Syntheses of Enantiopure Bifunctional 2-Guanidinobenzimidazole Cyclopentadienyl Ruthenium Complexes: Highly Enantioselective Organometallic Hydrogen Bond Donor Catalysts for CarbonCarbon Bond Forming Reactions
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2014 American Chemical Society. 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-NCH2(CH2)3CH2) are prepared in four steps from 2-aminobenzimidazole. Reactions with [(5-C5H5)Ru(CO)(NCCH3)2]+PF6- (5+PF6-) afford the chiral-at-metal chelates [(5-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+()-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.
author list (cited authors)
Mukherjee, T., Ganzmann, C., Bhuvanesh, N., & Gladysz, J. A.
complete list of authors
Mukherjee, Tathagata||Ganzmann, Carola||Bhuvanesh, Nattamai||Gladysz, John A