Exploiting imidate ligand effects in transition metal-mediated C-C bond forming processes

The effects of substituting (pseudo)halide for imidate ligands in Au(I) and Au(III) ([AuBr(NHC)] and [AuBr3(NHC)]), Ru(II) ([RuCl2(CHR)(L2)]) and Pd(II) ([Pd(OAc)2]) complexes has been investigated. The activity of these complexes as (pre)catalysts in enyne cycloisomerisation and propargylic nucleophilic substitution, diene ring-closing metathesis and ring-opening metathesis polymerisation and direct arylation reactions, respectively, has been determined.
[Au(N-imidate)(NHC)] and [AuBr2(N-imidate)(NHC)] complexes were prepared and the structure and bonding of the complexes examined spectroscopically and crystallographically. The [AuBr2(N-imidate)(NHC)] complexes, in combination with co-catalytic silver salts, were tested for activity in the cycloisomerisation of 1,5- and 1,6- enynes and found to be more effective than tribromide analogues. Kinetic analysis of the reactions showed subtle changes to the imidate structure had a pronounced effect on the activity of the complexes and the use of the silver salt Ag[Al(OC(CF3)3)4] as a co-catalyst greatly increased catalytic activity. The complexes were also found to catalyse a unique tandem nucleophilic substitution-cycloisomerisation of propargyl alcohols and allylsilanes. [AuBr2(N-tfs)(ItPe)] was found to be an effective precatalyst for this reaction whilst Au(III) tribromide and Au(I) complexes were ineffective. 1,3-Diarylbicyclo[3.1.0]hexenes products were found to undergo a post-reaction ambient temperature 1,3-carbon shift isomerisation.
The complex [Ru(N-tfs)2(o-iPrO-CHPh)(IMesH2)] was prepared and characterised spectroscopically and crystallographically. The complex was found to be inactive in the ring-closing metathesis and ring-opening metathesis polymerisation of alkenes. Attempts to selectively substitute chloride for imidate ligands derived from imides with higher pKa’s of 8.3-9.7 (in water) resulted in decomposition of the alkylidene or benzylidene ligand.
[Pd(imidate)2(MeCN)] and [Pd(imidate)2(THT)] complexes were prepared and analysed by NMR and infra-red spectroscopy. The complexes were tested for activity in the direct arylation of imidazole with iodoarenes without added base or neutral ligands. The activity of the complexes was to some degree dependant on the structure of the imidate ligand, possessing moderate activity in comparison with [Pd(OAc)2]. The activity of other palladium sources and conditions for this reaction were investigated and it was found that the formation of Pd nanoparticles may be key to reaction progression.