A highly active catalyst for the room-temperature amination and Suzuki coupling of aryl chlorides

Article abstract of DOI:10.1002/(SICI)1521-3773(19990816)38:16<2413::AID-ANIE2413>3.0.CO;2-H

A unique combination of steric and electronic properties appears to determine the effectiveness of phosphanyl-substituted biphenyls as ligands in palladium-catalyzed aminations and Suzuki coupling of aryl chlorides at room temperature [Eq. (1)]. The oxidative addition step is greatly accelerated, and transmetalation (or Pd-N bond formation) and reductive elimination processes are facilitated. Use of these ligands allows for Suzuki coupling at very low catalyst loadings (as little as 10^-6 mol % Pd). R'' = cyclohexyl, tert-butyl.

Full text of DOI:10.1002/(SICI)1521-3773(19990816)38:16<2413::AID-ANIE2413>3.0.CO;2-H

COMMUNICATIONS
[17] The reaction with NaOH as the additive was somewhat less clean than
the reaction with CsF.
A Highly Active Catalyst for the Room-
Temperature Amination and Suzuki Coupling
of Aryl Chlorides**
[18] General experimental: Under an atmosphere of argon or N2, a
solution of aryl chloride (1.0 mmol; in dioxane (0.5 ± 0.6 mL)) and a
solution of PtBu3 (0.060 mmol; in dioxane (0.5 ± 0.4 mL)) were added
in turn to a Schlenk tube charged with [Pd2(dba)3] (0.015 mmol) and
CsF (2.2 mmol). The organostannane (1.05 mmol) was then added by
syringe, and the Schlenk tube was sealed, placed in an 80 ± 1008C oil
bath, and stirred for 12 ± 48 h. The reaction mixture was then cooled to
room temperature, diluted with Et2O, and filtered through a pad of
silica gel. The silica gel was washed thoroughly with Et2O, and the
combined Et2O washings were concentrated by rotary evaporation.
The product was then purified by flash chromatography.
[19] Notes: a) These cross-coupling reactions do not appear to be highly
air- or moisture-sensitive. For example, they can be conducted in
reagent-grade dioxane through which argon has been bubbled. b) In
the absence of [Pd2(dba)3] or of PtBu3, no reaction (<2% conversion)
is observed. c) The reactions proceed to completion with only
1.1 equiv of CsF and with only 3.6% PtBu3, but more slowly than
under the conditions described in reference [18]. d) The reaction is
slower with PCy3 than with PtBu3, and it does not proceed in the
presence of electron-rich and sterically hindered tris(2,4,6-trimethoxy-
phenyl)phosphane. e) Cross-couplings in THF proceed with compa-
rable efficiency as in dioxane; reactions in toluene are somewhat
slower. f) [Pd(OAc)2] is inferior to [Pd2(dba)3] as a catalyst precursor.
g) Lower catalyst loadings may be used in these Stille couplings, at the
expense of slightly lower yields. For example, cross-coupling of 4-n-
butyl-1-chlorobenzene with tributyl(vinyl)tin in the presence of
0.25% [Pd2(dba)3] and 1.0% PtBu3 affords 4-n-butylstyrene in 67%
yield.
John P. Wolfe and Stephen L. Buchwald*
Palladium-catalyzed amination[1] and Suzuki coupling[2]
reactions have found widespread use in many areas of organic
synthesis. These methods permit the construction of Csp2 Csp2
bonds or Caryl N bonds which cannot be easily or efficiently
formed using classical transformations. Most procedures
commonly used for these processes employ triarylphos-
phane-based catalyst systems.[1, 2] While these catalysts are
readily available, they usually require elevated reaction
temperatures (usually 50 ± 1008C) to function efficiently, and
tend to be unreactive towards aryl chloride substrates.[3±5]
We recently reported that 2-dicyclohexylphosphanyl-2'-
dimethylaminobiphenyl (1, Cy ˆ cyclohexyl) was an excellent
ligand for palladium-catalyzed cross-coupling reactions of
aryl chlorides.[6] Although the Pd/1 catalyst system was
effective for the room-temperature Suzuki coupling of both
electron-rich and electron-deficient aryl chloride substrates,[7]
room-temperature catalytic aminations of aryl chlorides were
inefficient; only the highly activated 4-chlorobenzonitrile was
effectively transformed.
