Synthesis and catalytic application of palladium pyrazolin-3-ylidene complexes

Article abstract of DOI:10.1021/om0401096

Oxidative addition of a 3-halo 1,2-disubstituted pyrazolium salt to a palladium(0) complex leads to the formation of the first reported palladium pyrazolin-3-ylidene complex. Comparison of the new system with the analogous imidazolin-2-ylidene complex in Heck catalysis shows higher yields for the new system by a factor of 2.

Full text of DOI:10.1021/om0401096

6084
Organometallics 2004, 23, 6084-6086
Synthesis and Catalytic Application of Palladium
Pyrazolin-3-ylidene Complexes†
Jan Schu¨tz, Eberhardt Herdtweck, and Wolfgang A. Herrmann*
Anorganisch-chemisches Institut der Technischen Universita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85747 Garching bei Mu¨nchen, Germany
Received August 18, 2004
Summary: Oxidative addition of a 3-halo 1,2-disubsti-
tuted pyrazolium salt to a palladium(0) complex leads
to the formation of the first reported palladium pyrazo-
lin-3-ylidene complex. Comparison of the new system
with the analogous imidazolin-2-ylidene complex in Heck
catalysis shows higher yields for the new system by a
factor of 2.
Figure 1. Iododicarbonyl(azolinylidene)rhodium(I) com-
plexes 1b and 2b for comparison of their CO stretching
frequencies.
The Mizoroki-Heck reaction has proven to be a
versatile tool in organic synthesis that involves the
coupling of haloarenes and olefins.1 Recent develop-
ments have led to catalysts of mixed N-heterocyclic
carbene (NHC)/phosphine complexes of palladium(II).2
These catalysts are more easily reduced during catalysis
than bis-NHC complexes, while the Pd(0) complexes
generated undergo oxidative addition with aryl halides
less easily. NHC ligands are strong coordinating ligands
which undergo little to no dissociation from the metal
center in solution.3 The increased electron density
compared to phosphines on the palladium center pro-
vides an easier activation of the halogen-aryl bonding.
Most NHC palladium Mizoroki-Heck catalysts are
based on imidazolin-2-ylidenes or 1,2,4-triazolin-3-
ylidenes.4 Therefore, it seems appropriate to develop
alternative NHC ligands which promise to be stronger
σ-donors than the conventional imidazoline or triazoline
ligands. For this purpose a suitable carbene is pyrazolin-
3-ylidene. In a theoretical work it has been shown that
these carbenes are stronger σ-donors than imidazolin-
2-ylidenes.5 So far the free pyrazolin-3-ylidene has not
yet been reported. The difficulty in obtaining the free
carbene might derive solely from inductive effects,
because the carbene center in pyrazolin-3-ylidenes is
attached to just one electron-withdrawing N atom, in
contrast to imidazole or triazole systems with electron-
withdrawing N atoms on each side of the carbene
carbon. The difficulty in obtaining the free pyrazolin-
3-ylidene is consistent with the lower C-H acidity of
pyrazolium salts compared to imidazolium salts.6 So far
research has concentrated typically on imidazolin-2-
ylidenes or imidazolidin-2-ylidenes, and surprisingly the
pyrazolin-3-ylidenes have not attracted much interest.
The only known pyrazolin-3-ylidene complexes up
to now have been reported by O¨ fele et al.7 and much
later by Herrmann and co-workers.8 The syntheses of
carbene-carbonyl metal complexes by O¨ fele are only
applicable to carbonyl compounds and are not suitable
to obtain catalytic Heck-active pyrazolin-3-ylidene com-
plexes. Although the method described by Herrmann
which requires an alkoxide as a base to deprotonate the
pyrazolium salt is widely applicable for many imidazo-
lium salts, the conditions used to generate the carbene
seem only to be successful for complex 1a. All attempts
to apply this method to different metal centers or
different pyrazolium salts were unsuccessful.
