Kinetics of Spiro Meisenheimer Complex Formation from 3,6-dimethylcatechol 2,4,6-Trinitrophenyl Ether. Competition between a Trapping and a Preassociation Mechanism

Article abstract of DOI:10.1021/ja00224a038

The ionized form of catechol 2,4,6-trinitrophenyl ether was reported in 1976 to cyclize and form a spiro Meisenheimer complex with a rate constant in the order of 10⁹ s^-1 in 50percent Me2SO-50percent water.A stopped-flow and temperature-jump study of the cyclization of 3,6-dimethylcatechol 2,4,6-trinitrophenyl ether in the same solvent reported in this paper shows that the two methyl groups enhance the intramolecular nucleophilic addition rate still further.The results are consistent with the operation of two competing mechanisms: the first is a trapping mechanism that involves diffusion-controlled proton transfer between the phenolic hydroxyl and carboxylate ions and cyclization/ring opening occurring in the absence of catalysts; the second is a preassociation mechanism in which the cyclization/ring opening step involves buffer catalysis by hydrogen bonding.A reanalysis of the 1976 data for catechol 2,4,6-trinitrophenyl ether suggests that there is also a contribution by the preassociation mechanism but a very small one.The increase in the cyclization rate by the methyl groups in the 3,6-dimethylcatechol moiety is explained in terms of a steric effect that enhances the population of a more reactive conformation.This explanation is the same as that offered by Bunnett and Okamoto for the acceleration of the Smiles rearrangement of 2-hydroxy-2'-nitrodiphenyl sulfone to 2-(o-nitrophenoxy)benzenesulfinic acid, although the rate enhancement observed in the present work is much less dramatic than in the Smiles rearrangement.