Active catalysts for the Suzuki coupling: Palladium complexes of tetrahydropyrimid-2-ylidenes

Article abstract of DOI:10.1016/j.molcata.2005.08.046

An alternative way to synthesize a mixed N-heterocyclic carbenes (NHC) phosphine palladium(II) complex is described in this work. A free carbene, generated by deprotonation of a tetrahydro-pyrimidinium salt, was reacted with Pd(PPh₃)₂Cl₂ to yield the desired mixed NHC phosphine palladium(II) complex. In this work, we have combined the superior activity of mixed NHC phosphine palladium complexes in Suzuki-coupling reactions with the additional advantage of a stronger σ-donating ligand. The combination of these two effects results in an increased catalytic activity in Suzuki reactions than found for corresponding mixed imidazole-ylidene phosphine complexes. Using this new class of catalysts, TONs of approximately 1 Mio. after a reaction time of 14 h can be achieved. Desactivated bromoarenes could also be coupled efficiently, using quite a low amount of catalyst (0.005 mol%). Even aryl chlorides can be coupled: TONs of approximately 6000 can be reached after only 14 h without any detectable catalyst deterioration using only 0.01 mol% of catalyst.

Full text of DOI:10.1016/j.molcata.2005.08.046

Journal of Molecular Catalysis A: Chemical 245 (2006) 248–254
Active catalysts for the Suzuki coupling: Palladium complexes
of tetrahydropyrimid-2-ylidenesଝ
Sabine K. Schneider, Wolfgang A. Herrmann∗, Eberhardt Herdtweck
Anorganisch-chemisches Institut der Technischen Universita¨t Mu¨nchen, D-85747 Garching bei Mu¨nchen, Germany
Received 19 May 2005; received in revised form 23 August 2005; accepted 30 August 2005
Available online 9 November 2005
Abstract
An alternative way to synthesize a mixed N-heterocyclic carbenes (NHC) phosphine palladium(II) complex is described in this work. A free
carbene, generated by deprotonation of a tetrahydro-pyrimidinium salt, was reacted with Pd(PPh3)2Cl2 to yield the desired mixed NHC phosphine
palladium(II) complex.
In this work, we have combined the superior activity of mixed NHC phosphine palladium complexes in Suzuki-coupling reactions with the
additional advantage of a stronger ␴-donating ligand. The combination of these two effects results in an increased catalytic activity in Suzuki
reactions than found for corresponding mixed imidazole–ylidene phosphine complexes.
Using this new class of catalysts, TONs of approximately 1 Mio. after a reaction time of 14 h can be achieved. Desactivated bromoarenes could
also be coupled efficiently, using quite a low amount of catalyst (0.005 mol%). Even aryl chlorides can be coupled: TONs of approximately 6000
can be reached after only 14 h without any detectable catalyst deterioration using only 0.01 mol% of catalyst.
© 2005 Elsevier B.V. All rights reserved.
Keywords: N-heterocyclic carbene; Tetrahydropyrimid-2-ylidene; Palladium; Suzuki coupling; Carbon–carbon bond formation
1. Introduction
ity of palladium in various reactions including C,C-coupling has
been established [1]. Despite their activity as potential catalysts,
these complexes suffer from easy ligand dissociation, resulting
in deactivation [1b]. There have been many efforts to avoid this
unwanted degradation processes [7].
Herrmann et al. have repeatedly shown that the replacement
of a phosphine ligand by a stronger ␴-donating NHC ligand
alters the electron density on the palladium centre, thus facili-
tating the halogen aryl activation [5a]. Therefore, mixed NHC
phosphine complexes of palladium proved to be more efficient
than bis(carbene) or bis(phosphine) complexes in C,C-coupling
catalysis, especially in the Suzuki–Miyaura reaction [8]. To
maximize this effect, much interest is presently dedicated to
novel, strongly electron-donating N-heterocyclic carbene lig-
ands.
