A carbocyclic carbene as an efficient catalyst ligand for C-C coupling reactions

Article abstract of DOI:10.1002/anie.200503589

A new player on the field: Carbocyclic carbenes are potentially better control ligands than N-heterocyclic carbenes (NHCs) in catalyzed C-C coupling reactions. Pd^II complexes of cycloheptatrienylidene (see picture), which are readily prepared, thermally stable, and highly stable towards acid, are as effective as the familiar NHC complexes in Suzuki and Heck reactions or perform even better. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200503589

Angewandte
Chemie
Carbocyclic Carbenes
mononuclear carbene complexes 4 and 5 by way of the
binuclear complexes 2a,b (p-allyl) and 3a,b (s,p-carbene)
(Scheme 1). Complexes 4 and 5 were formed almost quanti-
DOI: 10.1002/anie.200503589
A Carbocyclic Carbene as an Efficient Catalyst
Ligand for CꢀC Coupling Reactions**
Wolfgang A. Herrmann,* Karl Öfele,
Sabine K. Schneider, Eberhardt Herdtweck, and
Stephan D. Hoffmann
Dedicated to Professor Richard F. Heck
on the occasion of his 75th birthday
[
1]
Starting from the prototypes I and II N-heterocyclic
carbenes (NHCs) have proved to be efficient control ligands
[
2–4]
in organometallic homogeneous catalysis.
It could not
Scheme 1. Synthesis of CHT–palladium complexes: a) CH Cl , ꢀ108C;
2
2
b) 408C (X=Cl) or 258C (X=Br); c) in C H , 1 equiv L=P(C H ) or
6
6
6
5 3
P(c-C H ) ; d) DMSO, ca. 1508C.
6
11 3
tatively when intermediates 2 and 3 were not isolated. The
intermediate 2a may be isolated by interruption of the
reaction at an early stage, but it rearranges into the carbene
complex 3a during the further course of the reaction and
during workup. The p-allyl complex 2a and the carbene
complex 4 were characterized by X-ray crystallography
have foreseen, but they are beginning to displace the classical
organophosphanes as ligands. Metal complexes of NHCs
generally have greater thermal and acid stability of their
metal complexes; they frequently exhibit higher catalytic
activities, are amenable to a broad range of structure
[
5,6]
(Figures 1 and 2).
The constitution of the complex
compounds in solution was established spectroscopically
[
2]
variation, and are also successful as immobilized variants.
Prominent examples of their application are the activation of
[2,4]
haloarenes and olefin metathesis. We have now found that,
as a control ligand, the simple, carbocyclic carbene III
(
cycloheptatrienylidene, CHT) is comparable with or even
superior to the well-established NHCs. We describe this here
with the example of the palladium complexes.
In a short reaction sequence 1,1-dichloro- and 1,1-
dibromocycloheptatrienes (1a and 1b, respectively) were
transformed in good yield with palladium black into the
[*] Prof. Dr. W. A. Herrmann, Dr. K. Öfele, Dipl.-Chem. S. K. Schneider,
Dr. E. Herdtweck
Lehrstuhl für Anorganische Chemie
Technische Universität München
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax: (+49)89-289-13473
E-mail: lit@arthur.anorg.chemie.tu-muenchen.de
Figure 1. ORTEP drawing of 2a in the crystal; thermal ellipsoids set at
5
2
0% probability. Selected bond lengths [Š] and angles [8]: Pd–Cl1
.4588(9), Pd–Cl1a 2.4545(10), Pd–C1 2.073(4), Pd–C2 2.183(5), Pd–
Dr. S. D. Hoffmann
Lehrstuhl für Anorganische Chemie
mit Schwerpunkt Neue Materialien
Technische Universität München
Lichtenbergstrasse 4, 85747 Garching (Germany)
C7 2.179(4), Pd–Cg 1.870, C1–C2 1.415(6), C1–C7 1.385(6), C2–C3
1
1
.450(7), C3–C4 1.353(7), C4–C5 1.402(7), C5–C6 1.328(6), C6–C7
.433(6); Cl1-Pd-Cl1a 87.58(3), Pd-Cl1-Pda 92.42(3), Cl1-Pd-Cg 134.10,
Cl1a-Pd-Cg 136.80. Symmetry operations for the equivalent atom
positions a: ꢀx, 1ꢀy, 1ꢀz. Cg defines the mass center of the allylic
system C1, C2, and C7.
