Reduction: Hydrogenation and Transfer Hydrogenation of Arenes and Heteroarenes

Heteroarenes and carbocyclic arenes are challenging substrates for catalytic asymmetric hydrogenation, because they are strongly stabilized by their own aromaticity. Overcoming this difficulty, various chiral catalysts have been developed for the dearomative hydrogenations to achieve high stereoselectivity. A broad range of heteroarenes, pyrroles, indoles, furans, thiophenes, pyridines, quinolines, quinoxalines etc. were transformed into the corresponding chiral heterocycles in high enantioselectivities. In most cases, the catalytic asymmetric reductions were conducted by using transition metal complexes as the chiral catalyst. Iridium, ruthenium, or rhodium were frequently chosen as the catalyst, while palladium, iron, gold, and metal elements are applicable to the asymmetric dearomative reduction. Furthermore, some organocatalysts are also useable for the enantioselective reductions of indoles, pyridines, quinolines, and quinoxalines. Condensed polycyclic carbocyclic arenes were also hydrogenated to the benzo-fused cyclohexanes with high enantiopurities through chiral ruthenium or rhodium catalyst.