Syntheses, structures, and catalytic activities of hemilabile thioether-functionalized NHC complexes

Article abstract of DOI:10.1021/om9010966

Four imidazolium (5a/b) and benzimidazolium (6a/b) salts with hemilabile alkyl-aryl thioether functions have been prepared via a straightforward and modular pathway in order to compare their reactivities toward palladation. Reaction of 5a/b with Pd(OAc)

Full text of DOI:10.1021/om9010966

Organometallics 2010, 29, 1479–1486 1479
DOI: 10.1021/om9010966
Syntheses, Structures, and Catalytic Activities of Hemilabile
Thioether-Functionalized NHC Complexes
Han Vinh Huynh,* Chun Hui Yeo, and Ying Xia Chew
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received December 21, 2009
Four imidazolium (5a/b) and benzimidazolium (6a/b) salts with hemilabile alkyl-aryl thioether
functions have been prepared via a straightforward and modular pathway in order to compare their
reactivities toward palladation. Reaction of 5a/b with Pd(OAc)2 gave complex product mixtures,
whereas 6a/b afforded the desired bis(benzimidazolin-2-ylidene) complexes 8a/b in good yields. The
difference in reactivities of benzimidazole versus imidazole derivatives was attributed to the presence
of additional acidic protons at C4/5 positions of the imidazolium ring, leading to competing and
unselective deprotonation reactions. The milder Ag-NHC transfer reaction, on the other hand,
provided either mono- or bis(imidazolin-2-ylidene) complexes (9 or 7a/b) in good yields depending on
the ligand:metal ratio. The interesting hemilability in monocarbene complex 9 was investigated by
spectroscopy and thioether displacement reaction with PPh3, yielding the mixed NHC-PPh3 complex
10 in high yields. An initial comparative catalytic study also reveals that the mixed-donor complex
10 exhibits the highest activity among the complexes tested.
Introduction
methyl-aryl thioether-functionalized imidazolin-2-ylidene
ligand.12 Herein we provide a more detailed account on the
modular design and coordination chemistry of alkyl-aryl
thioether-functionalized NHCs derived from imidazole and
benzimidazole as well as the applications of their PdII com-
plexes as catalyst precursors for the aqueous and aerobic
Suzuki-Miyaura cross-coupling reaction.
Donor-functionalization of N-heterocyclic carbenes (NHCs)1
has become an important theme in organometallic chemis-
try, attracting considerable attention in recent years. Ac-
cordingly, various potentially hemilabile and ditopic NHCs,
pincer-type NHCs, and their complexes have been explored
and recently reviewed.2 Notably, N-functionalization with
N-, O-, and P-donor groups is relatively common, whereas
carbene ligands bearing softer sulfur donors remain surpris-
ingly rare, possibly due to lack of suitable synthetic meth-
odologies, although some thiolato-NHCs,3-6 thioether-
NHCs,7-9 and thiophene-NHCs10,11 have been reported.
As a contribution to this little explored field, we have recently
communicated the hemilabile coordination behavior of a
Results and Discussion
Thioether-Functionalized Azolium Salts. The S-donor-
functionalized imidazolium and benzimidazolium salts 5
and 6 were synthesized in five steps according to Scheme 1.
The first step involved S-alkylation of commercially avail-
able thiosalicyclic acid with alkyl bromide to form the 2-
alkylmercaptobenzoic acids 1. The mercaptobenzoates 2
were subsequently formed through esterification of 1 in
methanol under acidic conditions. LiAlH4 reduction of 2
under anhydrous conditions and subsequent acidification
gave rise to the formation of the benzyl alcohols 3, which
then underwent substitution reaction with PBr3 to furnish
the benzyl bromides 4. N-Alkylation of N-methylimidazole
and N-methylbenzimidazole finally afforded the respective
carbene precursors in overall yields of 63% (5a), 64% (5b),
57% (6a), and 59% (6b) over five steps.
*Corresponding author. E-mail: chmhhv@nus.edu.sg.
(1) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122.
(2) (a) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251,
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ꢀ
The formation of the organic compounds in each step can
1
be nicely traced by H NMR spectroscopy. For example a
new singlet due to the OCH3 group is detected upfield at 3.92
(2a) or 3.95 ppm (2b) on formation of the esters 2. Upon
reduction to the alcohols 3, a more downfield singlet attrib-
uted to the benzylic CH2 protons can be observed at 4.70 (3a)
Organometallics 2006, 25, 1831.
(7) Seo, H.; Park, H.-J.; Kim, B. Y.; Lee, J. H.; Son, S. U.; Chung,
Y. K. Organometallics 2003, 22, 618.
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2007, 5069.
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ꢀ
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Lassaletta, J. M.; Fernandez, R. Organometallics 2007, 26, 2570.
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B. W.; White, A. H. J. Organomet. Chem. 2005, 690, 6133.
(11) McGuinness, D. S.; Suttil, J. A.; Gardiner, M. G.; Davies, N. W.
Organometallics 2008, 27, 4238.
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3833.
r
2010 American Chemical Society
Published on Web 02/18/2010
pubs.acs.org/Organometallics

1480 Organometallics, Vol. 29, No. 6, 2010
Huynh et al.
Scheme 1. Synthetic Pathway for Thioether-Functionalized
Azolium Salts
The formation of these bis(carbene) complexes is indicated
by a base peak in the ESI mass spectrum at m/z = 623 for the
[M - Br]þ complex fragment. Due to different solubilities,
cis- and trans-isomers can be separated by washing with
acetone. The remaining insoluble yellow powder (53%)
contains an approximately 1:1 rotameric mixture of trans-
7a, which cannot be further separated probably due to the
low rotational barrier. 1H and 13C NMR spectra in CDCl3 of
the latter show two sets of signals with Ccarbene resonances at
169.9 and 169.8 ppm, corresponding to the syn- and anti-
rotamers, respectively. Removal of the solvent from the
acetone filtrate and subsequent recrystallization from CHCl3
yielded an off-white powder of cis-7a (∼45%). Its 1H NMR
spectrum in CD3CN is dominated by broad signals indicat-
ing a restricted rotation of the Pd-Ccarbene bonds in the cis-
isomer. 13C NMR data, on the other hand, could not be
obtained due to insufficient solubility. However, an X-ray
diffraction analysis on single crystals of the solvate cis-
7a 2DMF, obtained by vapor diffusion of diethyl ether into
3
a concentrated DMF solution, confirms the cis-arrangement
of the NHC ligands in the essentially square-planar complex
with the bulky methylmercaptobenzyl groups anti to each
other (Figure 1). The two Pd-Ccarbene bonds of 1.982(3) and
˚
1.992(3) A and the Pd-Br bonds of 2.4856(4) and 2.4785(3)
˚
A are in the typical range found for cis-standing dibromo-
bis(carbene) PdII complexes.14
or 4.79 ppm (3b) instead. In addition, a broad signal due to
the OH proton was observed at 2.60 (3a) or 2.27 ppm (3b).
