Table 3 Poisoning and catalyst minimizing experiments of palladium
complex 3 catalyzed Suzuki–Miyaura reaction in watera
catalyst, which may be attributed to the steady Pd–C imposed
by the stronger s-donating property of benzimidazolylidene
compared to imidazolylidene and hardly broken to form Pd
nanoparticles during the reaction process.
Acknowledgements
Entry Catalyst (mol%) Additive or Note
Time (h) Yield (%)b
Financial support from Fudan University, the National Science
Foundation of China (No. 20902011), the Shanghai Municipal
Science and Technology Commission (Qimingxing Program No.
10QA1400500) and Shanghai Key Laboratory of Molecular Cata-
lysts and Innovative Materials, Department of Chemistry, Fudan
University (2010MCIMKF04), is gratefully acknowledged. We
thank Prof. Shu-Li You and Mingli Deng for technical assistance
(GC-MS and CEM Discover microwave instrument).
1
2
3
4
5
0.005
0.005
0.005
0.005
0.005
Hg (1 drop after 0 min)
Hg (1 drop after 15 min)
Hg (1 drop after 30 min)
Hg (1 drop after 45 min)
PVPy (300 eq. of
8
5
5
5
5
61
84
94
99
92
palladium)
6
7
0.0008
0.000008
TON: 1.188 ¥ 105
24
96
95
61
TON: 7.625 ¥ 106
a All reaction were carried out in water under air b Isolated yield.
Notes and references
1 For recent reviews, see: (a) F. E. Hahn and M. C. Jahnke, Angew. Chem.,
Int. Ed., 2008, 47, 3122; (b) “Recent developments in the organometallic
chemistry of N-heterocyclic carbenes”, ed. R. H. Crabtree, Coord. Chem.
Rev., 2007, 251, 595; (c) “N-Heterocyclic Carbenes in Transition Metal
Catalysis”, ed. F. Glorius, Top. Organomet. Chem., 2007, 21, 1–218;
(d) N. Marion, S. D´ıez-Gonza´lez and S. P. Nolan, Angew. Chem., Int.
Ed., 2007, 46, 2988; (e) O. Ku¨hl, Chem. Soc. Rev., 2007, 36, 592; (f) F.
E. Hahn, Angew. Chem., Int. Ed., 2006, 45, 1348.
2 (a) J. Dupont, C. S. Consorti and J. Spencer, Chem. Rev., 2005, 105,
2527; (b) M. E. van der Boom and D. Milstein, Chem. Rev., 2003, 103,
1759; (c) M. Albrecht and G. van Koten, Angew. Chem., Int. Ed., 2001,
40, 3750.
3 (a) D. Benito-Garagorri and K. Kirchner, Acc. Chem. Res., 2008, 41,
201; (b) D. Pugh and A. A. Danopoulos, Coord. Chem. Rev., 2007, 251,
610; (c) E. Peris and R. H. Crabtree, Coord. Chem. Rev., 2004, 248,
2239; (d) P. A. Chase, M. Lutz, A. L. Spek, G. P. M. van Klink and G.
van Koten, J. Mol. Catal. A: Chem., 2006, 254, 2; (e) “The Chemistry of
Pincer Compounds”, 1st. edn, ed. D. Morales-Morales, M. G. Jensen,
Elsevier, Amsterdam, 2007.
poly(4-vinylpridine)(PVPy) was also included in the catalyst
poisoning experiments.10a,16 Whereas, a 92% isolated yield further
confirmed the molecular catalyst played the “real role” in the
reaction, which is consistent with the results of the couplings
with heterocyclic bromoarenes. The different active species de-
rived from pyridine-bridged bis-benzimidazolylidene palladium
complex 3 and its imidazolium analogues 2b may arise from
the stronger s-donating property of benzimidazolylidene which
stabilized the Pd–C and avoided the formation of Pd nanoparticles
during the reflux.
