Calcium bromide is a polar covalent molecule

1. Calcium bromide is a polar covalent molecule A series of dihydropyrimidinone derivatives were synthesized by one pot ring condensation reaction of substituted benzaldehyde, ketone ester and urea/thiourea with calcium bromide as catalyst under microwave irradiation without solvent. The advantages of this method are solvent-free method, low catalyst loading, short reaction time, simple reaction process, easy access to catalyst, low price and easy operation. Using this method, we synthesized 25 different dihydropyrimidines with biological activity. Calcium bromide is easily soluble in water and absolute ethanol. It is thermally and chemically stable. The use of calcium bromide in organic synthesis is very rare. Microwave enhanced chemistry is based on the efficient heating of materials through the "microwave dielectric heating" effect. This phenomenon depends on the ability of specific materials (solvents or reagents) to absorb microwave energy and convert it into heat. The electrical components of electromagnetic fields cause heating through two main mechanisms: depolarization and ion conduction. The radiation of the sample at microwave frequency causes the alignment of dipoles or ions in an applied electric field. When field oscillation is applied, the dipole or ion field attempts to rearrange itself or with the alternating electric field. In this process, energy is lost in the form of heat through molecular friction and dielectric loss. The heat generated in this process is directly related to the power of the matrix, which can adjust itself according to the frequency of the given field. If the dipole does not have enough time to readjust with the applied field or reorient too quickly, heating will not occur. Due to the high electronegativity difference between calcium and bromine, calcium bromide is a polar covalent molecule. The binding electron pair of calcium and bromine is pulled to the bromine atom, forming double in the molecule. Because of this dipole, calcium bromide absorbs microwave energy and converts it into heat. The heat generated is used to cause the reaction between urea, benzaldehyde and ethyl acetoacetate. Taking the reaction of benzaldehyde with ethyl acetoacetate and urea as models, the reaction conditions of Biginelli condensation reaction catalyzed by calcium bromide under microwave irradiation were preliminarily studied. In a word, this paper reports a method for the efficient synthesis of dihydropyrimidinone and dihydropyrimidinone under microwave irradiation using calcium bromide as catalyst. This method has the advantages of mild reaction conditions, fast product formation, high yield,

2. cheap catalyst and easy availability. In addition, another advantage of this method is that it is compatible with the environment, has high catalyst efficiency after synthesis, and is easy to separate. The most important thing is that organic solvents are not used and microwave radiation is used as an alternative energy source, which conforms to the second and fifth principles of the twelve principles of green chemistry. Therefore, this method is conducive to the development of green technology. All reagents, chemicals and solvents are purchased from Loba, Merck, SRL and Sigma Aldrich. The microwave reaction was conducted with an unmodified household microwave oven (Onida MO20CJP27B) at 400W power level. Calcium bromide was purchased from Loba Chemical Company. Thin layer chromatography (TLC) (precoated silica gel 60 F254) was used to monitor the reaction process. The melting point was recorded by open capillary method and was not corrected. The infrared spectra of KBr pellets were recorded by Shimude FTIR. On Brooke DRX-300 Avance instrument, with DMSO d6 as solvent and TMS as internal standard, 1HNMR spectrum was obtained at 300MHZ frequency. All products are known compounds, and their authenticity is based on spectral data and comparison with real samples. General method for synthesizing dihydropyrimidinone and thione: stir the mixture of acetaldehyde (1mmol), ethyl acetoacetate (1mmol), urea or thiourea (1mmol) and calcium bromide (0.02mmol, 2mol%) in a reaction flask, and store in a microwave oven. The reaction mixture was irradiated in a 400W microwave oven for 2 minutes. The reaction was monitored by thin layer chromatography. After the reaction is completed, the reaction mixture is cooled, added into 20mL ice water to precipitate solids, filtered with a suction pump, and washed and dried with water. Pure samples were obtained by ethanol recrystallization.