This broad signal disappears when compounds 3 undergo
substitution reaction to form benzyl bromides 4. The forma-
tion of the imidazolium and benzimidazolium salts 5 and 6
was finally corroborated by characteristic downfield signals
at 10.37 (5a), 10.26, (5b), 11.25 (6a), and 11.34 ppm (6b) for
the NCHN protons. The identity of these carbene precursors
5 and 6 was further confirmed by their ESI mass spectra,
which in all cases show base peaks for the [M - Br]þ
fragments. While the benzimidazolium salts 6 are both
solids, the properties of their imidazolium analogues are
more prone to influences from the S-alkyl group. The use of
the more aliphatic S-isopropyl group results in a room-
temperature ionic liquid, whereas an S-methyl substituent
gives rise to a hygroscopic solid.
Palladium(II) Bis(Carbene) Complexes. The standard re-
action of Pd(OAc)2 with 2 equiv of azolium salt was initially
chosen in an attempt to prepare bis(carbene) PdII complexes
and to study the coordination behavior of thioether-functio-
nalized NHC ligands. However, heating a mixture of 5a and
Pd(OAc)2 in acetonitrile at 90 °C gave rise to a complex
product mixture from which only cis-7a could be isolated in a
low yield of 38%. The difficulty of carbene formation and
subsequent coordination to PdII under the relatively harsh
reaction conditions can be attributed to competing processes
such as (i) nonselective coordination of the soft sulfur atom,
(ii) competing deprotonation at C4/C5-positions, or (iii)
competing deprotonation of the benzylic protons. Thus the
alternative and milder Ag-carbene transfer method was
explored instead.13 Indeed, an isomeric mixture of bis-
(carbene) complexes cis-7a, trans-anti-7a, and trans-syn-7a
was obtained in 98% overall yield when [PdBr2(CH3CN)2]
was treated with Ag-carbene species generated in situ by
mixing Ag2O and 2 equiv of 5a in CH2Cl2 (Scheme 2).
A similar reaction sequence employing imidazolium salt
5b also gave an approximately 1:1 mixture of trans-anti-7b
and trans-syn-7b, as indicated by two sets of signals with
1
similar integrals in the H NMR spectrum and an ESI-MS
base peak at m/z = 679 for the [M - Br]þ cation. The two
rotamers equilibrate in solution, indicating a certain degree
of rotational freedom of the Pd-Ccarbene bonds. Surpris-
ingly, cis-isomers were not detected. The trans-configuration
is confirmed by 13C NMR spectroscopy, which shows the
presence of two closely spaced Ccarbene signals at 169.7 and
169.8 ppm for the two rotamers, respectively. Upon slow
evaporation of a concentrated CH2Cl2 solution, single crys-
tals of trans-anti-7b formed as the supposedly more favor-
able rotamer in the solid state (Figure 2). Compared to the
bond parameters in cis-7a the trans-configuration in trans-
anti-7b leads to a significant lengthening of the Pd-C bond
˚
to 2.046(6) A accompanied by a shortening of the Pd-Br
˚
bond to 2.4385(8) A, in line with the different trans-influ-
ences of the ligands involved. Notably, a remote change
seven bonds away from the PdII center can lead to the
preferred formation of trans-complexes.
The benzimidazolium salts 6a/b react differently and more
cleanly than their imidazole analogues due to the absence of
any acidic protons on the heterocycle. Thus the reaction with
Pd(OAc)2 could proceed smoothly in DMSO at 90 °C, giving
preferably the cis-configured complexes cis-8a/b in yields of
67% and 89%, respectively. The cis-configuration restricts
rotation of both N-benzyl and Pd-C bonds leading to
diastereotopy of the benzylic protons, which in turn resonate
as two doublets (6.07 and 5.67 ppm for cis-8a; 6.14 and 5.78
ppm for cis-8b) with geminal coupling constants of around
2J(H,H) = 17.0 Hz. The observation of only one AA0 spin-
system for each complex excludes the presence of cis-rotamers,
(14) (a) Baker, M. V.; Skelton, B. W.; White, A. H.; Williams, C. C.
J. Chem. Soc., Dalton Trans. 2001, 111. (b) Lee, H. M.; Lu, C. Y.; Chen, C.
Y.; Chen, W. L.; Lin, H. C.; Chiu, P. L.; Cheng, P. Y. Tetrahedron 2004, 60,
5807.
(13) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.;
Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561.

Article
Organometallics, Vol. 29, No. 6, 2010 1481
Scheme 2. Synthetic Pathway for PdII-Bis(carbene) Complexes
Figure 2. Molecular structure of complex trans-anti-7b showing
50% probability ellipsoids. Hydrogen atoms have been omitted
˚
for clarity. Selected bond lengths [A] and angles [deg]: Pd1-C1
Figure 1. Molecular structure of cis-7a 2DMF showing 50%
3
2.046(6), Pd1-Br1 2.4385(8), N1-C1 1.344(8), N2-C1
1.331(8); C1-Pd1-Br1 92.34(17), C1-Pd1-Br1A 87.66(17),
Br1-Pd1-Br1A 180.00(4), N1-C1-N2 105.1(6).
probability ellipsoids. Hydrogen atoms have been omitted for
˚
clarity. Selected bond lengths [A] and angles [deg]: Pd1-C1
1.982(3), Pd1-C13 1.992(3), Pd1-Br1 2.4856(4), Pd1-Br2
2.4785(3), C1-N1 1.350(4), C1-N2 1.348(4), C13-N3 1.347(4),
C13-N4 1.357(4); C1-Pd1-C13 92.84(13), C1-Pd1-Br2
86.57(9), C13-Pd1-Br1 87.06(9), Br1-Pd1-Br2 93.539(16),
N1-C1-N2 104.6(3), N3-C13-N4 104.8(3).
observed for the imidazolin-2-ylidene analogues (vide supra),
the trans-configuration of the two carbene ligands leads to
longer Pd-C [2.020(4) A] and shorter Pd-Br bonds
[2.4328(5) A] compared to those in cis-8a.
