Abstract
Many benzimidazole salts bearing a 3-phenylpropyl substituent (1a-1h) were synthesized and their structures were identified by $^{1}$H NMR, $^{13}$C NMR, and IR spectroscopic methods and elemental analysis. These N-heterocyclic carbene (NHC) precursors were used as a part of a catalytic system including Pd(OAc)$_{2}$ and the base in the Suzuki--Miyaura cross-coupling reaction under microwave irradiation. They were also used as catalysts in the cyclotrimerization of phenyl isocyanate to yield 1,3,5-triphenyl-1,3,5-triazinane-2,4,6-trione. It has been observed that benzimidazole salts made a positive contribution to both catalytic reactions as a NHC precursor.
Highlights
Most of the catalysts used for Suzuki– Miyaura reactions in previous studies have been focused on the use of palladium-phosphine or palladium-N-heterocyclic carbene (NHC) complexes and palladium nanoparticles. 32−41 microwave heating has been used successfully as an efficient heating technique in Suzuki–Miyaura type C-C coupling reactions. 33,42−52 much attention has been paid to finding out the most effective and environmentally benign catalyst or catalyst system
We aimed to explore the catalytic activities of a catalytic system including benzimidazole salts as electron-rich NHC precursors (1a–1h), Pd(OAc)[2 ], and base in the Suzuki–Miyaura C-C coupling reaction and the catalytic activity of in situ prepared NHC ligands in the cyclotrimerization reaction of phenyl isocyanate
Benzimidazole salts containing 3-phenylpropyl substituents (1a–1h) were prepared from the reactions of appropriate alkyl halides and 1-(3-phenylpropyl)benzimidazole. These salts were used as ligands in the presence of Pd(OAc) 2 in a catalytic system for Suzuki–Miyaura reactions. They were used as NHC precursors in the cyclotrimerization reaction of phenyl isocyanate
Summary
After the first report on N-heterocyclic carbenes (NHCs) in the early 1960s by Wanzlick and Schikora, a new window opened in organometallic chemistry. 1−11 Following the preparation of the first stable N-heterocyclic carbene by Arduengo et al in 1991, 12 they became powerful and were widely used as ligands in many organometallic compound syntheses and in many catalytic reactions. 13−16 Due to their lower toxicity, stronger σ -donor/lower π -acceptor character, and higher stability toward heat, moisture, and air, NHCs have become an alternative to phosphine-based ligands in organometallic chemistry. 5,11,17−19 Most of the transition metals, such as iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, yttrium, and samarium, can form stable complexes with NHC ligands. 3,5,20 Both NHCs and their organometallic derivatives have been used as catalysts in various organic reactions. 1−11 Following the preparation of the first stable N-heterocyclic carbene by Arduengo et al in 1991, 12 they became powerful and were widely used as ligands in many organometallic compound syntheses and in many catalytic reactions. 3,5,20 Both NHCs and their organometallic derivatives have been used as catalysts in various organic reactions These reactions include cross-coupling, olefin metathesis, transformation, reduction, rearrangement, telomerization, aryl amination, condensation, cyclotrimerization, polymerization, cyclization, addition, hydroformylation, dehydrogenation, borylation, and ring-opening metathesis polymerization. The Suzuki–Miyaura cross-coupling reaction is one of the most attractive methods for C-C bond-forming in organic syntheses. According to another report, 67 asymmetric 1,3,5-triazine-2,4,6-triones can be used as a novel class of gonadotropin-releasing hormone receptor antagonists
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