Theoretical Study on the Reaction Mechanism for Diels-Alder Cycloaddition of C50(D5h) with 1,3-Butadiene and 2,3-Disubstituted 1,3-Butadienes

The reaction mechanism for Diels-Alder cycloaddition of C-50(D-5h) with 1,3-butadiene and 2, 3-disubstituted 1,3-butadienes has been investigated using the semiempirical AMI method. The calculated results show that: (1) All the Diels-Alder cycloaddition reactions take place via a concerted process. The length of the forming C-C bonds in transition states is in the range of 0.2293-0.2527 nm. (2) The activation energy(15.90 kJ/mol) for the cycloaddition of C-50(D-5h) with 1,3-butadiene is low. Moreover, this reaction is high exothermic of 402.50 kJ/mol. Therefore, it is predicted that not only kinetically but also thermodynamically the cycloaddition of C-50(D-5h) with 1, 3-butadiene is easy to take place at room temperature. The C-50(D-5h) is more reactive than C-60 and C-70. (3) Different substituted groups on 1,3-butadiene have different effect on the reaction activation energy of the cycloaddition of C-50(D-5h) with substituted 1,3-butadiene. Weak electron-withdrawing or weak electron-donating groups have small effect on the reaction activation energy. While strong electron-withdrawing or strong electron-donating groups have outstanding effect on the reaction activation energy: Strong electron-donating groups obviously lower and strong electron-withdrawing group dramatically increase the reaction activation energy, respectively. (4) All the reverse reaction activation energies for the Diels-Alder cycloaddition of C-50(D-5h) with 1,3-butadiene and the substituted 1,3-butadienes are very high and in a range of 416.51-425.93 kJ/mol, indicating that the product of the Diels-Alder cycloadditions is very stable kinetically.