˚
˚
which have been observed for thiophene-functionalized de-
rivatives.15 This is further substantiated by 13C NMR spectra
showing only one carbene signal for each cis-bis(carbene)
complex at 175.1 ppm (cis-8a) and 174.4 ppm (cis-8b),
respectively. X-ray diffraction studies on single crystals of
cis-8a grown from a concentrated CH3CN solution con-
firmed the sterically more favorable cis-anti-arrangement
of the complex similar to that of cis-7a. The Pd-C and
Palladium(II) Monocarbene Complexes. In an initial at-
tempt to enforce thioether coordination through abstraction
of the bromo ligands, trans-7b was treated with 2 equiv of
AgBF4. Disappointingly, this reaction gave rise to an in-
tractable dark reaction mixture possibly due to decom-
position induced by AgI-promoted thioether cleavage.16
Alternatively, it was anticipated that a monocarbene
complex with two stabilizing bromo ligands should also
allow thioether coordination. Thus, we have attempted the
Ag-carbene transfer reaction in a 1:1 ligand to Pd ratio
(Scheme 3).12
˚
Pd-Br bonds of 1.977(4) and 2.4652(5) A are comparable to
those found in the thiophene-functionalized analogue
(Figure 3).15
Attempts to obtain single crystals of cis-8b on the other
hand proved unsuccessful. Instead cis-trans isomerization
occurs upon prolonged standing, affording only a few single
crystals, which were analyzed as trans-anti-8b (Figure 4). As
The 1H NMR spectrum of the reaction product in CDCl3
shows very broad signals at ambient temperature. Upon
cooling to -30 °C, two doublets at 5.00 and 5.85 ppm
were resolved for the nonequivalent benzylic protons, which
(15) Huynh, H. V.; Chew, Y. X. Inorg. Chim. Acta 2009, in press. DOI:
10.1016/j.ica.2009.02.035
(16) Hiskey, R. G.; Adams, J. B. J. Org. Chem. 1966, 31, 2178.

1482 Organometallics, Vol. 29, No. 6, 2010
Huynh et al.
Figure 4. Molecular structure of complex trans-anti-8b CH2Cl2
3
showing 50% probability ellipsoids. Hydrogen atoms have been
˚
omitted for clarity. Selected bond lengths [A] and angles [deg]:
Pd1-C1 2.020(4), Pd1-Br1 2.4328(5), N1-C1 1.355(5), N1-C2
1.391(4); C1-Pd1-C1A 180.0(3), Br1-Pd1-Br1A 180.0, C1-
Pd1-Br1 89.98(11), C1A-Pd1-Br1 90.02(11), N2)-C1-N1
105.9(3).
Figure 3. Molecular structure of complex cis-8a showing 50%
probability ellipsoids. Hydrogen atoms have been omitted for
clarity. Selected bond lengths [A] and angles [deg]: Pd1-C1
˚
1.977(4), Pd1-Br1 2.4652(5), C1-N1 1.354(4), C1-N2
1.368(5), N1-C2 1.379(5), N1-C8 1.460(5), N2-C7 1.394(5),
N2-C9 1.451(4), C2-C7 1.381(5); C1-Pd1-C1A 95.6(2),
Br1-Pd1-Br1A 94.07(3), C1-Pd1-Br1 85.96(10), C1A-Pd1-
Br1 170.40(10), C1-Pd1-Br1A 170.40(10), C1A-Pd1-Br1A
85.96(10), N1-C1-N2 105.5(3).
2J(H,H) = 14.3 Hz. This observation is in line with a hindered
rotation of the Pd-Ccarbene bond due to the steric bulk of the
phosphine ligand and indirectly confirms the cis arrange-
ment of the NHC and PPh3 ligands. The 13C NMR signal for
the carbenoid carbon appears at 162.7 ppm as a singlet, and
coupling to the cis-standing 31P nucleus could not be resolved
under the given conditions. However, the presence of the
phosphine donor is evidenced by a resonance at 27.13 ppm in
the 31P NMR spectrum. Single crystals obtained from a
saturated CD3CN solution were subjected to an X-ray
diffraction analysis, and the molecular structure of 10 is
shown in Figure 6. The metal center is coordinated by one
NHC, one phosphine, and two bromo ligands in a distorted
square-planar fashion. The former two are found in a cis
arrangement, which is thermodynamically favored due to the
transphobia effect of phosphines.17 Furthermore, it can be
clearly seen that the steric bulk of the phosphine ligand
prevents a free rotation of the Pd-Ccarbene bond.
indirectly indicates coordination of the sulfur function. In
addition, two sets of signals are observed for the N-CH3 and
the S-CH3 groups, pointing to a rather complex mixture
(vide infra). The identity of complex 9 was finally confirmed
by X-ray diffraction. The complex adopts a distorted square-
planar geometry with the carbene and sulfur donors in cis-
position, resulting in a seven-membered chelate ring, which is
in a boat-like conformation (Figure 5). The coordination of
the sulfur atom leads to a significantly small dihedral angle of
65.155(77)° between the carbene ring plane and the PdCSBr2
coordination plane. The longer Pd-Br2 bond [2.4792(4) vs
˚
2.4508(4) A for Pd-Br1] trans to the carbene reflects its
Suzuki-Miyaura Catalysis. In order to compare the cat-
alytic performance of bis- versus monocarbene complexes,
we tested five selected complexes with S-Me donor groups
for their catalytic activities in the Suzuki-Miyaura coupling
of aryl bromides and chlorides in/on pure water under
aerobic conditions. The results summarized in Table 1 show
that all complexes are highly active in the coupling of
activated 4-bromobenzaldehyde, giving quantitative yields
already at ambient temperature (entries 1-5). Furthermore,
the coupling of 4-bromoacetophenone reveals that among
bis(carbene) species cis-7a and cis-8a perform slightly better
than the trans-analogue trans-7a (entries 6, 8 vs 7). The
superiority of cis- versus trans-complexes has also been
observed for the Mizoroki-Heck reaction.18
strong trans-influence.
1
The complicated H NMR spectrum of complex 9 was
mainly attributed to two dynamic processes in solution. The
first process involves flipping of the seven-membered ring,
which slows down upon cooling, giving rise to the inequi-
valent benzylic protons observed at low temperature. The
second process involves reversible de- and recoordination of
the sulfur donor in a hemilabile fashion. Decoordination
results in a formally unsaturated metal center, which stabi-
lizes itself upon dimerization. The two dynamic processes are
in equilibrium with at least five species (Scheme 4). The
situation is further complicated due to the fact that the
thioether donor becomes chiral upon coordination.
To confirm the hemilabile behavior of the thioether-NHC
ligand, one equivalent (based on Pd) of PPh3 was added to a
solution of 9 in CH3CN. The stronger phosphine donor is
expected to cleave potential dimeric complexes as well as the
weaker Pd-S(Ar)R bond in 9. Indeed, the addition of PPh3
leads to a clean and well-resolved 1H NMR spectrum
corroborating the formation of the mixed NHC-phosphine
complex 10 as the sole product. Two doublets are observed
at 5.76 and 4.71 ppm, respectively, for the diastereoto-
pic benzylic protons with a geminal coupling constant of
The best performance, however, was exhibited by the
mixed NHC-phosphine complex 10. The coupling of deactivated
4-bromoanisole generally gave very low yields of <30%. How-
ever, it can be quantitatively coupled by all complexes at elevated
(17) (a) Han, Y.; Lee, L. J.; Huynh, H. V. Organometallics 2009, 28,
2778. (b) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. Organometallics
2009, 28, 5395.
(18) Huynh, H. V.; Ho, J. H. H.; Neo, T. C.; Koh, L. L. J. Organomet.
Chem. 2005, 690, 3854.