The acyclic bis(dialkylamino)carbenes published by Alder
et al. [9] are the strongest ␴-donating carbene ligands known
so far [10]. However, their metal complexes suffer from
peculiar deactivation phenomena [10]. For this reason, the
six-membered tetrahydropyrimid-2-ylidenes were investigated
since they expect to be comparable in their ␴-donor strength
[10], but may form more stable metal complexes.
Palladium complexes have widely been used as catalysts for
carbon–carbon bond forming reactions [1]. These reactions are
key steps in many syntheses of organic chemicals, natural prod-
ucts, aswellasinavarietyofindustrialprocesses [1]. Oneimpor-
tant example of this type of catalysis is the Suzuki–Miyaura
reaction [2].
Palladium(II) complexes of N-heterocyclic carbenes (NHC)
are well known, and various routes to obtain them have been
published [3]. Often times they are thermally stabile and dis-
play high dissociation energies regarding the Pd–NHC bond [4].
These complexes have been successfully applied to Heck-type
C,C-coupling reactions [5].
Phosphine complexes of palladium(II) are even better known
and easily prepared from palladium(II) salts and an excess of
phosphine ligand [6]. With these complexes, the catalytic activ-
ଝ
N-heterocyclic carbenes, 41. Mitteilung. – 40. Mitteilung: Lit. [16].
Corresponding author. Tel.: +49 89 289 13080; fax: +49 89 289 13473.
∗
E-mail address: lit@arthur.anorg.chemie.tu-muenchen.de
(W.A. Herrmann).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.08.046

S.K. Schneider et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 248–254
249
13C{1H} NMR (67.9 MHz, CDCl3): δ = 182.1 (t, J = 5.6 Hz,
NCN), 134.7–128.9 (m, ArC), 60.2 (NCHMe2), 40.8 (NCH2),
29.7 (CH3), 19.2 (CCH2C).
It could be shown in literature that ruthenium complexes with
isopropyl and mesityl substituted tetrahydropyrimid-2-ylidene
ligands are active catalysts in olefin metathesis. Grubbs-type cat-
alysts [11] as well as Grubbs–Hoveyda-type catalysts [12] were
reported in this regard. Also certain silver(I)–carbene complexes
with this particular ligand were recently reported [13].
In the present paper, we report that tetrahydropyrimid-2-
ylidene ligands form exceptionally active palladium(II) catalysts
for the Suzuki–Miyaura reaction.
31P{1H} NMR (109.4 MHz, CDCl3): δ = 22.2 (s).
MS (FAB-MS): m/z (relative intensity) 833 [M]+ (5); 572
[M − PPh3]+ (12); 536 [M – PPh3−Cl]+ (4).
2.3. cis-Dichloro(1,3-diisopropyltetrahydropyrimid-2-
ylidene)(triphenylphosphine)
palladium(II) 4
2. Experimental part
200 mg (0.23 mmol) of 3 were dissolved in 40 ml of toluene
and stirred for 2 h at 120 ◦C yielding a white precipitate. The
precipitated product was filtered off and was washed with 25 ml
of toluene. Drying in vacuo affords 129.7 mg (yield: 93%) of a
colourless solid, which could be identified as 4.
2.1. General comments
NMR spectra (1H, 13C) were recorded either on a Jeol JMX-
GX 400, on a Jeol JMX-GX 270 or on a Bruker DPX 400
instrument. Chemical shifts are given in ppm. The spectra are
calibrated to the residual protons of the solvent (1H) or to
the 13-carbon signals of the solvent (13C). NMR multiplicities
are abbreviated as follows: s = singlet, d = doublet, q = quintet,
m = multiplet. Coupling constants J are given in Hz. FAB-MS
1H NMR (399.8 MHz, CDCl3): δ = 7.96–7.35 (m, 15H, ArH),
5.84 (septett, 2H, J = 7.2 Hz, NCHMe2), 3.21 (m, 2H, NCH2),
2.71 (m, 2H, NCH2), 1.73 (m, 2H, CCH2C), 1.56 (d, 6H, CH3),
0.43 (d, 6H, CH3).
