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 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 [(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.