Article
Organometallics, Vol. 29, No. 6, 2010 1483
Scheme 3. Synthetic Pathway for PdII-Monocarbene Complexes
and 31P NMR spectra were recorded on a Bruker ACF 300
spectrometer, and the chemical shifts (δ) were internally refer-
enced to the residual solvent signals relative to tetramethylsilane
(1H, 13C) or externally to 85% H3PO4 (31P). Mass spectra were
measured using a Finnigan MAT LCQ (ESI) spectrometer.
Elemental analyses were performed on a Perkin-Elmer PE
2400 elemental analyzer at the Department of Chemistry, Na-
tional University of Singapore.
2-Methylmercaptobenzoic Acid (1a) and 2-Isopropylmercapto-
benzoic Acid (1b). Aqueous NaOH (10 mL of a 25% solution,
62.5 mmol) was added to a solution of thiosalicyclic acid (3.084 g,
20.0 mmol) in methanol (55 mL). The reaction mixture was
stirred for 20 min at room temperature, and methyl iodide
(2.75 mL, 44.0 mmol for 1a) or isopropyl bromide (4.20 mL,
44.9 mmol for 1b) was added. The reaction mixture was heated
under reflux overnight at 70 °C. The reaction mixture was
allowed to cool to room temperature, and the solvent was
removed in vacuo. The residue was dissolved in deionized water
(50 mL) and acidified with concentrated HCl (37%). The
mixture was extracted with diethyl ether (1 ꢀ 50 mL, 3 ꢀ 30 mL),
and the combined organic layers were dried over MgSO4 and
filtered. Removal of the solvent in vacuo yielded the product as
Figure 5. Molecular structure of 9 showing 50% probability
˚
ellipsoids. Selected bond lengths [A] and angles [deg]: Pd1-C1
1.971(3), Pd1-S1 2.3079(3), Pd1-Br1 2.4508(4), Pd1-Br2
2.4792(4), C1-N1 1.343(4), C1-N2 1.335(4); C1-Pd1-S1
93.65(9), C1-Pd1-Br1 86.53(8), S1-Pd1-Br2 86.94(2), Br1-
Pd1-Br2 93.070(13), N1-C1-N2 105.8(3).
temperatures and with the addition of [N(n-C4H9)4]Br (entries
11-15).19 It is believed that such ammonium salts help to
stabilize reactive intermediates through formation of ion-pairs
and hampering their aggregation.18 Disappointingly, the cou-
pling of more challenging aryl chlorides under the same condi-
tions gave rather poor results for most complexes with the
exception of mixed NHC-phosphine complex 10. It is note-
worthy that 10 gave the best yields of 52% and 44%, respectively
(entries 20 and 25), which is superior to previously reported PdII-
monocarbene complexes under similar conditions.19
1
an off-white powder. 1a, 3.196 g (19.00 mmol), 95% yield. H
NMR (300 MHz, CDCl3): δ 8.15 (dd, 3J(H,H) = 7.9 Hz, 4J(H,
H) = 1.5 Hz, 1 H, Ar-H), 7.53 (td, 3J(H,H) = 7.7 Hz, 4J(H,H) =
1.6 Hz, 1 H, Ar-H), 7.30 (d, 3J(H,H) = 8.1 Hz, 1 H, Ar-H),
7.20 (td, 3J(H,H) = 7.6 Hz, 4J(H,H) = 1.1 Hz, 1 H, Ar-H), 2.48
(s, 3 H, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 171.4
(COOH), 144.4, 133.3, 132.5, 125.5, 124.4, 123.5 (Ar-C), 15.6
(CH3). 1b, 3.870 g (19.72 mmol), 99% yield. 1H NMR (300
MHz, CDCl3): δ 8.15 (dd, 3J(H,H) = 7.8 Hz, 4J(H,H) = 1.1 Hz,
1 H, Ar-H), 7.52-7.43 (m, 2 H, Ar-H), 7.28-7.23 (m, 1 H,
Ar-H), 3.51 (sep, 3J(H,H) = 6.6 Hz, 1 H, CH), 1.39 (d, 3J(H,H) =
6.6 Hz, 6 H, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 171.2
(COOH), 140.6, 133.6, 133.3, 129.9, 129.1, 125.9 (Ar-C), 37.4
(CH), 23.3 (CH3).
Conclusion
We have presented a straightforward and modular syn-
thetic pathway to thioether-functionalized N-heterocyclic
carbene ligands derived from imidazole and benzimidazole.
Their coordination chemistry with respect to the formation
of mono- versus bis(carbene) complexes have been studied in
detail, revealing their hemilabile nature. Moreover, an initial
catalytic investigation demonstrated that the mixed NHC-
phosphine complex 10 showed overall the best performance.
Research in our laboratory is currently underway to extend
the coordination chemistry of these hemilabile ligands to
other late transition metals and to study their potential
applications in catalysis.
Methyl 2-Methylmercaptobenzoate (2a) and Methyl 2-Isopro-
pylmercaptobenzoate (2b). Concentrated H2SO4 (1 mL) was
added to a solution of 1a (3.1 g, 18.43 mmol) or 1b (3.8 g,
19.36 mmol) in methanol (30 mL). The reaction mixture was
heated under reflux at 70 °C for 3 h and allowed to cool to
ambient temperature. Deionized water (50 mL) was added, and
the mixture was extracted with diethyl ether (1 ꢀ 50 mL, 3 ꢀ 20
mL). The combined organic layers were dried over MgSO4 and
filtered. Removal of the solvent in vacuo yielded the product as
an off-white powder (2a) or yellow liquid (2b). 2a, 3.090 g (16.96
1
mmol), 92% yield. H NMR (300 MHz, CDCl3): δ 8.00 (dd,
4
3J(H,H) = 7.9 Hz, J(H,H) = 1.5 Hz, 1 H, Ar-H), 7.48 (td,
4
3J(H,H) = 7.7 Hz, J(H,H) = 1.6 Hz, 1 H, Ar-H), 7.28 (d,
3
3J(H,H) = 7.7 Hz, 1 H, Ar-H), 7.16 (td, J(H,H) = 7.6 Hz,
Experimental Section
4J(H,H) = 1.2 Hz, 1 H, Ar-H), 3.92 (s, 3 H, OCH3), 2.46 (s, 3 H,
SCH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 166.8 (COOH),
143.2, 132.4, 131.2, 126.7, 124.3, 123.3 (Ar-C), 51.9 (OCH3),
General Considerations. Unless otherwise noted all operations
were performed without taking precautions to exclude air and
moisture. All solvents and chemicals were used as received
1
15.5 (SCH3). 2b, 3.663 g (17.42 mmol), 90% yield. H NMR
1
without any further treatment if not noted otherwise. H, 13C,
(300 MHz, CDCl3): δ 7.92 (dd, 3J(H,H) = 7.7 Hz, d, 4J(H,H) =
1.5 Hz, 1 H, Ar-H), 7.43-7.29 (m, 2 H, Ar-H), 7.20 (td, 3J(H,
H) = 7.2 Hz, d, 4J(H,H) = 2.0 Hz, 1 H, Ar-H), 3.95 (s, 3 H,
OCH3), 3.56(sep, 3J(H,H) = 6.8 Hz, 1 H, CH), 1.41 (d, 3J(H,H) =
6.6 Hz, 6 H, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 167.1
(19) (a) Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Organome-
tallics 2006, 25, 3267. (b) Han, Y.; Hong, Y.-T.; Huynh, H. V. J. Organomet.
