Novel N-heterocyclic carbine ligands for use in supported catalysis

Abstract

Ligands involving CNC structures have been of intense interest in research. With an aim to investigating this area, a modular synthesis was developed using condensation reactions between a base unit of 2,6-dichloroisonicotinic acid and N-alkyl imidazoles toprovide a range of 2,6-bis(imidazolium) salts. From this point several methods wereavailable to incorporate an active metal centre, however, state of the art microwaveaccelerated synthesis was found to be the most successful technique in forming the tridentate palladium complexes. Several strategies were investigated to attach these complexes to polymer resins, exploring a range of linking groups, coupling procedures and resins. The most effective strategy involved forming an acid functionalised palladium complex to allow loading onto an amino-functionalised resin. Although several reagents were investigated it was found that the commercially available reagent PyBop® was the most effective in forming the stable amide bond from the palladium complex to the polymer resin. By using an excess of coupling reagent and an excess of the acid functionalised palladium complex complete loading onto an amino functionalised resin could be achieved.The supported complexes were found to be highly stable catalysts in Heck, SuzukiMiyauraand Stille reactions, and capable of cross-coupling a range of aryl iodides in very high yields. The active catalysts showed very little leaching of palladium (ICPMS) and could be recycled up to fourteen times with no loss of activity. Long reaction times were overcome using tetra-n-butylammonium bromide as an additive or by using microwave irradiation. Following these catalytic studies and with an aim to using less active electrophiles, several strategies were investigated to develop enhanced activity catalysts. These strategies involved replacing one of the N-Heterocyclic carbene ligands with either an "inert" bulky group or with another alternative ligand. Full details of this research are presented in chapters 2 to 5.