Paper
Catalysis Science & Technology
1
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Lett., 2013, 54, 3457.
Conclusions
In conclusion, two new palladium(II) carbene complexes were
synthesized and studied for their catalytic activities in the
Suzuki reaction. Their comparative behaviour in toluene,
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2 2 2
CH Cl and H O were assessed. Both 1 and 2 were able to
activate C–I, C–Br, and C–Cl bonds in polar and nonpolar
21 S. Wei, Z. Ma, P. Wang, Z. Dong and J. Ma, J. Mol. Catal. A:
Chem., 2013, 370, 175.
aprotic solvents, as well as in H O. However, it was shown
2
that catalyst decomposition was prevalent in toluene and
H O, yielding new, presumably heterogeneous, yet equally
2
22 J. Flapper and J. N. H. Reek, Angew. Chem., Int. Ed., 2007,
46, 8590.
active catalyst species. In CH Cl , the molecular catalysts
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2012, 717, 141.
24 C. Crisóstomo-Lucas, R. A. Toscano and D. Morales-Morales,
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27 K. Yu, W. Sommer, J. Richardson, M. Weck and C. Jones,
Adv. Synth. Catal., 2005, 347, 161.
28 G. Kyriakou, S. K. Beaumont, S. M. Humphrey, C. Antonetti
and R. M. Lambert, ChemCatChem, 2010, 2, 1444.
29 T. D. Sheppard, Org. Biomol. Chem., 2009, 7, 1043.
30 R. R. Butorac, S. S. Al-Deyab and A. H. Cowley, Molecules,
2011, 16, 2285.
2
2
were largely stable and recyclable. In toluene solution, both
forms of the catalyst were found to co-exist. Kinetic studies
revealed that the average activation energy for dissociation of
−
1
the C–Br bond in organic solvents was 166 kJ mol com-
−
1
pared to only 113 kJ mol for H O. The latter result infers
2
that the heterogeneous species generated from the original
Pd(II) carbene complexes were more reactive for the Suzuki
coupling reaction, and that catalyst decomposition was not
actually detrimental in the Suzuki–Miyaura cross-coupling
reaction. The results of this study exemplify how well-defined
molecular and less well-defined heterogeneous Pd species can
provide misleadingly similar results in solution-phase catalysis.
3
1 S. A. Svejda, M. Brookhart and C. Hill, Organometallics,
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Acknowledgements
1
3
3
2 T. Tu, W. Fang and J. Jiang, Chem. Commun., 2011, 47, 12358.
3 W. Fang, J. Jiang, Y. Xu, J. Zhou and T. Tu, Tetrahedron,
The authors thank Ms. Stephany Garcìa for supplying the
Pd–PVP catalysts and Dr. Vincent M. Lynch for assisting in
X-ray crystallography, and the Robert A. Welch Foundation
2
013, 69, 673.
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3
3
(F-1738, F-003) for funding.
2
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1464 | Catal. Sci. Technol., 2014, 4, 1456–1464
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