Rhodium‐Catalyzed ON‐OFF Switchable Hydrogenation Using a Molecular Shuttle Based on a [2]Rotaxane with a Phosphine Ligand
Yuping Tang, Yi‐Er Luo, Junfeng Xiang, Yan‐Mei He, Qing‐Hua Fan
Abstract
Abstract 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.