d Surprisingly, when the Pd–Br bonds trans to
the NSHC and those trans to the phosphine ligands are compared,
no clear trend for the trans influence can be found. For example,
in complexes 3–6, the Pd–Br bonds trans to the phosphine were
found to be longer than those trans to the carbene ligand, maybe
implying a slightly stronger trans influence for the phosphine
ligands. However, the opposite trend was observed for complexes
7 and 8, although the latter complexes merely differ from 3 and 4
only by one nitrogen/carbon atom. Finally, no coordination of the
pyridyl moiety was found for complexes 7 and 8 in the solid-state.
Since complexes 3, 5 and 7 were found to be the better
precatalysts, they were subjected to the coupling of a wider range
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Fig. 2 ORPTEP view of the molecules of complexes 3–8 with 50% thermal ellipsoids and labelling scheme; hydrogen atoms and the second independent
molecule of 4 are omitted for clarity.
of substrates in order to examine their general versatility. Further-
more, the use of unactivated bromides allows us to compare the
Conclusions
activity of catalysts toward the more resistive coupling. Under the
given conditions, 4-bromotoluene can be coupled with good yields
of 83–100% (runs 17–19). Moreover, complex 3 demonstrates its
superiority over the other mixed carbene–phosphine complexes
in the coupling of 4-bromoanisole (runs 21–23). Activated aryl
chlorides such as 4-chlorobenzaldehyde can also be coupled,
although only under prolonged reaction time. Here, complexes 5
and 6 bearing the more basic and bulky PCy3 ligand perform best
(runs 25–28). However, they are slightly less effective as compared
to [PdBr2(PPh3)2] (run 29).
We have achieved a ready entry to new mixed carbene–phosphine
Pd(II) complexes bearing N,S-heterocyclic carbene ligands by
cleavage of dimeric monocarbene species. The complexes are
catalytically active in the Suzuki–Miyaura reaction. The synthetic
route presented herein allows an easy extension to other mixed
carbene–co-ligand systems. This system presents no serious prob-
lems, in terms of formation of isomers (geometric and nuclearity),
or competing reactivity at sulfur. However, efforts to bring the
pendant pyridyl (in 7 and 8) to the metal site through coordination
via a bromide extraction reaction (with Ag+) led to unidentified
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Table 2 Suzuki–Miyaura coupling reactionsacatalyzed by complexes 3–8
in different substrates
a Hewlett Packard 5973 MS detector. Elemental analyses were
performed on a Perkin-Elmer PE 2400 elemental analyzer at the
Department of Chemistry, National University of Singapore.
3-Propylbenzothiazolium bromide (A)
A mixture of benzothiazole (3.71 g, 27.5 mmol) and 1-
bromopropane (3.38 g, 27.5 mmol) was stirred at 120 ◦C overnight.
The orange solid thus obtained was washed several times with
Et2O. The solid was continuously washed with ethyl acetate until a
yellow solid was obtained. Diffusion of Et2O into a concentrated
CH2Cl2 solution yielded transparent crystals suitable for X-ray
Run
[Pd]
X
R
t/h
Yieldb(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
3
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
4-COCH3
4-COCH3
4-COCH3
4-COCH3
4-COCH3
4-COCH3
4-COCH3
4-COCH3
4-CHO
4-CHO
4-CHO
4-CHO
4-CHO
4-CHO
4-CHO
4-CHO
4-CH3
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
43
43
43
43
43
43
43
43
66
66
66
66
66
100
100
100
98
100
100
71
100
100
82
100
86
100
85
88
100
100
83
100
100
96
4
diffraction studies. Yield: 5.55 g (21.5 mmol, 78%). 1H NMR
5
3
6
(500 MHz, CDCl3): d 12.09 (s, 1H, NCH), 8.43 (d, JHH
=
8.20 Hz, 1H, Ar–H), 8.11 (d, 3JHH = 8.85 Hz, 1H, Ar–H), 7.89 (t,
7
8
3
1H, Ar–H), 7.80 (t, 1H, Ar–H), 5.12 (t, 2H, JHH = 7.25 Hz,
cis-[PdBr2(NSHC)2]
CH2CH2CH3), 2.17 (ps sext, 2H, CH2CH2CH3), 1.08 (t, 3H,
[PdBr2(PPh3)2]
3JHH = 7.25Hz, CH2CH2CH3). 13C{ H} NMR(125 MHz, CDCl3):
1
3
4
d 164.9 (NCH), 140.2, 131.4, 130.2, 129.1, 125.3, 116.5 (Ar–C),
54.7 (CH2CH2CH3), 22.9 (CH2CH2CH3), 10.9 (CH2CH2CH3).
