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

2-Guanidinobenzimidazole ( GBI ) derivatives with a NHR group in place of NH 2 (R: a, CH 2 Ph; b, ( S C )-CH(CH 3 )Ph; c, ( R C R C )-CH(CH 2 ) 4 CHNMe 2; d, ( R C R C )-CH(CH 2 ) 4 CH- NCH 2 (CH 2 ) 3 CH 2 ) are prepared in four steps from 2-aminobenzimidazole. Reactions with [(η 5 -C 5 H 5 )Ru(CO)(NCCH 3 ) 2 ] + PF 6 – ( 5 + PF 6 – ) afford the chiral-at-metal chelates [(η 5 -C 5 H 5 )Ru(CO)( GBI -R)] + PF 6 – ( 6a – d + PF 6 –, 39–77%), which have been characterized by NMR ( 1 H, 13 C, 31 P, 19 F) and other spectroscopy. The Ru,C configurational diastereomers of the dimethylamino containing complex 6c + PF 6 – separate upon alumina chromatography ( R Ru R C R C, >99:01 dr; S Ru R C R C, <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 ( R Ru R C R C )- 6c + (Δ)-TRISPHAT – (TRISPHAT – = P( o -C 6 Cl 4 O 2 ) 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.