Structural analysis of two tetraketones and theoretical investigation of the reactions involved in their preparation

Abstract

The 2,2'-((5-(4-bromophenyl)furan-2-yl)methylene) bis (5,5-dimethylcyclohexane-1,3-dione) (3) and 2,2'-((5-(4-chlorophenyl)furan-2-yl)methylene) bis (5,5-dimethylcyclohexane-1,3-dione) (4) were prepared in, respectively, 63% and 59% yield, via ZrOCl2⋅8H2O catalyzed condensation reactions between dimedone and appropriate aldehydes. Their structures were investigated by IR, NMR, and X-ray spectroscopy techniques. The asymmetric unit of tetraketone 3 is composed of just one molecule, while two almost identical crystallographically independent molecules of compound 4 are present there. Compound 3 is conformationally similar to both molecules of 4. The diketone rings assume a half-chair conformation with the flaps oriented toward the same side of the substituent at C1. Each diketone ring is featured by an electronic delocalization path encompassed through the keto-enol moiety. All bond lengths inside this conjugated system are intermediate between those of pure double and single bonds. Furthermore, the furan plane of the substituent at C1 is almost parallel to the bond axes bridging the diketone rings as a consequence of steric hindrance effects between the heterocycle moiety and two hydrogen bonded oxygens. The enol forms of compounds 3 and 4 were noticed via IR and NMR spectroscopies. Furthermore, thermodynamics parameters were calculated in order to interpret the experimental results. In this line, theoretical findings reveal that electronic and solvent effects play an important role in the chemical reactions involved in the preparation of tetraketones.