Comparison of the ΔEa of different reactions.
For the whole reaction process, there are three elementary reactions and three transition states. The removal of H+ and Cl- from reactant 1a through transition state TS1 is carried out in triethylamine solvent. The cyclization of intermediate med2 to 3a through transition state TS3 is an intramolecular reaction. On the other hand, the reaction between intermediates 1a’ and 2a’ to form intermediate med1 through transition state TS2 is an intermolecular reaction that is greatly affected by a substituent. Therefore, elementary reaction 2 was further investigated by selecting reactants with different substituents.
Because the reaction is LUMO controlled, changing the substituent on reactant 1a has a greater effect. The reactions (elementary reaction 2) of 1a , 1b , 1f , 1g ,1j , 1m , 1n , and 1o with 2awere studied. Reactants 1a , 1b , 1f ,1g , 1j , and 1m contain a phenyl group with H, CH3, OCH3, F, Cl, and Br substituents, respectively. On the other hand, 1n is thiophene substituted and 1o is alkyl substituted. The molecular configurations of the reactants and products were optimized, and IRC tracing was carried out. The calculation shows that the reaction path of dienophile1a does not change whether the substituent is an electron-donating or electron-withdrawing group. Figures 15(a–h) shows the IRC curves for the different substituted reactants.