Heck-type carbon-carbon coupling
N-Heterocyclic Carbenes as Ligands in Homogeneous Catalysis
N-heterocyclic carbenes have recently been established in homogeneous catalysis to complete and extend the capabilities of the ubiquitous phosphines. They are yielded by deprotonation of the corresponding imidazolium salt with NaH in a mixture of liquid ammonia and thf. This route is suitable NHC with a large range of N-substituents. These ligands have successfully been tested in different C-C-coupling reactions.
IntroductionTransition metal catalyzed C-C bond formation is a focal issue in both organic synthesis and polymer chemistry. One of these C-C coupling processes is olefin metathesis encompassing reactions such as ring-opening metathesis polymerisation (ROMP), acyclic diene metathesis reaction (ADMET), ring-closing metathesis (RCM) and metathesis of acyclic olefins. In this area the very active but also highy moisture and air sensitive molybdenum system developed by Schrock and Grubbs’ less active but highly tolerant ruthenium-phosphine complexes have conveyed much progress.
We developed three generations of ruthenium alkyilidene complexes bearing NHC ligands. In our first approach both phosphine ligands of Grubbs’ complex were replaced by NHC ligands. The resulting complexes show a high stability towards air and moisture but a slightly decreased activity compared with the bisphosphine complex.
Since the dissociation of one ligand is postulated in the established mechanism by Herisson and Chauvin further approaches in catalyst design were aimed to combine the stable NHC ligand with a labile ligand. Supported by DFT calculations the second generation of catalysts realized this principle by combining one NHC ligand with one remaining phosphine ligand.
For the third and most active generation of catalysts easily dissociating transition metal fragments were chosen
as labile ligands.
The increase of activity by magnitudes in all kinds of olefin metathesis reactions confirmed the correctness of this principle of combining one strongly bound NHC ligand with one weakly bound ligand.
IntroductionOne of the most important C-C-coupling reactions for the preparation of biaryls is the coupling of phenylboronic acids with halogenated aryls: the Suzuki crosscoupling. It is catalyzed mainly by group 10 metals, often stabilized by ligands like phosphines.
Our ResearchSince palladium complexes with bulky trialkylphosphines are good catalysts for the Suzuki crosscoupling and since NHC resemble basic phosphines we synthesized bis(NHC)palladium(0) complexes. These complexes are easily accessible by phosphine exchange starting from bis(tri-o-tolyl)phosphine palladium(0). Our research evaluated that these systems exhibit the highest turnover frequency (TOF) in the Suzuki crosscoupling of chloroarenes known up to date.
Christian Gstöttmayr (Suzuki crosscoupling)
Books: B. Cornils, W.A. Herrmann (eds.)
Applied Homogeneous Catalysis with Oragnometallic Compounds Part I+II
VCH, Weinheim 1996
K. J. Ivin
Academic Press, London, 1983
Starting point: W. A. Herrmann, C. Köcher
Angew. Chem. Int. Ed.
1997, 36, 2162
1997, 109, 2256.
T. Weskamp, V.P.W. Böhm, W.A. Herrmann
J. Organomet. Chem.
2000, 600, 12.
Dissertations at the Institute of Inorganic Chemistry: Olefin Metathesis:
W. C. Schattenmann 1998
T. Weskamp 1999
F. J. Kohl 2000
Heck Type Reactions:
J. Fischer 1996
C.P. Reisinger 1997
V.P.W. Böhm 2000
For publications of our group please see the Publications Section.