Chem. 2008, 693, 3159.

1484 Organometallics, Vol. 29, No. 6, 2010
Huynh et al.
Scheme 4. Dynamic Processes Due to the Hemilabile Ligand in Complex 9
Table 1. Suzuki-Miyaura Cross-Coupling Reactionsa in/on
Water
entry catalyst
aryl halide
t [h] temp [°C] yield [%]b
1
2
3
4
5
6
7
8
9
cis-7a 4-bromobenzaldehyde
trans-7a 4-bromobenzaldehyde
8
8
8
8
8
8
8
8
8
8
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
>99
>99
>99
>99
>99
98
84
90
85
>99
>99
>99
>99
>99
>99
7
11
3
8
52
cis-8a
9
10
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromoacetophenone
cis-7a
trans-7a 4-bromoacetophenone
cis-8a
9
4-bromoacetophenone
4-bromoacetophenone
4-bromoacetophenone
4-bromoanisole
Figure 6. Molecular structure of 10 showing 50% probability
˚
10 10
11c cis-7a
ellipsoids. Selected bond lengths [A] and angles [deg]: Pd1-C1
21 85
21 85
21 85
21 85
21 85
21 85
21 85
21 85
21 85
21 85
1.986(3), Pd1-P1 2.2642(8), Pd1-Br1 2.4775(4), Pd1-Br2
2.4785(3), C1-N1 1.349(4), C1-N2 1.350(4); C1-Pd1-P1
92.18(9), C1-Pd1-Br1 86.41(9), P1-Pd1-Br2 91.03(2),
Br1-Pd1-Br2 90.763(15), N1-C1-N2 105.1(3).
12c trans-7a 4-bromoanisole
13c cis-8a
4-bromoanisole
4-bromoanisole
14c
9
15c 10
4-bromoanisole
4-chlorobenzaldehyde
16c cis-7a
17c trans-7a 4-chlorobenzaldehyde
18c cis-8a
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-chlorobenzaldehyde
(COOH), 140.3, 131.8, 130.9, 129.3, 127.6, 124.1 (Ar-C), 52.0
(OCH3), 35.4 (CH), 22.6 (CH3).
19c
9
20c 10
2-Methylmercaptobenzyl Alcohol (3a) and 2-Isopropylmercap-
tobenzyl Alcohol (3b). 2a (3 g, 16.46 mmol) or 2b (3 g,
17.12 mmol) was dissolved in anhydrous THF (20 mL) and
added dropwise to a suspension of LiAlH4 (1.2 g, 31.62 mmol) in
anhydrous THF (30 mL). The reaction mixture was heated
under reflux overnight at 75 °C. Deionized water was then
added dropwise to hydrolyze excess LiAlH4, and the resulting
suspension was acidified with sulfuric acid. The mixture was
extracted with diethyl ether (1 ꢀ 50 mL, 3 ꢀ 20 mL). The
combined organic layers were dried over MgSO4 and filtered.
Removal of the solvent in vacuo yielded the product as a yellow
liquid. 3a, 2.462 g (15.97 mmol), 97% yield. 1H NMR (300 MHz,
CDCl3): δ 7.36-7.12 (m, 4 H, Ar-H), 4.70 (s, 2 H, CH2), 2.60
(brs, 1 H, OH), 2.44 (s, 3 H, CH3). 13C{1H} NMR (75.47 MHz,
CDCl3): δ 138.9, 136.4, 128.1, 127.6, 126.2, 125.3 (Ar-C), 62.8
21c cis-7a
4-chloroacetophenone 21 85
3
9
23
25
44
22c trans-7a 4-chloroacetophenone 21 85
23c cis-8a
24c
25c 10
4-chloroacetophenone 21 85
4-chloroacetophenone 21 85
4-chloroacetophenone 21 85
9
a Reaction conditions: 1 mmol of aryl halide; 1.5 mmol of phenyl-
boronic acid; 3 mL of water; 2 equiv of K2CO3; 1 mol % of catalyst.
b Yields were determined by 1H NMR spectroscopy for an average of
two runs. c With addition of 1.5 equiv of [N(n-C4H9)4]Br.
dropwise to a solution of 3a (2.4 g, 15.56 mmol) or 3b (3 g, 16.46
mmol) in diethyl ether (10 mL) at 0 °C. The solution was stirred
for 1 h at ambient temperature, and methanol (1.5 mL) was then
added. The mixture was diluted with deionized water (50 mL),
and the mixture was extracted with diethyl ether (1 ꢀ 50 mL, 3 ꢀ
20 mL). The combined organic layers were dried over MgSO4
and filtered. Removal of the solvent in vacuo yielded the product
as a yellow liquid. 4a, 3.243 g (14.94 mmol), 96% yield. 1H NMR
(300 MHz, CDCl3): δ 7.34-7.09 (m, 4 H, Ar-H), 4.63 (s, 2 H,
CH2), 2.49 (s, 3 H, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ
139.0, 136.0, 130.8, 129.7, 127.3, 125.8 (Ar-C), 32.4 (CH2), 16.5
(CH3). 4b, 3.915 g (15.97 mmol), 97% yield. 1H NMR (300 MHz,
1
(CH2), 25.5 (SCH3). 3b, 3.027 g (16.61 mmol), 97% yield. H
NMR (300 MHz, CDCl3): δ 7.46-7.24 (m, 4 H, Ar-H), 4.79 (s,
2 H, CH2), 3.39 (sep, 3J(H,H) = 6.6 Hz, 1 H, CH), 2.27 (brs, 1 H,
3
OH), 1.30 (d, J(H,H) = 6.6 Hz, 6 H, CH3). 13C{1H} NMR
(75.47 MHz, CDCl3): δ 142.5, 134.0, 133.1, 128.7, 128.3, 127.6
(Ar-C), 64.2 (CH2), 39.1 (CH), 23.5 (CH3).
