New ferrocenyl-substituted heterocycles. Formation under Biginelli conditions, DFT modelling, and structure determination

A series of novel ferrocene-containing-dihydropyrimidines (DHPs) were prepared by three-component Biginelli reactions of formylferrocene, 1,3-dioxo-components and thiourea catalyzed by boric acid and ytterbium triflate, respectively. When cyclic-1,3-diones were employed as dioxo component in the reactions promoted by boric acid, besides one expected 4-ferrocenyl-2-thioxoquinazoline, 9-ferrocenyl-2H-xanthene-1,8-dione and 9-ferrocenylcyclopenta[b]chromen-8-ones could also be isolated as products. By means of control reactions and B3LYP/6-31 G(d) modelling the formation of the chromenone was interpreted by hetero-Diels–Alder addition involving the Knoevenagel intermediate and the cyclopentadiene resulted in situ from acid-catalyzed decomposition of formylferrocene. The enhanced tendency of the acyclic dioxo components to undergo Biginelli reaction avoiding cycloaddition was reasoned by the formation of Knoevenagel intermediates capable of chelating proton or Lewis acids. The structures of the new compounds were established by IR and NMR spectroscopy, including HMQC, HMBC, DEPT and DNOE measurements. Some structural characteristics were disclosed by B3LYP/6-31 G(d) method.