| Bisthiourea-catalyzed asymmetric Diels-Alder reactions have potential in obtaining the carbazolespirooxindole derivatives. However, the mechanism of these reactions have not been completely elucidated. In this thesis, we have investigated the enantioselectivity of bisthiourea-catalyzed asymmetric Diels-Alder reaction with Density Functional Theory (DFT). Moreover, Natural Bond orbital (NBO) theory and Atom In Molecule (AIM) theory were used to discuss the active mechanism of thiourea and to understand the active site and active pathway between thiourea and reactants. The main research contains two parts:1. Hydrogen bond between thiourea and carbonyl dienophile. The hydrogen bonds of complexes of thiourea and carbonyl dienophile have been studied by DFT (M06-2X/6-31+G(d,p)). The results reveal that carbonyl dienophile’s substituent could affect the strength of hydrogen bond. AIM analysis suggests that the electron density change of red-shifted N-H…O hydrogen bond is responsible for the change of the strength of hydrogen bond and the red shift, the stronger the electron density, the greater the hydrogen bond. The more difference of two carbon atoms’s electron density is more advantageous. Meanwhile, NBO analysis shows that intramolecular hyperconjugations is the key factor leading to the red-shifted hydrogen bond.2. Bisthiourea-catalyzed Asymmetric Diels-Alder reactions. The steroselectivity of asymmetric Diels-Alder reaction of 3-vinylindoles with methylenedolinones catalyzed by a C2-symmetric bisthiourea organocatalyst was studied. A theoretical rationale was developed through the combined use of ONIOM(M06-2X/6-31+G(d,p):AMl) caculations. The results suggest that the stereoselectivity involves the reaction of a bisthiourea-methylenedolinone complex with the uncoordinated 3-vinylindoles, and this reaction proceeds via a concerted mechanism through an asynchronous transition state (TS), which is in agreement with the experimental findings. Meanwahile, the intramolecular hydrogen bond between the dienophile and the diene is found to improve the exo selectivity. The sense of asymmetric induction is well-rationalized by the analysis of the relative Gibbs energies of bisthiourea-conplexed TSs, the TS of the main product is (1S,2S,3S). while the stereocontrol capabilities by bisthiourea in the reaction can be qualitatively established on the basis of the structural features of their corresponding TSs. |