Subsequent studies demonstrated that the bulky phosphane
2 was a more effective ligand than 1 in palladium-catalyzed
C O bond forming reactions, presumably due to its ability to
increase the rate of reductive elimination in these proces-
ses.[5g, 8] Furthermore, experiments designed to determine
whether the amino group on 2 was necessary for effective
catalysis revealed that for some substrate combinations the
desamino ligand 4 was as effective as 2, prompting us to
examine the use of 4 in amination processes.[9]
[20] Metal-catalyzed Cross-coupling Reactions (Eds.: F. Diederich, P. J.
Stang), WILEY-VCH, New York, 1998.
[21] In the Stille cross-couplings of the other organostannanes illustrated
in Table 3, essentially no butyl transfer is observed (<2%).
[22] For a general discussion of the problem of separating reaction
products from organotin residues, see: D. Crich, S. Sun, J. Org. Chem.
1996, 61, 7200 ± 7201.
[23] M. Hoshino, P. Degenkolb, D. P. Curran, J. Org. Chem. 1997, 62, 8341 ±
8349; D. P. Curran, Angew. Chem. 1998, 110, 1230 ± 1255; Angew.
Chem. Int. Ed. 1998, 37, 1174 ± 1196.
[24] a) Addition of fluoride (e.g., KF) after a reaction is complete is a
common method for removing organotin halide impurities: D.
Milstein, J. K. Stille, J. Am. Chem. Soc. 1978, 100, 3636 ± 3638; J. E.
Liebner, J. Jacobus, J. Org. Chem. 1979, 44, 449 ± 450. b) Stille and
Scott have reported that the addition of CsF to cross-coupling
reactions of vinyl triflates with organotin compounds leads to ꢀ80%
removal of tin: W. J. Scott, J. K. Stille, J. Am. Chem. Soc. 1986, 108,
3033 ± 3040. c) Under our conditions, we do not detect any Bu3SnCl at
the end of the reaction.
P(tBu)2
P(tBu)2
PCy2
PCy2
Me2N
Me2N
1
3
4
2
[*] Prof. Dr. S. L. Buchwald, Dr. J. P. Wolfe
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, MA, 02139 (USA)
Fax: (‡1)617-253-3297
E-mail: sbuchwal@mit.edu
[**] We gratefully acknowledge the National Institutes of Health
(GM58160 and GM34917) and the National Cancer Institute (Train-
ing grant NCI no. CIT32CA09112), who provided financial support
for this work. We also thank Pfizer, Merck, and Novartis for
additional unrestricted support. J.P.W. is a recipient of a fellowship
from the Organic Division of the American Chemical Society
sponsored by Schering-Plough, for which he is grateful. We thank
Dr. Ken Kamikawa for performing preliminary experiments on the
room-temperature catalytic amination of aryl chlorides, Dr. Bryant
Yang for performing the experiments depicted as entries 1 and 2 of
Table 2, and Dr. Robert Singer for performing the experiment
depicted in entry 3 of Table 2.
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/angewandte/ or from the author.
Angew. Chem. Int. Ed. 1999, 38, No. 16
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3816-2413 $ 17.50+.50/0
2413

COMMUNICATIONS
Table 1. Room-temperature catalytic amination of aryl chlorides.[a]
As shown in Table 1, mixtures of
palladium acetate and
4 effectively
Entry Halide
Amine
Product
Mol% Pd
t [h]
Yield [%]
catalyzed the room-temperature amina-
tion of a variety of aryl chloride sub-
strates, including substrates which are
electron-rich and/or ortho-substituted.
