Triptycene-Based Chiral and meso-N-Heterocyclic Carbene Ligands and Metal Complexes

Based on 1-amino-4-hydroxy-triptycene, new saturated and unsaturated triptycene-NHC (N-heterocyclic carbene) ligands were synthesized from glyoxal-derived diimines. The respective carbenes were converted into metal complexes [(NHC) MX] (M=Cu, Ag, Au; X=Cl, Br) and [(NHC) MCl(cod)] (M=Rh, Ir; cod=1,5-cyclooctadiene) in good yields. The new azolium salts and metal complexes suffer from limited solubility in common organic solvents. Consequently, the introduction of solubilizing groups (such as 2-ethylhexyl or 1-hexyl by O-alkylation) is essential to render the complexes soluble. The triptycene unit infers special steric properties onto the metal complexes that enable the steric shielding of selected areas close to the metal center. Next, chiral and meso-triptycene based N-heterocyclic carbene ligands were prepared. The key step in the synthesis of the chiral ligand is the Buchwald-Hartwig amination of 1-bromo-4-butoxy-triptycene with (1S,2S)-1,2-diphenyl-1,2-diaminoethane, followed by cyclization to the azolinium salt with HC(OEt)(3). The analogous reaction with meso-1,2-diphenyl-1,2-diaminoethane provides the respective meso-azolinium salt. Both the chiral and meso-azolinium salts were converted into metal complexes including [(NHC) AuCl], [(NHC) RhCl(cod)], [(NHC) IrCl(cod)], and [(NHC) PdCl(allyl)]. An in situ prepared chiral copper complex was tested in the enantioselective borylation of alpha,beta-unsaturated esters and found to give an excellent enantiomeric ratio (er close to 90:10).