Computational Investigations of Enantioselection in Carbon-Carbon Bond Forming Reactions of Ruthenium Guanidinobenzimidazole Second Coordination Sphere Hydrogen Bond Donor Catalysts

The NH2 group of 2-guanidinobenzimidazole (GBI) can be replaced by (RCRC)-NHCH(CH2)4CHNMe2 and elaborated to the enantiopure chelate salts (SRuRCRC)-[(η5-C5H5)Ru(CO)(GBICH(CH2)4CHNMe2)]+PF6- ((SRuRCRC)-2+PF6-) and (RRuRCRC)-2+PF6-. These catalyze highly enantioselective additions of 1,3-dicarbonyl compounds to nitroalkenes. The mechanism and basis for enantioselection are probed by DFT calculations. First, the parent GBI complex [(η5-C5H5)Ru(CO)(GBI)]+PF6- (1+PF6-) is examined. This species has only ruthenium-centered chirality and must be used with a trialkylamine, as it lacks the internal base of 2+PF6-. The dicarbonyl compound initially hydrogen bonds to the NH triad of the GBI ligand, but the transition states leading to each product enantiomer are essentially equal in energy. In contrast, after similar bonding of the dicarbonyl compound to (SRuRCRC)- or (RRuRCRC)-2+PF6-, a proton is transferred to the:NMe2 moiety, giving an enolate and a HNMe2+ group. The latter mediates the introduction of trans-β-nitrostyrene such that one enolate πface attacks the Csi≠CrePh face to give an addition product with an R configuration, in agreement with experiment. Thus, the configurations of the catalyst carbon stereocenters control the product stereochemistry. Interactions in competing transition states are analyzed.