Secondary amines were found to be
effective coupling partners, and primary
amines were successfully arylated with
ortho-substituted aryl halides. Reac-
tions of primary amines with aryl hal-
ides that lack ortho substituents failed to
proceed to completion under these
conditions,[10] and substrates containing
base-sensitive functional groups could
not be transformed due to the ineffi-
ciency of the room-temperature reac-
tions in the presence of bases weaker
than NaOtBu (e.g. K3PO4).[10] In a few
cases the amination reactions of aryl
chlorides were effected using low cata-
lyst loadings (0.05 mol% Pd) with 3 or 4
as ligand at 1008C (Table 1, entries 1, 2).
However, this protocol of low catalyst
amount is not yet general.[10]
Me
N
H
N
1
1.0
0.005
19
19
98
Me
Cl
Ph
Me
95[b]
Me
HN
Ph
2
3
1.0
0.05/3
20
26
94
89[b]
O
N
O
Me
2.0
1.0
2.0
18
21
18
81
98
99
HNBu2
NBu2
Me
Me
Me
Me
HN
Me
4
Cl
N
Me
Me
H2NBn
5
N(H)Bn
N
MeO
Cl
Cl
HN
O
O
O
6
MeO
2.0
1.0
20
15
90
86
HN
NC
MeO
7
8
NC
N
O
MeO
Cl
N(H)Bn
H2NBn
1.0
1.0
14
16
99
97
MeO
MeO
MeO
MeO
Me
H
Cl
N
N
9
Ph
Me
Ph
Catalysts based on ligand 4 were also
effective for the room-temperature Su-
zuki coupling of aryl chlorides using
1.0 ± 1.5 mol% Pd in the presence of a
stoichiometric amount of KF. These
conditions tolerate the presence of a
wide variety of functional groups, and
provide the desired products in excel-
lent yields (Table 2).[11]
[a] Reaction conditions: 1.0 equiv of aryl chloride, 1.2 equiv of amine, 1.4 equiv of NaOtBu, 1 ±
2 mol% Pd(OAc)2, 2 ± 4 mol% 4, toluene (1 mL per mmol of halide), room temperature. Reaction
times t have not been minimized. The yields given represent yields of isolated product (average of two
or more experiments) estimated to be ꢁ95% pure by 1H NMR spectroscopy and GC analysis (known
compounds) or combustion analysis (new compounds). [b] The reaction was run at 1008C using
[Pd2(dba)3] in place of Pd(OAc)2. Bn ˆ benzyl; dba ˆ trans,trans-dibenzylideneacetone.
Table 2. Room-temperature Suzuki coupling of aryl chlorides.[a]
Entry Halide
Boronic acid
Product
Mol% Pd t [h] Yield [%]
Use of catalysts based on 3 or 4 at
1008C allowed for effective Suzuki
coupling at low catalyst loadings; higher
turnover numbers were usually ob-
tained with 3 (Table 3).[12] The coupling
of 4-bromoacetophenone with phenyl-
boronic acid (entry 3) was achieved with
10 6 mol% Pd (108 turnovers),[13] al-
though a control reaction conducted in
the absence of phosphane ligands pro-
ceeded to completion with 10 3 mol%
Pd(OAc)2, suggesting that reactions of
this substrate combination are particu-
larly facile.[14] Aryl chlorides were effec-
tively coupled with 0.02 ± 0.05 mol%
Pd, the lowest catalyst loadings reported
thus far for the Suzuki coupling of aryl
chlorides.[13a]
Me
Cl
B(OH)2
OMe
Me
1
1
24
21
95
92
MeO
OMe
CN
MeO
Cl
B(OH)2 MeO
B(OH)2
2
1.5
OMe
Cl
3
4
1
1
1
24
20
2
96
91
91
CN
B(OH)2
Cl
Cl
Me(O)C
B(OH)2
MeO2C
5
C(O)Me
MeO2C
[a] Reaction conditions: 1.0 equiv of aryl chloride, 1.5 equiv of boronic acid, 3.0 equiv of KF, cat.
