Confinement induced catalytic activity in a Diels-Alder reaction: comparison among various CB[n], n = 6–8, cavitands

The impact of the size of the confining regime on the thermodynamic and kinetic outcome of a representative Diels-Alder reaction between ethylene and 1,3 butadiene has been investigated in silico. To this end, two organic hosts namely cucurbit[6]uril (CB[6]) and cucurbit[8]uril (CB[8]) have been considered in order to impose confinement on the reactants/transition state/product of the concerned reaction. The obtained results have been compared with the recently reported (Chakraborty et al. ChemPhysChem 18:2162-2170, 2017) corresponding case of the same reaction happening inside cucurbit[7]uril (CB[7]). Results indicate that as compared to the reaction of ethylene and 1,3 butadiene inside CB[7], both CB[6] and CB[8] cavitands slow down the same reaction at 298.15 K and 1 atm. It appears that the size of the cavitand plays a crucial role in affecting the kinetic outcome of the considered reaction. While CB[7] can enforce productive alignment of the reactants inside its cavity thereby facilitating the reaction, neither CB[6] nor CB[8] can perform the same task as effectively. This situation bears qualitative resemblance with the cases of enzyme catalyzed reactions.