engleski

5,6-Dihydroxy-4,6-decadiene-3,8-dione

The title compound was prepared on treating ethylmethylketone with diethyloxalate as stable crystalline substance of low melting point. The compound can appear either as keto or enol tautomer structure. Because the type of the tautomer could not be established unequivocally by IR spectroscopy an X-ray structural analysis was undertaken. The crystal structure of two analogous "tetraketones", having attached phenyl and p-tolyl as terminal groups, have been reported previously [B. Kaitner, G. Jovanovski and I. Janev, Acta Cryst. C48 (1992) 127-129]. Different terminal groups, positioned symmetrically on the opposite sides of the otherwise identical central tetraketone moiety, do not affect the keto-enol tautomerism as well as the position of hydrogen atom in O-H…O intramolecular hydrogen bonds. The central fragment was found to be the same with OH groups placed closer to the imposed centre of inversion between C1 and C1*. It should be pointed out that there are no delocalisation of pi-electrons in any of three structures and position of single bonds and double C-C bonds is unambiguously determined. Nevertheless, the molecules represent a special type of tetradentate ligand related closely to b-diketones. The unsaturated central moiety of the chain containing tetraketone fragment is planar with maximum deviation 1.8(2) pm for C3. The structural study equivocally showed the compound is in the keto-enol form. The position of hydrogen atoms in intramolecular hydrogen bonding was unambiguously determined as a well defined electron density. Very short O1-H…O2 (O1…O2 255.9 pm) hydrogen bond, dictated by the molecular stereochemistry, has got the dimensions: O1-H 92.3 pm, O2…H 172.7 pm, and O1-H…O2 148.4 degrees. There are no contacts between neighbouring molecules shorter than the sum of van der Waals radii. The absence of intramolecular attractions accounts for the partial sublimation of the specimen during the data collection procedure (14 % lowering of standard reflections intensities). The angle between the least-squares planes calculated through the C1, C2, C3, O1, O2 unsaturated part of the chain and C3, C4, C5 atoms amounts 10.7(2) degrees.

tetraketone; intramolecular hydrogen bond; X-ray structure analysis

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