Browsing by Author "Carvalho, M. Fernanda N.N."
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- Cu(II) complexes derived from N-carboxymethyl and N-carboxyethyl amino acids as catalysts for asymmetric oxidative coupling of 2-naphtholPublication . Adão, Pedro; Teixeira, Carlos M.; Carvalho, M. Fernanda N.N.; Kuznetsov, Maxim L.; Gomes, Clara S.B.; Pessoa, João CostaThe synthesis, characterization and catalytic performance of chiral Cu(II) complexes derived from N-carboxymethylated and N-carboxyethylated amino acids is reported. The ligand precursors are prepared by single step N-alkylation of the sodium salts of the appropriate chiral amino acid with either sodium chloroacetate or sodium 3-chloropropionate in water. The Cu(II) complexes are obtained upon reaction of Cu(CH3COO)2 with the aqueous or alcoholic suspension of the suitable ligand under vigorous stirring or ultrasound irradiation at room temperature. The Cu(II) compounds are characterised by EPR, UV–vis, circular dichroism and ESI-MS. The molecular structures of two of the prepared complexes are also obtained by single-crystal X-ray diffraction analysis. The catalytic activity of the complexes in the asymmetric oxidative coupling of 2-naphthol is described. All compounds exhibit moderate activity, selectivity and enantioselectivity in ethanol/water mixtures, under aerobic conditions and using potassium iodide as additive. The yields of 1,1′-bi-2-naphthol (BINOL) reached 50% under the optimal conditions, while enantiomeric excesses reached ca. 48%. The effect of variables such as ligand substituents, solvent, temperature and additives on the catalytic activity is also described. In the absence of a base, the complexes only show catalytic activity in the presence of alkali metal iodide such as KI. Details of the oxidative coupling mechanism are studied using spectroscopic and electrochemical methodologies.
- L-Phenylalanine derived tripodal vanadium complexes as catalysts for the asymmetric reductive coupling of benzaldehydePublication . Teixeira, Carlos M.; Adão, Pedro; Carvalho, M. Fernanda N.N.; Gomes, Clara S.B.; Pessoa, João CostaTwo tripodal vanadium complexes derived from modified L-phenylalanine were prepared and characterised. Both compounds were tested for their catalytic activity in the reductive coupling of benzaldehyde. Overall, the complexes are capable of catalysing the reductive coupling of benzaldehyde in the presence of metallic zinc as co-reductant and alkylammonium or alkylpyridinium acetate salts in ethanol under mild aerobic conditions. While benzyl alcohol is generally the major product, the yield of hydrobenzoin reaches ca. 38% under these conditions. Enantioselectivities reach 39% and there is a preferential formation of the trans diastereoisomer of hydrobenzoin. Attempts to study some aspects of the underlying mechanism were made. It was found that metallic zinc is capable of reducing the complexes to VIII species, which are likely to be the active catalytic species. The methodology described may set up a basis for the development of catalytic systems for the asymmetric synthesis of hydrobenzoins under mild conditions, not requiring chlorosilanes as electrophilic reagents for the regeneration of the catalyst.