Iodo(η4-1,5-COD)(1,2-dimethylpyrazoline-5-ylidene)-
rhodium(I) (1a) has been synthesized in order to obtain
the corresponding dicarbonyl complex 1b (Figure 1). The
CO stretching frequencies are sensitive to the electron
density at the metal. Hence, comparison of the CO
stretching frequencies of complex 1b with those from
the analogous imidazolin-2-ylidene complex 2b allow us
to evaluate the donor strength of the pyrazolin-3-ylidene
in contrast to the corresponding imidazolin-2-ylidene.
This method was recently performed by Herrmann et
al. and Bertrand et al. for a variety of carbenes.9
* To whom correspondence should be addressed. Tel: (+49)-
8928913080, Fax: (+49)8928913473. E-mail: lit@arthur.anorg.chemie.tu-
muenchen.de.
† N-Heterocyclic Carbenes. Part 38. Part 37: Schu¨tz, J.; Herrmann,
W. A. J. Organomet. Chem. 2004, 689, 2995.
(1) Review: (a) Herrmann, W. A. In Homogeneous Catalysis with
Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; Wiley-
VCH: Weinheim, Germany, 2000; p 712. (b) Heck, R. F., Trost, B. M.,
Fleming, I., Eds. Comprehensive Organic Synthesis; Pergamon: Oxford,
U.K., 1991; Vol. 4, p 883. (c) Beletskaya, I. P.; Cheprakov, A. V. Chem.
Rev. 2000, 100, 3009.
(2) (a) Herrmann, W. A.; Bo¨hm, V. P. W.; Gsto¨ttmayr, C. W. K.;
Grosche, M.; Reisinger, C.-P.; Weskamp, T. J. Organomet. Chem. 2001,
617, 616. (b) Yang, C.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511.
(3) (a) Lappert, M. J. J. Organomet. Chem. 1975, 100, 139. Re-
views: (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41(8), 1290.
(c) Herrmann, W. A.; Weskamp, T.; Bo¨hm, V. P. W. Adv. Organomet.
Chem. 2001, 48, 1. (d) Weskamp, T.; Bo¨hm, V. P. W.; Herrmann, W.
A. J. Organomet. Chem. 2000, 600, 12. (e) Herrmann, W. A.; Ko¨cher,
C. Angew. Chem., Int. Ed. 1997, 36, 2162.
(6) Olofson, R. A.; Thompson, W. R.; Michelman, J. S. J. Am. Chem.
Soc. 1964, 86(9), 1865.
(7) (a) Mu¨ller, J.; O¨ fele, K.; Krebs, G. J. Organomet. Chem. 1974,
82, 383. (b) O¨ fele, K.; Roos, E.; Herberhold, M. Z. Naturforsch. 1976,
31b, 1070.
(8) Ko¨cher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532,
261.
(9) (a) Herrmann, W. A.; O¨ fele, K.; von Preysing, D.; Herdtweck, E.
J. Organomet. Chem. 2003, 684, 235. (b) Denk, K.; Sirsch, P.;
Herrmann, W. A. J. Organomet. Chem. 2002, 649, 219. (c) Lavallo, V.;
Mafhouz, J.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G.
J. Am. Chem. Soc. 2004, 126, 8670.
(4) Herrmann, W. A.; O¨ fele, K.; von Preysing, D.; Schneider, S. K.
J. Organomet. Chem. 2003, 687, 229.
(5) Tafipolsky, M.; Scherer, W.; O¨ fele, K.; Artus, G.; Pedersen, B.;
Herrmann, W. A.; McGrady, G. S. J. Am. Chem. Soc. 2002, 124, 5865.
10.1021/om0401096 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/16/2004

Communications
Organometallics, Vol. 23, No. 26, 2004 6085
Table 1. Mizoroki-Heck Reaction: Coupling of
Different Aryl Halides and Styrene Using
Complexes 5a,b and 6 as Catalystsa
entry
R
X
Pd (mol %)
catalyst
yield (%)b
1
2
C(O)CH3
C(O)CH3
C(O)CH3
H
Br
Br
Cl
Br
Br
Br
Br
Cl
Br
Br
Br
Br
Cl
1.0
1.0
1.0
1.0
1.0
0.1
1.0
1.0
1.0
1.0
0.1
1.0
1.0
5a
5b
5b
5a
5b
5b
6
98
98
9
94
86
79
40
6
69
59
53
26
0
3
4
5
H
6
H
7
H
8
H
5b
5a
5b
5b
6
9
OCH3
OCH3
OCH3
OCH3
OCH3
10
11
12
13
Figure 2. Molecular structure of 5b.
