Palladium and gold N-heterocyclic carbene complexes: synthesis and catalytic applications

Abstract

Palladium and gold-N-heterocyclic carbene (NHC) complexes have been studied in detail, with focus on their synthesis and applications. Palladium(II)-NHC complexes are known to be highly catalytically active complexes for numerous organic reactions, specifically [PdCl(ƞ3-R-allyl)(NHC)] complexes are widely used in crosscoupling chemistry. Based on this, an optimised, more facile and most importantly air-stable synthesis was developed (Chapter 2). The preparation of a new class of Pd(II)-NHC complexes, [NHC·H][Pd(ƞ3-R-allyl)Cl2], was optimised and applied to a wide range of NHC ligands (Chapter 2). Furthermore, the catalytic activity of [NHC·H][Pd(ƞ3-R-allyl)Cl2] was intensely studied in crosscoupling chemistry (Chapter 3-6). An environmentally friendly protocol was established for the Suzuki-Miyaura reaction using [NHC·H][Pd(ƞ3-R-allyl)Cl2] in low catalyst loading. Both, the synthesis of the pre-catalysts as well as the catalysis can be carried out under air and the mild conditions and enabled a diverse substrate scope (Chapter 3). The formation of C-N bonds is widely studied and of importance due to their appearance in a wide range of molecules, particular in pharmaceutical reagents and natural products. [IPr*·H][Pd(ƞ3-cin)Cl2] was found to be a highly catalytically active pre-catalyst in the Buchwald- Hartwig amination as well as in ketone arylation (Chapter 4). Low catalyst loading, the use of a green solvent and high yields created an efficient protocol. An extensive study of the effect of different R-allyl ligands was performed for the Mizoroki-Heck reaction (Chapter 5). Different [NHC·H][Pd(ƞ3-R-allyl)Cl2] pre-catalysts were tested and the catalysis was optimised resulting in very low catalyst loading (0.1 mol%). Biologically interesting heteroatom-containing aryl bromides were successfully coupled with styrene. The catalytic activity of [NHC·H][Pd(ƞ3-R-allyl)Cl2] was further extended to the C-H activation of terminal alkynes with aryl bromides (Chapter 6). The catalysis tolerated substrates containing electron donating and withdrawing substituents. Furthermore the dimerization of terminal alkynes to 1,3-enynes was investigated resulting in excellent yields. Transition-Novel Au(I)-NHC complexes, namely [Au(Bpin)(NHC)] were synthesised and fully characterised. In total, four complexes bearing different NHCs were synthesised and their reactivity studied, revealing unique characteristics (Chapter 7).