Developing azole-based ambient condition Pd(II) catalysts for C-C coupling: trends and study of electronic / rigidity features of 2-(thiophene-2-yl)-1H-imidazoles on catalyst activity

Palladium catalysed carbon- coupling reactions have provided seamless solutions and invaluable to otherwise difficult or impossible organic synthetic challenges ^[1–4]. Furthermore, it is also a common practice to add a palladium salt and a prescribed ligand into reaction mixtures during application of C-C catalytic coupling strategies ^[5,6]. However, studies of catalytic behaviours for varying possible catalyst or pre-catalyst species self-assembled during in-situ generation of catalyst are scarce. In particular, comparative study of effects of pre-catalyst molecular variations such as cis- or trans-coordination disposition, donor systems variation, etc. on catalytic outcomes is generally scarce ^[7]. A series of monodentate 2-(thiophen-2-yl)-1H-imidazole ligands (L1 – L8), 2,4,5-tri phenyl oxazole (L9) and 2-(1H-imidazol-2-yl)pyridines L10 – L11 were prepared in search for improved azole-based palladium coupling catalysts as well as for systematic study of molecular variation effects on catalytic efficiencies of their palladium complexes. These phosphine-free ligands formed diverse dichloropalladium complexes in three coordination forms, which are chloro-bridged dimers (PdL)₂, mono-ligand species with trans-solvent coordination PdL-solvent and trans-bis-ligand complexes PdL₂. The triphenylphosphine complex PdI₂(PPh₃)₂ was also studied for comparative purposes. Ligand pK_a estimations and palladium complex single crystal data were analysed and correlations were observed between ligand donor strengths, x-ray structural properties and catalytic efficiencies.

Author(s): Abiodun Omokehinde Eseola

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