Scheme 1. Synthesis of 5a (R ) Ph) and 5b (R )
5b
Me)
a Conditions: 1 equiv of aryl bromide, 1.5 equiv of styrene, 1.5
equiv of Na(OAc), dimethylacetamide (DMAc), T ) 130 °C, t ) 14
h. b GC yields using diethylene glycol di-n-butyl ether as the
internal standard.
The catalytic activity of 5a,b was investigated in
detail for the Pd-catalyzed Mizoroki-Heck olefination
of aryl halides with olefins and compared to the analo-
gous known complex diiodo(1,3-dimethylimidazolin-2-
ylidene)(triphenylphosphine)palladium(II) (6).2,12 Ex-
periments with bromoarenes and styrene show that
these novel Pd-pyrazolin-3-ylidene complexes are active
catalysts (Table 1). For example, 4-bromoacetophenone
is coupled with styrene to trans-stilbene in the presence
of 1 mol % of 5a,b and 1.5 equiv of NaOAc in 98% yield
at 130 °C in 14 h in both cases. Deactivated haloarenes
such as bromoanisole show the expected lower yields,
but there is an increased activity observed for the
bulkier NHC 5a, as is described in the literature for
bulky imidazolin-2-ylidene ligands in Pd complexes.4
The change of the catalyst concentration from 1 to 0.1
mol % using bromoarenes seems not to affect the yields
The observed frequencies for complex 1b are 2066 and
1993 cm-1, in contrast to 2073 and 1999 cm-1 for
complex 2b. Thus, pyrazolin-3-ylidenes induce a sig-
nificant higher electron density at the rhodium center
than the corresponding imidazolin-2-ylidenes.
It has been shown that oxidative addition of a low-
valent metal precursor to a carbon-halogen bond is
possible.10 Due to the difficulty in obtaining the free
pyrazolin-3-ylidene, the oxidative addition of a low-
valent metal to a 3-halo 1,2-disubstituted pyrazolium
salt opens the way to various pyrazolin-3-ylidene com-
plexes.
(11) Crystal structure analysis of compound 5b: C25H27Cl2N2PPd,
Mr ) 563.78, colorless fragment (0.13 × 0.28 × 0.30 mm3), orthorhom-
bic, Pbca (No. 61), a ) 10.6000(1) Å, b ) 16.0669(1) Å, c ) 27.8350(2)
Å, V ) 4740.56(6) Å3, Z ) 8, dcalcd ) 1.580 g cm-3, F000 ) 2288, µ )
1.092 mm-1. Preliminary examination and data collection were carried
out on a kappa-CCD device (NONIUS MACH3) with an Oxford
Cryosystems cooling system at the window of a rotating anode
(NONIUS FR591) with graphite-monochromated Mo KR radiation (λ
) 0.710 73 Å). Data collection was performed at 173 K within the θ
range of 1.46° < θ < 25.33°. A total of 110 029 reflections were
integrated. Raw data were corrected for Lorentz, and polarization and,
arising from the scaling procedure, for latent decay and absorption
effects. After merging (Rint ) 0.041), 4336 (3965 with Io > 2σ(Io))
independent reflections remained, and all were used to refine 388
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 found and refined with individual isotropic displacement
parameters. Full-matrix least-squares refinements were carried out
Treatment of Pd(PPh3)4 with an equimolar amount
of the pyrazolium salt 3a,b in refluxing dichloroethane
leads to the first palladium pyrazolin-3-ylidene complex
4a,b in 70% yield with traces of the complex 5a,b. To
form the neutral palladium dichloride complex 5a,b, a
suspension of complex 4a,b in toluene is heated for an
additional 1 h (Scheme 1). The formation of 5a,b can
be monitored by 31P NMR spectroscopy. In the case of
4b and 5b, respectively, the signal at δ(P) 22.7 ppm
decreases while the signal at δ(P) 28.8 ppm slowly
increases and 1 equiv of PPh3 is simultaneously re-
leased.
