Design and synthesis of cyclometallated sulphur-n-heterocyclic carbene complexes and their application in catalysis
Date of Issue2012
School of Physical and Mathematical Sciences
This thesis describes the design and development of new chiral cyclometallated sulphur functionalized N-Heterocyclic carbene complexes and their catalytic potential studies on C-N and C-O bond forming reactions. In Chapter 1, a brief discussion on the history of N-Heterocyclic carbenes, its recent development of functionalized NHCs complexes and their catalytic potential activity in asymmetric catalysis is presented. Further, this chapter is concluded with describing the aim of this project. Chapter 2, describes the synthesis of several S-functionalized cyclopalladated NHC racemic complexes with a potential chiral centre on the α-carbon. This in turn results in the formation of a five membered ring chelate. The ligands were easily obtained when the substituted thiolates react with the common intermediate namely, 1-(bromo(phenyl)methyl)-3-tert-butyl-1H-imidazol-3-ium bromide, and 1-(bromo(phenyl)methyl)-3-mesityl-1H-imidazol-3-ium bromide and their corresponding palladium(II) complexes were fully characterized. Analysis on ring rigidity of neutral and cationic sulphur-NHC palladacycles was carried out with the help of XRD, 2D-ROESY, VT NMR techniques. A detailed study on the role of the new cationic π-allylpalladium(II) sulphur-NHC complexes, as a catalyst, in allylic amination reaction was established. Chapter 3, describes the new methodology which was developed to synthesize chiral five membered Sulphur-NHC bidentate metal complexes via 18 fractional crystallization method. The racemic ligands and its corresponding palladacycles synthesized in chapter 2 were unable to resolve either in the form of diastereomeric salt formation or amino acid method, therefore the diastereomeric sulphur ligands were achieved from the common intermediate 3-(bromo(phenyl)methyl)-1-((R)-1-phenylethyl)-1H-imidazol-3-ium synthesized from (R)-1-(1-phenylethyl)-1H-imidazole. The rigidity of neutral chiral cyclometallated sulphur-NHC complexes obtained from the silver complexes were completely analysed by XRD, 2D-ROESY, VT NMR techniques. Evidently, the chiral cyclometallated sulphur-N-Heterocyclic carbene complexes with (Rs*,S*,R*) and (Ss*,R*,R*) remained locked in λ and latter in δ ring conformation in the solid as well as the solution state. The significant feature of these new complexes invariably display the Ph(α) substituent taking an axial position either above or below the coordinating plane, due to the relative trans disposition of the sulphur atom substituent. These complexes were found to be reactive in [2,3]-sigmatropic rearrangement of (E)-N,N-dibenzylbut-2-en-1-amine.