Pd(OAc)2, cat. 4 (two ligands per Pd center), THF (1 mL per mmol of aryl chloride), room
temperature. Reaction times t have not been minimized.
Although the reasons for the high
reductive elimination,[8] and examination of models reveals
that the o-phenyl moiety may be oriented such that a
favorable interaction between the aromatic p system and
the metal orbitals occurs.[16] This interaction may also orient
the arene ring of the substrate perpendicular to the N Pd
bond, placing it in the stereoelectronically optimum arrange-
activity of catalysts supported by 3 and 4 are not well
understood at this time, we believe that several structural
features of the ligands are of importance. The electron-rich
phosphane facilitates oxidative addition of the aryl chloride[15]
and binds tightly to the metal to prevent precipitation of the
catalyst. The steric encumbrance of the ligands promotes
2414
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3816-2414 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1999, 38, No. 16

COMMUNICATIONS
Table 3. Suzuki coupling at low catalyst loading.[a]
Herrmann, C.-P. Reisinger, M. Spiegler, J.
Organomet. Chem. 1998, 557, 93 ± 96.
c) Trudell, Nolan et al. have recently re-
ported the Suzuki coupling of aryl chlorides
using bulky, heterocyclic carbene ligands:
C. Zhang, J. Huang, M. L. Trudell, S. P.
Nolan, J. Org. Chem. 1999, 64, 3804 ± 3805.
[5] Recent work has led to the use of bulky,
electron-rich phosphanes as supporting li-
gands for palladium-catalyzed aminations,
diaryl ether formation, and Suzuki coupling
of aryl chloride substrates. These catalyst
systems, however, still require elevated
reaction temperatures, and Suzuki coupling
reactions of electron-rich aryl chlorides are
often ineffective. For catalytic amination
reactions, see ref. [6] and a) M. Nishiyama,
T. Yamamoto, Y. Koie, Tetrahedron Lett.
1998, 39, 617 ± 620; b) T. Yamamoto, M.
Nishiyama, Y. Koie, Tetrahedron Lett. 1998,
39, 2367 ± 2370; c) N. P. Reddy, M. Tanaka,
Tetrahedron Lett. 1997, 38, 4807 ± 4810;
d) B. C. Hamann, J. F. Hartwig, J. Am.
Chem. Soc. 1998, 120, 7369 ± 7370; e) X. H.
Bei, A. S. Guram, H. W. Turner, W. H.
Weinberg, Tetrahedron Lett. 1999, 40,
1237 ± 1240. For diaryl ether formation see
f) G. Mann, C. Incarvito, A. L. Rheingold,
J. F. Hartwig, J. Am. Chem. Soc. 1999, 121,
3224 ± 3225; g) A. Aranyos, D. W. Old, A.
Kiyomori, J. P. Wolfe, J. P. Sadighi, S. L.
Entry Halide
Boronic acid
Product
Mol% Pd Ligand t [h] Yield [%]
Br
B(OH)2
2
1
2 Â 10
5 Â 10
4
3
26
16
92
93
3
tBu
tBu
B(OH)2
Me
Br
Br
3
2
5 Â 10
3
20
96
tBu
Me
tBu
O
3
3
6
1 Â 10
1 Â 10
1 Â 10
4
±
4
19
19
24
96[e]
100[b]
91[c]
B(OH)2
B(OH)2
3
4
O
Me
Me
Cl
Cl
1
2
1 Â 10
5 Â 10
4
3
25
25
95
94[d]
Me
Me
B(OH)2
2
O
5
2 Â 10
4
23
92
O
Me
Me
[a] Reaction conditions: 1.0 equiv of aryl halide, 1.5 equiv of boronic acid, 2.0 equiv of K3PO4, cat.
Pd(OAc)2, cat. ligand (two ligands per Pd center), toluene (3 mL per mmol of halide), 1008C.