13C{1H} NMR (100.5 MHz, CDCl3): δ = 183.6 (d, J = 28.8 Hz,
NCN), 135.2–128.5 (m, ArC), 59.5, 59.2, 29.6, 19.7, 18.8.
31P{1H} NMR (161.8 MHz, CDCl3): δ = 26.6 (s).
MS (FAB-MS): m/z (relative intensity) 571 [M − Cl]+ (9); 535
[M − 2Cl]+ (3).
¨
spectra were measured at the TU Munchen Mass Spectrometry
Laboratory on a Finnigan MAT 90 mass spectrometer (xenon/p-
nitrobenzyl alcohol). Elemental analyses were carried out by
the Microanalytical Laboratory at the TU Munchen. GC spectra
were measured on a Hewlett-Packard gas chromatograph GC
6890 A equipped with a FID.
¨
Unless otherwise stated, all manipulations were carried out
using standard Schlenk techniques. All solvents for use in an
inert atmosphere were purified by standard procedures and dis-
tilled under nitrogen immediately prior to use. Other chemicals
were obtained from commercial sources and used without fur-
ther purification.
Anal. Calcd. for 4 × C28H35Cl2N2PPd + 1 × CHCl3: C,
53.20; H, 5.57; N, 4.39. Found: C, 53.14; H, 5.31; N, 4.14.
2.4. Suzuki reaction
A total of 1.2 equivalents of phenylboronic acid (293 mg,
2.4 mmol) and 1.5 equivalents of potassium carbonate (415 mg,
3 mmol) are placed in a Schlenk tube equipped with a stirring
bar. The vessel is put under an argon atmosphere and 1.0 equiva-
lentofarylhalide(2.0 mmol, e.g. 374 mgbromoanisole), 100 mg
diethyleneglycol-di-n-butylether and 2 ml degassed xylene are
added. After thermostating at 130 ◦C for 10 min, the required
amount of the catalyst stock solution is added against a positive
stream of argon. To finish the reaction, the mixture is allowed
to cool to room temperature and 3 ml of water are added. The
water phase is extracted three times with 2 ml of diethylether and
the organic phases are dried with MgSO4. The reaction products
were identified and quantified by a Hewlett-Packard gas chro-
matograph GC 6890 A equipped with an FID detector, using
diethylenglykole-di-n-butylether as internal standard.
2.2. trans-Chloro(1,3-diisopropyltetrahydropyrimid-2-
ylidene)bis(triphenylphosphine)palladium(II)-
chloride 3
In 30 ml of THF 250 mg (0.97 mmol, 1 eq.) 1, 43 mg
(0.38 mmol, 0.4 eq.) KOtBu and 425 mg (17.7 mmol, 18.2 eq.)
NaH were mixed and stirred for 2 h at room temperature. The
formed free carbene was extracted with 15 ml n-hexane, which
after removal of the solvent, remains as a colourless oil. This oil
is solved in 15 ml of dry THF, and then canulated to a suspension
of 375 mg (0.35 mmol) trans-Pd(PPh3)2Cl2 in THF.
After stirring for 3 h, the solvent was removed in vacuo and
the residue was washed with 50 ml of hexane. The crude prod-
uct is extracted with 50 ml of acetonitrile. After removal of the
solvent, the crude product is solved in dichloromethane and
precipitated with pentane. Product: colourless powder: 270 mg
(yield: 89%).
2.4.1. Catalyst stock solution
0.02 mmol of catalyst were solved (3 or 4) in 10 ml of
dimethylacetamide (DMAc) and stored in the freezer. The
concentration was chosen in order to make sure that for our
experimental setup 0.1 ml of the stock solution equals a cata-
lyst/substrate ratio of 0.01 mol% catalyst. For the experiments
withextremelylowcatalystconcentrations, theinitialstocksolu-
tion was further diluted with DMAc.
1H NMR (270.2 MHz, CDCl3): δ = 7.70–7.04 (m, 30H, ArH),
5.35 (septett, 2H, J = 7.0 Hz, NCHMe2), 3.14 (t, 4H, J = 5.6 Hz,
NCH2), 2.20 (quintet, 2H, J = 5.6 Hz, CCH2C), 0.42 (d, 12H,
J = 7.0 Hz, CH3).

