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

摘要

2-Guanidinobenzimidazole (GBI) derivatives with a NHR group in place of NH2 (R: a, CH2Ph; b, (SC)-CH(CH3)Ph; c, (RCRC)-CH(CH2)(4)CHNMe2; d, (RCRC)-CH(CH2)(4)CH-NCH2(CH2)(3)CH2) are prepared in four steps from 2-aminobenzimidazole. Reactions with [(eta(5)-C5H5)Ru(CO)(NCCH3)(2)]+PF6 (5+PF6) afford the chiral-at-metal chelates [(eta(5)-C5H5)Ru(CO)(GBI-R)](+) PF6 (6ad+PF6, 3977%), which have been characterized by NMR (H-1, C-13, P-31, F-19) 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(+)(Delta)-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 (9099% 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.