MS (ESI, positive mode) m/z (%): 178 (100) [M − Br]+. Anal.
Calc for C10H12BrNS (M = 258.18): C, 46.52; H, 4.68; N, 5.43; S,
12.42. Found: C, 46.18; H, 4.76; N, 5.51; S, 12.81.
5
6
7
8
cis-[PdBr2(NSHC)2]
[PdBr2(PPh3)2]
3
5
4-CH3
4-CH3
4-CH3
Dibromo(l-dibromo)bis(3-propylbenzothiazolin-2-
ylidene)dipalladium(II) (2)
7
[PdBr2(PPh3)2]
3
4-OCH3
4-OCH3
4-OCH3
4-OCH3
4-CHO
4-CHO
4-CHO
4-CHO
4-CHO
Complex 2 was prepared in analogy to 1 from A (518 mg, 2 mmol)
5
80
80
100
84
73
93
91
100
7
and Pd(OAc)2 (225 mg, 1 mmol). Yield: 337 mg (0.38 mmol,
[PdBr2(PPh3)2]
1
76%). H NMR (300 MHz, DMSO-d6): d 8.13 (br, m, 4H, Ar–
3
H), 7.59 (m, 4H, Ar–H), 5.07 (br, t, 4H, CH2CH2CH3), 2.17 (br,
4
m, 4H, CH2CH2CH3), 1.08 (t, 6H, 3JHH = 6.03 Hz, CH2CH2CH3).
5
1
13C{ H} NMR (75 MHz, DMSO-d6): d 142.0, 135.2, 127.2, 125.4,
6
[PdBr2(PPh3)2]
122.5, 114.8 (Ar–C), 54.8 (CH2CH2CH3), 21.4 (CH2CH2CH3),
11.07 (CH2CH2CH3). MS (FAB, positive ions) m/z (%): 807 (10)
[M − Br]+. Anal. Calc C20H22Br4N2Pd2S2 (M = 886.98): C, 27.08;
H, 2.50; N, 3.16; S, 7.23. Found: C, 26.27; H, 2.90; N, 3.05; S, 7.89.
a 1 mmol of Ar–X, 2 mmol of Cs2CO3, 1.5 mmol of Ph–B(OH)2. b GC-MS
determination by using n-dodecane as the internal standard.
mixtures. The exact role of heterofunctionality in the catalytic
performance also needs to be better defined. Ongoing experiments
are directed toward these goals.
cis-Dibromo(3-benzylbenzothiazolin-2-ylidene)-
(triphenylphosphine)palladium(II) (3)
A mixture of 1 (983 mg, 1 mmol) and PPh3 (525 mg, 2 mmol)
was suspended in CH2Cl2 (5 mL) and stirred overnight at r.t.
Et2O (15 mL) was added to give a yellow precipitate which was
filtered and washed with Et2O. Yield: 905 mg (1.20 mmol, 60%).
1H NMR (500 MHz, CDCl3): d 7.71–7.70 (m, 1H, Ar–H), 7.63–
7.59 (m, 7H, Ar–H). 7.54–7.52 (m, 1H, Ar–H), 7.49–7.47 (m,
1H, Ar–H), 7.37–7.34 (m, 4H, Ar–H), 7.31–7.28 (m, 1H, Ar–H),
7.25–7.22 (m, 8H, Ar–H), 7.16–7.15 (m, 1H, Ar–H), 6.53 (d, 1H,
2JHH = 15.10 Hz, NCHH), 5.00 (d, 1H, 2JHH = 15.10 Hz, NCHH).
