Synthesis of proline-based N-heterocyclic carbene ligands.

The synthesis of proline-based N-heterocyclic carbene ligands is reported. An approach to a variety of proline-containing imidazolium NHC precursors is presented. The proline amino acids are shown to be compatible with peptide synthetic methods by incorporation into tripeptides. Following peptide synthesis, the carbene is generated and bound to rhodium.