Abstract
In this thesis, ab initio molecular orbital theory was used to study the mechanisms of the Diels-Alder reaction of masked o-benzoquinone with mono-substituted alkenes, followed by the Diels-Alder reactions of the masked o-benzoquinone with cyclopentadiene, furan, pyrrole and thiophene.In the first section, the transition structures of each Diels-Alder reaction by the masked o-benzoquinone with ethylene, propene, methyl vinyl ether, methyl vinyl ketone and butadiene, were optimized individually, using HF/3-21G, HF/6-31G*, MP2/3-21G and MP2/6-31G* levels of theories. The single-point energetics employing the aforementioned optimized geometries were done using MP3 and MP4SDQ methods. The highest level of energy calculation utilized was MP4SDQ//MP2/6-31G*. The charge distribution of each species was also analyzed. The result showed that the different properties exist in each transition structure of these Diels-Alder reactions with each dienophile, and lead to different dominant control of stereo-selectivities and regio-selectivities in the reaction mechanisms.In the second section, the transition structures of each Diels-Alder reaction by the masked o-benzoquinone with the heterocyclics, such as furan, pyrrole and thiophene, were optimized individually, at the level of HF/3-21G, followed by the single-point energy calculation at MP4SDQ/6-31G* level. The results showed that the most transition structures process highly asynchronous concerted reaction. The charge transfer dominates while the cyclopentadiene, furan and pyrrole act as the dienophiles. The main transition structures obtained by the calculations were reconfirmed using intrinsic reaction coordinate (IRC) method. According to the thermodynamic data analysis, the free energies of the [2 + 4] reactions are too large to make the reactions spontaneous although in the [2 + 4] reactions in which furan, pyrrole and thiophene act as the dienophiles, the activation energies are equivalent to those of the main [4 + 2] reactions.