A Modular Furanoside Thioether-Phosphite/Phosphinite/Phosphine Ligand Library for Asymmetric Iridium-Catalyzed Hydrogenation of Minimally Functionalized Olefins: Scope and Limitations

A highly modular furanoside thioether-phosphite/phosphinite/phosphine ligand library has been synthesized for the iridium-catalyzed asymmetric hydrogenation of minimally functionalized olefins. These ligands can be prepared efficiently from easily accessible D-(+)-xylose. We found that their effectiveness at transferring the chiral information in the product can be tuned by correctly choosing the ligand components. Enantioselectivities were therefore excellent (ees up to 99%) in a wide range of E- and Z-trisubstituted alkenes using 5-deoxyribofuranoside thioether-phosphite ligands. It should be pointed out that these catalysts are also very tolerant to the presence of a neighbouring polar group. For 1,1-disubstituted substrates, both enantiomers of the hydrogenation product can be obtained in high enantioselectivities simply by changing the configuration of the biaryl phosphite moiety. The asymmetric hydrogenation was also performed using propylene carbonate as solvent, which allowed the iridium catalysts to be reused while maintaining the excellent enantioselectivities.