Chiral C₂ Symmetric NHC: Synthesis And Catalytic Properties

The chemistry of N-heterocyclic carbene (NHC) and its metal complexes is a new and active research area in the last two decades. NHC, a novel type of ligand, can coordinate with transition metal ions, forming complexes with good stability and unique quality in many catalytic reactions. In recent years, their catalytic properties in asymmetric version have been highlighted, both with academic interest and technical importance.In this thesis, we have designed five- membered-ring, and six-membered-ring NHC precursors with a C2 symmetry, where substitutents attached to nitrogen atoms of the ring have potentially coordinative oxygen atoms. Therefore, these NHC might act as multi-dentated ligand when coordinating with metal ions, and exhibit different, hopefully more excellent catalytic abilities, when compared with the current popular mono-dentate NHC ligands. Due to their special coordinative property and topology, they might be different, or even as an alternative, to the traditional phosphorous ligand.The following routes were employed in this thesis, for the synthesis of imidazolium salts, imidazolinium salts and 3,4,5,6-tetrahydropyrimidinium salts, the precursor of C2-symmetric NHC: (1) amides obtained from chiral amino alcohol (ether) and diethyl oxalate were reduced with LiAlH4, yielding diamines, the latter ones reacted with ethyl orthoformate and ammonium tetrafluoroborate (or ammonium chloride etc.), furnishing seven imidazolinium salts; (2) diamines derived from the reaction between amino alcohols and 1,2-dibromoethane reacted with ethyl orthoformate and ammonium tetrafluoroborate (ammonium chloride etc.), providing three imidazolinium salt; (3) the hydroxy group in alkyl lactate (mandelate) was converted into the corresponding ethers, and then the formed esters were reduced to alcohols, and the alcohol was converted to bromide; finally, the bromides reacted with imidazole giving six imidazolium salts; (4) one imidazolium salt was produced via the reaction of styrene oxide with imidazole under pressure; (5) two imidazolium salts were obtained via the cyclization of diimine derived from amine and glyoxal, with paraformaldehyde in the presence of trimethylsilyl chloride; (6) a similar reaction to route (2), but 1,3–dibromopropane was used and six six-membered ring 3,4,5,6-tetrahydropyrimidinium salts were obtained.Reaction of amino alcohols with glyoxal was also investigate, however, spiro bisoxazolidine compounds, instead of the expected diamines, were produced.The synthesized imidazolium salts were used as the NHC precursor in catalytic reactions. Cu-NHC compounds formed in situ from imidazolium (imidazolinium) salts were applied to the asymmetric conjugate addition of Et2Zn to cyclohexenone and cyclopentenone. In the presence of imidazolinium salts, high chemical yield (up to 99%) and reasonable ee (up to 78%) were achieved. It is interesting that using different bases may result in the switch of the configuration of the major product.The imidazolium salts were also applied to the Cu-promoted conjugate reduction ofα,β-unsaturated ketones with silane. Catalysts derived from imidazolium salts showed good catalytic properties and selectivity. The reaction could be finished in just 5 min, and only polar C-C double bond was reduced.