Rhodium-Catalyzed ON-OFF Switchable Hydrogenation Using a Molecular Shuttle Based on a [2]Rotaxane with a Phosphine Ligand.

A novel pH-responsive molecular shuttle based on a [2]rotaxane with a phosphine ligand has been designed and synthesized. In the rhodium-catalyzed hydrogenation of α,β-dehydroamino acid esters and aryl enamides, ON/OFF-switchable catalysis was accomplished with high ON/OFF ratios by adjusting the movements of the rotaxane wheels located at the catalyst terminals with acid/base. Mechanistic studies using NMR spectroscopy and quasi in situ X-ray photoelectron spectroscopy revealed that Rh III -hydride species are possibly formed in a H 2 atmosphere when the catalyst is in the OFF state. During the reaction, a heterolytic activation of dihydrogen occurs by the interlocked rotaxane dibenzylamine and Rh I catalytic center acting as a frustrated Lewis pair. Subsequent homolytic splitting of dihydrogen with the newly formed Rh I -hydride species generates Rh III -hydride species. These findings show that a substrate-selective hydrogenation can be achieved by using the OFF-state catalyst.