[
**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie (fellowship for S.K.S.).
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
Figure 2. ORTEP drawing of 4 in the crystal; thermal ellipsoids set at
5
0% probability. Selected bond lengths [Š] and angles [8]: Pd–C1
1
.968(2), Pd–Cl1 2.3697(6), Pd–Cl2 2.3884(7), Pd–P 2.2483(6); Cl1-Pd-
Cl2 91.83(2), Cl1-Pd-P 175.89(2), Cl1-Pd-C1 85.14(7), Cl2-Pd-P
2.27(2), Cl2-Pd-C1 174.55(6), P-Pd-C1 90.76(7).
9
Scheme 2. Mesomeric resonance structures of CHT(6p)–metal com-
plexes.
(
see the Experimental Section). The bromo derivative 2b
reacts immediately to form the carbene complex 3b and
therefore cannot be isolated.
moacetophenone and n-butyl acrylate underwent CꢀC cou-
ꢀ
4
The CꢀC bonds of the cyclocarbene in complex 4 are
pling at 1458C with only 10 mol% of catalyst 4, which
6
almost all the same length (1.39 ꢁ 0.02Š), which suggests
extensive 6p delocalization similar to that of the cyclohepta-
trienyl cation. The delocalization of the p system includes the
carbene p orbital similar to that in the NHC ligands I and II so
that p(M!C) backbonding contributes only minimally to the
stabilization of the CHT complexes. Hence the cyclohepta-
trienylium resonance form (a in Scheme 2) is also favored in
corresponds to turnover numbers (TON) of the order of 10 at
3
ꢀ1
turnover frequencies (TOF) of @ 10 h (Table 1, entry 1).
ꢀ
1
Reaction of bromobenzene was also achieved (10 mol%
4
cat) in 90% yield and with TON ꢂ 10 (Table 1, entry 2). In all
examples of the Heck coupling reaction investigated the NHC
[
10]
reference catalyst 6 was inferior (Figure 3a). Unlike the
NHC catalyst 6, 4 did not require an induction period.
An analogous picture emerges for typical Suzuki coupling
[
7]
the literature for the low-valent metal complexes. This is in
ꢀ
3
agreement with the PdꢀC distance in 4 which at 1.968(2) Š
reactions (Figure 3b). With catalyst 4 (10 mol%) bromo-
[
1,2]
lies in the range typical for the NHCs I and II.
Clearly the
benzene underwent quantitative coupling with phenylboronic
C H6 carbene is essentially a s-
7
donor ligand.
We used the route shown in
Scheme 1 to prepare palladium
complexes of cycloheptatrienyli-
dene for the first time. This syn-
thesis did not require special pre-
cursors for air- and temperature-
sensitive metal(0) complexes such
as [Pt{P(C H ) } ] as was previously
Table 1: Catalyzed Heck coupling.
[
a]
[a]
Entry
R
X
Cat.
PR’3
mol% Pd
t [h]
Conv. [%]
Yield [%]
TON
ꢀ4
6
1
2
3
4
C(O)CH
H
OCH3
OCH3
Br
Br
Br
Br
4
4
4
4
–
–
–
–
–
–
–
–
10
14
14
14
14
35
104
14
35
104
14
100
92
43
59
66
91
30
63
81
76
100
90
42
58
65
89
28
58
76
75
10
9000
420
58
6
5 3 3
3
[
8]
0.1
0.1
1
described by Jones et al. Since our
method has several steps (p-allyl
intermediate!), we avoid formation
of heptafulvalene, which would
occur from the reductive coupling
65
89
28
58
76
75
[
b]
5
6
C(O)CH3
C(O)CH3
Cl
Cl
4
1
1
[
7a]
of the ligand precursor.
