Cation-switchable transposition and hydrogenation of alkenes via interconnected reaction mechanisms

Abstract

Cation-switchable catalysis is a promising strategy for modulating reaction rates or selectivity using external stimuli, with potential applications including polymer synthesis and tandem catalysis. This study establishes a deeper understanding of the mechanistic origins of cation rate promotion in systems where a hemilabile crown ether affixed to the catalyst serves as a cation receptor site, which in turn enables reactivity extensions of this platform to introduce cation-switchable alkene hydrogenation. The mechanistic study hinges on the synthesis of two previously unreported iridium(III) complexes, bearing either bis(2-methoxyethyl)amine or diethylamine groups, which could be compared in catalysis with and without cation promoters. Only the catalyst containing a crown ether exhibits switchable catalysis, providing support for a “substrate gating” mechanism in which cations tune the hemilability of the crown ether donors to enable alkene binding. The generality of the substrate gating mechanism is established through the extension of catalytic reactivity to cation-switchable alkene hydrogenation. The crown-ether-containing catalyst was found to be an effective hydrogenation catalyst capable of reducing internal and trisubstituted olefins. The isomerization and hydrogenation reactions occur at similar rates in some cases, and kinetic and labeling studies provide insight into the implications of competitive reactivity.