Reaction times t have not been minimized. All reactions proceeded to completion unless otherwise
noted. [b] Yield according to GC. [c] Result of two experiments, one proceeded to only 99%
conversion. [d] The reaction proceeded to 99% conversion. [e] Pd2(dba)3 used in place of Pd(OAc)2.
ment for reductive elimination to take place.[17] The combi-
nation of these effects serve to accelerate oxidative addition
without inhibition of transmetalation or reductive elimina-
tion.
Buchwald, J. Am. Chem. Soc. 1999, 121, 4369 ± 4378. For Suzuki
coupling see ref. [6] and h) W. Shen, Tetrahedron Lett. 1997, 38, 5575 ±
5578; i) N. A. Bumagin, V. V. Bykov, Tetrahedron 1997, 53, 14437 ±
14450; j) M. B. Mitchell, P. J. Wallbank, Tetrahedron Lett. 1991, 32,
2273 ± 2276; k) F. Firooznia, C. Gude, K. Chan, Y. Satoh, Tetrahedron
Lett. 1998, 39, 3985 ± 3988; l) B. Cornils, Org. Proc. Res. Dev. 1998, 2,
121 ± 127. m) Fu and Littke have recently reported the Suzuki coupling
of electron-rich aryl chlorides using palladium complexes with P(tBu)3
as the supporting ligand: A. F. Littke, G. C. Fu, Angew. Chem. 1998,
110, 3586 ± 3587; Angew. Chem. Int. Ed. 1998, 37, 3387 ± 3388; n) X.
Bei, T. Crevier, A. S. Guram, B. Jandeleit, T. S. Powers, H. W. Turner,
T. Uno, W. H. Weinberg, Tetrahedron Lett. 1999, 40, 3855 ± 3858.
[6] D. W. Old, J. P. Wolfe, S. L. Buchwald, J. Am. Chem. Soc. 1998, 120,
9722 ± 9723.
[7] The few previously reported methods for room-temperature Suzuki
couplings frequently require toxic additives and do not function for
aryl chloride substrates: J. C. Anderson, H. Namli, C. A. Roberts,
Tetrahedron 1997, 53, 15123 ± 15134, and references therein.
[8] Bulky ligands have been shown to accelerate other palladium-
catalyzed cross-coupling reactions: a) V. Farina in Comprehensive
Organometallic Chemistry, Vol. 12, 2nd ed., Pergamon, Oxford, 1995,
pp. 161 ± 240; b) J. F. Hartwig, S. Richards, D. Baranano, F. Paul, J.
Am. Chem. Soc. 1996, 118, 3626 ± 3633.
In conclusion, we have developed a new, highly active
catalyst system based on ligand 4 for the palladium-catalyzed
amination and Suzuki coupling of aryl chlorides at room
temperature and at low catalyst loading. Although 4 provides
better results for room-temperature reactions, 3 is frequently
more effective for reactions with low levels of the palladium
catalyst and for Suzuki coupling reactions of very hindered
substrates.[12] The mild reaction conditions employed for these
transformations provide further evidence that the oxidative
addition of aryl chlorides to complexes of Pd0 can be induced
to occur at low temperatures when catalysts which possess
suitable steric and electronic properties are used.
Received: May 7, 1999 [Z13382IE]
German version: Angew. Chem. 1999, 111, 2570 ± 2573
[9] Ligands 3 and 4 are air-stable, crystalline solids which are prepared in
one step. These ligands are now commercially available from Strem
Chemical Co.
Keywords: aminations ´ biaryls ´ catalysts ´ ligand effects ´
palladium
[10] While the scope of room-temperature aminations of aryl chlorides and
aminations at low catalyst loadings is somewhat limited, a much
broader range of substrates are efficiently coupled at higher temper-
atures (80 ± 1008C) using 0.5 ± 1.0 mol% Pd. Reactions of functional-
ized substrates may be carried out using K3PO4 in place of NaOtBu at
80 ± 1008C. These results will be reported in full papers.
