Abstract
Abiodun Omokehinde Eseola
Abstract
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 pKa estimations and palladium complex single crystal data were analysed and correlations were observed between ligand donor strengths, x-ray structural properties and catalytic efficiencies.