Table 1 Comparison of yields for catalysts 2–4 for the reaction of PhI with
styrene in refluxing DEA to form trans-stilbene (under Ar with 0.2 mol%
catalyst and NaOAc)a
DMSO solution. Data collection: Siemens Smart CCD diffractometer
(l = 0.71073 Å). The structure was solved by direct methods and was
refined using the SHELXTL 5.1 software package. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were assigned to ideal
positions and refined using a riding model. CCDC 182/1880. See http:/
format. The diffraction frames were integrated using the SAINT17 package
and corrected for absorption with SADABS.18
§ General Heck procedure: NaOAc (360 mg, 4.4 mmol) and the catalyst
were placed in a 3-necked flask fitted with a reflux condenser and degassed.
Aryl halide (4 mmol), styrene (640 mL, 5.6 mmol), and solvent (DMA or
DEA, 5 mL), were added under Ar or air. The reaction vessel was placed
into an oil bath preheated to the desired temperature. Aliquots (200 mL)
were removed after fixed times and added to 10 mL CH2Cl2. The organic
layer was extracted five times with 10 mL portions of water and dried with
MgSO4. The mixture was then filtered and the CH2Cl2 removed in vacuo.
The residue was dissolved in CDCl3 or CD2Cl2 and analyzed by 1H NMR
(400 MHz).
Entry
Catalyst 0.5 h
1 h
1.5 h
2 h
4 h
1b
2
3
2
3
4
48, 494 58, 298 67, 230 73, 188 85, 109
36, 370 48, 247 62, 213 72, 185 84, 108
50, 514 61, 314 67, 230 75, 193 90, 116
a Reported as: yield (%), TOF [mol product/(mol Pd 3 h)]. Yield
determined by 1H NMR based on amount of product vs. amount of starting
material remaining. b Average of two runs.
Table 2 Heck reaction between aryl halides and styrene to form trans-
stilbene. All reactions carried out in refluxing DMA with NaOAc as base
Stilbene TOF
Entry
(air/Ar) Aryl halide
2
Reaction yield
[mol prod./
(mol Pd)(h)]
1 R. H. Crabtree, The Organometallic Chemistry of the Transition Metals,
3rd edn., Wiley, New York, 2001.
(mol %) time/h
(%)a
2 P. Garrou, Chem. Rev., 1981, 81, 229.
1 (Ar)
2 (air)
3 (air)
4 (air)
5 (air)
PhBr
PhBr
PhI
PhI
PhI
5
5
5
1
1
1
1
1
20
20
> 99
> 99
> 99
89
33
75
20
20
20
89
16.500
15
3 M. J. Burk and R. H. Crabtree, J. Am. Chem. Soc., 1987, 109, 8025.
4 A. J. Arduengo, III, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361; A. J. Arduengo, III, Acc. Chem. Res., 1999, 32, 913.
5 K. Öfele, W. A. Herrmann, D. Mihalios, M. Elison, E. Herdtweck, W.
Scherer and J. Mink, J. Organomet. Chem., 1993, 459, 177.
6 (a) V. P. W. Böhm, C. W. K. Gstottmayr, T. Weskamp and W. A.
Herrmann, J. Organomet. Chem., 2000, 595, 186; (b) J. Schwarz, V. P.
W. Böhm, M. G. Gardiner, M. Grosche, W. A. Herrmann, W. Hieringer
and G. Raudaschl-Sieber, Chem. Eur. J., 2000, 6, 1773; (c) T.
Weskamp, V. P. W. Böhm and W. A. Herrmann, J. Organomet. Chem.,
1999, 585, 348; (d) W. A. Herrmann, C.-P. Reisinger and M. Spiegler,
J. Organomet. Chem., 1998, 557, 93; (e) W. A. Herrmann, M. Elison, J.
Fischer, C. Köcher and G. R. J. Artus, Angew. Chem., Int. Ed. Engl.,
1995, 34, 2371; (f) C. Zhang and M. L. Trudell, Tetrahedron Lett., 2000,
41, 595; (g) C. Zhang, J. Huang, M. L. Trudell and S. P. Nolan, J. Org.
Chem., 1999, 64, 3804; (h) J. Huang and S. P. Nolan, J. Am. Chem. Soc.,
1999, 121, 9889; (i) D. S. McGuiness and K. J. Cavell, Organometallics,
2000, 19, 741; (j) D. S. McGuiness, K. J. Cavell, B. W. Skelton and
A. H. White, Organometallics, 1999, 18, 1596.
0.0001
5
6 (Ar)b p-(CHO)C6H4Cl
a Yield determined by 1H NMR based on amount of product vs. amount of
starting material remaining. b In the presence of n-Bu4NBr (20 mol% vs.
Pd).
apparatus open to the air (Table 2, entry 3). A yield of 89% can
still be obtained in only 1 h if 1 mol% of 2 is used (Table 2, entry
4). To see if the TOF could be improved at low loading, we find
that as little as 1024 mol% of 2 still gives a TOF of 16.500 after
20 h under air (Table 2, entry 5). Aryl chlorides react more
slowly (Table 2, entry 6). Other alkenes react satisfactorily—for
example, n-butyl acrylate and PhI give the Heck product in 99%
yield after 1 h in refluxing DMA with 1 mol% catalyst.
