8a-hexahydroimidazo[1,5-a]pyridine (1), has been prepared from 2-(aminomethyl)piperidine and N,N-dimethylformamide dimethylacetal. The reactions of three imidazolidinium salts with [Rh(acac)(COD)] and [Ir(µ-OMe)(COD)] 2 were found to afford rhodium and iridium complexes of N-heterocyclic carbenes, respectively. These complexes have been tested as catalysts in the transfer hydrogenation of cyclohexanone and acetophenone using 2-propanol as hydrogen source.
Cleavage reactions of [PdBr(2)(NHC)](2) with two equiv. of pyridine derivatives (L), having one or two carboxylic acid groups [L = NC(5)H(4)-2-COOH, NC(5)H(4)-3-COOH, NC(5)H(4)-4-COOH, or NC(5)H(3)-2,6-(COOH)(2) in chloroform or DMSO, respectively, afforded the monomeric mixed ligand complexes trans-[PdBr(2)(NHC)L] (1-4) which, due to deprotonation of the carboxylic acid functionality, are water soluble in KOH. The catalytic activity of complexes 1-4 in aqueous Suzuki-Miyaura cross-coupling reactions were evaluated and compared. The dicarboxylic functionality enhances the catalyst reactivity and stability and the carboxylate derived from 4 could be easily recovered and reused for several cycles under the mild reaction conditions.
A saturated Pd-N-heterocyclic complex was immobilized on an amorphous silica. The complex itself is of very high thermal stability. However, TEM observations, hot filtration, reusability, and poisoning tests all revealed that the complex acted only as a precatalyst to highly active Pd species in Mizoroki-Heck reactions when immobilized. The complex appears more stable when used under homogeneous reaction conditions. The immobilized complex afforded high turnover numbers, 10 4 -10 5 . The higher turnover frequencies were realized at the lower Pd concentrations, which is a characteristic property of ligand-free Pd catalyzed reactions.
Benzimidazol-2-ylidene(NHC) derivatives of iridium(I) and rhodium(I) bearing electron-rich and/or functional substituents R and RЈ at the nitrogen atoms of the heterocycle [IrBr(cod)(NHC)] (a) or [RhBr(cod)(NHC)] (b) (R = 2,3,5,6-tetramethylbenzyl; RЈ = methoxyethyl, 1a,b; R = RЈ = 2,3,5,6-tetramethylbenzyl, 2a,b; R = pentamethylbenzyl, RЈ = methoxyethyl, 3a,b; R = RЈ = pentamethylbenzyl, 4a,b) were prepared. All compounds were fully characterized by 1 H and 13 C NMR spectroscopy, mass spectrometry (MALDI), and ele-
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