Over the last three years, several chiral rutheniumbased metathesis catalysts have been synthesized. These bear chiral NHC ligands and have been employed successfully in catalyzing ARCM (Asymmetric Ring Closing Metathesis) and AROM (Asymmetric Ring Opening Metathesis) reactions providing enantioselection beyond 90 % ee for various substrates. Particularly interesting are catalyst with back-bonding hydroxy moieties which promote metathesis in air and nondried solvents. In our research, we target the design of an array of novel, chiral NHC ligands with back-bonding hydroxyl moieties in generally applicable synthetic sequences of few steps. Ru-based metathesis catalysts of the new ligands are expected to display high enantioselection in ARCM and AROM reactions, as well as being fairly inert towards molecular oxygen and moisture. Due to the straightforward synthesis, we now can explore the possibility of ligand tuning adjusting steric and electronic parameters to obtain maximum catalytic performance. Starting from enantiomerically resolved trans-1,2- cyclohexanediamine, we initially targeted tridentate, C2- symmetric NHC-ligand precursors with two backbonding hydroxy groups which are available via reductive amination and subsequent ring closing reaction with triethyl-orthoformate. The synthesis of dissymmetric, bidentate ligand precursors was attempted in a sequential derivatization of the diamine. The optimization of the second step of the reaction sequence, a Pd-mediated arylation reaction or reductive amination remains under current investigation. Both precursors can be readily used for catalyst preparation in a one-pot ligand exchange reaction with first generation Grubbs’ catalyst derivatives.
- NHC ligands,
- Asymmetric ring closing metathesis,
- Asymmetric ring opening metathesis
Available at: http://works.bepress.com/hans-joerg_schanz/90/