Experimental
Unless otherwise stated, all manipulations were performed
without taking precautions to exclude air and moisture.
All solvents were used as received. 3-Benzylbenzothiazolium
bromide and dibromo(l-dibromo)bis(3-benzylbenzothiazolin-2-
ylidene)dipalladium(II) were prepared according to the literature.8
R
Benzothiazole was purchased from Sigma-Aldrichꢀ and distilled
R
prior to use. Pd(OAc)2 was purchased from Sigma-Aldrichꢀ and
31P{ H} NMR (202.43 MHz, CDCl3): d 27.3 (s, 1P, PPh3). 13C{ H}
NMR (125 MHz, CDCl3): d 208.4 (NCS), 167.8, 142.2, 136.5 (Ar–
C), 134.5 (d, 2/3JCP = 10.90 Hz, Ar–C), 132.6, 132.5 (Ar–C), 131.1
1
1
used as received. H, 13C and 31P NMR spectra were recorded
1
on Bruker ACF 300 and Bruker AMX 500 spectrometers, and
the chemical shifts (d) were internally referenced by the residual
solvent signals relative to tetramethylsilane (1H, 13C) or externally
to 85% H3PO4 (31P). Mass spectra were obtained using a Finnigan
MAT LCQ (ESI) and Finnigan MAT 95XL-T (FAB). The yields
of C–C coupling products were determined by using Hewlett-
Packard Series 6890 GC (Santa Clara, CA, USA) coupled to
4
(d, JCP = 2.74 Hz, Ar–C), 130.8, 130.0, 129.6, 128.9 (Ar–C),
128.8 (d, 4JCP = 2.73 Hz, Ar–C), 128.4 (d, 2/3JCP = 10.90 Hz, Ar–
C), 128.2, 126.6, 125.07, 121.7, 115.2 (Ar–C), 59.5 (NCH2). MS
(ESI, positive mode) m/z (%): 674 (100) [M − Br]+. Anal. Calc
for C32H26Br2NPPdS (M = 753.82): C, 50.99; H, 3.48; N, 1.86; S,
4.25. Found: C, 49.59; H, 3.32; N, 1.38; S, 4.56.
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cis-Dibromo(3-propylbenzothiazolin-2-ylidene)-
(triphenylphosphine)palladium(II) (4)
(M = 723.92): C, 46.46; H, 6.13; N, 1.93; S, 4.43. Found: C, 46.43;
H, 6.35; N, 2.05; S, 4.31.
Complex 4 was prepared in analogy to 3 from 2 (887 mg, 1 mmol)
and PPh3 (525 mg, 2 mmol). Yield: 762 mg (1.08 mmol, 54%). 1H
NMR (500 MHz, CDCl3): d 7.68–7.64 (m, 6H, Ar–H), 7.61 (d,
cis-Dibromo(3-benzylbenzothiazolin-2-ylidene)-
(2-diphenylphosphanylpyridine)palladium(II) (7)
3
1H, JHH = 7.55 Hz, Ar–H). 7.43–7.41 (m, 1H, Ar–H), 7.36–
Complex 7 was prepared in analogy to 3 from 1 (983 mg, 1 mmol)
and PPh2Py (527 mg, 2 mmol). The white precipitate obtained was
filtered and washed with Et2O. Yield: 906 mg (1.20 mmol, 60%).