–
PCy3
The carbocyclic carbene com-
plex 4 gave good results in numer-
[
b]
3a
ous variants of the Heck and Suzuki [a] GC yield with diethylene glycol di-n-butyl ether as the internal standard. [b] Addition of [nBu N]Br
4
[
9]
coupling reactions. Thus p-bro- (0.2 equiv.), T=1508C.
3
860 www.angewandte.org
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Angewandte
Chemie
trans-dibenzylideneacetone) and imidazolium salts, the struc-
[
11]
tures of which were not defined.
With respect to their ready synthesis and high thermal and
air stability, the catalyst complexes of cycloheptatrienylidene
are at least comparable to the NHC complexes. The
tremendous robustness of the CHT catalysts is demonstrated
inter alia in that the complex 4 used in the catalyst experi-
ments could be subjected to temperatures of more than 1458C
over five days without a trace of decomposition. Complex 3a
can even be purified by stirring with concentrated HCl.
We attribute the efficiency of the new class of catalysts
essentially to a low metal coordination achieved with high
thermal stability such that the metal–carbene unit is retained
in the catalytic cycle. Whether and to what extent the carbon–
metal bond determines the catalytic properties is the subject
of kinetic and quantum mechanical investigations. The
catalysts may be widely varied both constitutionally and
structurally. It is expected above all that the substituents at
positions 2and 7 next to the carbene center exercise a far
greater effect on the metal environment than is the case in the
NHC five-membered-ring system. In that way we hope to
solve the problem of the Heck coupling of deactivated
chloroarenes which have failed with all known, molecularly
[
12]
defined, homogeneous catalysts.
Figure 3. Conversion(U)–time(t) plot for the a) Heck coupling of
bromobenzene with n-butyl acrylate and the b) Suzuki coupling of p-
bromoanisole with phenylboronic acid; in each case catalysts 4 (
c; 0.1 mol%) and 6 (*, g; 0.1 mol%) are compared.
&
,
Experimental Section
2
a: Pd black (1.0 g, 9.40 mmol) and 1a (1.47 g, 9.13 mmol) were
stirred in 25 mL of dichloromethane for 6 h at ꢀ108C. The suspension
was filtered through a D4 frit, the residue was rinsed with dichloro-
methane (6 ” 20 mL), and the combined rinses were concentrated
under reduced pressure to a volume of 20 mL. The solution was
cooled to ꢀ188C, and the resulting red-brown crystals were filtered
acid at 1308C and the conversion rates (TON) were of the
4
6
3
order of 10 to 10 ; with chloroarenes TONs of up to > 10
were achieved, for example with p-chloroacetophenone
Table 2, entry 4). Once again catalyst 6 does not approach
(
1
off and dried in vacuo. Yield: 0.21 g (9%). H NMR (399.8 MHz,
the carbocyclic variant 4.
1
3
1
CD Cl ): d = 6.39 (m, 2H), 5.59 (m, 2H), 5.30 ppm (m, 2H); C{ H}
2
2
In Suzuki coupling reactions with chloroarenes catalyst 5
was more effective than the most active NHC–phosphane
system previously known, which is produced in situ from 7
NMR (100.5 MHz, CD Cl ): d = 116.2, 101.7, 89.5 ppm; elemental
2
2
analysis (%) calcd for C H Cl Pd : C 31.43, H 2.26, Pd 39.78; found:
1
4
12
4
2
C 31.76, H 2.31, Pd 39.36.
[
10]
and PCy (Table 2, entries 5–8). Better results for chloro-
3a: Pd black (1.45 g, 13.63 mmol) and 1a (2.15 g, 13.35 mmol)
were stirred in dichloromethane (30 mL) for 24 h at ꢀ108C. The
product mixture was then extracted in two portions for 24 h in a small
Soxhlet apparatus with 50 mL of boiling dichloromethane in each
case. The combined extracts were concentrated under reduced
pressure to 40 mL and cooled to ꢀ188C. The precipitated orange-
colored crystals were filtered off, washed with dichloromethane
3
arenes were found previously only with optimized in situ
catalysts derived from Pd(OAc) or [Pd (dba) ] (dba = trans,-
2
2
3
Table 2: Catalyzed Suzuki coupling.
