Showing papers by "Rita G. Hazell published in 2003"

Catalytic Asymmetric Mannich Reactions of Glycine Derivatives with Imines. A New Approach to Optically Active α,β-Diamino Acid Derivatives

Abstract: Imines of glycine alkyl esters react with imines in a diastereo- and highly enantioselective Mannich reaction in the presence of chiral copper(I) complexes as the catalyst to give optically active α,β-diamino acid derivatives. A series of imines of glycine esters derived from glycine and aromatic carbonyl compounds has been screened as substrates for the Mannich reaction with different imines in the presence of various combinations of metal salts and chiral ligands. The benzophenone imine of glycine esters was found to react with N-protected imines in a diastereoselective fashion giving functionalized α,β-diamino acid esters with excellent enantioselectivities. The most effective chiral catalysts are chiral copper(I) complexes having phosphino-oxazoline (P,N)-ligands, and among these ligands, those derived from (1R,2S)-dihydroxy-1,2,3,4-tetrahydronaphthalene gave the best results. The scope of this new catalytic asymmetric reaction of the benzophenone imine glycine esters is demonstrated for the reaction ...

The First Catalytic Highly Enantioselective Alkylation of Ketimines—A Novel Approach to Optically Active Quaternary α‐Amino Acids

Abstract: A series of novel ketimines with intrinsic protecting group anchoring was synthesized and allowed to react with various silylketene acetals in the presence of 5–10 mol % of a chiral Zn(OTf)2-(R,R)-Ph-pybox-aqua complex. The corresponding optically active quaternary α-amino acid derivatives were obtained in high yields and with enantioselectivities ranging from 34 % up to 95 % ee. The catalyst was studied by 1H NMR spectroscopy and X-ray crystallography, and a dynamic equilibrium of two species was identified in solution. These are a homo-chiral 1:2 metal–ligand complex and a 1:1 metal–ligand complex, of which the latter is expected to be the actual catalyst of the diastereo- and enantioselective reaction. A strong positive nonlinear effect was observed due to the formation of a catalytically inactive 1:2 metal–ligand hetero-chiral complex. On the basis of DFT calculations and the absolute stereochemistry of the products, simultaneous coordination of the imino electrophile and a single molecule of H2O to the chiral Lewis acid complex is proposed. Coordination of the imine-nitrogen atom in the axial position of an octahedral complex can account for the facial selectivity as well as the diastereoselectivity observed.

Stepwise construction of mono-, di- and tri-nuclear 2 ∶ 1, 1 ∶ 2, 2 ∶ 3 ligand ∶ mixed-metal complexes using a bis-tridentate bridging ligand

Abstract: Extended molecular structures have been constructed with the bis-tridentate ligand N,N,N′,N′-tetrakis(2-pyridylmethyl)benzene-1,4-diamine (tpbd), and the bis-tridentate, metal ‘complex’ ligand, [Ru(tpbd)2](PF6)2 (1) as building blocks. The ligand tpbd and complex 1 react analogously with Cu(II) salts to yield the corresponding di- and tri-nuclear aqua complexes, [Cu2(tpbd)(H2O)4](ClO4)4 (2a) and [Cu2Ru(tpbd)2(H2O)4](BF4)4(PF6)2·2H2O (3a). The labile water ligands in these complexes have been replaced by other solvents or 2,2′-bipyridine (bipy). In the latter case the mixed ligand complexes [Cu2(tpbd)(bipy)2](PF6)4·H2O (2b) and [Cu2Ru(BF4)2(tpbd)2(bipy)2](PF6)4·2H2O (3b) respectively are obtained. These reactions represent prototype substitutions for the controlled stepwise evolution of even larger molecular entities. Recrystallisation of 3a in 2% NaBF4 methanol/water solution, resulted in crystals of [Cu2Ru(tpbd)2(H2O)4](BF4)4(PF6)2·8H2O (3a′). Recrystallisation of 3b in 2% NaBF4 CH3CN/water solution gave [Cu2Ru(BF4)2(tpbd)2(bipy)2](BF4)2(PF6)2·H2O (3b′). The X-ray structures of the mononuclear and trinuclear ruthenium-containing complexes (1, 3a′, 3b′) show cis–fac six-coordinate geometries around the Ru atoms. As a consequence the phenylene groups of the tpbd ligands are approximately perpendicular to each other. In the trinuclear Ru–Cu2 complexes the copper atoms are located anti to each other about the [(tpbd)Ru(tpbd)]2+ building block. The flexibility of tpbd is demonstrated by the geometries of the copper coordinated ends of tpbd, which are coordinated meridionally in 3a′, but facially in 3b′. Solution spectroscopy, and the detection of appropriate ions by mass spectrometry, show that the dicopper and trinuclear copper–ruthenium complexes are present in solution. UV-Visible spectroscopy, EPR and cyclic voltammetry indicate that there is insignificant electronic communication between the metal centers in the di- and tri-nuclear complexes since salient metal-based features are additive.