Metal-Templated Hydrogen Bond Donors as “Organocatalysts” for Carbon–Carbon Bond Forming Reactions: Syntheses, Structures, and Reactivities of 2‑Guanidinobenzimidazole Cyclopentadienyl Ruthenium Complexes

• Published in: Organometallics 2014-12, Vol.33 (23), p.6709-6722
• Main Authors: Scherer, Alexander, Mukherjee, Tathagata, Hampel, Frank, Gladysz, John A
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om500704u

The reaction of 2-guanidinobenzimidazole (GBI) and (η5-C5H5)­Ru­(PPh3)2(Cl) in refluxing toluene gives the chelate [(η5-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 [(η5-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 – ((Δ)-TRISPHAT–; 81%). The reaction of [(η5-C5H5)­Ru­(CO)­(NCCH3)2]+PF6 – and GBI also gives 2 +PF6 – (81%). The pentamethylcyclopentadienyl analogues [(η5-C5Me5)­Ru­(CO)­(GBI)]+X– (3 +X–; X– = Cl–, BF4 –, PF6 –, BArf –; 61–84%) are prepared from (η5-C5Me5)­Ru­(PPh3)2(Cl), GBI, and CO followed (for the last three) by anion metatheses. An indenyl complex [(η5-C9H7)­Ru­(PPh3)­(GBI)]+Cl– (96%) is prepared from (η5-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-β-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 –.