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Forschungsarbeit

Carbocyclic Carbenes

von Christian Taubmann

 

 

Carbocyclic Carbenes

N-Heterocyclic carbenes (NHCs), proving to be much more than simple phosphane ligand substitutes, have transformed modern catalysis. In the course of our research with and beyond NHCs in organometallic chemistry and catalysis, a simple carbocyclic carbene, cycloheptatrienylidene (CHT), was found to be comparable or even superior as a supporting ligand to well established NHC systems. Ongoing efforts have been extended to explore and optimize this new class of catalysts also to include the smallest carbocyclic carbene ligand cyclopropenylidene and evaluate both the seven and the three-membered ring systems with the example of their easily accessible palladium complexes in C-C as well as in C-N coupling reactions.

Significant differences in the structural features of the three- and seven-membered ring system could be revealed by X-ray crystallography (Figure 1). A stronger delta-donor character of the CHT ligand compared to that of the 2,3-dipenylcyclopropenylidene ligand is demonstrated by a slightly longer Pd–C1 distance and a considerably weakened Pd–Cl2 bond (stronger trans-influence). Moreover, almost equal C–C bond lengths in the CHT ligand suggest extensive 6 phi delocalization similar to that of the cycloheptatrienyl cation, whereas a distinctive cyclopropenylidene structure seems to be present in the three-membered ring ligand.

 

 

Different structural features in carbocyclic carbene complexes[Bildunterschrift / Subline]: Figure 1: Different structural features in carbocyclic carbene complexes. [left side: cis-dichloro(2,3-diphenylcyclopropenylidene)(triphenylphosphane)palladium(II); right side: cis-dichloro(cycloheptatrienylidene)(triphenylphosphane)palladium(II)]

Palladium catalysts bearing ancillary carbocyclic carbene ligands were shown to be highly active in C-C and C-N coupling reactions (Figure 2). In comparison to the three-membered ring system faster initiation and greater yields could be obtained using the CHT system. Catalytic studies in cooperation with Evangeline Tosh revealed that both systems do not suffer from an induction period.

C-N coupling by a palladium pre-catalyst supported by a CHT ligand[Bildunterschrift / Subline]: Figure 2: C-N coupling by a palladium pre-catalyst supported by a CHT ligand.

Work is underway to further investigate electronically and sterically optimized CHT ligands, and to elucidate the role of the carbocyclic carbene ligand in the catalytic cycle.

Christian Taubmann during a poster session at the ACS meeting in New Orleans in March 2008[Bildunterschrift / Subline]: Christian Taubmann during a poster session at the ACS meeting in New Orleans in March 2008.