2-Methylmercaptobenzyl Bromide (4a) and 2-Isopropylmer-
captobenzyl Bromide (4b). PBr3 (0.84 mL, 9.00 mmol) was added

Article
Organometallics, Vol. 29, No. 6, 2010 1485
CDCl3): δ 7.48-7.18 (m, 4 H, Ar-H), 4.74 (s, 2 H, CH2), 3.46
Complexes cis-7a, trans-anti-7a, and trans-syn-7a. Ag2O
(0.116 g, 0.5 mmol) was added to a solution of 5a (0.299 g,
1 mmol) in CH2Cl2 (5 mL) and stirred overnight at ambient
temperature. The reaction mixture was then filtered and added
to a solution of trans-[PdBr2(CH3CN)2] (0.133 g, 0.5 mmol) in
CH3CN. The suspension was stirred overnight at ambient
temperature and filtered to remove AgBr. The volatiles were
then removed in vacuo, and the residue was washed with diethyl
ether. Acetone was added, upon which a yellow precipitate
formed upon stirring. The yellow precipitate was filtered,
washed with small amounts of acetone, and dried in vacuo,
affording a mixture of trans-anti-7a and trans-syn-7a. The
solvent in the acetone filtrate was removed in vacuo, and CHCl3
was added to the off-white residue. Upon standing and slow
evaporation of CHCl3 cis-7a precipitates. cis-7a, 0.157 g
(0.223 mmol), 45% yield. trans-anti-7a þ trans-syn-7a, 0.187 g
(0.266 mmol), 53% yield. Anal. Calcd for C24H28Br2N4S2Pd: C,
41.01; H, 4.02; 7.97. Found: C, 40.30; H, 4.07; N, 8.00. 1H NMR
(300 MHz, CDCl3): δ 7.59-6.74 (m, 24 H, Ar-H), 5.90 (s, 4 H,
CH2), 5.71 (s, 4 H, CH2), 4.13 (s, 6 H, NCH3), 3.99 (s, 6 H,
NCH3), 2.52 (s, 6 H, SCH3), 2.37 (s, 6 H, SCH3). 13C{1H} NMR
(75.47 MHz, CDCl3): 169.9 (NCN), 169.8 (NCN), 137.3, 137.1,
134.7, 134.5, 130.5, 130.4, 128.9, 128.7, 126.6, 126.3, 126.1, 125.9
(Ar-C), 122.5, 122.4, 121.2, 121.0 (NCH), 51.6 (CH2), 51.5
(CH2), 38.2 (NCH3), 38.1 (NCH3), 16.5 (SCH3), 16.3 (SCH3).
MS (ESI, positive ions) m/z (%): 623 (100) [M - Br]þ.
Complexes trans-anti-7b and trans-syn-7b. Ag2O (0.116 g,
0.5 mmol) was added to a solution of 5b (0.327 g, 1 mmol) in
CH2Cl2 (5 mL) and stirred overnight at ambient temperature.
The reaction mixture was then filtered and added to a solution of
trans-[PdBr2(CH3CN)2] (0.133 g, 0.5 mmol) in CH3CN. The
reaction mixture was stirred overnight at ambient temperature
and filtered to remove AgBr. The solvent of the filtrate was
removed in vacuo and the residue washed with diethyl ether and
methanol. trans-anti-7b selectively crystallized from a CH2Cl2
solution. trans-anti-7b þ trans-syn-7b, 0.207 g (0.273 mmol),
54% yield. Anal. Calcd for C28H36Br2N4S2Pd: C, 44.31; H, 4.78;
N, 7.38. Found: C, 44.80; H, 4.90; N, 7.36. 1H NMR (300 MHz,
CDCl3): δ 7.61-6.72 (m, 24 H, Ar-H), 6.01 (s, 4 H, CH2), 5.88
(s, 4 H, CH2), 4.15 (s, 6 H, NCH3), 3.99 (s, 6 H, NCH3), 3.41 (sep,
3J(H,H) = 6.7 Hz, 2 H, CH), 3.27 (sep, 3J(H,H) = 6.6 Hz, 2 H,
CH), 1.34 (d, 3J(H,H) = 6.6 Hz, 12 H, CH3), 1.22 (d, 3J(H,H) =
6.8 Hz, 12 H, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): 169.8
(NCN), 169.7 (NCN), 138.3, 138.2, 134.4, 134.3, 133.2, 133.1,
130.8, 130.6, 128.5, 128.4, 128.1, 128.0 (Ar-C), 122.6, 121.0
(NCH), 51.9 (CH2), 51.8 (CH2), 39.2 (NCH3), 39.1 (NCH3), 38.2
(CH), 38.1 (CH), 23.3 (CH3), 23.2 (CH3). MS (ESI, positive
ions) m/z (%): 679 (100) [M - Br]þ.
3
3
(sep, J(H,H) = 6.6 Hz, 1 H, CH), 1.32 (d, J(H,H) = 6.6 Hz,
6 H, CH3).
Imidazolium Salts 5a and 5b. 4a or 4b (10 mmol) was added to
a solution of 1-methylimidazole (0.80 mL, 10 mmol) in toluene.
The reaction mixture was stirred overnight at 80 °C. An
immiscible layer was formed and the toluene was decanted.
The remaining yellow oil was washed with toluene and diethyl
ether and subsequently dried in vacuo. 5a solidifies in the
refrigerator overnight, giving a white hygroscopic solid, whereas
5b was obtained as viscous yellow oil. 5a, 2.878 g (9.62 mmol),
78% yield. 1H NMR (300 MHz, CDCl3): δ 10.37 (s, 1 H,
NCHN), 7.65 (dd, 3J(H,H) = 7.5 Hz, 4J(H,H) = 1.2 Hz, 1 H,
Ar-H), 7.43-7.22 (m, 5 H, Ar-H þ CHImd), 5.64 (s, 2 H, CH2),
4.10 (s, 3 H, NCH3), 2.50 (s, 3 H, SCH3). 13C{1H} NMR (75.47
MHz, CDCl3): δ 138.4 (NCN), 137.4, 131.2, 130.7, 130.5, 127.0,
126.2 (Ar-C), 123.4, 121.8 (CHImd), 51.1 (CH2), 36.8 (NCH3),
16.1 (SCH3). MS (ESI, positive ions) m/z (%): 219 (100) [M -
Br]þ. 5b, 3.118 g (9.53 mmol), 76% yield. 1H NMR (300 MHz,
CDCl3): δ 10.26 (s, 1 H, NCHN), 7.72 (dd, 3J(H,H) = 7.4 Hz,
4J(H,H) = 1.7 Hz, 1 H, Ar-H), 7.55-7.16 (m, 5 H, Ar-H þ
CHImd), 5.72 (s, 2 H, CH2), 4.10 (s, 3 H, NCH3), 3.40 (sep, 3J(H,
H) = 6.6 Hz, 1 H, CH), 1.29 (d, 3J(H,H) = 6.7 Hz, 6 H, CH3).
13C{1H} NMR (75.47 MHz, CDCl3): δ 137.2 (NCN), 135.6,
134.1, 133.0, 131.3, 130.1, 128.1 (Ar-C), 123.4, 121.7 (CHImd),
51.3 (CH2), 39.4 (NCH3), 36.8 (CH), 22.9 (CH3). MS (ESI,
positive ions) m/z (%): 247 (100) [M - Br]þ.
