Merzouk, Mahboub (2008) Synthesis of chiral iminoalkyl functionalised N-Heterocyclic carbenes and their use in asymmetric catalysis. (PhD thesis), Kingston University, .
Abstract
The steric and electronic properties of N-Heterocyclic carbenes and their use as ligands in asymmetric catalysis are reviewed. Key features of enantioselective palladium-catalysed allylie substitution and copper-catalysed conjugate addition reaction are discussed. A modular design approach was applied to the synthesis of iminoalkyl imidazol-2-ylidenes to enable simple structural modifications and fine-tuning catalytic performance for selected reactions. A range of novel chiral imino alkyl alkylimidazolium salts and imino alkyl arylimidazolium salts have been prepared via the N-alkylation of substituted imidazoles with iminoalkyl bromides in moderate to good yields. The chiral iminoalkyl bromides were synthesised from amino acids using a five-step procedure, involving reduction, N-BOC protection, bromination, N-BOC deprotection and imination. The synthesis of chiral iminoalkyl alkylimidazolium salts derived from glycine, alanine, leucine, valine and phenylalanine and N-methyl-, N-benzyl-, N-phenyl and N-mesitylimidazoles is reported. The preparation of both benzylideneamine- and benzylhydrylideneamine derivatives was achieved. One example of an imidazolin-2-ylidene ligand precursor, derived from a dihydroimidazole is also reported. Silver iminoalkyl imidazol-Z-ylidene complexes were prepared by deprotonation of the corresponding imidazolium salts with silver(I) oxide. An X-ray crystal structure of example is reported; it shows the monodentate co-ordination of the iminoalkyl complexes failed. Nonetheless, the silver complexes were successfully employed as earbene transfer reagents for the generation of imino alkyl imidazol-2-ylidene palladium catalysts for use in asymmetrie allylie substitution. The application and performance of the small library of imino alkyl imidäzol-z-ylidene ligands in asymmetric palladium-catalysed allylie substitution is reported and discussed. Catalytic testing demonstrated that variation of the imidazole substituent had a greater effect on enantioselectivity than changing the alkyl substituent at the stereogenic centre on the ligand backbone. The highest enantiomeric excess obtained for the substitution of 1,3-diphenylprop-3-enyl acetate with dimethyl malonate was 53%. The ligands were also screened for enantioselectivity towards copper catalysed conjugate addition of diethyl zinc to cyclohexenone. Performance for this reaction was poor with a highest enantiomeric excess of 18% being obtained.
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