Density functional study on intramolecular dehydro Diels-Alder reactions of conjugated enynes

The effects of several substituents X on the intramolecular dehydro Diels-Alder reactions of conjugated enynes (1R-9R) were investigated at the B3LYP/6-311G(d,p) level of theory. Geometry optimizations and harmonic frequency calculations were performed for all reactants, transition structures, as well as products. It has shown that all transition structures are asynchronous, with forming C4C8 bond length being shorter than C1C9 distance except for 9TS. 7R with the protonated amino (X=NH2+) function displays a lower barrier than 6R with the amino (X=NH) function. The intramolecular [4+2] cycloaddition with X=O has the lowest reaction barrier and the highest exothermicity.