Benzimidazolium Salt 6a. 4a (0.637 g, 2.93 mmol) was added
to a solution of 1-methylbenzimidazole (0.338 g, 2.56 mmol) in
toluene (5 mL). The reaction mixture was stirred overnight at
90 °C. The white precipitate formed was filtered and washed
with toluene and diethyl ether. The product was dried in vacuo,
giving a white powder. 6a, 0.623 g (1.78 mmol), 70% yield. 1H
NMR (300 MHz, CDCl3): δ 11.25 (s, 1 H, NCHN), 7.69-7.51
(m, 5 H, Ar-H), 7.40-7.31 (m, 2 H, Ar-H), 7.25-7.20 (m, 1 H,
Ar-H), 5.88 (s, 2 H, CH2), 4.31 (s, 3 H, NCH3), 2.51 (s, 3 H,
SCH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 144.4 (NCHN),
138.6, 132.7, 131.8, 131.0, 130.9, 130.8, 128.1, 127.8, 127.8,
127.0, 114.4, 113.4 (Ar-C), 50.0 (CH2), 34.6 (NCH3), 17.2
(SCH3). MS (ESI): m/z = 269 [M - Br]þ.
Benzimidazolium Salt 6b. 4b (0.544 g, 2.22 mmol) was added
to a solution of 1-methylbenzimidazole (0.268 g, 2.03 mmol) in
toluene (5 mL). The reaction mixture was stirred overnight at
90 °C. The volatiles were removed in vacuo, and the viscous
brown mixture obtained was washed with copious amounts of
diethyl ether. Upon standing in diethyl ether, a white precipitate
formed. The diethyl ether was decanted, and the product was
dried in vacuo to obtain a white powder. 6b, 0.536 g (1.42 mmol),
70% yield. 1H NMR (300 MHz, CDCl3): δ 11.34 (s, 1 H,
NCHN), 7.69-7.49 (m, 6 H, Ar-H), 7.35-7.27 (m, 2 H,
Ar-H), 6.01 (s, 2 H, CH2), 4.30 (s, 3 H, NCH3), 3.36 (sep,
3J(H,H) = 6.7 Hz, 1 H, SCH), 1.28 (d, 3J(H,H) = 6.7 Hz, 6 H,
SCH(CH3)2). 13C{1H} NMR (75.47 MHz, CDCl3): δ 144.3
(NCHN), 135.6, 134.8, 134.6, 132.7, 131.7, 130.9, 130.5, 129.1,
127.8, 114.7, 113.3 (Ar-C, due to accidental overlap only 11
signals are observed), 50.1 (CH2), 40.5 (NCH3), 34.5 (SCH),
23.7 (CH3). MS (ESI): m/z = 297 [M - Br]þ.
Complex cis-7a. Pd(OAc)2 (0.112 g, 0.5 mmol) was added to a
solution of 5a (0.299 g, 1 mmol) in CH3CN (60 mL). The
reaction mixture was stirred overnight at 70 °C and then filtered.
The solvent of the filtrate was removed in vacuo, and the residue
was washed several times with CH2Cl2, affording a white solid of
cis-7a. Due to insufficient solubility of cis-7a, the 1H NMR
spectrum shows broad signals and the 13C NMR spectrum could
not be obtained. cis-7a, 0.136 g (0.193 mmol), 38% yield. Anal.
Calcd for C24H28Br2N4S2Pd: C, 41.01; H, 4.02; 7.97. Found: C,
41.10; H, 3.97; N, 8.02. 1H NMR (300 MHz, CD3CN): δ
7.36-6.82 (m, br, 12 H, Ar-H), 5.45 (brs, 4 H, CH2), 3.87
(brs, 6 H, NCH3), 2.55 (brs, 6 H, SCH3). MS (ESI, positive ions)
m/z (%): 623 (100) [M - Br]þ.
Complex cis-8a. A mixture of salt 6a (0.180 g, 0.52 mmol) and
Pd(OAc)2 (0.056 g, 0.25 mmol) was dissolved in DMSO (10 mL)
and stirred overnight at 90 °C. A clear yellow solution was
obtained. The solvent was removed by vacuum distillation, and
the resulting residue was dissolved in CH2Cl2 (15 mL) and
washed with H2O (4 ꢀ 15 mL). Drying of the organic phase
over Na2SO4 followed by removal of the solvent in vacuo gave a
yellow solid. Subsequent washing with diethyl ether produced a
crystalline yellow powder. Slow evaporation of a saturated
acetonitrile solution afforded the product as yellow crystals.
cis-8a, 0.176 g (0.22 mmol), 89% yield. Anal. Calcd for
C32H32Br2N4S2Pd: C, 47.86; H, 4.02; N, 6.98. Found: C,
48.03; H, 4.07; N, 6.90. 1H NMR (300 MHz, CDCl3): δ
7.24-6.88 (m, 12 H, Ar-H), 6.26-6.00 (m, 4 H, Ar-H), 6.07
(d, 2J(H,H) = 17.2 Hz, 2 H, NCH2), 5.67 (d, 2J(H,H) = 17.2 Hz,
2 H, NCH2), 4.08 (s, 6 H, NCH3), 2.62 (s, 6 H, SCH3). 13C{1H}
NMR (75.47 MHz, CDCl3): δ 175.1 (NCN), 136.4, 135.6, 134.1,
131.8, 128.9, 125.6, 125.1, 124.3, 124.3, 111.6, 111.2 (Ar-C),
50.0 (NCH2), 36.5 (NCH3), 16.2 (SCH3). MS (ESI): m/z = 723
[M - Br]þ.
Complex cis-8b. A mixture of salt 6b (0.415 g, 1.10 mmol) and
Pd(OAc)2 (0.112 g, 0.50 mmol) was dissolved in DMSO (15 mL)

1486 Organometallics, Vol. 29, No. 6, 2010
Huynh et al.
and stirred overnight at 90 °C. The light brown solution turned
yellow after a few hours. The reaction mixture was filtered,
and the solvent of the filtrate was removed by vacuum distilla-
tion. The resulting residue was dissolved in CH2Cl2 (30 mL)
and washed with H2O (4 ꢀ 20 mL). Drying of the organic
phase over Na2SO4 followed by removal of the solvent in
vacuo afforded the compound as a yellow powder. The cis-
complex was isolated by column chromatography using a
ethyl acetate/hexane (5:1) eluting solvent mixture (Rf = 0.32).
