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A hydroxocobalt(III) complex (1a), has been obtained by reaction of bis(dehydroacetato)ethylenediimine (H2dhaen) with cobalt(II) hydroxide or acetate in the presence of air. Addition of a second complexing agent leads to the formation of a series of mixed-ligand complexes (2)-(26) having either the trans- or cis-β configuration. In the cis-β complexes, the quadridentate ligand dhaen adopts a nonplanar conformation. Configurations are distinguishable from characteristic differences in the electronic and n.m.r. spectra.

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.

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.

Substituent effects in para-substituted 5-arylmethylenehydantoins have been studied by correlations of 1H and 13C chemical shifts with Hammett constants. In addition to the expected correlations for the 1H chemical shift of the vinyl proton and the 13C chemical shift of the β-carbon C-5, good linear correlations are also obtained for the chemical shifts of the N-1 and N-3 protons which are respectively four and five atoms away from the benzene ring as well as for the α-carbon C-6.

The 1H and 13C n.m.r. spectra of 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione (I) and 3,5-diacetyltetrahydropyran-2,4,6-trione (III) and their Schiff's bases were studied in CDCl3 over a range of temperatures for elucidation of possible tautomerism. Whereas compound (I) appears to exist in only one form, at least two forms have been detected for compound (III). The Schiff's bases exist predominantly in keto-amine forms and the rotational barrier due to substantial double bond character of the exocyclic carbon to carbon bonds allows ready detection of rotamers. Structures of the predominant rotamers are deduced from the 1Hand 13C spectra and the assignments of NH proton signals are based on the relative strengths of the intramolecular hydrogen-bonds. The chemical shifts of NH protons correlate linearly with the pKa of the amine precursors. The 15N spectra of some 15N-enriched Schiff's bases were obtained in CDCl3 and in (CD3)2SO and the considerable downfield shifts of the 15N resonances, compared with those of typical amides, are indicative of extensive electron delocalisation.

The effects of 5-acyl substituents on the tautomerism of pyran-2,4-diones are revealed by comparison of the u.v.,i.r., 1H, and 13C n.m.r. spectral properties of 5-acetyl-6-methyl-,5-acetyl-6-phenyl-,5-benzoyl-6-methyl-, and 3,5-diacetyl-6-methyl-pyran-2,4-diones (3)–(6), with those of 6-methyl-(1) and 3-acetyl-6-methyl-pyran-2,4-done (2). The differences in the behaviour of the 5-acyl and 3-acyl derivatives towards primary amines are also discussed. Compound (6) reacts with aniline to form 5-acetyl-2,6-dimethyl-1-phenyl-4-oxopyridine-3-carboxylic acid (7).

The i.r., u.v., and 1H- and 13C-n.m.r. spectra of some acetylpyran-2,6-diones are discussed in relation to their tautomeric structures and compared with those of the acetylpyran-2,4-diones. Their reactions with primary amines to give Schiff bases, salts, pyridones or open-chain products as well as a thermal rearrangement of the Schiff bases are reported.