The synthesis, catalytic investigation, and theoretical rationalisation of unsymmetrical pincer palladacycles

• Main Author: Roffe, Gavin William
• Published: University of Sussex 2016
• Subjects: 547; QD0241 Organic chemistry
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.693205

This thesis presents the synthesis of a number of unsymmetrical SCN, N'CN, PCN and PCS pincer palladacycles. A new synthetic route has been designed towards unsymmetrical pincer ligands, involving a key Suzuki-Miyaura coupling, yielding the characteristic biaryl ligand backbone presented. New unsymmetrical SCN pincer ligands, containing a pyridine and a thioether ligand were synthesised. A number of substituents were used on the thioether ligand arm, with various steric demand and electron donating characteristics. These unsymmetrical ligands underwent C-H bond activation with palladium (II) salts, yielding the corresponding palladacycles. In order to investigate the effect of changing the sulphur donor atom, the thioether ligand arm was replaced by amines in the synthesis of N'CN pincer palladacycles, and phosphinites in the synthesis of PCN pincer palladacycles. Also changing the pyridine donor arm to a thioether was investigated, yielding a PCS pincer palladacycle. The palladacycles were tested in a number of catalytic applications: Suzuki-Miyaura coupling of sterically demanding and electronically deactivated aryl bromides; cross coupling of arylboronic acids and vinyl epoxides; and catalytic aldol condensations; revealing differences between the SCN, N'CN and PCN pincer palladacycles. In order to investigate the differences in catalytic activity, density functional theory was employed. A palladacycle formation pathway containing a key C-H bond activation step was investigated for an SCN pincer palladacycle, revealing differences in energy barriers of the C-H bond activation step depending on whetherthe thioether or the pyridine arm coordinates to the PdCl2 first. Next, the activation pathway of the palladacycles in the Suzuki-Miyaura coupling reaction was studied, identifying key transmetallation and reductive elimination steps. Differences in the overall thermodynamics and kinetics provide explanations for differences in catalytic activity. The results show that slower release of the catalytically active Pd(0) species yield a better precatalyst in the Suzuki-Miyaura coupling reaction, due to the lower propensity to form catalytically inactive palladium black. Overall this thesis provides a novel synthetic route to a family of unsymmetrical pincer palladacycles; their testing in catalytic activity in several applications, revealing differences in catalytic activity; and a theoretical study into key mechanisms, C-H bond activation, and catalyst activation in the Suzuki-Miyaura coupling reaction, which provide a rationalisation to the varying catalytic activities of the unsymmetrical pincer palladacycles.