cis-8b, 0.286 g (0.33 mmol), 67% yield. Anal. Calcd for
C36H40Br2N4S2Pd: C, 50.33; H, 4.69; N, 6.52. Found:
C, 49.98; H, 4.47; N, 6.40. 1H NMR (300 MHz, CDCl3):
δ 7.41 (dd, 3J(H,H) = 7.4 Hz, 4J(H,H) = 0.8 Hz, 2 H, Ar-H),
7.22-7.17 (m, 2 H, Ar-H), 7.11-6.98 (m, 6 H, Ar-H), 6.82
(d, 3J(H,H) = 8.0 Hz, 2 H, Ar-H), 6.31 (t, 3J(H,H) =
7.4 Hz, 2 H, Ar-H), 6.14 (d, 2J(H,H) = 17.0 Hz, 2 H,
NCH2), 6.05 (d, 3J(H,H) = 7.5 Hz, 2 H, Ar-H), 5.78 (d,
2J(H,H) = 17.0 Hz, 2 H, NCH2), 4.07 (s, 6 H, NCH3), 3.55
(sep, 3J(H,H) = 6.6 Hz, 2 H, SCH), 1.42 (d, 3J(H,H) = 6.7 Hz,
6 H, SCH(CH3)), 1.40 (d, 3J(H,H) = 6.6 Hz, 6 H, SCH(CH3)).
13C{1H} NMR (75.47 MHz, CDCl3): δ 174.4 (NCN), 135.0, 134.1,
133.5, 133.4, 130.7, 128.1, 126.1, 125.1, 123.6, 123.5, 110.9, 110.4
(Ar-C), 50.0 (NCH2), 38.1 (NCH3), 35.8 (SCH), 23.1 (SCH3),
22.9 (SCH3). MS (ESI): m/z = 779 [M - Br]þ.
Complex 9. Ag2O (0.116 g, 0.5 mmol) was added to a solution
of 5a (0.299 g, 1 mmol) in CH2Cl2 (5 mL) and stirred overnight
at ambient temperature. The reaction mixture was then filtered
directly into a solution of trans-[PdBr2(CH3CN)2] (0.266 g,
1 mmol) in CH3CN. The reaction mixture was stirred overnight
at ambient temperature and filtered to remove AgBr. The
volatiles were then removed in vacuo. The residue was dissolved
in CH3CN and added to a solution of diethyl ether. The
suspension was filtered, and the residue was washed with ethyl
acetate. Crystals were obtained from the vapor diffusion of
diethyl ether into a DMF solution. The 1H NMR spectrum
showed broad signals that were not well resolved. 13C NMR was
not recorded due to the presence of a complicated product
mixture (vide supra). 9, 0.382 g (0.788 mmol), 79% yield. Anal.
Calcd for C12H14Br2N4SPd: C, 29.75; H, 2.91; N, 5.78. Found:
C, 29.30; H, 2.47; N, 5.40. 1H NMR (300 MHz, CDCl3): δ
7.50-6.54 (m, br, 6 H, Ar-H), 5.93 (brs, 2 H, CH2), 4.04 (brs,
3 H, NCH3), 2.83 (brs, 3 H, SCH3).
5.76 (d, 2J(H,H) = 14.3 Hz, 1 H, CHH), 4.71 (d, 2J(H,H) = 14.3
Hz, 1 H, CHH), 3.61 (s, 3 H, NCH3), 2.42 (s, 3 H, SCH3). 31
P
NMR (121 MHz, CDCl3): δ 27.1 (s, PPh3). 13C{1H} NMR
(75.47 MHz, CDCl3): δ 162.7 (NCN), 137.5 (Ar-C), 134.3 (d,
J(P,C) = 11.0 Hz, Ar-C), 132.3 (Ar-C), 131.2 (d, 1J(P,C) =
35.7, Ar-C), 131.0 (d, 4J(P,C) = 2.7 Hz, Ar-C), 130.2, 129.5
(Ar-C), 128.5 (d, 2/3J(P,C) = 11.0 Hz, Ar-C), 126.7, 126.3,
122.8, 121.3 (Ar-C), 51.4 (CH2), 37.9 (NCH3), 16.4 (SCH3). MS
(ESI, positive ions) m/z (%): 667 (100) [M - Br]þ.
General Procedure for the Suzuki-Miyaura Coupling. In a
typical run, a test tube was charged with a mixture of aryl halide
(1.0 mmol), phenylboronic acid (1.2 mmol), potassium carbo-
nate (2 mmol), precatalyst, and [N(n-C4H9)4]Br (1.5 mmol) (for
entries 11-25 in Table 1). To the mixture was added H2O (3
mL). The reaction mixture was vigorously stirred at the appro-
priate temperature (RT or 85 °C). After the desired reaction
time, the solution was allowed to cool. Then 10 mL of dichloro-
methane was added to the reaction mixture, and the organic
phase was extracted with water (6 ꢀ 5 mL) and dried over
MgSO4. The solvent was removed by evaporation to give a
crude product, which was analyzed by 1H NMR spectroscopy.
X-ray Diffraction Studies. X-ray data were collected with a
Bruker AXS SMART APEX diffractometer, using Mo KR
radiation with the SMART suite of programs.20 Data were
processed and corrected for Lorentz and polarization effects
with SAINT21 and for absorption effect with SADABS.22
Structural solution and refinement were carried out with the
SHELXTL suite of programs.23 The structure was solved by
direct methods to locate the heavy atoms, followed by difference
maps for the light, non-hydrogen atoms. All hydrogen atoms
were put at calculated positions. All non-hydrogen atoms were
generally given anisotropic displacement parameters in the final
model. A summary of the most important crystallographic data
is given in the Supporting Information. CCDC-610655-610657
contain the supplementary crystallographic data for complexes
9, 10, and cis-7a 2DMF.12 These data can be obtained free of
3
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgment. We thank the National University
of Singapore for financial support (Grant No. R 143-000-
327-133) and the CMMAC staff of our department for
technical assistance.
Complex 10. PPh3 (0.077 g, 0.29 mmol) was added to a
solution of 9 (0.129 g, 0.27 mmol) in CH3CN (5 mL). The
reaction mixture was stirred for 0.5 h at ambient temperature.
The volatiles were then removed in vacuo. The residue was
dissolved in CH2Cl2 and the mixture was filtered. The solvent
of the filtrate was removed and the residue washed with small
amounts of acetone. Upon drying in vacuo, complex 10 was
isolated as a yellow powder. 10, 0.129 g (0.173 mmol), 64%
yield. Anal. Calcd for C30H29Br2N2PSPd: C, 48.25; H, 3.91; N,
Supporting Information Available: Crystallographic data for
cis-7a 2DMF, trans-anti-7b, cis-8a, trans-anti-8b, 9, and 10 as
3
CIF files. This material is available free of charge via the Internet
at http://pubs.acs.org.
(20) SMART version 5.628; Bruker AXS Inc.: Madison, WI, 2001.
(21) SAINTþ version 6.22a; Bruker AXS Inc.: Madison, WI, 2001.
1
3.75. Found: C, 48.30; H, 3.97; N, 3.69. H NMR (300 MHz,
€
(22) Sheldrick, G. W. SADABS version 2.10; University of Gottingen,
3
CDCl3): δ 7.66-7.05 (m, 19 H, Ar-H), 6.56 (d, J(H,H) =
2001.
(23) SHELXTL version 6.14; Bruker AXS Inc.: Madison, WI, 2000.
1.9 Hz, 1 H, CHImd), 6.40 (d, 3J(H,H) = 1.9 Hz, 1 H, CHImd),