A Theoretical Study On The 1,3-Dipolar Cycloaddition Of Nitrones With Mono Substituted Alkenes

1,3-dipolar cycloaddition reaction is the most useful reaction in fundmental theory of organic chemistry and organic synthesis .The 1,3-dipolar cycloaddition of nitrones with alkenes is one of the most studied reactions, among all the cycloadditions. It provides the most efficient method to prepare isoxazolidines that are widely used as starting compounds in organic synthesis, in particular for building natural products. The reaction of nitrones with mono substituted alkenes has a good regioselectivity but no steroselectivity . In the context of our studies we started a computational investigation in the mechanism of this reaction using ab initio molecular orbital theory in detail, we provide theoretical proof for learn the mechanism of this reaction.In the past years 1,3-dipolar cycloaddition reactions have been studied only limitted for little molecule systems . They only use frontier orbital theory to explain the mechanism , predict the selectivity and products of the reaction. In our thesis we studied the bigger system at B3LYP/6-31G and MP2/6-31G levels by using ab initio molecular orbital theory. Namely the mechanism and regioselectivity of 1,3-dipolar cycloaddition of C,N-dimethyl nitron and propylene and 1,3-dipolar cycloaddition of C-phenyl-N-methyl nitrone and nitroethylene. Geometry optimizations and frequency analysis of stationary points, including reactants, products and transition states are preformed. The minimum energy reaction paths (MEP) in both forward and reverse directions are obtained by the intrinsic reaction coordinate theory (IRC). We built a pattern and grasp the process of 1,3-dipolar cycloaddition of nitrones with alkenes deeply.Basing on the above results , the reaction of mono substituted alkenes has a good regioselectivity but no steroselectivity,5-substituted-isooxazoline was predominately yield as the product of this type of reaction. The calculation results show that the regioselectivity of this kind of reaction doesn't only concluded by the energy barriers but also by the energy gap of the H-bond complexes.