Subsequent workup of the filtered suspension af-
forded the complexes 5a,b in yields between 55 and
60%. Colorless crystals of 5b suitable for X-ray diffrac-
tion studies were obtained from an ethanol/diethyl ether
solution at ambient temperature (Figure 2).11
2
2
by minimizing ∑w(Fo - Fc2
) and converged with R1 ) 0.0225 (Io >
2σ(Io)), wR2 ) 0.0535 (all data), GOF ) 1.094, and a shift/error of
<0.001. The final difference Fourier map shows no striking features
(∆emin/max ) +0.34/-0.37 e Å-3). Crystallographic data (excluding
structure factors) for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-255379 (5b). Copies of the data
can be obtained free of charge on application to the CCDC, 12 Union
Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223-336-033; e-mail,
deposit@ccdc.cam.ac.uk.
(10) (a) Fraser, P. J.; Roper, W. R.; Stone, F. G. A. J. Chem. Soc.,
Dalton Trans. 1974, 102. (b) McGuinness, D. S.; Cavell, K. J.; Yates,
B. F.; Skelton, B. W.; White, A. H. J. Am. Chem. Soc. 2001, 123, 8317.
(c) Gru¨ndemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree,
R. H. Dalton 2002, 2163. (d) Fu¨rstner, A.; Seidel, G.; Kremzow, D.;
Lehmann, C. W. Organometallics 2003, 22, 907.
(12) (a) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971,
44, 581. (b) Heck, R. F.; Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320.

6086 Organometallics, Vol. 23, No. 26, 2004
Communications
To evaluate the differences in activity, the coupling
of bromobenzene with styrene was followed over time
(Figure 3). The imidazole system 6 shows an initial
activity being lower than that of complex 5b. Thus, it
appears that the induction period is evidently larger for
a conventional imidazole system such as 6 than for the
new pyrazole system 5b.
This report demonstrates the synthetic route to
pyrazolin-3-ylidene complexes which may introduce this
NHC into homogeneous catalysis. The two complexes
are the first palladium pyrazolin-3-ylidene complexes
reported in the literature. It is very probable that for
any catalysts containing an imidazolin-2-ylidene which
promise to be more active with strong donating ligands,
substitution of the imidazolin-2-ylidene by pyrazolin-
3-ylidene will result in a significant increase in activity
of the catalyst. Its superior role as compared to standard
imidazolin-2-ylidene complexes in Heck catalysis was
demonstrated as being due to its nucleophilic carbene
center acting as a strong σ-donor ligand.
Figure 3. Time-yield comparison of 5b and 6 in the
Mizoroki-Heck reaction of bromobenzene and styrene.
significantly. However, chloroarenes do not appear to
be active enough for these catalysts. Most interesting
is the comparison to the analogous imidazolin-2-ylidene
complex 6. The different halogens on the palladium may
also influence the activity of the complex.
To our knowledge, there has been no such consider-
able difference in activity observed thus far for the
change of the halogens on an NHC palladium Heck
catalyst. The increase of the yield by a factor of 2 is
therefore mostly ascribable to pyrazolin-3-ylidene as a
stronger donor (see entries 10 and 12 in Table 1). This
shows clearly the superiority of the strong donating
pyrazolin-3-ylidene to the weaker imidazolin-2-ylidene
donors.
Acknowledgment. We thank Dr. Karl O¨ fele for his
continued interest, regular discussions, and experimen-
tal assistance.
Supporting Information Available: Text giving experi-
mental procedures for the synthesis of pyrazolium salts 4a,b
and the new palladium complexes together with the analytical
and spectral data of all new compounds and structural data
for complex 5b. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM0401096