Studies On Cascade Oxa-Michael-Henry Reaction Via Ball Milling And Fluorescent Properties Of 3-Nitro-2H-chromenes

Benzopyran oxygen-containing six-membered heterocycles have been widely found in nature and synthetic pharmaceutical molecule. They have significant biological activities as well as excellent optical properties. The study of preparation and properties of these kinds of compounds is significant to the field of biological medicine, material science and natural products synthesis. This thesis focused on the development of efficient and green methods to synthesize a series of 3-nitro-2H-chromene derivatives via mechanical ball milling. Solvent effect, concentration effect and the substituent electronic effects to the influence of their fluorescence properties were studied. These results are the base of studying small molecules detection and heavy metal ions recognition in practical application in clinical, biological and environment. The results clearly showed that:1. This thesis has developed a general procedure for the production of nineteen 3-nitro-2H-chromene derivatives via the cascade oxa-Michael-Henry reaction of salicylaldehyde derivatives with β-nitrostyrenes promoted by solvent-free ball milling. As a consequence, the reaction conditions were determined with respect to base and its loading, ratio of reagents, ball-milling time and frequency and grinding aid. A range of different salicylaldehyde derivatives and β-nitrostyrenes had been investigated in an effort to evaluate the scope of this cascade oxa-Michael-Henry process. Most of the products can be obtained in high yields, even 97%, under the optimal reaction conditions. Their structures were characterized by 1HNMR,13CNMR, MS accroding to the experimental datas and literature releated. According to previous reports, the suggested reaction mechanism for this sequence involves oxa-Michael/Henry/dehydration steps. The cascade reaction was also performed on a gram scale, which resulted in 80% yield. This method offers significant advantages particularly in terms of high yields, short reaction times, mild conditions, eco-friendliness and simple work up, and has good prospects in the future development of industrial applications.2. Generally,3-nitro-2H-chromene derivatives were kinds of ICT (intramolecular charge transfer) fluorescent substance and its fluorescence properties were mainly depended on the push and pull system formed by the electron withdrawing substituents in position 3 and electron donating substituents in position 6 or 7. The fluorescence intensity of 3-nitro-2H-chromene was stronger in weak polar solvent, like dichloromethane and toluene. The maximum UV/vis absorption wavelengths were at 364～414 nm and the maximum emission wavelength were at 468～630 nm and stokes shifts were at 121～190nm. A remarkable red shift was observed in the emission spectrum. of fluorescence upon increasing the solvent polarity. Substituents in the position 6 caused great impact on fluorescence properties, namely, the electron-withdrawing group (F, Br) in the position 6 caused blue shift as well as fluorescence reduction and the donor groups (CH3, OCH3) caused red shift as well as fluorescence enhancement to the corresponding fluorescence emission spectrum of 3-nitro-2H-chromene. A good linear relationship was established between concentration and fluorescence intensity. However, the fluorescence quantum yield of the product was relatively low, which may be due to the fluorescence quenching effect of nitro group and the reduced coplanarity of benzopyran ring and benzene ring caused by the rotation of intramolecular carbon-carbon single bond.