Highly enantio- and s-trans C=C bond selective catalytic hydrogenation of cyclic enones: Alternative synthesis of (-)-menthol

A highly enantioselective catalytic hydrogenation of cyclic enones was achieved by using the combination of a cationic Rh¹ complex, (S)-5,5′-bis{di(3,5-di-tert-butyl-4-methoxyphenylphosphino)}-4, 4′-bi-1,3-benzodioxole (DTBM-SEGPHOS), and (CH₂CH ₂PPh₃Br)₂. The presence of an s-cis C=C bond isopropylidene moiety on the cyclic enone influenced the enantioselectivity of the hydrogenation. Thus, the hydrogenation of 3-alkyl-6-isopropylidene-2- cyclohexen-1-one, which contains both s-cis and s-trans enones, proceeded in excellent enantioselectivity (up to 98% ee). To obtain high enantio- and s-trans selectivities, the addition of a halogen source to the cationic Rh complex was the essential step. With the key step of the s-trans selective asymmetric hydrogenation of piperitenone, we demonstrated a new synthetic method for optically pure (-)-menthol via three atom-economical hydrogenations. Moreover, we found that the complete s-trans and s-cis C=C bond selective reactions were also realized by the proper choice of both the chiral ligands and halides.