250
S.K. Schneider et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 248–254
2.5. Crystallography
be split using a phosphine to yield the desired product. Unfor-
tunately, the well-known reaction to synthesize imidazolylidene
palladium(II) complexes by treating an imidazolium salt with
Pd(OAc)2, KOtBu and NaI in acetonitrile [8] failed when it was
applied to the saturated 6-membered tetrahydropyrimidinium
salts, probably due to the low acidity of the azolium-H. The
reaction of the free carbene 2, derived according to Alder et
al. by reacting the tetrahydropyrimidinium salt with NaH and
catalytic amounts of KOtBu [15] with Pd(PPh3)2Cl2 in THF,
affordedcompound3. Thiscompoundcouldthenbetransformed
to compound 4 by refluxing a toluene solution of 3 (Scheme 1).
Compound 3 already contains traces of 4. The latter complex is
already formed during the reaction of the free carbene 2 with
the palladium precursor.
The reaction of 2 with common Pd(II) precursors like
Pd(CH3CN)2Cl2 failed, as the free carbenes of the six-
membered ring carbenes are strong reducing agents as compared
to their unsaturated, five-membered ring analogues [11]. For
example, the Pd(CH3CN)2Cl2 was reduced to palladium black
upon addition of the free carbene to a suspension of the palla-
dium(II) precursor in THF.
2.5.1. Crystal structure analysis of compound 4
C28H35Cl2N2PPd, 3(CHCl3), Mr = 965.97, colourless
plate (0.25 mm × 0.51 mm × 0.81 mm), orthorhombic, Pca21
˚
(No.: 29), a = 19.3191(2), b = 11.2720(1), c = 38.2844(4) A,
3
V = 8337.00(14) A , Z = 8, dcalc = 1.539 g cm−3, F0 0 0 = 3888,
˚
µ = 1.214 mm−1. Preliminary examination and data collection
were carried out on a kappa-CCD device (NONIUS MACH3)
with an Oxford Cryosystems cooling device at the window of
a rotating anode (NONIUS FR591) with graphite monochro-
˚
mated Mo K␣ radiation (λ = 0.71073 A). Data collection were
performed at 123 K within the Θ range of 1.06◦ < Θ < 25.35◦.
A total of 108,810 reflections were integrated, corrected for
Lorentz, polarization, and arising from the scaling procedure,
corrected for latent decay and absorption effects. After merging
(Rint = 0.028), 14,593 [13,126: Io > 2σ(Io)] independent reflec-
tions remained and all were used to refine 838 parameters. The
structure was solved by a combination of direct methods and
difference-Fourier syntheses. All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen
atoms were placed in ideal positions (riding model). Full-matrix
Complexes 3 and 4 are soluble in polar solvents like
dichloromethane or chloroform. However, they are insoluble in
non-polar solvents like n-pentane, n-hexane and diethylether.
Compound 3 is soluble in hot toluene and 4 is insoluble.
4 can be obtained after recrystallization from chloroform/n-
pentane as colourless plates. The molecular structure of 4 was
determined by X-ray diffraction techniques.
least-squares refinements were carried out by minimizing
ꢀ
w(F2 − Fc2)2 and converged with R1 = 0.0282 [Io > 2σ(Io)],
wR2 =o0.0662 [all data], GOF = 1.028 and shift/error < 0.001.
The final difference-Fourier map shows no striking features
(ꢀemin/max = +0.71/−0.51 e A−3). The unit cell contains two
˚
crystallographically independent molecules A and B and six
1
All compounds were identified by H and 13C NMR spec-
solvent molecules CHCl3 [17].
troscopy, elemental analysis and FAB-MS.
