Asymmetric reduction of methoxy substituted β-tetralones using transfer hydrogenation

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Asymmetric reductions of methoxy substituted 2-tetralones were studied. The asymmetric transfer hydrogenation reaction developed by Noyori using chiral η 6-arene–ruthenium complexes (arene = p-cymene or benzene) was found to efficiently reduce various methoxy substituted 2-tetralones with >80% ee. Their enantiomeric excesses were found to depend on the position of the methoxy group and the types of the arene complexes. The conditions were identified for the asymmetric reduction of 8-methoxy-2-tetralone to the corresponding 2-tetralol in 98% ee, which was notably higher in enantioselectivity than the other methoxy substituted 2-tetralones.