Studies On The Synthesis Of3-Substitutide Coumarins Catalyzed By L-lysine In Water And Its Solvent Effect

Most organic reactions were occurred in solvents. Selecting appropriate solvent can enhance reaction rate and improve yield, so the study of solvent effect has been received extensive attention from chemists.In recent years, environmental pollution has become an issue of serious international concerns, therefore the term "green chemistry" was raised by chemists, with the purpose of minimizing or eliminating environment pollution. Green solvent technique plays an important role in green chemistry. The solvents were widely used in the chemical reaction and the separation of products. However, a lot of toxic volatile organic solvents have brought serious environmental problem and threatened our human health. As we know, water is the most inexpensive and environmentally benign solvent, the properties of non-flammable and non-volatile make water popular as solvent in organic chemistry.First of all, we carried out the reaction of various salicylaldehyde with several active methylene such as ethyl acetoacetate, diethyl malonate and Meldrum acid to synthesize3-substitutide coumarins in the presence of L-lysine in water at the benign reaction condition and attained good yield in short reaction time. It indicated that L-lysine was extremely important and indispensable in the reaction; Without the L-lysine, the reaction did not happen; and with L-lysine, the yield of reaction will be improved. It is also shown that the condensation reaction of salicylaldehyde, which possesses donating group on the aromatic compound, with the active methylene substance can be carried out in higher yield than that with electron withdrawing group. In addition, the system of water containing L-lysine could be recycled and reused at least five times without noticeably decreasing the productivity.Secondly, we focused on the effect of different solvents in the Knoevenagel condensation reaction. The reactions were conducted in six distinct traditional solvents under the same other conditions, they are water, dimethylsulfoxide, acetonitrile, dichloromethane, tetrahydrofuran, cyclohexane respectively. It was showed that the yield was improved with the enhancement of polarity of solvent.Finally, all calculation were performed with Gaussian03program and the geometries of all the stationary points were optimized at the B3LYP/6-31G*level of Density Functional Theory (DFT). The total reaction energies and solvation energies of the reactants and the product were computed using a Polarized Continuum Model and the calculation results were in good accordance with the experiment outcomes:1) The solvent played a crucial roles that the total reaction energies and solvation energies of the reactants and product decreased following the enhancement of polarity of solvent. Meanwhile the△G298of this reaction was also declined when the polarity of solvent was increased, which was in good accordance with the experimental results except for the solvent water. In the experiment, water obtained the highest yield, however, the result was not the same in our calculation. In our opinion, there may be two reasons, For one thing, the product can be isolated from the system timely, as a result, the equilibrium was changed, the yield was increased too. For another, we didn’t consider the influence of hydrogen bond and hydrophobic effect in the water, which can extremely improve the reaction.2) The△G298of the Knoevenagel condensation reaction of salicylaldehydes, which possesses donating group on the aromatic compound, with the active methylene substance was lower than that with electron withdrawing group.3) The result of calculation at B3LYP/6-31+G**level was as similar as that at B3LYP/6-31G*.