The carbene carbon signal of 3 appears at 182.1 ppm as a
triplet with a coupling constant of ca. 5.6 Hz, indicating two
phosphine groups in a trans arrangement. The two phosphine
groups, are thus equivalent and show a single resonance at
22.2 ppm. The protons of the isopropyl groups also give rise
to only one set of signals, as expected for a trans arrangement
of the two-phosphine ligands. The carbene carbon signal of 4
appears at 183.6 ppm as doublet with a coupling constant of
about 28.8 Hz indicating only one phosphine group. The 31P
NMR spectrum contains only one resonance with a chemical
shift of 26.6 ppm. The protons of the isopropyl groups show two
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of the tetrahydropyrimidinium salt 1 is
achieved according to Saba et al. [14] by reaction of N,N-
diisopropylpropane diamine with triethyl orthoformiate and
ammonium tetrafluoroborate. In order to make a mixed phos-
phine carbene complex, a halobridged derivative according to
Herrmann et al. [8] had to be prepared, which can subsequently
Scheme 1. Synthetic procedure; Ph = C6H5, iPr = i-C3H7, tBu = t-C4H9.

S.K. Schneider et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 248–254
251
Table 1
different sets of signals, also indicating the cis arrangement of
◦
˚
Selected bond lengths [A] and bond angles [ ] in 4 and 5 [X = Cl (A,B), X = I
(5)]
the phosphine and the carbene ligand. This cis arrangement is
also in line with the X-ray crystal structure of complex 4.
For the Pd(II) carbene complex 3, the most prominent peak
in the mass spectrum is the [M − PPh3]+ fragment. The molec-
ular peak [M]+ with m/z = 571 is also present. A third peak at
535 [M − PPh3−Cl]+ indicates the molecular structure of 3. The
peaks at 571 [M − Cl]+ and at 535 [M − 2Cl]+ are also detected
in the FAB-MS of 4. The molecular peak is missing which is in
line with the fact, that 4 is neutral, and therefore the loss of an
anion is necessary to reach a fragment which is detectable in the
FAB-MS. In both spectra (for 3 and 4), the isotope patterns are
in accord with palladium carbene fragments.
4·3CHCl3
5 [8]
A
B
Pd CCarbene
Pd
2.003(4)
2.2648(9)
2.3660(8)
2.3649(9)
1.339(5)
1.328(5)
1.999(4)
2.2642(9)
2.3660(8)
2.3647(8)
1.338(5)
1.338(5)
2.031(4)
P
2.334(1)
2.6325(8)
2.6175(8)
1.362
Pd X1
Pd X2
N1 CCarbene
N2 CCarbene
1.361
CCarbene Pd X1
CCarbene Pd X2
CCarbene Pd
X2 Pd
N1 CCarbene N2
CR1 N1 CCarbene
CR2 N2 CCarbene
176.60(10)
87.25(10)
92.09(10)
179.33(3)
120.5(3)
176.45(10)
86.92(10)
92.40(10)
179.18(3)
120.1(3)
86.43
89.81
175.5
90.19
105.7(3)
130.4
P
P
3.2. X-ray crystal structure of the six-membered
N-heterocyclic carbene complex 4
121.4(3)
121.0(3)
120.8(3)
121.1(3)
130.6
The molecular structure of the square-planar Pd(II) complex
4 in the solid state is shown in Fig. 1. Selected bond lengths
and angles are depicted in Table 1. To our knowledge, this is
the first X-ray structure determination of a mixed phosphine
tetrahydropyrimid-2-ylidene palladium(II) complex. It is espe-
cially noteworthy to mention that 4 is one of the very few cases of
mixed carbene–phosphine complexes with a cis-arrangement of
the two ligands. In most of the known imidazol-based exam-
ples, the phosphine and the carbene are in trans position to
each other [8]. Another example of such an arrangement is
a corresponding dimethylpyrazol-2-ylidene palladium(II) com-
plex recently reported by our group [16]. The cis-structure of
4 is in accord with the solution-phase geometry, cf. the above
NMR data.
five-membered to the six-membered ring geometry causes a
widening of the NCcarbeneN-bond angle from ca. 106◦ in 5
to 120.5◦ in 4. This results in a compression of the ␣-angle
CRNCcarbene from 131◦ in the imidazole complex [8] to 121◦
˚
in 4 (Table 1; Fig. 2). The Ccarbene–Pd distance of 2.00 A in 4
is slightly shorter than the one reported for the corresponding
˚
imidazol-2-ylidene compound (2.03 A) (Table 1; [8]).
