Chiral-at-Metal Ruthenium Complexes with Guanidinobenzimidazole and Pentaphenylcyclopentadienyl Ligands: Synthesis, Resolution, and Preliminary Screening as Enantioselective Second Coordination Sphere Hydrogen Bond Donor Catalysts
Tathagata Mukherjee, Subrata Ghosh, Taveechai Wititsuwannakul, Nattamai Bhuvanesh, John A. Gladysz
Abstract
Pentaphenylcyclopentadienyl ruthenium 2-guanidinobenzimidazole (GBI = HN═C(NHH′)NHC(NHR)═NR′) chelate complexes are accessed by treating (η5-C5Ph5)Ru(CO)2(Br) with Me3NO, GBI, and Ag+PF6–. Chromatographic workups give [(η5-C5Ph5)Ru(CO)(GBI)]+PF6– (2+PF6–, 70%; silica gel) or 2+BArf– (69%; alumina, then Na+BArf– (BArf– = B(3,5-C6H3(CF3)2)4–)). Treatment of 2+PF6– with K+t-BuO– deprotonates the GBI ligand to give the neutral species (η5-C5Ph5)Ru(CO)(GBI–H) (3; 73%). Complexes 2+PF6–, 2+BArf–, and 3 have been characterized by NMR, X-ray crystallography, and other methods. Protonation of 3 with enantiopure (P)-1,1′-binaphthyl-2,2′-diyl hydrogen phosphate ((P)-H-4) gives the diastereomeric salts (RRu/SRu)-2+(P)-4– (93%); (SRu)-2+(P)-4– selectively precipitates from cold toluene/hexanes (35% or 70% of theory), and addition of Na+BArf– gives (SRu)-2+BArf– (71%). The absolute configuration is provisionally assigned by CD spectroscopy. (SRu)-2+BArf– is an effective hydrogen bond donor catalyst for a variety of addition reactions by virtue of a synperiplanar NH triad involving the noncoordinating NH groups (some reactions with racemic 3 are also explored), but the enantioselectivities are uniformly low. Nonetheless, this provides insight regarding the mechanism of asymmetric induction for related catalysts with GBI-based carbon stereocenters.