Sau-Fun Tan
National University of Singapore
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Transition Metal Chemistry | 1984
Sau-Fun Tan; Kok-Peng Ang; Hari Lakshmi Jayachandran
SummaryTri-and quadri-dentate Schiff bases have been synthesized from the reaction of dehydroacetic acid with diamines, aminoacids, aminophenols and aminoalcohols. The copper(II) and some nickel(II) and palladium(II) chelates of these ligands as well as copper(II) complexes of bidentate Schiff bases of dehydroacetic acid with anilines have been prepared and characterised by electronic, i.r. and n.m.r. spectral measurements and magnetic moments.
Transition Metal Chemistry | 1988
Sau-Fun Tan; Kok-Peng Ang
SummaryPartial displacement of the symmetrical bis-Schiff bases (1)–(4) of ethylenediamine and salicyladehyde, o-hydroxyacetophenone, acetylacetone or benzoylacetone by dehydroacetic acid have led to the formation and isolation of the unsymmetrical bis-Schiff base ligands (6)–(9). Attempted partial displacements of the symmetrical bis-Schiff base (5) of dehydroacetic acid or (1)–(4) by each of the other four carbonyl compounds have been unsuccessful. The Ni(II) and Cu(II) complexes of (6)–(9) have been prepared and characterized.
Journal of The Chemical Society-perkin Transactions 1 | 1986
Sau-Fun Tan; Kok-Peng Ang; Yoke-Fan Fong
Both (Z)- and (E)-5-arylmethylene-1-methylhydantoins have been obtained directly in substantial proportions from condensations of 1-methylhydantoin with aromatic aldehydes. However, the products of similar condensations of hydantoin or 3-methylhydantoins consist almost entirely of the Z-isomers. The configurations of all the 5-arylmethylenehydantoins prepared can be unambiguously assigned by consideration of their 1H and 13C n.m.r., i.r., and u.v. spectra. The mass spectra of the isomers show similar fragmentation patterns.
Transition Metal Chemistry | 1990
Sau-Fun Tan; Kok-Peng Ang; Lai-Har Ng; Alwyn G. Davies; Dan Oprescu
SummaryThe cobalt(II) complex of an unsymmetrical bis-Schiff base. Co(dha, salen), has been prepared. In the presence of pyridine, it forms both monomeric superoxo and dimeric peroxo adducts with dioxygen. At −10°C and lower temperatures, the adduct formed in pyridine solution is mainly superoxo as shown by electronic and e.s.r. spectra as well as volumetric measurement of oxygen uptake. The solid adduct precipitated by addition of toluene is predominantly peroxo as shown by elemental analysis, i.r. spectrum, magnetic and thermogravimetric measurements. Comparison of the oxygen affinities of this complex and of the symmetrical Co(salen) is discussed.
Transition Metal Chemistry | 1991
Sau-Fun Tan; Pak-Hing Leung; Wun-Chey Sin
SummaryThe CoII complex derived from the unsymmetricalbis-Schiff base ligand, 3-[1-[[2-[[(2-hydroxyphenyl)methylene]-amino]ethyl]amino]ethylidene]-6-methyl-2H-pyran-2,4(3H) dione H2(dha, salen), is a catalyst for the autoxidation of aromatic aldehydes. Under ambient condition Co(dha, salen) catalyses the oxidation ofp-anisaldehyde top-anisic acid in quantitative yield (250 turnovers). The presence of Co(dha, salen) also accelerates the co-oxidation ofp-anisaldehyde and styrene (88 turnovers). In the co-oxidation process, styrene is selectively converted to styrene oxide as the major product.
Journal of The Chemical Society-perkin Transactions 1 | 1988
Sau-Fun Tan; Kok-Peng Ang; Yoke-Fan Fong; Harilakshmi Jayachandran
The pK values of (Z)-5-arylmethylenehydrantoins, (Z)-5-arylmethylene-3-methylhydantoins, and (Z)- and (E)-5-arylmethylene-1-methylhydantoins with various para-substituents in the phenyl ring have been measured. The effects of structure and stereochemistry on acidity are discussed. For each series, a good linear correlation is obtained between the pK values and Hammett constants σ and σ– in spite of the considerable distance of the site of deprotonation from the phenyl ring. The ρ values show that substituent effects are significantly greater on the acidity of N(1)–H than on that of N(3)–H, and are also greater for the E- than the Z-isomers of the 5-arylmethylene-1-methylhydantoins.
Journal of The Chemical Society-perkin Transactions 1 | 1983
Sau-Fun Tan; Kok-Peng Ang; Harilakshmi Jayachandran
pK Determination in 80%(w/w) dimethyl sulphoxide–water solvent at 25 °C has been used as a means of studying the tautomerism in a series of Schiffs bases derived from some C-acylhydroxypyrones. While their much lower acidities compared with the corresponding parent compounds indicate a predominance of ketoamine tautomers, their higher acidities compared with amides and other nitrogen acids suggest significant contributions from dipolar structures. Correlation of the wide range of pK values with the structures is discussed.
Journal of The Chemical Society-perkin Transactions 1 | 1983
Sau-Fun Tan; Kok-Peng Ang; Harilakshmi Jayachandran
The ionization constants of some hydroxypyrones have been measured spectrophotometrically at 25 °C in water and in 80%(w/w) dimethyl sulphoxide–water. A few of them are exceptionally strongly acidic. The pKa values of these compounds are discussed in relation to their molecular structures. A plot of the pKa in water against the corresponding pKa in 80%(w/w) dimethyl sulphoxide–water is linear, although ΔpKa varies from negative values for the weaker acids to positive values for the stronger acids. Possible solvent effects have been suggested.
Journal of The Chemical Society-perkin Transactions 1 | 1988
Sau-Fun Tan; Kok-Peng Ang; Gee-Fung How
Standard free energy differences between the Z- and E-isomers of 14 5-arylmethylenehydantoins have been estimated by thermal equilibration in dimethyl sulphoxide. The Z-configuration is more stable for the N-unsubstituted compounds while the E-configuration is preferred for those N-1methyl derivatives with only para-substituents in the aryl group. The ΔG° values vary with the nature of the para-substituents. For those N-1 methyl derivatives with two ortho-substituents in the aryl group, the configurational preference is dependent on the electronic effects of these aryl substituents, suggesting the presence of non-bonded aromatic π⋯ Me interactions. The factors influencing the relative stabilities of Z- and E-isomers are discussed.
Journal of The Chemical Society-perkin Transactions 1 | 1979
Kok-Peng Ang; Sau-Fun Tan
The ionization constants of some hydroxy-pyrones have been measured in aqueous solutions by a spectrophotometric method and compared with those of analogous phenols. The strikingly high acidity of 3,5-diacetyl-4,6-dihydroxy-2-pyrone has been explained by the formation of a symmetrical and extensively resonance-stabilized monoanion through ionization of the C-6 hydroxy-group.