7.32 (m, 5H, Ar–H), 7.25–7.22 (m, 6H, Ar–H), 4.81 (dt, 1H,
2
3JHH = 12.5 Hz, JHH = 5.4 Hz, CHHCH2CH3), 4.05 (dt, 1H,
2
3JHH = 12.5 Hz, JHH = 5.4 Hz, CHHCH2CH3), 2.41 (m, 1H,
1H NMR (500 MHz, CDCl3): d 8.20 (d, 1H, JHH = 5.05 Hz,
3
CH2CHHCH3), 1.79 (m, 1H, CH2CHHCH3), 1.07 (t, 3H, 3JHH
=
Ar–H), 7.97–7.93 (m, 2H, Ar–H), 7.73–7.69 (m, 2H, Ar–H), 7.60–
7.57 (m, 3H, Ar–H), 7.51–7.49 (m, 2H, Ar–H), 7.47–7.40 (m,
1H, Ar–H), 7.36–7.32 (m, 2H, Ar–H), 7.31–7.20 (m, 8H, Ar–H),
7.15–7.13 (m, 1H, Ar–H), 7.10–7.07 (m, 1H, Ar–H), 6.78 (d, 1H,
2JHH = 15.45 Hz, NCHH), 5.26 (d, 1H, 2JHH = 15.45 Hz, NCHH).
1
7.58 Hz, CH2CH2CH3). 31P{ H} NMR (202.43 MHz, CDCl3):
1
d 27.7 (s, 1P, PPh3). 13C{ H} NMR (125 MHz, CDCl3): d 206.3
2/3
(NCS), 142.0, 136.4 (Ar–C), 134.3 (d,
J
= 10.94 Hz, Ar–
CP
4
C), 131.1 (d, JCP = 2.74 Hz, Ar–C), 129.9, 129.5 (Ar–C), 128.3
2/3
1
31P{ H} NMR (202.43 MHz, CDCl3): d 24.5 (s, 1P, PPh2Py).
(d,
J
= 10.93 Hz, Ar–C), 126.7, 125.1, 121.9, 113.8 (Ar–C),
CP
1
13C{ H} NMR (125 MHz, CDCl3): d 207.7 (NCS), 156.0, 155.4
57.3 (CH2CH2CH3), 21.3 (CH2CH2CH3), 11.6 (CH2CH2CH3).
MS (ESI, positive mode) m/z (%): 658 (100) [M − Br + MeOH]+.
Anal. Calc for C28H26Br2NPPdS (M = 705.78): C, 47.65; H, 3.71;
N, 1.98; S, 4.54. Found: C, 47.06; H, 3.52; N, 1.79; S, 4.89.
(Ar–C), 149.5 (d, 1/2JCP = 16.39 Hz, Ar–C), 142.2, 136.5 (Ar–C),
135.99 (d, 3JCP = 8.19 Hz, Ar–C), 135.93 (d, 2/3JCP = 10.90 Hz, Ar–
C), 135.8 (Ar–C), 134.2 (d, 2/3JCP = 10.9 Hz, Ar–C), 133.0, 131.5
(Ar–C), 131.0 (d, 4JCP = 2.74 Hz, Ar–C), 130.5, 130.3, 130.2, 129.8,
128.9, 128.8, 128.5, 128.4 (Ar–C), 128.2 (d, 2/3JCP = 10.9 Hz, Ar–
cis-Dibromo(3-benzylbenzothiazolin-2-ylidene)-
(tricyclohexylphosphine)palladium(II) (5)
4
C), 127.9, 126.5, 124.9 (Ar–C), 124.1 (d, JCP = 2.74 Hz, Ar–C),
121.6, 115.3 (Ar–C), 60.2 (NCH2). MS (ESI, positive mode) m/z
(%): 675 (100) [M − Br]+. Anal. Calc for C31H25Br2N2PPdS (M =
754.81): C, 49.33; H, 3.34; N, 3.71; S, 4.25. Found: C, 48.93; H,
3.11; N, 3.63; S, 3.98.
