The synthesis of chiral C2-symmetric N-heterocyclic carbene complexes
The quaternisation of 1-substituted-azoles with a, w-dihaloalkanes furnished the diazolium salts. Conversion of the salts by deprotonation in the presence of sulphur generated the corresponding dithiones. A series of propylene diimidazolium salts and thiones gave an insight into the ease of handling, hygroscopic nature and yields associated with these compound. Propylene bridged diimidazol-2-ylidenes and ethylene bridged imidazolidin-2-ylidenes were never isolated from deprotonation of the salts or reductive desulphurisation of the thiones. In situ reactions with the salts and palladium acetate resulted in decomposition. The synthesis of both enantiomers of 4,5-bis(bromomethyl)-2,2-dimethyl-1,3-dioxolane from tartaric acid enabled the preparation of chiral bridged diazolium salts and thiones. Neither the diimidazol-2-ylidenes nor the dibenzimidazol-2-ylidenes were isolated due to electronically stabilised carbenes being incompatible with the bridge. The diimidazolidin-2-thiones gave the free dicarbene when the N-substituent was the 2,6-diisopropylphenyl group with the 2,4,6-trimethylphenyl derivative giving the dimer. Ligand exchange complexation with the diimidazolidin-2-ylidene and dimer failed to provide a means to ruthenium and palladium chiral chelating complexes due to steric congestion. In situ studies of the salts gave the nine membered cis 4,5-bis(1-cyclohexylbenzimidazol-2-ylidene-3-methyl)-2,2-dimethyl-1,3-dioxolane palladium dibromide. Utilising both enantiomers of 4,5-bis(bromoethyl)-2,2-dimethyl-1,3-dioxolane, that were prepared through asymmetric dihydroxylation, allowed the synthesis of eleven membered ruthenium benzylidene complexes from stable solutions of diimidazol-2-ylidenes. Square planar trans palladium dibromide complexes were prepared through in situ methods using the 1-cyclohexyl and 1-phenylbenzimidazolium salts.