Theoretical Studies On Cyclization Mechanism And Regioselectivities Of Selected Organic Reactions

The theoretical studies on the mechanisms for the Copper(I) catalyzed Kinugasa reaction and the regioselectivity of reaction between tropone and olefins were studied in the thesis.The first part of this thesis presented a theoretical study on the reaction mechanisms for the Copper catalyzedβ-lactam synthesis through Kinugasa reaction. The reaction is proceeding through a reaction between nitrone and two alkynyl-copper(I) complexes to generate a six-membered ring intermediate. It was followed that the six-membered ring intermediate was isomerized to the copper substituted intermediate oxazoline. It was found to be the rate-limiting step that the copper substituted oxazoline then decomposed into ketene and imine. Then the newly formed imine re-coordinated to the copper center. There was the enol-keto tautomerism, and the enol was converted into the more stableβ-lactam. The barrier for cis-β-lactam was found to be even lower than the barrier for trans-β-lactam by 10.5 kJ/mol. Therefore, the principal product of cis-β-lactam was obtained finally. The calculated energy barrier of this Kinugasa reaction is only 102.1 kJ/mol. Based on the dinuclear copper(I) model, calculations of monocopper(I) model were performed at the B3LYP level of theory. The different step of monocopper(I) model was that the metallacycle species were five-membered. DFT studies indicate that the second copper center facilitates the formation of the cupracycle and stabilizes the metallacycle intermediate itself, so the dinuclear copper(I) model was more credible.The second part of the thesis presented a theoretical study on the reaction mechanism and reaction regioselectivity of tropone and olefins. Our calculated results suggest that tropone and ethylene were prone to undergo a Diels-Alder addition process, and the barrier for the [4+2] addition was calculated to be 107.1 kJ/mol; while tropone and ketene dimethyl acetal were prone to undergo a [8+2] addition process, and the barrier for the addition was calculated to be 67.4 kJ/mol. Frontier molecular orbital (FMO) analysis were also done to explain how the catalyst affected the reaction. Our investigations indicate that the catalyst BF3 can lower the lowest unoccupied molecular orbital of tropone and the substituent group MeO can raise up the highest occupied molecular orbital of olefins, so the barrier was reduced greatly.