Triaryl-like kitphos phosphines in gold and palladium catalysis

Abstract

Bulky electron-rich phosphines have been extensively used for transition metal catalysis in organic synthesis to great success. In particular, cyclohexyl- and tert-butyl- substituted phosphines have been predominately used. Conversely, the use of less electron-rich phenyl-substituted phosphines has been largely overshadowed by their more electron-rich counterparts, with much of the current literature stipulating the need for electron-rich ligands. The Doherty-Knight group has recently demonstrated that complexes of phenyl-substituted KITPHOS phosphines outperform their more electron rich counterparts in gold-catalysed transformations. This project aimed to extend the range of available KITPHOS ligands and to elucidate the abilities of these triaryl-like phosphines in a range of catalytic processes. In this thesis it has been demonstrated that these phosphines are highly efficient in the gold-catalysed cycloisomerisation of propargyl carboxamides to give methylene oxazolines (scheme A). Gold(I) complexes of these phosphines have also been shown to be highly effective in the cycloisomerisation of 2-[(2-alkynyl)phenyl]alcohols to isochromenes and capable of catalysing the formation of iminoisocoumarins exclusively from the relevant 2-(phenylethynyl)benzamides.

Scheme A. Gold-catalysed cycloisomerisation of propargyl carboxamides to give

methylene oxazolines. It has also been demonstrated that cyclopalladated pre-catalyst complexes bearing these KITPHOS phosphines are efficient in Suzuki-Miyaura cross coupling reactions involving a range of aryl- and heteroaryl bromides. These complexes are particularly effective in reactions involving either diethyl o-bromobenzenephosphonate or phenyl- and naphthylphosphonate-based boronic esters, giving excellent yields of the biaryl- phosphonate compounds using catalyst loadings of 0.1 – 2.0 mol% (scheme B). It has also been demonstrated that the phenyl-substituted KITPHOS phosphine complex II was remarkably more effective in the ethoxycarbonylation of styrene, than its cyclohexyl-substituted counterpart.