Enantioselective Hydrogenations of a Terpenic Acrylic Acid Catalyzed by Rhodium Complexes of Chiral Diphosphines

Article abstract of DOI:10.1021/jo01306a032

Hydrogenations of (E)-3,7-dimethylocta-2,6-dienoic acid ((E)-1) catalyzed by rhodium complexes of the chiral diphosphine (1R,2R)-bis<(diphenylphosphino)methyl>cyclobutane (5) in chloride-free methanol (23 deg C, 2-3 atm of H2) were rapid (<*> 300 turnovers per hour) and gave (S)-(-)-3,7-dimethyloct-6-enoic acid ((S)-(-)-3) in ca. 40-42percent ee.Under the same conditions, (Z)-1 was converted to (R)-(+)-3 in 45percent ee.The Rh-5-catalyzed hydrogenation of (E)-1 in chloride-free methanol was strongly promoted by triethylamine, with maximum rates of ca. 2500 turnovers per hour.The product ena ntiomeric excess was not changed by addition of triethylamine.A study of the amine-promoted hydrogenation of (E)-1 is reported.It is concluded that the carboxylate anion of (E)-1 complexes with rhodium more strongly than does the parent acid and that the resulting rhodium(I) substrate complex adds H2 more rapidly in the deprotonated form.