Factors Controlling the Reactivity of Strained-Alkyne Embedded Cycloparaphenylenes.

The factors controlling the reactivity of the strained-alkyne embedded cycloparaphenylenes have been computationally explored by means of Density Functional Theory calculations. To this end, the Diels-Alder cycloaddition reaction involving cyclopentadiene and these macrocyclic systems has been selected in order to understand the influence of the strained nature of the alkyne in their structures as well as the size of the system on their reactivity. It is found that the cycloaddition reactions involving those macrocycles having more strained alkynes not only are more exothermic and exhibit lower activation barriers but also are associated with earlier transition states. The combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis method suggests that the enhanced reactivity of bent alkynes, as compared to linear C≡C triple bonds, finds its origin not only in the lower deformation energy required to adopt the corresponding transition state structure but also in the stronger interaction energy between the deformed reactants.