Supramolecular kinetic effects by pillararenes: the synergism between spatiotemporal and preorganization concepts in decarboxylation reactions?

New Journal of Chemistry Pub Date: 2021-03-18 DOI: 10.1039/D1NJ00551K

Abstract

A study about spontaneous decarboxylation reactions of 3-carboxy-1,2-benzisoxazole (CBI) nitrated derivatives (6-NitroCBI and 5,6-DinitroCBI) compared with supramolecular kinetic effects promoted by two cationic pillararenes (P5A and P6A) has been carried out and brings contributions to advance current theories related to supramolecular and enzymatic catalysis/inhibition. A higher energy barrier to spontaneous hydrolysis of 6-NitroCBI relative to that of 5,6-DinitroCBI (ΔΔG? = 2.98 kcal mol?1) was determined experimentally, however similar transitions states were observed for both substrates. The host–guest complexes generated catalytic effects up to 7.1-fold on kobs, except for 6-NitroCBI?P5A which is the only system that presented unprecedent inhibitory kinetic effects. The data (kinetics, 1H NMR titration and molecular dynamic simulations) elucidated the kinetic effects of the four complexes as a result of their geometries, more specifically, by modifying 3 main factors: (1) number of hydrogen bonds between solvent and carboxyl groups of the CBIs, (2) polarization of the O–N bond of the isoxazole ring and (3) reduction of the inductive effect of NO2 groups by field effects. The data from the present study encourage the use of both spatiotemporal and preorganization theories during interpretations related to supramolecular and enzymatic catalysis.

Graphical abstract: Supramolecular kinetic effects by pillararenes: the synergism between spatiotemporal and preorganization concepts in decarboxylation reactions
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