3.3. Catalysis
Phosphine-based systems and NHC-based complexes are
among the most active catalysts for the Suzuki–Miyaura reac-
tion. Despite the high activities reported for phosphine-based
systems [1], NHC-based catalysts are much more stable under
The complexes 4 and 5 (Fig. 2) enable us to compare
the structural differences between imidazolin-2-ylidene and
tetrahydropyrimid-2-ylidene complexes. The transition from the
Fig. 1. ORTEP style plot of molecule A in the solid state of compound 4. Thermal ellipsoids are drawn at the 50% probability level.

252
S.K. Schneider et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 248–254
Fig. 2. NHC-complexes for comparison.
the relevant reaction conditions; they also can be recycled and
reused. Moreover, higher reaction temperatures are an option of
the NHC-based catalysts.
The data obtained for the Suzuki–Miyaura reaction of aryl
bromides andchlorides withphenylboronic acid are summarized
inTable2. Theresultsshowthat4isindeedaanexcellentcatalyst
forthecouplingofactivatedaswellasdeactivatedarylbromides;
turnover numbers of 1 Mio. can be reached after 14 h (entries 3
and 4, Table 2).
Deactivated bromoarenes can also be coupled with low
amounts of catalysts. The optimum concentration of catalyst
was minimized in this case to 0.005 mol% (entry 11), this is
unexpected in case of deactivated bromoarenes [5a].
Even aryl chlorides can be coupled with only 0.01 mol%
of catalyst 4 (entry 21, Table 2) no catalyst deterioration is
observed. TONs of 6000 after only 14 h can be reached.
Catalyst 4 reveals to be superior to its congener 3. How-
ever, the differences are not dramatic. The difference is possibly
caused by the lower stability of 3 under the catalytic conditions
applied. Pd black occurred faster when 3 instead of 4 was used
as catalyst.
It is known that the use of alkylphosphines instead of
arylphosphines strongly enhances the activity of such mixed
NHC phosphine catalysts, especially in the Suzuki reaction
[8,10]. We expect the tricyclohexylphosphine derivative to
give even more improved activities. Experiments are under
way.
To put the results in a proper context, time conversion experi-
ments were performed to compare the activity of 4 with the cor-
responding imidazolylidene palladium(II) complex 6 (Fig. 2),
which was generated according to Herrmann et al. [8].
In order to evaluate the differences in activity, the coupling
of 4-chloroacetophenone with phenylboronic acid was followed
over time (Fig. 3).
It can be seen that 4 is more active than 6. It is remarkable that
the initial activity of 4 is much higher than for 6, cf. Figs. 2 and 3.
There is no induction period, thus indicating a quick formation
of the catalytically active Pd0 species. Complex 6 clearly shows
a lower starting activity combined with a weaker overall perfor-
mance. This is probably due to the stronger ␴-donating ligand
present in 4, which might help in the reduction step forming
the active species. We explain the superior performance of 4 as
Fig. 3. Time-conversion diagram of the Suzuki–Miyaura reaction.

S.K. Schneider et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 248–254
253
Table 2
Suzuki–Miyaura coupling
References
[1] (a) J. Tsuji, Palladium Reagents and Catalysts, Wiley, Chichester, 1995;
(b) W.A. Herrmann, in: B. Cornils, W.A. Herrmann (Eds.), Applied
Homogeneous Catalysis with Organometallic Compounds, second ed.,
VCH, Weinheim, 2002.
[2] (a) N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457;
(b) A. Suzuki, Pure Appl. Chem. 63 (1991) 419.
