Mechanism and Origins of Enantioselectivities in Spirobiindane-Based Hypervalent Iodine(III)-Induced Asymmetric Dearomatizing Spirolactonizations.

Abstract

The work of Kita et al. on asymmetric oxidative dearomatization of naphthol carboxylic acids to spirolactones mediated/catalyzed by a novel, conformationally rigid μ-oxo-bridged hypervalent iodine(III) species is a landmark discovery in enantioselective iodine(III) catalysis [Kita, Y.; et al. Angew. Chem., Int. Ed. 2008, 47, 3787. DOI: 10.1002/anie.200800464 ; J. Am. Chem. Soc. 2013, 135, 4558. DOI: 10.1021/ja401074u ]. We have investigated the detailed mechanism of this important transformation using density functional theory. Calculations revealed that proton transfer from the pendant carboxylic acid of naphthols to the bridging oxygen atom or the ligand of iodine(III) species, which enhances the nucleophilicity of the carboxylic oxygen and the nucleofugality of the iodoarene, is crucial for the dearomatizing spirolactonization. Halogen bonding between the resulting carboxylate and the electron-deficient iodine(III) center further stabilizes the dearomatizing spirolactonization transition states. Calculations also revealed a long-neglected cleaved μ-oxo iodine(III) species that is more reactive but less selective than the μ-oxo-bridged hypervalent iodine(III) species itself for the oxidative dearomatization of naphthols. The coexistence of two sequential dearomatizing spirolactonization processes in the reaction system results in a lower enantioselectivity. A new stereochemical model that is able to reproduce and rationalize the observed apparent enantioselectivities is proposed.