Benzimidazolium‐Pyrazole‐Palladium(II) Complexes: New and Efficient Catalysts for Suzuki, Heck and Sonogashira Reactions

• Published in: Advanced synthesis & catalysis 2008-10, Vol.350 (14‐15), p.2391-2400
• Main Authors: Li, Fuwei, Hor, T. S. Andy
• Format: Article
• Language: English
• Subjects: cross‐coupling; hybrid ligands; N‐heterocyclic carbenes; palladium; water channel; X‐ray structure determination
• ISSN: 1615-4150; 1615-4169
• DOI: 10.1002/adsc.200800356

Three unsymmetrical benzimidazolium‐pyrazole N‐N ligands 2‐(1‐propylbenzimidazolylmethyl)‐3,5‐di‐R‐pyrazole (R=H, Me, t‐Bu) have been conveniently prepared and structurally analyzed. The solid lattice packing of the R=Me compound at 223 K reveals one‐dimensional “zig‐zag” water chains stabilized by organic molecular channels through N⋅⋅⋅HO and OH⋅⋅⋅O bonding. These hybrid ligands add to palladium(II) to give high yields of air‐stable complexes that are fully characterized by NMR, ESI, and X‐ray single‐crystal crystallography. They are active Suzuki catalysts at room temperature towards cross‐coupling of unactivated aryl bromides and 5‐ or 6‐membered heteroaryl bromides with arylboronic acids with turnover frequencies (TOF) reaching as high as 60,000 h−1. Their catalytic efficiency is significantly better than that of the CN carbene‐imidazole analogue. These catalysts are also active in Heck and Sonogashira cross‐coupling reactions of aryl bromides giving the desired products in good yields. These results suggested that these Pd(II) complexes with N‐based hybrid ligands are versatile and efficient catalysts for different types of cross‐coupling reactions under aerobic conditions.