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 Rh(I) complex, (S)-5,5'-bis{di(3,5-di-tert-butyl-4-methoxyphenylphosphino)}-4,4'-bi-1,3-benzodioxole (DTBM-SEGPHOS), and (CH2CH2PPh3Br)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.
Ohshima
Takashi
T
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan.
Tadaoka
Hiroshi
H
Hori
Kiyoto
K
Sayo
Noboru
N
Mashima
Kazushi
K
D000475:Alkenes / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D002384:Catalysis; D006865:Hydrogenation; D007659:Ketones / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods / Q000592:standards; D008610:Menthol / Q000138:chemical synthesis* / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry; D012015:Reference Standards; D012238:Rhodium / Q000737:chemistry; D013237:Stereoisomerism
D000475:Alkenes; D007659:Ketones; D008024:Ligands; D009942:Organometallic Compounds; D008610:Menthol; D002244:Carbon; D012238:Rhodium