(
(
(
10 mL), and dried in vacuo. Yield: 2.25 g (64%). M.p. 150–1608C
1
decomp). H NMR (399.8 MHz, [D ]DMSO): d = 5.36 (m, 2H), 5.22
6
1
3
1
m, 2H), 5.03 ppm (d, J = 12.0 Hz, 2H); C{ H} NMR (100.5 MHz,
D ]DMSO): d = 185.8 (s, carbene C), 140.2, 135.4, 133.8; IR (Nujol):
[
6
ꢀ1
Entry
R
X
Cat. PR’3
mol% Yield TON
n˜ = 346 (terminal halogen atom), 313, 282 cm (bridging halogen
[
a]
Pd
[%]
atoms). Elemental analysis (%) calcd for C H Cl Pd : C 31.43, H
1
4
12
4
2
2
.26, Pd 39.78; found: C 30.96, H 2.31, Pd 39.08.
: A solution of complex 3a (0.45 g, 0.815 mmol) and P(C H )
5 3
ꢀ
3[b]
4[b]
5
1
2
3
4
5
6
7
8
H
H
Br
Br
Br
4
4
4
4
–
–
–
–
10
10
100
89
43
11
100
42
10
8.9”10
4300
4
ꢀ
5
6
(0.44 g, 1.68 mmol) in C H (20 mL) was first warmed to 808C with
[
b]
6 6
OCH3
C(O)CH3 Cl
H
H
OCH3
OCH
0.01
0.01
1
1
1
stirring and then stirred for a further 20 h at 258C. The resulting
yellow microcrystalline needles were filtered off, washed with C H6
1100
100
42
93
69
6
Cl 3a
Cl
Cl 3a
Cl
PCy3
+PCy3
PCy3
(
5 ” 10 mL), purified by recrystallization from CH CN, and dried in
3
+
[
10]
7
vacuo. Yield: 0.80 g (93%). MS (FAB-MS): m/z: 493 [MꢀCl] (100).
93
69
Elemental analysis (%) calcd for C H Cl PdP: C 56.68, H 4.00, Pd
[
3
10]
[c]
25 21
2
3
7
+PCy
1
20.09; found: C 56.42, H 3.79, Pd 19.5. Compound 4 was obtained
[
a] GC yield with diethylene glycol di-n-butyl ether as the internal
almost quantitatively when, as described in the above method, 1a was
first allowed to react with Pd black in dichloromethane at ꢀ108C and
standard. [b] K CO as base. [c] Yield after 32 h.
2
3
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Communications
4
2
8 h at room temperature, and then with PPh in C H at 808C and
4 h at room temperature.
Catalyzed reactions: a) Heck coupling: A Schlenk flask that had
Herrmann, H.-C. Militzer, U. Scholz, C. Gstöttmayr
(Bayer AG), EP 1308157A1, 2003.
3
6
6
[5] X-ray crystal structure analysis of 2a: C H Cl Pd , M = 534.88,
1
4
12
4
2
r
3
¯
been evacuated and filled with argon several times was charged with
sodium acetate (3.0 mmol), aryl halide (2.0 mmol), and the internal
standard diethylene glycol di-n-butyl ether (100 mg). Then n-butyl
acrylate (3 mmol) and degassed N,N-dimethylacetamide (DMA;
red fragment (0.03 ” 0.20 ” 0.51 mm ), triclinic, P1, a = 5.0196(6),
b = 7.2075(8), c = 11.5141(15) Š, a = 86.792(10), b = 80.650(10),
g = 80.080(9)8, V= 404.72(9) Š , Z = 1, 1calcd = 2.195 gcm
F000 = 256, m = 2.886 mm . The preliminary investigation and
data collection were carried out with a IPDS-2T diffractometer
(Stoe & Cie) at the window of a rotating-anode generator
(NONIUS FR591) with MoKa radiation (graphite monochroma-
tor, l = 0.71073 Š) at 293 K in the range of 4.538 < V< 25.358.
