Experimental and theoretical study on a one-pot, three-component route to 3,4-dihydropyrimidin-2(1H)-ones/thiones TiCl3OTf-[bmim]Cl

Understanding the Lack of Reactivity of 2,4-Dihydroxybenzaldehyde Towards the Biginelli Adduct Using Density Functional Theory Molecular Modeling

The Biginelli reaction is a multicomponent reaction for obtaining dihydropyrimidinthiones quickly, with multiple substitution patterns. The reaction mechanism remains unclear. Three possible pathways proposed for the reaction are the iminium route, an enamine intermediate, and the Knoevenagel pathway. However, when thiourea was used, no theoretical calculations were reported. Thus, based on the literature, the iminium pathway was used to obtain evidence explaining the lack of reactivity of 2,4-dihydroxybenzaldehyde towards the Biginelli adduct, compared with 4-hydroxybenzaldehyde. This computational study, carried out using the B3LYP/6-31++G(d,p) level of theory, showed an increment of 150 kJ/mol in the activation energy of the slowest pathway, due to the presence of a hydroxyl group in position 2 (ortho) of the aromatic aldehyde, decreasing its reactivity. Natural bond orbital (NBO) calculations suggest that the determinant steps are simultaneous, i.e., the polarization of the carbonyl group and its corresponding protonation by the hydrogen of the SH fragment of the thiourea tautomer. The activation enthalpy values suggest that the nucleophile attack takes place later on the compound 2,4-dihydroxybenzaldehyde compared to 4-hydroxybenzaldehyde-TS, confirming that the OH group in position 2 hinders the condensation reaction.

NH3(CH2)6NH3SiF6 catalyzed highly efficient synthesis of benzimidazoles, benzoxazoles, benzothiazoles, quinoxalines and pyrimidin-2-ones/thiones

Herein, we describe a simple, highly efficient and environmentally friendly protocol for the synthesis of benzimidazoles, benzoxazoles, benzothiazoles, 3,4-dihydropyrimidin-2-ones/ thiones and quinoxalines derivatives using hybrid crystal NH3(CH2)6NH3SiF6 as a catalyst. Use of recyclable catalyst, easy work-up procedure, excellent yields, short reaction times and scalability are the important practical features of the present protocol.