Indexed on: 12 Jun '16Published on: 12 Jun '16Published in: Journal of the American Chemical Society
We report density functional theory (M06-2X) studies of five dehydro-Diels-Alder (DDA) reactions. For these and the parent reaction, the stepwise mechanisms have similar barriers, whereas the barriers of the concerted mechanisms differ significantly. The reactivity of DDA reactions is controlled by distortion energy. The concerted and stepwise mechanisms of the hexadehydro-Diels-Alder (HDDA) reaction are competitive with activation barriers of ~36 kcal/mol. This is because a large distortion energy (~43 kcal/mol) is required to achieve the concerted transition state geometry. MD simulations reveal that productive concerted trajectories display a strong angle bending oscillation (~25(o) oscillation amplitude), while the stepwise trajectories show only a chaotic pattern and less pronounced bending vibrations.