Catalysis by [Ga ₄L ₆] ¹²⁻ Metallocage on the Nazarov Cyclization: The Basicity of Complexed Alcohol is Key

Abstract

The Nazarov cyclization is investigated in solution and within K ₁₂[Ga ₄L ₆] supramolecular organometallic cage by means of computational methods. The reaction needs acidic condition in solution but works at neutral pH in the presence of the metallocage. The reaction steps for the process are analogous in both media: (a) protonation of the alcohol group, (b) water loss and (c) cyclization. The relative Gibbs energies of all the steps are affected by changing the environment from solvent to the metallocage. The first step in the mechanism, the alcohol protonation, turns out to be the most critical one for the acceleration of the reaction inside the metallocage. In order to calculate the relative stability of protonated alcohol inside the cavity, we propose a computational scheme for the calculation of basicity for species inside cavities and can be of general use. These results are in excellent agreement with the experiments, identifying key steps of catalysis and providing an in‐depth understanding of the impact of the metallocage on all the reaction steps.

Abstract

The Nazarov cyclization process performed inside a metallocage is much faster than in solution. This is mainly due to the change of basicity of the substrate (an alcohol) once encapsulated.