Efficient one-pot synthesis and dehydrogenation of tricyclic dihydropyrimidines catalyzed by OMS-2-SO3H, and application of the functional-chromophore products as colorimetric chemosensors

An efficient and convenient one-pot multicomponent reaction (MCR) for the synthesis and dehydrogenation of tricyclic dihydropyrimidine derivatives, catalyzed by –SO₃H functionalized octahedral manganese oxide molecular sieves (OMS-2-SO₃H) as a novel solid acid catalyst, is reported. All of the organic products and the catalyst were unambiguously characterized with conventional techniques including Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction analysis (XRD), ¹H NMR, and ¹³C NMR spectroscopy. The targeted dehydrogenated chromophore compounds were successfully used as colorimetric chemosensors for detection of transition metals in aqueous solution. For example, 1-[4-(4-hydroxy-3-methoxy-phenyl)-2-methyl-benzo[4,5]imidazo[1,2-a]pyrimidin-3-yl]-ethanone (7d), exhibited high sensitivity and selectivity toward detection of Cr³⁺ over a panel of other transition metal cations. The interference of foreign ions was found to be negligible. It was found that a 1 : 1 complex of Cr³⁺ and 7d is responsible for the color change of the solution from ochre to brown. These newly devised chemosensors can also exhibit significant wavelength shifts (up to 100 nm) when used as pH indicators. 7d for example, showed a vivid and sharp color change from pink to yellow in the pH range of 4 to 6.

Hyperconjugated products of dihydropyrimidines may act as colorimetric chemosensors.