[3] (a) W.A. Herrmann, Angew. Chem. 114 (2002) 1342;
W.A. Herrmann, Angew. Chem. Int. Ed. Engl. 41 (2002) 1290;
(b) W.A. Herrmann, C. Ko¨cher, Angew. Chem. 109 (1997) 2256;
W.A. Herrmann, C. Ko¨cher, Angew. Chem. Int. Ed. Engl. 36 (1997)
2162;
Entry
R
X
Pd (mol%) Catalyst Yield (%) TON
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
C(O)CH3 Br 0.01
C(O)CH3 Br 0.001
C(O)CH3 Br 0.0001
C(O)CH3 Br 10−5
C(O)CH3 Br 0.001
C(O)CH3 Br 0.0001
H
H
H
H
4
4
4
4
3
3
4
4
4
4
3
3
4
4
4
4
4
3
3
4
4
3
100
100
100
10
100
63
100
100
100
3
100
80
91
80
89
63
100
88
85
87
60
104
105
106
106
(c) W.A. Herrmann, M. Elison, J. Fischer, C. Ko¨cher, Chem. Eur. J. 2
(1996) 772;
(d) W.A. Herrmann, J. Schwarz, M.G. Gardiner, M. Spiegler, J.
Organomet. Chem. 575 (1999) 80.
105
6.3 × 105
Br 0.01
Br 0.001
Br 0.0001
Br 10−5
Br 0.001
Br 0.0001
Br 0.1
Br 0.01
Br 0.005
Br 0.001
Br 0.01b
Br 0.1
104
105
[4] J. Schwarz, V.P.W. Bo¨hm, M.G. Gardiner, M. Grosche, W.A. Herrmann,
W. Hieringer, G. Raudaschl-Sieber, Chem. Eur. J. 6 (2000) 1773.
[5] (a) W.A. Herrmann, M. Elison, J. Fischer, C. Ko¨cher, G.R.J. Artus,
Angew. Chem. 107 (1995) 2602;
106
3 × 105
H
H
105
8 × 105
9.1 × 102
8 × 103
1.78 × 104
6.3 × 104
104
W.A. Herrmann, M. Elison, J. Fischer, C. Ko¨cher, G.R.J. Artus, Angew.
Chem. Int. Ed. Engl. 34 (1995) 2371;
OMe
OMe
OMe
OMe
OMe
OMe
OMe
¨
(b) W.A. Herrmann, K. Ofele, D.v. Preysing, S.K. Schneider, J.
Organomet. Chem. 687 (2003) 229;
(c) D. Enders, H. Gielen, G. Raabe, J. Runsink, J.H. Teles, Chem. Ber.
129 (1996) 1483;
8.8 × 103
8.5 × 104
8.7 × 103
6 × 104
6.5 × 103
(d) D.S. McGuinness, M.J. Green, K.J. Cavell, B.W. Skelton, A.H.
White, J. Organomet. Chem. 572 (1999) 239.
[6] F. Ozawa, in: S. Komiya (Ed.), Synthesis of Organometallic Compounds,
Wiley, Sussex, 1997, p. 249 ff.
Br 0.01
C(O)CH3 Cl 0.1a
C(O)CH3 Cl 0.01a
C(O)CH3 Cl 0.1a
[7] Examples;
65
¨
(a) M. Beller, H. Fischer, W.A. Herrmann, K. Ofele, C. Broßmer, Angew.
a
Cs2CO3 as the base.
Chem. 107 (1995) 1992;
M. Beller, H. Fischer, W.A. Herrmann, K. Ofele, C. Broßmer, Angew.
¨
Chem. Int. Ed. Engl. 34 (1995) 1848;
compared to 6 with the stronger ␴-donating properties of the
tetrahydropyrimid-2-ylidene ligands.
(b) A.F. Littke, G.C. Fu, Angew. Chem. 110 (1998) 3586;
A.F. Littke, G.C. Fu, Angew. Chem. Int. Ed. Engl. 37 (1998) 3387;
(c) D.W. Old, J.P. Wolfe, S.L. Buchwald, J. Am. Chem. Soc. 120 (1998)
9722.
4. Conclusion
[8] W.A. Herrmann, V.P.W. Bo¨hm, C.W.K. Gsto¨ttmayer, M. Grosche, C.-P.
