Abstract
Pincer complexes have amassed a considerable amount of interest as alternative catalysts due to their unique balance between stability and reactivity. Unsurprisingly, pincer complexes of certain transition metals have gained favor as catalysts in certain reactions. However, certain classes of pincer complexes such as neutral palindromic N^N^N palladium pincer complexes which consists of a central pyridyl unit with aliphatic amine side arms are underrepresented in the literature. Only a few of them are commonly reported, namely: the bis(oxazolinyl)pyridines, 2,6-bis(imino)pyridines and terpyridines. In this regard, we were encouraged to synthesize and characterize a series of cationic palladium(II) N^N^N pincer complexes that are based on a ridged pyridine backbone. This was then followed by the systematic study of the catalytic activity of these complexes in the Suzuki-Miyaura reaction, the Mizoroki-Heck reaction and in the selective conversion of levulinic acid (LA) to γ-valerolactone (GVL). A library of N^N^N pincer ligands and their corresponding novel cationic palladium(II) pincer complexes C1-C4 were successfully synthesized via the treatment of ligands L1-L4 with [PdCl2(MeCN)2]. The ligands, L1-L4, were in turn synthesized from the treatment of 2,6-bis(bromomethyl)pyridine with an appropriate secondary amine in the presence of K2CO3. The structures of the ligands and complexes were confirmed through the use of characterization techniques which employed analytical methods such as NMR, IR, HRMS and single crystal XRD. Thereafter, our focus turned to the catalytic evaluation of these complexes in the Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions as well as in the selective synthesis of γ-valerolactone from levulinic acid. Complexes C1-C4 were evaluated as pre-catalysts in the Suzuki-Miyaura and Mizoroki- Heck cross-coupling reactions. Relatively mild conditions, reasonably cheap reagents and an environmentally benign solvent (H2O) was used in the Suzuki-Miyaura cross-coupling reaction. With C4 as a pre-catalyst, the catalyst process possibly occurred through a combination of a heterogenous Pd(0)/Pd(II) pathway and a homogenous Pd(II)/P(IV) pathway as partial inhibition was observed in the mercury drop test. Our catalyst could not be reused and was compatible with various boronic acids and aryl bromides only...
M.Sc. (Chemistry)