Complex 5 was prepared in analogy to 3 from 1 (983 mg, 1 mmol)
and PCy3 (561 mg, 2 mmol). The yellow precipitate was obtained
by filtration. Yield: 1.02 g (1.32 mmol, 66%). 1H NMR (500 MHz,
3
CDCl3): d 7.86 (d, 1H, JHH = 8.15 Hz, Ar–H), 7.44–7.31 (m,
8H, Ar–H), 6.69 (d, 2H, 2JHH = 15.75 Hz, NCHH), 6.14 (d, 2H,
2JHH = 15.75 Hz, NCHH), 2.25–2.12 (m, 6H, CH2), 1.80 (broad s,
7H, CH2), 1.66–1.58 (m, 10H, CH2), 1.25–1.12 (m, 7H, CH2), 0.91
cis-Dibromo(3-propylbenzothiazolin-2-ylidene)-
(2-diphenylphosphanylpyridine)palladium(II) (8)
1
(br, s, 3H, PCH). 31P{ H} NMR (202.43 MHz, CDCl3): d 44.7
Complex 8 was prepared in analogy to 3 from 2 (887 mg, 1 mmol)
and PPh2Py (527 mg, 2 mmol). The white precipitate obtained was
filtered and washed with Et2O. Yield: 1.02 g (1.44 mmol, 72%).
1
(s, 1P, PCy3). 13C{ H} NMR (125 MHz, CDCl3): d 208.7 (NCS),
142.8, 135.6, 132.8, 128.8, 128.4, 127.6, 127.1, 125.6, 122.0, 115.8
(Ar–C), 61.1 (NCH2), 36.6 (d, 1JCP = 21.86 Hz, PCH), 30.5, 29.8
1H NMR (500 MHz, CDCl3): d 8.23 (d, 1H, JHH = 4.40 Hz,
3
2/3
(CH2), 27.5 (d,
J
= 10.9 Hz, CH2), 26.0 (CH2). MS (ESI,
Ar–H), 7.97–7.92 (m, 2H, Ar–H), 7.75–7.70 (m, 2H, Ar–H), 7.61–
7.55 (m, 3H, Ar–H), 7.45–7.39 (m, 3H, Ar–H), 7.36–7.32 (m,
3H, Ar–H), 7.27–7.25 (m, 1H, Ar–H), 7.22–7.19 (m, 2H, Ar–H),
CP
positive mode) m/z (%): 692 (100) [M − Br]+. Anal. Calc for
C32H44Br2NPPdS (M = 771.97): C, 49.79; H, 5.74; N, 1.81; S, 4.15.
Found: C, 49.37; H, 5.78; N, 1.89; S, 3.95.
7.10–7.07 (m, 1H, Ar–H), 5.04 (dt, 1H, 2JHH = 12.61 Hz, 3JHH
=
2
3
4.93 Hz, CHHCH2CH3), 4.28 (dt, 1H, JHH = 12.61 Hz, JHH
=
4.93 Hz, CHHCH2CH3), 2.39 (m, 1H, CH2CHHCH3), 1.88 (m,
1H, CH2CHHCH3), 1.07 (t, 3H, 3JHH = 7.25 Hz, CH2CH2CH3).
cis-Dibromo(3-propylbenzothiazolin-2-ylidene)-
(tricyclohexylphosphine)palladium(II) (6)
1
31P{ H} NMR (202.43 MHz, CDCl3): d 24.6 (s, 1P, PPh2Py).
