Cationic iridium complexes with chiral P,Nligands and tetrakis [3,5-(trifluoromethyl)phenyl]borate (BAr F ) as the counterion are efficient homogeneous catalysts for the enantioselective hydrogenation of olefins. The complexes are readily prepared, air-stable, and easy to handle. In contrast to chiral rhodium-and ruthenium-phosphine catalysts, they do not require the presence of a polar coordinating group near the C C bond. In the hydrogenation of unfunctionalized arylolefins, high enantioselectivities of > 95% ee with turnover numbers of up to 5000 and turnover frequencies of > 5000 h À1 have been achieved.

Asymmetric hydrogenation of olefins is one of the most useful reactions for the synthesis of optically active compounds, especially in industry. However, the application range of the catalysts developed so far is limited to alkenes with a coordinating functional group or an aryl substituent next to the double bond. We have found a class of chiral iridium catalysts that give high enantioselectivity in the hydrogenation of unfunctionalized, trialkyl-substituted olefins. Because these catalysts do not require the presence of any particular functional group or aryl substituent in the substrate, they considerably broaden the scope of asymmetric hydrogenation.

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Four iridium complexes with achiral phosphino-oxazoline (PHOX) ligands were readily prepared in two steps starting from commercially available phenyloxazolines. The air-stable complexes with tetrakisA C H T U N G T R E N N U N G [3,5-bis(trifluoromethyl)phenyl]borate (BAr F ) as counterion showed high reactivity in the hydrogenation of a range of tri-and tetrasubstituted olefins. The best results were obtained with an iridium complex (11) derived from a dicyclohexylphosphino-oxazoline ligand bearing no additional substituents in the oxazoline ring. With several substrates, which gave only low conversion with the Crabtree catalyst, [Ir(Py

We recently reported a class of chiral iridium catalysts derived from pyridylphosphine ligands 1, which for the first time have allowed highly selective asymmetric hydrogenation of unfunctionalized trialkyl-substituted C=C bonds.[1, 2] Unlike rhodium or ruthenium diphosphine complexes, these catalysts do not require any special coordinating group next to the C = C bond. Enantiofacial selection by the catalysts in this case results from discrimination between the H atom and a sterically more demanding alkyl group at the monosubstituted olefinic C atom. Consequently, cis and trans olefins are converted into products of opposite configuration.[1] Thus, the sense of asymmetric induction can be controlled by proper choice of the double bond geometry. In this way, two or more stereogenic centers can be introduced with the desired relative and absolute configuration by hydrogenation of a di-or polyene.[3]Herein we report the asymmetric hydrogenation of farnesol and O-protected derivatives to demonstrate the potential of this strategy. This class of substrates was chosen because efficient routes to all four cis/trans isomers were available, [5] and we were interested in comparing the reactivity of the two trialkyl-substituted C=C bonds with the allylic alcohol unit in its free and protected form. Furthermore, hexahydrofarnesol, which can be readily prepared by this route in any of the four possible stereoisomeric forms, [6] is an important building block for the syntheses of vitamins E and K [7] or related antioxidants, [8] and insect pheromones. [9] It also has an important function as a constituent of ether lipids in archea [10] and was identified as a precursor of many terpenoid compounds in plants [11] and geological sediments.[12]