5867 Reflections integrated with TWIN program, data LP-
corrected, and corrected for decomposition, starting from the
3
ꢀ3
,
ꢀ1
2
mL) were added, and the reaction mixture was heated to 145 8C.
When the reaction temperature had been reached the catalyst
solution was added against stream of argon. At the end of the
reaction the reaction solution was cooled to 258C, treated with 1n aq.
HCl (1 mL), and extracted with dichloromethane (3 ” 2mL). The
organic phase was dried over MgSO . Conversions and yields were
4
determined by GC analysis.
scaling. Numerical absorptions correction (Tmin = 0.544, Tmax =
b) Suzuki coupling: A Schlenk flask that had been evacuated and
filled with argon several times was charged with potassium or cesium
carbonate (3.0 mmol), aryl halide (2.0 mmol), phenylboronic acid
0.812) after optimization of the crystal form. All 1168 independ-
ent reflections (Rint = 0.039) [1081: I > 2s(I )] used for the
o
o
refinement of the 102parameters, structure solution by direct
methods and difference Fourier syntheses. All non-hydrogen
atoms refined with anisotropic displacement parameters, hydro-
gens at calculated positions with a CꢀH distance of 0.93 Š and
(2.4 mmol), and the internal standard diethylene glycol di-n-butyl
ether (100 mg). Then degassed xylene (2mL) was added, and the
reaction mixture was heated to 1308C. When the reaction temper-
ature had been reached the catalyst solution was added against a
stream of argon. At the end of the reaction the reaction solution was
cooled to 258C, treated with water (3 mL), and extracted with diethyl
U
= 1.2U
refined, structure refinement (method of least
iso(H)
eq(C)
2
2 2
squares on ꢀw(F ꢀF ) ) converged with R1 = 0.0206 [I >
o
c
o
2s(I )], wR2 = 0.0492[all data], GOF = 1.041, and shift/error
o
ether (3 ” 2mL). The organic phase was dried over MgSO . Con-
versions and yields were determined GC analysis.
ratio < 0.001. The final difference Fourier map showed only
4
ꢀ
3
small deviations (Demin/max = + 0.31/ꢀ0.30 eŠ ). Smaller extinc-
tion effects were corrected with SHELXL-97 (G. M. Sheldrick,
SHELXL-97, Program for Refinement of Crystal Structures,
Universität Göttingen, 1997) [e = 0.013(2)]. The molecule lies on
an inversion center. The resulting partial disorder
[0.830(4):0.170(4)]of the chlorine atoms C12/C13 was observed
and resolved. CCDC-683874 contains the detailed crystallo-
graphic data for this publication. The data may be obtained free
of charge from the Cambridge Crystallographic Data Centre
under www.ccdc.cam.ac.uk/data_request/cif.
Catalyst solutions for the catalysts 4 and 6: A solution of catalyst
(0.02mmol) in DMA (10 mL) was stored in the freezer. The
concentration was selected such that 0.1 mL of the solution corre-
sponds to a catalyst/substrate ratio of 0.01 mol% catalyst. For
experiments with extremely low catalyst concentrations the catalyst
solution was diluted further. Catalyst solutions for the catalysts 5 and
7
: The solutions were prepared by stirring the phosphane with 3a or 7
(P/Pd ratio 1:1) in DMA (0.5 mL) for 10 min at 258C.
Received: October 11, 2005
Published online: May 3, 2006
[6] X-ray crystal structure analysis of 4·CH
3
CN: C27
H
24Cl
2
NPPd,
3
¯
M = 570.76, yellow needles (0.05 ” 0.10 ” 0.66 mm ), triclinic, P1
r
(No. 2), a = 9.7796(2), b = 10.5189(2), c = 12.5658(3) Š, a =
3
9
9.2431(7), b = 93.8821(6), g = 90.2501(6)8, V= 1272.79(5) Š ,
Keywords: CꢀC coupling · carbenes · carbocycles ·
ꢀ
3
ꢀ1
.
