Metal-Templated Hydrogen Bond Donors as Organocatalysts for CarbonCarbon Bond Forming Reactions: Syntheses, Structures, and Reactivities of 2-Guanidinobenzimidazole Cyclopentadienyl Ruthenium Complexes
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2014 American Chemical Society. The reaction of 2-guanidinobenzimidazole (GBI) and (5-C5H5)Ru(PPh3)2(Cl) in refluxing toluene gives the chelate [(5-C5H5)Ru(PPh3)(GBI)]+Cl- (1+Cl-; 96%). Subsequent anion metatheses yield the BF4-, PF6-, and BArf- (B(3,5-C6H3(CF3)2)4-) salts (77-85%). Reactions with CO give the carbonyl complexes [(5-C5H5)Ru(CO)(GBI)]+X- (2+X-; X- = Cl-, BF4-, PF6-, BArf-; 87-92%). The last three salts can also be obtained by anion metatheses of 2+Cl- (77-87%), as can one with the chiral enantiopure anion P(o-C6Cl4O2)3- (()-TRISPHAT-; 81%). The reaction of [(5-C5H5)Ru(CO)(NCCH3)2]+PF6- and GBI also gives 2+PF6- (81%). The pentamethylcyclopentadienyl analogues [(5-C5Me5)Ru(CO)(GBI)]+X- (3+X-; X- = Cl-, BF4-, PF6-, BArf-; 61-84%) are prepared from (5-C5Me5)Ru(PPh3)2(Cl), GBI, and CO followed (for the last three) by anion metatheses. An indenyl complex [(5-C9H7)Ru(PPh3)(GBI)]+Cl- (96%) is prepared from (5-C9H7)Ru(PPh3)2(Cl) and GBI. All complexes are characterized by NMR (1H, 13C, 31P, 19F, 11B), with 2D spectra aiding assignments. Crystal structures of 1+PF6-CH2Cl2 and 1+BArf-CH2Cl2 are determined; the anion is hydrogen bonded to the cation in the former. Complexes 1-3+X- are evaluated as catalysts (10 mol %, RT) for condensations of indoles and trans--nitrostyrene. The chloride salts are ineffective (0-5% yields, 48-60 h), but the BArf- salts exhibit excellent reactivities (97-46% yields, 1-48 h), with the BF4- and PF6- salts intermediate. Evidence for hydrogen bonding of the nitro group to the GBI ligand is presented. GBI shows no catalytic activity; a BArf- salt of methylated GBI is active, but much less so than 2-3+BArf-.
author list (cited authors)
Scherer, A., Mukherjee, T., Hampel, F., & Gladysz, J. A.
complete list of authors
Scherer, Alexander||Mukherjee, Tathagata||Hampel, Frank||Gladysz, John A