7 S. R. Stauffer, S. Lee, J. P. Stambuli, S. I. Hauck and J. F. Hartwig, Org.
Lett., 2000, 2, 1423; J. Huang, G. Grasa and S. P. Nolan, Org. Lett.,
1999, 1, 1307.
In view of recent studies that find evidence that the active
species can be metallic palladium,13 we checked 2–4 for
heterogeneity by the Hg drop test.14 Heck catalysis with 2–4
was unaffected by the presence of Hg, and no induction period
is observed for 2, so a homogeneous active species is likely.
On the standard model of the Heck reaction, with
[Pd(0){PR3}2] as the key intermediate, a pincer carbene might
seem to be a poor choice, even if the carbene is an acceptable
replacement for the tertiary phosphine of the standard system.
8 C. W. Bielawski and R. H. Grubbs, Angew. Chem., Int. Ed., 2000, 39,
2903; M. Scholl, S. Ding, C. W. Lee and R. H. Grubbs, Org. Lett., 1999,
1, 953; T. Weskamp, F. J. Kohl, W. Hieringer, D. Gleigh and W. A.
Herrmann, Angew. Chem., Int. Ed., 1999, 38, 2416; U. Frenzel, T.
Weskamp, F. J. Kohl, W. C. Schattenman, O. Nuyken and W. A.
Herrmann, J. Organomet. Chem., 1999, 586, 263; J. Huang, E. D.
Stevens, S. P. Nolan and J. L. Petersen, J. Am. Chem. Soc., 1999, 121,
2674; J. K. Huang, H. J. Schanz, E. D. Stevens and S. P. Nolan,
Organometallics, 1999, 18, 5375; S. B. Garber, J. S. Kingsbury, B. L.
Gray and A. H. Hoveyda, J. Am. Chem. Soc., 2000, 122, 8168.
9 C. M. Jensen, Chem. Commun., 1999, 2443; C. J. Moulton and B. L.
Shaw, J. Chem. Soc., Dalton Trans., 1976, 1020.
10 (a) D. Morales-Morales, R. Redón, C. Yung and C. M. Jensen, Chem.
Commun., 2000, 1619; (b) D. Morales-Morales, C. Grause, K. Kasaoka,
R. Redón, R. E. Cramer and C. M. Jensen, Inorg. Chim. Acta, 2000,
300–302, 958; (c) M. Ohff, A. Ohff, M. E. van der Boom and D.
Milstein, J. Am. Chem. Soc., 1997, 119, 11687; (d) I. P. Beletskaya,
A. V. Chuchurjukin, H. P. Dijkstra, G. P. M. van Klink and G. van
Koten, Tetrahedron Lett., 2000, 41, 1075; (e) D. E. Bergbreiter, P. L.
Osburn and Y. S. Liu, J. Am. Chem. Soc., 1999, 121, 9531.
11 J. C. C. Chen and I. J. B. Lin, J. Chem. Soc., Dalton Trans., 2000,
839.
On
the
Amatore–Jutand
model,15
however,
[Pd(0)(OAc){PR3}2]2 is the key intermediate. Our work
supports this model if the pyridine part of the pincer ligand is
considered as replacing the OAc group. It is true that
[Pd(0)XL2]2 would normally be expected to adopt a trigonal
geometry, but the presumed intermediate Pd(0) form of the
metal being d10, there should be no strong penalty to adopt the
pincer geometry. Eisenstein and Clot16 are currently looking at
such mechanistic issues in detail.
The results are of interest not so much as an advance in Heck
catalysis—other catalysts can be better6,10—but as an indication
that chelating carbenes can provide ligand systems that give
high catalytic activity with excellent stability, even in air. This
approach should be widely applicable to the development of
non-phosphine late metal homogeneous catalysis.
12 W. A. Herrmann, C. Brossmer, C.-P. Reisinger, T. H. Riermeier, K.
Öfele and M. Beller, Chem. Eur. J., 1997, 3, 1357.
13 M. Nowotny, U. Hanefeld, H. van Koningsveld and T. Maschmeyer,
Chem Commun., 2000, 1877; M. T. Reetz and E. Westermann, Angew.
Chem., Int. Ed., 2000, 39, 165.
We thank US DOE (J. A. L.) and NSF (R. H. C.) for funding
and Matthew Torres for some preliminary observations.
14 D. R. Anton and R. H. Crabtree, Organometallics, 1983, 2, 855; P.
Foley, R. DiCosimo and G. M. Whitesides, J. Am. Chem. Soc., 1980,
102, 6713.
Notes and references
‡ Crystal data for 2: C11H7Br2N5Pd·H2O, monoclinic, space group P2(1)/n,
a = 8.7782(4), b = 14.3181(6), c = 13.5684(6) Å, a = 90, b =
107.1600(10), g = 90°, Z = 4, D = 2.011 g cm23, m = 6.044 mm21, 5300
measured reflections, 1467 [R(int) = 0.0310] independent reflections, R =
0.0225 [F > 2s(F)]. Crystals were grown by diffusion of CH2Cl2 into a
15 C. Amatore and A. Jutand, Acc. Chem. Res., 2000, 33, 314.
16 O. Eisenstein and E. Clot, personal communication, 2000.
17 SAINT, version 5.0, Bruker Analytical X-ray Systems, Madison, WI.
18 G. M. Sheldrick, SADABS empirical absorption program, University of
Go¨ttingen, 1996.
202
Chem. Commun., 2001, 201–202