Pyrazole and benzothiazole palladacycles: Stable and efficient catalysts for carbon-carbon bond formation

Article abstract of DOI:10.1039/b005452f

Cyclopalladated phosphine free pyrazole complexes 1 and benzothiazole complexes 2 are excellent catalysts for the Heck vinylation of aryl iodides and termolecular queuing cascades, leading to turnover numbers of > 10⁶ in some cases, at moderate

Full text of DOI:10.1039/b005452f

Pyrazole and benzothiazole palladacycles: stable and efficient catalysts for
carbon–carbon bond formation
Xinjie Gai,a Ronald Grigg,*a M. Imran Ramzan,a Visuvanathar Sridharan,a Simon Collardb and
Jayne E. Muirb
a Molecular Innovation, Diversity and Automated Synthesis (MIDAS) Centre, School of Chemistry, Leeds University,
Leeds, UK LS2 9JT. E-mail: R.Grigg@chem.leeds.ac.uk
b Johnson Matthey, Orchard Road, Royston, Herts, UK SG8 5HE
Received (in Cambridge, UK) 1st August 2000, Accepted 11th September 2000
First published as an Advance Article on the web 2nd October 2000
Table 1 Heck coupling of iodobenzene with n-butyl acrylate using pyrazole
Cyclopalladated phosphine free pyrazole complexes 1 and
and benzothiazole palladacycles at 100 °C in DMF
benzothiazole complexes 2 are excellent catalysts for the
Heck vinylation of aryl iodides and termolecular queuing
Catalyst
Conversiona
(%)
cascades, leading to turnover numbers of > 106 in some
cases, at moderate temperatures; these catalysts show high
thermal stability and are not sensitive to moisture and air.
Entry
(1 mol %)
T/°C
t/min
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
2a
2b
2c
2f
100
100
100
100
100
100
110
110
100
110
60
15
90
45
15
100
100
100
100
100
100
95
93
100
100
Palladium salts/complexes are exceptionally versatile and
robust catalysts for the construction of carbon–carbon and
carbon–heteroatom bonds. Recently a number of novel phos-
phapalladacycles1–7 as well as cyclometallated imine com-
plexes8 have been used in the Heck coupling of aryl halides and
acrylates with TONs (turnover numbers) of 105–106. Su-
zuki1,5,8,9 and Stille cross couplings1,5 have also been performed
using some of these palladacycle catalysts with varying TONs
of 102–106. These catalysts exhibit higher air and thermal
60
180
180
30
10
180
a Conversion estimated by NMR spectroscopy.
stability compared with conventional Pd(
Pd(PPh3)4.
0) catalysts e.g.
entries 4 and 9). Exchanging the R1 and R2 methyl groups for an
electron withdrawing trifluoromethyl group in 1 also perturbs
the rate. Thus replacing the C(3) methyl group in 1a by CF3
results in a more active catalyst (Table 1, entries 1 and 2; entries
4 and 5) whilst replacing the C(5) methyl group does not
produce rate acceleration (Table 1, entries 1 and 3; entries 4 and
6). The role of the fluoro and trifluoromethyl substituents is to
perturb the C–Pd covalent bond, and N–Pd dative bond. When
these substituents are optimally located they facilitate a
controlled breakdown of the Pd(II) complexes 1 and 2
furnishing the active catalytic species. These palladacycles
The use of phosphine free, nitrogen based ligands in the Heck
reaction is a neglected area. We report here that palladacycles 1
and 210 are outstanding catalysts for the Heck reaction and
could operate via a Pd(II)/Pd(IV)
12 or Pd(0)/Pd(II) catalytic cycle.
It appears probable, on our current evidence that the active
species are Pd(0) nanoparticles.13,14
Next we briefly studied the effects of base in the Heck
reaction employing milder conditions. These results are sum-
marised in Table 2.
Although in some cases competitive reduction of the aryl
iodide bond was observed, the use of sodium and potassium
formate was found to accelerate the rate of the reaction. When
potassium carbonate was employed as the base, the product was
furnished in 80% isolated yield (entry 5) after 2 h. However
lower isolated yields of product (65–70%) were obtained after
TONs of > 106 can be achieved at 70–110 °C. Palladium
complexes 1 and 2 are easily prepared, show high thermal
stability and are not sensitive to moisture or air. Recently
Milstein and coworkers reported non-phosphine cyclopalla-
dated imine complexes for Heck reactions.8
Initially we explored the Heck reaction between iodobenzene
and n-butyl acrylate (Scheme 1) in the presence of 1 mol%
catalyst in DMF at 100 °C (Table 1).
Introducing fluorine (2I and p donor) substituents in the
heterocyclic ligand para and/or ortho to the C–Pd bond
increases the rate of the reaction (Table 1, entries 1, 4 and 9).
Dual o- and p-fluoro substituents enhance this trend (Table 1,
Table 2 Effect of base on the Heck coupling of iodobenzene with n-butyl
acrylate using pyrazole and benzothiazole palladacycles at 70 °C in DMF
Catalyst
(1 mol%)
Base
(2 eq.)
Conversiona
(%)
Entry
t/h
1
2
3
4
5
6
7
1b
1b
1b
1b
1e
1e
1e
HCO2Na
K2CO3
HCO2K
MeCO2Na
K2CO3
1
2
1
1
2
1
1
100
61
100
29
93
100
100
HCO2Na
HCO2K
a Conversion estimated by NMR spectroscopy.
Scheme 1
DOI: 10.1039/b005452f
Chem. Commun., 2000, 2053–2054
This journal is © The Royal Society of Chemistry 2000
2053