[1] a) J. P. Wolfe, S. Wagaw, J.-F. Marcoux, S. L. Buchwald, Acc. Chem.
Res. 1998, 31, 805 ± 818; b) J. F. Hartwig, Angew. Chem. 1998, 110,
2154 ± 2177; Angew. Chem. Int. Ed. 1998, 37, 2046 ± 2067; c) B. H.
Yang, S. L. Buchwald, J. Organomet. Chem. 1999, 576, 125 ± 146.
[2] A. Suzuki in Metal-Catalyzed Cross-Coupling Reactions (Eds.: F.
Diederich, P. J. Stang), WILEY-VCH, Weinheim, 1998, chap. 2.
[3] V. V. Grushin, H. Alper, Chem. Rev. 1994, 94, 1047 ± 1062.
[4] a) The Herrmann/Beller palladacycle catalyst has been demonstrated
to be effective for some C C and C N bond forming reactions of aryl
chlorides at 1358C: T. H. Riermeier, A. Zapf, M. Beller, Top. Catal.
1997, 4, 301 ± 309, and references therein. b) Herrmann has demon-
strated the Suzuki coupling of 4-chloroacetophenone using palladium
complexes bearing chelating, heterocyclic carbene ligands: W. A.
[11] The scope and limitations of Suzuki couplings which employ 3 or 4 will
be the subject of a full paper.
[12] All reactions proceed to completion unless otherwise noted.
[13] Beller, Herrmann et al. and Bedford et al. have reported catalysts
which provide turnovers of 7.4 Â 104 and 1 Â 106, respectively, for this
reaction: a) M. Beller, H. Fischer, W. A. Herrmann, K. Öfele, C.
Brossmer, Angew. Chem. 1995, 107, 1992 ± 1993; Angew. Chem. Int.
Ed. Engl. 1995, 34, 1848 ± 1849; b) D. A. Albisson, R. B. Bedford, S. E.
Lawrence, P. N. Scully, Chem. Commun. 1998, 2095 ± 2096.
Angew. Chem. Int. Ed. 1999, 38, No. 16
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3816-2415 $ 17.50+.50/0
2415

COMMUNICATIONS
the phosphane system 1.[5] Herein, we show that it is the
combination of NHCs with coordinatively more labile ligands
on the ruthenium center that allows NHCs to develop their
full potential in this class of catalysts.
[14] Under these conditions, Suzuki coupling reactions of other substrates
give little or no products in the absence of phosphane ligands.
[15] a) G. O. Spessard, G. L. Meissler, Organometallic Chemistry, Prentice-
Hall, Upper Saddle River, NJ, 1996, pp. 171 ± 175; b) M. Portnoy, D.
Milstein, Organometallics 1993, 12, 1665 ± 1673.
[16] Metal ± p interactions have been observed in other palladium
complexes: a) H. Ossor, M. Pfeffer, J. T. B. H. Jastrzebski, C. H. Stam,
Inorg. Chem. 1987, 26, 1169 ± 1171; b) L. R. Falvello, J. Fornies, R.
Navarro, V. Sicilia, M. Tomas, Angew. Chem. 1990, 102, 952 ± 954;
Angew. Chem. Int. Ed. Engl. 1990, 29, 891 ± 893; c) C.-S. Li, C.-H.
Cheng, F.-L. Liao, S.-L. Wang, J. Chem. Soc. Chem. Commun. 1991,
710 ± 711; d) S. Kannan, A. J. James, P. R. Sharp, J. Am. Chem. Soc.
1998, 120, 215 ± 216.
In the catalytic cycle of olefin metathesis the mechanistic
scheme for 1 postulates the dissociation of a phosphane ligand
as the key step in the dominant reaction pathway.[6] Theoret-
ical investigations of Group 11 transition metal NHC com-
plexes, which suggest a strong metal ± NHC bond,[7] raise the
question as to whether this mechanism can be transferred to
metathesis catalysts of type 2. To address this problem we
calculated the dissociation energies of NHC and phosphanes
for ruthenium ± alkylidene model compounds (Figure 1) ac-
[17] Biaryl-forming reductive elimination from PtII has been postulated to
occur via a transition state in which both aryl groups are perpendicular
to the coordination plane: P. S. Braterman, R. J. Cross, G. B. Young, J.
