Studies on gold(i) and gold(iii) alcohol functionalised NHC complexes

Abstract

Five pairs of novel chiral alcohol functionalised gold(I) and gold(III) NHC complexes derived from chiral amino alcohols, were synthesized and characterised (NMR, IR, HRMS). Single crystal X-ray diffraction data of gold(I) and gold(III) complexes are reported and discussed. The chiral imidazolium preligands were readily synthesized via the oxalamides, subsequent reduction and final orthoformate condensation. An improved method was used for generation of gold(I) NHC complexes (up to 92%) and further oxidation afforded the corresponding gold(III) NHC complexes (up to 99%). All the Au(I) and Au(III) NHC complexes proved far more catalytically active in a 1,6-enyne alkoxycyclization test reaction than our previously tested N,N- and P,N-ligated Au(III) complexes. Comparative gold(I) and gold(III) catalytic studies demonstrated different catalytic ability, depending on the NHC ligand flexibility and bulkiness. Excellent yields (92–99%) of target alkoxycyclization product were obtained with both gold(I) and gold(III) complexes with the bulky N¹-Mes-N²-ethanol based NHC ligand.