Catalytic Asymmetric Hydrogenation of Arenes

Enantioselective hydrogenations of arenes, including heteroarenes, will offer a fruitful methodology for creating stereogenic centers on chiral 5- or 6 -membered rings. The resulting chiral cyclic structures are frequently seen in useful biologically active compounds. However, the asymmetric hydrogenation of arenes, particularly benzene ring, is a formidable target in synthetic organic chemistry, because the unsaturated bonds in arenes are highly stabilized with their own aromaticity. This account describes our study on the hydrogenations with asymmetric catalysis of heteroarenes and carbocyclic arenes. Previously, we had accounted the highly enantioselective hydrogenation of indoles with the chiral PhTRAP-rhodium and ruthenium catalyst. Here, we found that the chiral ruthenium catalyst is useful for the asymmetric hydrogenation of various 5-membered nitrogen-containing heteroarenes, such as pyrroles, imidazoles, oxazoles, and azaindoles. Furthermore, some fused carbocyclic arenes can be transformed into the chiral cyclohexanes with good enantiomeric excesses through the PhTRAP-ruthenium catalyst. Although the ruthenium complex failed to catalyze the hydrogenations of pyrimidines and isoxazoles, these heteroarenes were converted to the corresponding chiral heterocycles with high enantiopurities through chiral iridium catalyses.