Chem. Soc. Dalton Trans. 1 1977, 1892 ± 1897.
Figure 1. Model compounds for the calculation of ligand dissociation
energies.
Highly Active Ruthenium Catalysts for Olefin
Metathesis: The Synergy of N-Heterocyclic
Carbenes and Coordinatively Labile Ligands**
cording to Equation (1) by density functional (DFT) meth-
ods.[8, 9] The results compiled in Table 1 demonstrate that the
ligand dissociation energies ascend in the series PH3 <
PMe3 < NHC.[10a] As a consequence of the higher coordina-
Thomas Weskamp, Florian J. Kohl,
Wolfgang Hieringer, Dieter Gleich, and
Wolfgang A. Herrmann*
tion energy the dicarbene complexes
2
should
N-Heterocyclic carbenes (NHCs) have been established in
homogeneous catalysis to complement and extend the capa-
bilities of the ubiquitous phosphanes.[1, 2] In olefin meta-
thesis[3] ruthenium alkylidene compounds 2[4] bearing two
NHC ligands exhibit a catalytic activity comparable to that of
Table 1. Calculated ligand dissociation energies DE [kcalmol 1] for the
model compounds as depicted in Equation (1).[a]
Model compound
DE for PH3 DE for PMe3 DE for NHC
1m (L1 ˆ L2 ˆ PH3)
18.2 (19.4)
±
±
18.7 (15.8)
±
±
±
±
1n (L1 ˆ L2 ˆ PMe3)
27.0 (25.8)
±
±
2m (L1 ˆ L2 ˆ NHC)
3m (L1 ˆ PH3; L2 ˆ NHC)
3n (L1 ˆ PMe3; L2 ˆ NHC)
45.0 (42.2)
46.9 (49.7)
42.0 (43.4)
26.0 (24.9)
[a] Ligand dissociation energies without ethylene coordination are given in
brackets.
disfavor a dissociative pathway similar to that of 1.[6] A mixed
NHC/phosphane complex of type 3, however, reveals a
phosphane dissociation energy in the same order of magni-
tude as 1. Therefore, 3 should be able to populate the
dissociative pathway[6] just as readily as 1. In contrast to 1,
however, a phosphane-free species A is considered as the key
intermediate in the catalytic cycle.
[*] Prof. Dr. W. A. Herrmann, Dipl.-Chem. T. Weskamp,
Dipl.-Chem. F. J. Kohl, Dipl.-Chem. W. Hieringer,
Dipl.-Chem. D. Gleich
Anorganisch-chemisches Institut der
Technischen Universität München
Lichtenbergstrasse 4, D-85747 Garching (Germany)
Fax: (‡49)89-289-13473
E-mail: lit@arthur.anorg.chemie.tu-muenchen.de
[**] This work received generous support from the Fonds der Chemischen
Industrie (PhD fellowship to T.W.), the Bayerische Forschungsstif-
tung (Bayerischer Forschungsverbund Katalyse, FORKAT), the
Leibniz-Rechenzentrum München, the Deutsche Forschungsgemein-
schaft, Aventis R&T, and Degussa AG (loans of RuCl3). Assistance by
Ania Jarnicka and Juliana Marcussi Alves is gratefully acknowledged.
The air-stable NHC/phosphane complexes 3a ± c are acces-
sible in excellent yields by adding 1.2 equivalents of the
appropriate NHC to a solution of 1 in THF.[11] Low temper-
ature is crucial for the selectivity of the phosphane/NHC
substitution reaction. At room temperature the selectivity is
2416
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3816-2416 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1999, 38, No. 16