Reisinger, T. Weskamp, J. Organomet. Chem. 617–618 (2001) 616.
[9] R.W. Alder, P.R. Allen, M. Murray, A.G. Orpen, Angew. Chem. Int. Ed.
Engl. 35 (1996) 1121.
Tetrahydropyrimid-2-ylidenes form excellent palladium cat-
alysts for the Suzuki coupling, particularly when the N-
heterocyclic carbenes are combined with phosphines at the
metal centre. Especially remarkable is the fact that even chloro
arenes work well with the new catalysts, with concentrations
of the latter in the order of 0.01 mol% being fully sufficient.
A further optimization of NHC-derived C,C-coupling catalysts
seems possible. As already mentioned above, this optimiza-
tion could probably be achieved by employing catalysts bearing
stronger basic phosphanes, e.g. tricyclohexylphosphane in the
reported Suzuki coupling. Different palladium precursors like
Pd(PCy3)2Cl2 have to be investigated in this regard. Also the
variation of the N-substituent in the carbene ligand may alter
the activity of these catalysts in a positive way too. Active work
in this area is on the way.
¨
[10] W.A. Herrmann, K. Ofele, D.v. Preysing, E. Herdtweck, J. Organomet.
Chem. 684 (2003) 235.
[11] (a) M. Sakamoto, S. Okada, Y. Tsunokai, S. Ikeda, W.A. Herrmann, K.
¨
Ofele, Nippon Zeon Co. Ltd, JP 2003089689, 2003;
¨
(b) W.A. Herrmann, K. Ofele, S. Okada, S. Ikeda, M. Sakamoto, Y.
Tsunokai, WO 03027079, 2005;
(c) J. Yun, E.R. Marinez, R.H. Grubbs, Organometallics 23 (2004) 4172.
[12] L. Yang, M. Mayr, K. Wurst, M.R. Buchmeiser, Chem. Eur. J. 10 (2004)
5761.
¨
[13] (a) W.A. Herrmann, S.K. Schneider, K. Ofele, M. Sakamoto, E.
Herdtweck, J. Organomet. Chem. 689 (2004) 2441;
(b) M. Mayr, K. Wurst, K.-H. Ongania, M.R. Buchmeiser, Chem. Eur.
J. 10 (2004) 1256;
(c) M. Mayr, M.R. Buchmeiser, Macromol. Rapid Commun. 25 (2004)
231.
[14] S. Saba, A.-M. Brescia, M.K. Kaloustian, Tetrahedron Lett. 32 (1991)
5031.
[15] R.W. Alder, M.E. Blake, C. Bortolotti, S. Bufali, C.P. Butts, E. Linehan,
J.M. Oliva, A.G. Orpen, M.J. Quale, Chem. Commun. (1999) 241.
[16] G.D. Frey, W.A. Herrmann, J. Organomet. Chem. 690 (2005), in
press.
[17] (a) Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication no. CCDC-281521
Acknowledgment
This work was generously supported by the Fonds der
Chemischen Industrie (studentship for S.K.S.) and the Deutsche
Forschungsgemeinschaft. The authors thank Dr. Karl Ofele for
¨
helpful discussions.

254
S.K. Schneider et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 248–254
(4). Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336
033; e-mail: deposit@ccdc.cam.ac.uk);
(e) International Tables for Crystallography, vol. C, Tables 6.1.1.4,
4.2.6.8 and 4.2.4.2, in: A.J.C., Wilson, (Ed.), Kluwer Academic Pub-
lishers, Dordrecht, The Netherlands, 1992;
(b) Data Collection Software for Nonius kappa-CCD devices, Delft, The
Netherlands, 2001;
(f) A.L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht
University, Utrecht, The Netherlands, 2001;
(c) Z. Otwinowski, W. Minor, Methods in Enzymology 276 (1997) 307ff;
(d) A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M.C.
Burla, G. Polidori, M. Camalli, SIR92, J. Appl. Crystallogr. 27 (1994)
435;
(g) G. M. Sheldrick, SHELXL-97, Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1998.