As a practical protocol, the possibility of minimizing and
recycling of the catalyst is the key issue. Decreasing the catalyst
loading to 8 ppm, the model reaction resulted in a 98% isolated
yield which extended the reaction time to 24 h (TON: 1.188 ¥ 105,
Table 3, entry 6). Even with 8 ppb catalyst loading, the reaction still
carried out very smoothly and a 61% isolated yield was observed
for 7aa after 96 h (Table 3, entry 7). With these attractive results
in hand, our attention was then focused on the recovery and
reusability of the catalyst. Because of the insolubility of most
products 7a–o in water, the solid biaryls were easily isolated by
simply filtering after reaction and the filtrate containing catalyst
3 could be reused for the next catalytic cycle. By adding the
substrates and base in the filtrate of the model reaction, there
was no obvious loss of catalytic activity (up to three times) and
almost quantitative yields were observed (See ESI†). Although,
no palladium black appeared, the yield of the further run
dropped dramatically which may be caused by catalyst loss during
filtration.
4 C. Fleckenstein, S. Roy, S. Leutha¨uber and H. Plenio, Chem. Commun.,
2007, 2870.
5 (a) A. Chanda and V. V. Fokin, Chem. Rev., 2009, 109, 725;
(b) “Aqueous-Phase Organometallic Chemistry”, ed. B. Cornils, W. A.
Herrmann, Wiley-VCH, Weinheim, 2004.
6 (a) T. Tu, X. Bao, W. Assenmacher, H. Peterlik, J. Daniels and K. H.
Do¨tz, Chem.–Eur. J., 2009, 15, 1853; (b) T. Tu, W. Assenmacher, H.
Peterlik, R. Weisbarth, M. Nieger and K. H. Do¨tz, Angew. Chem., Int.
Ed., 2007, 46, 6368.
7 T. Tu, W. Assenmacher, H. Peterlik, G. Schnakenburg and K. H. Do¨tz,
Angew. Chem., Int. Ed., 2008, 47, 7127.
8 (a) T. Tu, H. Mao, C. Herbert, M. Xu and K. H. Do¨tz, Chem. Commun.,
2009, DOI: 10.1039/c0cc03107k; (b) T. Tu, J. Malineni, X. Bao and K.
H. Do¨tz, Adv. Synth. Catal., 2009, 351, 1029; (c) T. Tu, J. Malineni and
K. H. Do¨tz, Adv. Synth. Catal., 2008, 350, 1791.
9 P. G. Steel and W. T. Teasdale, Tetrahedron Lett., 2004, 45, 8977.
10 (a) B. Ine´s, R. SanMartin, M. J. Moure and E. Dom´ınguez, Adv. Synth.
Catal., 2009, 351, 2124; (b) F. Churruca, R. SanMartin, B. Ine´s, I.
Tellitu and E. Dom´ınguez, Adv. Synth. Catal., 2006, 348, 1836.
11 C. M. Amb and S. C. Rasmussen, J. Org. Chem., 2006, 71, 4696.
12 (a) S. Zhang, S. Yang, J. Lan, S. Yang and J. You, Chem. Commun.,
2008, 6170; (b) Z Xi, F. Liu, Y. Zhou and W. Chen, Tetrahedron, 2008,
64, 4254.
13 J. A. Loch, M. Albrecht, E. Peris, J. Mata, J. W. Faller and R. H.
Crabtree, Organometallics, 2002, 21, 700.
14 J.-P. Corbet and G. Mignani, Chem. Rev., 2006, 106, 2651.
15 (a) E. Peris and R. H. Crabtree, Coord. Chem. Rev., 2004, 248, 2239;
(b) E. Peris, J. A. Loch, J. Mata and R. H. Crabtree, Chem. Commun.,
2001, 201.
In conclusion, a robust hydrophilic pyridine-bridged bis-
benzimidazolylidene pincer palladium complex 3, which is readily
accessible from inexpensive and commercial precursors, demon-
strated excellent catalytic activity towards Suzuki–Miyaura cou-
pling reactions and tolerated a variety of functional groups varying
in their electronic and steric properties in aqueous media with
a catalyst loading down to the ppm scale. As a practical and
environmentally-friendly protocol, the products could be easily
separated after reaction by simple filtration and the filtrate could
be reused as a catalyst several times. In contrast to imdazolium
analogues, the palladium pincer complex 3 acts as a molecular
16 K. Yu, W. Sommer, J. M. Richardson, M. Weck and C. W. Jones, Adv.
Synth. Catal., 2005, 347, 161.
10600 | Dalton Trans., 2010, 39, 10598–10600
This journal is
The Royal Society of Chemistry 2010
©