Evaluation of platinum group metal Lewis acid complexes of the novel biaryl-like R2-CATPHOS diphosphines in catalysis

Abstract

Recently, cationic ‘coordinately unsaturated’ square planar transition metal group Lewis acid complexes of the type [M(Ligand)]2+ (M = Pt, Pd,) have been shown to exhibit a number of advantageous features, which include well defined coordination geometries and wide functional group tolerance. They have proven to be efficient catalysts for a host of important enantioselective transformations including Diels-Alder reactions and 1,3-dipolar cycloadditions, as well as cycloisomerisations and various cyclisations. The supporting ligands and the metal centre each assert significant effects on the reactivity of the catalyst where, in particular, those complexes based on biaryl diphosphines such as BINAP, BIPHEP, MeO-BIPHEP, and NUPHOS are established as highly efficient catalysts for various couplings and cyclisations. The Doherty group has recently synthesised the novel biaryl-like R2-CATPHOS class of diphosphines and showed enantiopure (S)-Me2-CATPHOS to be markedly efficient ligand for rhodium catalysed asymmetric hydrogenation of dehydroamino esters and (E)-β-aryl-β-(enamido)phosphonates. This thesis reports details of a systematic evaluation of the efficiency of Lewis acid platinum and palladium metal complexes of enantiopure (S)-Me2-CATPHOS as catalysts for the asymmetric carbonyl-ene and Friedel-Crafts reaction and a comparison with their BINAP-based counterparts. The performance of R2-CATPHOS (R = H, Me, MeO) diphosphines as ligands for the palladium catalysed Suzuki-Miyaura cross-coupling, the Buchwald-Hartwig amination as well as a tandem carbopalladation-carbonylation sequence has also been investigated. Gold(I) complexes of R2-CATPHOS have also been shown to form efficient Lewis acid catalysts for the cycloisomerisation of a range of propargylamides. A range of platinum, palladium, and gold(I) precatalysts have been prepared and spectroscopically characterised, and catalytic conditions optimised in order to establish the extent to which R2-CATPHOS biaryl-like diphosphines are surrogates for their more conventional biaryl based counterparts such as BINAP and BIPHEP. The performance of catalysts based on R2-CATPHOS has also been compared for selected reactions to establish the influence of R on catalyst efficiency. This thesis evaluates the performance of R2-CATPHOS-based Lewis acid catalysts in chiral and achiral transformations with not only palladium and platinum but also with relatively new bis-gold catalysts that have only recently emerged in catalysis and offer great research potentials due to the unique properties of gold.