Syntheses of Enantiopure Bifunctional 2-Guanidinobenzimidazole Cyclopentadienyl Ruthenium Complexes: Highly Enantioselective Organometallic Hydrogen Bond Donor Catalysts for Carbon–Carbon Bond Forming Reactions (original) (raw)

2-Guanidinobenzimidazole (GBI) derivatives with a NHR group in place of NH2 (R: a, CH2Ph; b, (SC)-CH(CH3)Ph; c, (RCRC)-CH(CH2)4CHNMe2; d, (RCRC)-CH(CH2)4CH-tiebar above startNCH2(CH2)3tiebar above endCH2) are prepared in four steps from 2-aminobenzimidazole. Reactions with [(η5-C5H5)Ru(CO)(NCCH3)2]+PF6– (5+PF6–) afford the chiral-at-metal chelates [(η5-C5H5)Ru(CO)(GBI-R)]+ PF6– (6a–d+PF6–, 39–77%), which have been characterized by NMR (1H, 13C, 31P, 19F) and other spectroscopy. The Ru,C configurational diastereomers of the dimethylamino containing complex 6c+PF6– separate upon alumina chromatography (RRuRCRC, >99:01 dr; SRuRCRC, <2:98 dr) but slowly epimerize at ruthenium in solution at room temperature. Configurations have been assigned by CD spectra and the crystal structure of (RRuRCRC)-6c+(Δ)-TRISPHAT– (TRISPHAT– = P(o-C6Cl4O2)3–). The latter reveals hydrogen bonding between two of the GBI-R NH linkages and the TRISPHAT oxygen atoms. Both diastereomers catalyze (10 mol %, room temperature) additions of malonate esters to nitroalkenes in high yields and enantioselectivities (90–99% ee for aryl-substituted nitroalkenes). The dominant product configurations are identical, showing the chiral ruthenium center to have little influence. The free GBI-R ligand exhibits only modest activity. Unlike most transition-metal-catalyzed reactions, there is no direct interaction of the substrate with the ruthenium; rather, evidence is presented for “second coordination sphere promoted catalysis” mediated by hydrogen bonds derived from two NH groups that chelation preorganizes in a synperiplanar conformation.