Complex 6 was prepared in analogy to 3 from 2 (887 mg, 1 mmol)
and PCy3 (561 mg, 2 mmol). The yellow precipitate was obtained
and filtered. Yield: 1.01 g (1.40 mmol, 70%). 1H NMR (500 MHz,
1
13C{ H} NMR (125 MHz, CDCl3): d 205.3 (NCS), 156.0, 155.3
1/2
(Ar–C), 149.50 (d,
J
= 17.31 Hz, Ar–C), 142.0, 136.2 (Ar–
CP
C), 135.9 (d, 3JCP = 9.56 Hz, Ar–C), 134.2 (d, 2/3JCP = 10.03 Hz,
3
CDCl3): d 7.86 (d, 1H, JHH = 8.20 Hz, Ar–H), 7.66 (d, 1H,
Ar–C), 131.5 (d, 4JCP = 1.81 Hz, Ar–C), 131.0 (d, 4JCP = 1.83 Hz,
3
3JHH = 8.85 Hz, Ar–H), 7.57 (t, 1H, JHH = 7.73 Hz, Ar–H),
Ar–C), 130.5, 130.3, 130.2, 129.8, 128.8 (Ar–C), 128.4 (d, 2/3JCP
=
3
2
7.49 (t, 1H, JHH = 7.60 Hz, Ar–H), 5.39 (dt, 1H, JHH = 12.45,
11.85 Hz, Ar–C), 128.1 (d, 2/3JCP = 10.93 Hz, Ar–C), 126.6, 124.9
3JHH = 4.63 Hz, CHHCH2CH3), 4.46 (dt, 1H, 2JHH = 12.45, 3JHH
=
4
(Ar–C), 124.1 (d, JCP = 2.73 Hz, Ar–C), 121.8, 113.9 (Ar–C),
4.63 Hz, CHHCH2CH3), 2.72 (m, 1H, CH2CHHCH3), 1.95 (m,
1H, CH2CHHCH3), 2.19 (br s, 7H, CH2), 1.80–1.62 (m, 23H,
57.4 (CH2CH2CH3), 21.2 (CH2CH2CH3), 11.5 (CH2CH2CH3).
MS (ESI, positive mode) m/z (%): 627 (100) [M − Br]+. Anal.
Calc for C27H25Br2N2PPdS (M = 706.77): C, 45.88; H, 3.57; N,
3.96; S, 4.54. Found: C, 45.32; H, 3.52; N, 4.01; S, 4.35.
3
CH2), 1.16 (t, 3H, JHH = 7.25 Hz, CH2CH2CH3), 0.86–0.83 (m,
3H, PCH). 31P{ H} NMR (202.43 MHz, CDCl3): d 43.7 (s, 1P,
1
PCy3). 13C{ H} NMR (125 MHz, CDCl3): d 206.0 (NCS), 142.3,
1
135.8, 127.2, 125.6, 122.3, 114.2 (Ar–C), 59.1 (CH2CH2CH3), 36.5
General procedure for the Suzuki–Miyaura reaction
1
2/3
(d, JCP = 21.90 Hz, PCH), 30.7, 29.6 (CH2), 27.6 (d,
J
=
CP
8.20 Hz, CH2), 27.5 (d, 2/3JCP = 6.375 Hz, CH2), 26.9 (CH2), 20.7
(CH2CH2CH3), 11.7 (CH2CH2CH3). MS (ESI, positive mode)
m/z (%): 644 (100) [M − Br]+. Anal. Calc for C28H44Br2NPPdS
Cs2CO3 (2 mmol), aryl halide (1 mmol), phenyl boronic acid
(1.5 mmol) and dodecane (1 mmol as internal standard) were
placed in a reaction flask equipped with a stirring bar. Toluene
704 | Dalton Trans., 2008, 699–706
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©

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(5 mL) was introduced and the resulting suspension was heated
to 100 ◦C for 10 min before the catalyst was added. After the
desired reaction times, the reaction mixture was allowed to cool to
ambient temperature. Water was added and the aqueous phase was
extracted with CH2Cl2 (3 × 4 mL). The combined organic phases
were dried over MgSO4, filtered and the solution was analyzed by
GC-MS.
CMMAC NUS for technical support. H. V. H. is grateful to
the Alexander von Humboldt Foundation for a Feodor Lynen
Research Fellowship. S. K. Y. thanks NUS for the research
scholarship and K. E. Neo for discussion.
References
1 (a) C. Ko¨cher and W. A. Herrmann, J. Organomet. Chem., 1997, 532,
261; (b) W. A. Herrmann, L. J. Gooßen and M. Spiegler, J. Organomet.
Chem., 1997, 547, 357; (c) C. J. Mathews, P. J. Smith, T. Welton, A. J. P.
White and D. J. Williams, Organometallics, 2001, 20, 3848; (d) H. V.