Z = 2, 1calcd = 1.489 gcm , F000 = 576, m = 1.017 mm . R1 =
homogeneous catalysis · palladium
0
.0290 [I > 2s(I )], wR2 = 0.0555 [all data], GOF = 1.116,
o o
shift/error ratio < 0.001. CCDC-603873 contains the detailed
crystallographic data for this publication. The data may be
obtained free of charge from the Cambridge Crystallographic
Data Centre under www.ccdc.cam.ac.uk/data_request/cif.
[
1] a) W. A. Herrmann, M. Elison, J. Fischer, C. Köcher, G. R. J.
Artus, Angew. Chem. 1995, 107, 2602; Angew. Chem. Int. Ed.
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Chem. 1997, 109, 2256; Angew. Chem. Int. Ed. Engl. 1997, 36,
[
[
[
7] a) N. T. Allison, Y. Kawada, W. M. Jones, J. Am. Chem. Soc.
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Chem. Rev. 2000, 100, 39.
114, 10991; b) Z. Lu, W. M. Jones, W. R. Winchester, Organo-
[
[
[
2] Review article : a) W. A. Herrmann, Angew. Chem. 2002, 114,
metallics 1993, 12, 1344.
1342; Angew. Chem. Int. Ed. 2002, 41, 1290; b) W. A. Herrmann,
9] Current literature, e.g. a) W. A. Herrmann, K. Öfele, D.
von Preysing, S. K. Schneider, J. Organomet. Chem. 2003, 687,
T. Weskamp, V. P. W. Böhm, Adv. Organomet. Chem. 2002, 48, 1.
3] Since the first publication on CꢀC coupling reactions catalyzed
229; b) W. A. Herrmann in Applied Homogeneous Catalysis with
[
1a]
by NHC complexes in 1995 more than 200 original publica-
tions from 15 research groups have appeared.
Organometallic Compounds, 2nd ed. (Eds.: B. Cornils, W. A.
Herrmann), Wiley-VCH, Weinheim, 2002, p. 591 – 598; W. A.
Herrmann in Applied Homogeneous Catalysis with Organome-
tallic Compounds, 2nd ed. (Eds.: B. Cornils, W. A. Herrmann),
Wiley-VCH, Weinheim, 2002, p. 775 – 793; W. A. Herrmann in
Applied Homogeneous Catalysis with Organometallic Com-
pounds, 2nd ed. (Eds.: B. Cornils, W. A. Herrmann), Wiley-
VCH, Weinheim, 2002, p. 822 – 828.
4] a) W. A. Herrmann, J. Fischer, M. Elison, C. Köcher, K. Öfele
(
Hoechst AG), DE 4447066.5A1, 1996, EP 0721953A1, 1996
Chem. Abstr. 1996, 125, 143019y]; b) W. A. Herrmann, J.
Fischer, M. Elison, C. Köcher (Hoechst AG), DE
447068.1A1, 1996, EP 0719758A1, 1996, US 5.703.269, 1997
Chem. Abstr. 1996, 125, 167571y]; c) W. A. Herrmann, M.
Elison, J. Fischer, C. Köcher (Celanese GmbH), DE
[
4
[
[
10] W. A. Herrmann, V. P. W. Böhm, C. W. K. Gstöttmayer, M.
4
1
4
1
447067.3A1, 1996, EP 0719753A1, 1995 [Chem. Abstr. 1996,
25, 167338c]; d) W. A. Herrmann (Hoechst AG), DE
447070.3A1, 1996, EP 0721951A1, 1996 [Chem. Abstr. 1996,
25, 143016v]; e) W. A. Herrmann, C. Köcher, L. Goossen
Grosche, C.-P. Reisinger, T. Weskamp, J. Organomet. Chem.
2001, 617–618, 616.
[
[
11] G. A. Grasa, M. S. Viciu, J. Huang, C. Zhang, M. L. Trudell, S. P.
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(
Hoechst AG), DE 1961090.8A1, 1997, EP 888308, 1997, WO
734875, 1997 [Chem. Abstr. 1997, 127, 318962v]; f) W. A.
9
3
862
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Angew. Chem. Int. Ed. 2006, 45, 3859 –3862