Abstract
This thesis covers an investigation of the use of pyrazolyl palladium complexes as Heck and Suzuki coupling catalysts. It is organised into four chapters. Chapter 1 is a review of the relevant literature and therefore does not cover any new work. Chapters 2 and 3 describe the synthesis and characterization of palladium complexes and testing as Heck and Suzuki coupling catalysts respectively. Chapter 4 is a summary of conclusions and future work. The subsequent section provides the main findings in chapters 2, 3 and 4.
Chapter 2 is on the synthesis and characterization of palladium complexes that were used to catalyse the Heck and the Suzuki coupling reactions. The poly(pyrazol-1-ylmethyl)benzene ligands were prepared from reactions in which bis(bromomethyl)benzene with bromo substituents in different positions were reacted with either 3,5-dimethylpyrazole or 3,5-ditertiarybutylpyrazole. The products formed from these reactions were: {(3,5-Me2pzCH2)4-1,2,4,5-C6H4} (L1), {(3,5-Me2pzCH2)2}2-1,4-C6H4 (L2), {(3,5-tBu2pzCH2)2-1,4-C6H4} (L3), {(3,5-Me2pzCH2)2}2-1,3-C6H4 (L4), and {(3,5-Me2pzCH2)2}2-1,2-C6H4 (L4). Compounds L1-L5 were subsequently used to prepare their corresponding palladium complexes C1-C8, by reacting the poly(pyrazol-1-ylmethyl)benzene ligands L1-L5 with [PdCl2(NCMe)2] or [PdClMe(COD)] to form the tetranuclear palladium complex [{Pd2Cl(3,5-Me2pzCH2)4-1,2,4,5-C6H4}2] (C1), or dinuclear palladium complexes [{Pd2(μ-Cl)2X2(3,5-Me2pzCH2)2-1,4-C6H4}] (X = Cl (C2), Me (C6)), [{Pd2(μ-Cl)2Cl2(3,5-tBu2pzCH2)2-1,4-C6H4}] (C3), [{Pd2ClX(3,5-Me2pzCH2)2-1,3-C6H4}2] (X = Cl (C4), Me (C8), [{Pd2ClX(3,5-Me2pzCH2)2-1,2-C6H4}2] {X = Cl (C5), Me (C7)}, All the compounds were characterized by multinuclear NMR and elemental analysis.
The structures of C1, C5 and C7 were confirmed by single crystal X-ray structural analysis.
Chapters 3 and 4 describe the use of new palladium complexes prepared in this project as catalysts in Heck and Suzuki cross coupling reactions. The complexes efficiently catalysed the Heck coupling reactions which involved the coupling of iodobenzene and butylacrylate to produce trans-butyl cinnamate at 80 oC, with over 80 % conversion found within 6 h, and over 90 % within 24 h. The Suzuki coupling reactions between iodobenzene and phenylboronic acid were also performed at 80 oC. The Suzuki coupling reactions were not as efficient as the Heck coupling reactions and conversions of more than 70 % could only be reached after 24 h. Complex C6 gave the highest conversions in both the Heck and the Suzuki coupling reactions, with Heck coupling conversions of 100 % within 6 h and Suzuki coupling conversion of 73 % within 24. The major significant finding in using these palladium complexes in the two coupling reactions is that they perform both reactions at a much lower temperature (80 oC) compared to the normal temperatures of 120-160 oC used in such reaction.
Professor James Darkwa