Huynh, Y. Han, J. H. H. Ho and G. K. Tan, Organometallics, 2006, 25,
3267.
2 (a) O. Navarro, R. A. Kelly, III and S. P. Nolan, J. Am. Chem.
Soc., 2003, 125, 16194; (b) M. S. Viciu, R. A. Kelly, III, E. D.
Stevens, F. Naud, M. Studer and S. P. Nolan, Org. Lett., 2003, 5,
1479.
3 G. D. Frey, J. Schu¨tz, E. Herdtweck and W. A. Herrmann,
Organometallics, 2005, 24, 4416.
4 C. W. K. Gsto¨ttmayr, V. P. W. Bo¨hm, E. Herdtweck, M. Groschem and
W. A. Herrmann, Angew. Chem., Int. Ed., 2000, 41, 1363.
5 L. R. Titcomb, S. Caddick, F. G. N. Cloke, D. J. Wilson and D.
Mckerrecher, Chem. Commun., 2001, 1388.
X-Ray diffraction studies
Suitable crystals were mounted on quartz fibers and X-ray data
collected on a Bruker AXS APEX diffractometer, equipped with a
CCD detector, using graphite-monochromated Mo-Ka radiation
˚
(k = 0.71073 A). The collecting frames of date, indexing reflec-
tion and determination of lattice parameters and polarization
effects were performed with the SMART suite programs.12 The
integration of intensity of reflections and scaling was performed
by SAINT. The empirical absorption correction was performed
by SADABS.13 The space group determination, structure solution
and least-squares refinements on |F|2 were carried out with the
SHELXTL.14 The structures were solved by direct methods to
locate the heavy atoms, followed by difference maps for the light
non-hydrogen atoms. Anisotropic thermal parameters were refined
for the rest of the non-hydrogen atoms. The hydrogen atoms were
placed in their ideal positions. A selected summary of crystal data
for complexes 3–8 are summarized in Table 3. The two terminal
carbon atoms of the propyl substituents of both molecules of A
are disordered into two positions of occupancy ratio 55/45. The
three cyclohexyl rings of complex 5 have relatively high thermal
parameters and one of them is disordered into two positions at a
50 : 50 ratio.
6 D. Kremzow, G. Seidel, C. W. Lehmann and A. Fu¨rstner, Chem.–Eur. J.,
2005, 11, 1833.
7 J. C. C. Chen and I. J. B. Lin, Organometallics, 2000, 19, 5113.
8 (a) S. K. Yen, L. L. Koh, F. E. Hahn, H. V. Huynh and T. S. A. Hor,
Organometallics, 2006, 25, 5105; (b) S. K. Yen, L. L. Koh, H. V. Huynh
and T. S. A. Hor, Dalton Trans., 2007, 3952.
9 H. V. Huynh, C. H. Yeo and G. K. Tan, Chem. Commun., 2006,
3833.
10 S. P. Nolan, N-Heterocyclic Carbenes in Synthesis, Wiley-VCH, Wein-
heim, Germany, 2006.
11 (a) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290; (b) D.
Bourissou, O. Guerret, F. P. Gabba¨ı and G. Bertrand, Chem. Rev.,
2000, 100, 39; (c) F. E. Hahn, Angew. Chem., Int. Ed., 2006, 45,
1348.
12 SMART (Version 5.631) & SAINT (Version 6.63) Software Reference
Manuals, Bruker AXS GmbH, Karlsruhe, Germany, 2000.
13 G. M. Sheldrick, SADABS: a Software for Empirical Absorption
Correction, University of Go¨ttingen, Go¨ttingen, Germany, 2001.
14 SHELXTL Reference Manual, Version 6.10, Bruker AXS GmbH,
Karlsruhe, Germany, 2000.
Acknowledgements
We thank the National University of Singapore for financial
support (Grant R-143-000-277-305). We thank the staff at the
706 | Dalton Trans., 2008, 699–706
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The Royal Society of Chemistry 2008
©