Table 3 Heck coupling of iodobenzene (1 mmol) with n-butyl acrylate (2 mmol) using pyrazole and benzothiazole palladacycles for 48 h at 90–110 °C in
DMF
Entry
Catalyst/mmol
Base (2 mmol)
T/°C
Conversiona (%)
TONb
Yieldc (%)
1
2
3
4
5
6
7
8
9
1b (5 3 1027
1b (5 3 1027
)
)
)
HCO2Na
MeCO2Na or MeCO2Cs
HCO2Na
MeCO2Na
HCO2Na
HCO2Na
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
100
100
100
100
100
100
110
90
75
< 10
74
0
93
100
100
78
98
98
1.5 3 106
—
—
—
—
—
—
50
78
—
—
—
—
1e (5 3 1027
1e (5 3 1027
1.48 3 106
—
)
)
1f (5 3 1027
1a (5 3 1027
1a (5 3 1027
1.8 3 106
2 3 106
2 3 106
7.8 3 105
9.8 3 104
9.8 3 104
1 3 105
)
)
2c (1 3 1026
)
2b (1 3 1025
2e (1 3 1025
2f (1 3 1025
)
90
90
90
10
11
)
)
100
a Conversion estimated by NMR spectroscopy. b TON based on consumption of iodobenzene. c Isolated yields.
1 h when either sodium or potassium formate were used (entries
1, 3, 6 and 7). Sodium acetate was the least effective of the bases
evaluated (entry 4), with only 29% conversion after 1 h.
Turnover numbers are summarised in Table 3 for the Heck
reaction of iodobenzene (1 mmol) and n-butyl acrylate (2
mmol) using either HCO2Na (2 mmol) or K2CO3 (2 mmol) as
base in DMF at 90–110 °C for 48 h.
We thank Leeds University, the ORS (X. G.) and Johnson
Matthey for support.
Notes and references
1 For a recent review, see: W. A. Herrmann, V. P. W. Bohm and C.-P.
Reisinger, J. Organomet. Chem., 1999, 576, 23.
2 W. A. Herrmann, C. Brossmer, K. Ofele, C.-P. Riesinger, T. Priermeier,
M. Beller and H. Fischer, Angew. Chem., Int. Ed. Engl., 1995, 34, 1844;
M. Beller, H. Fischer, W. A. Herrmann, K. Ofele and C. Brossmer,
Angew. Chem., Int. Ed. Engl., 1995, 34, 1848.
3 M. Ohff, A. Ohff, M. E. Van der Boom and D. Milstein, J. Am. Chem.
Soc., 1997, 119, 11687; K. Kiewel, Y. Liu, D. E. Bergbreiter and G. A.
Sulikowski, Tetrahedron Lett., 1999, 40, 8945.
Although TONs of up to 2 3 106 can be achieved when
employing sodium formate as base (entry 6) competitive
reduction of the aryl iodide bond results in lower isolated yield
of Heck product (50%). Employing potassium carbonate as base
resulted in a TON of 2 3 106 (entry 7) and 78% isolated
yield.
Finally we have briefly explored the scope of these catalysts
in a three-component cascade process.15 Typical examples are
shown in Scheme 2.
4 B. L. Shaw, S. D. Perera and E. E. Staley, Chem. Commum., 1998,
2095.
5 D. A. Albisson, R. B. Bedford, S. E. Lawrence and P. N. Scully, Chem.
Commun., 1998, 2095.
6 F. Miyazaki, K. Yamaguchi and M. Shibasaki, Tetrahedron Lett., 1999,
40, 7379.
7 I. P. Beletskaya, A. V. Churchurjukin, H. P. Dijkstra, G. P. M. Van
Klink and G. VanKoten, Tetrahedron Lett., 2000, 41, 1075.
8 M. Ohff, A. Ohff and D. Milstein, Chem. Commun., 1999, 357; H.
Weissman and D. Milstein, Chem. Commun., 1999, 1901; D. A. Alonso,
C. Najera and M. C. Pacheco, Org. Lett., 2000, 13, 1823.
9 W. A. Herrmann, M. Elison, J. Fischer, C. Kocher and G. R. J. Artus,
Angew. Chem., Int. Ed. Engl., 1995, 34, 2371; W. A. Herrmann, C.-P.
Reisinger and M. J. Splieger, J. Organomet. Chem., 1998, 557, 93.
10 1a–f and 2a–f are new compounds prepared from the ligand (1 eq.) and
Pd(OAc)2 (1 eq.) in acetic acid and 100–120 °C over 2–5 h. The ligands
were prepared by conventional ring synthetic methods for pyrazoles and
benzothiazoles.11
11 Pyrazoles: J. Elguero, in Comprehensive Heterocyclic Chemistry, ed.
A. R. Katritsky and C. W. Rees, Pergamon, 1st edn., 1984, vol. 5, p. 167
et seq; benzothiazoles: J. Metzger, Comprehensive Heterocyclic
Chemistry, ed. A. R. Katritsky and C. W. Rees, Pergamon, 1st edn.,
1984, vol. 6, p. 235 et seq.
12 B. L. Shaw, New J. Chem., 1998, 77.
13 D. G. Blackmond, J. S. Bradly, J. Lebers and U. Specht, Langmuir,
1999, 15, 7621.
Scheme 2
In conclusion, we have developed a range of non-phosphine
palladacycles which are efficient catalysts with high TONs for
Heck reactions under mild conditions and with relatively short
reaction times. Studies on other Heck substrates and the
mechanism are in progress.
14 M. Reetz and A. Westermann, Angew. Chem., Int. Ed., 2000, 39, 165.
15 R. Grigg and V. Sridharan, J. Organomet. Chem., 1999, 576, 65.
2054
Chem. Commun., 2000, 2053–2054