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]
Chiral iridium complexes with bicyclic pyridine-based N,P ligands have emerged as efficient catalysts for the enantioselective hydrogenation of unfunctionalized trialkyl-substituted olefins. Optimization of the reaction conditions by variation of the solvent, pressure, and temperature led to enantiomeric excesses of up to 99%. Three pure alkenes, (E)-2-cyclohexyl-2-butene and (E)- and (Z)-3,4-dimethyl-2-pentene were converted into the corresponding chiral alkanes with 97%, 94%, and 93% ee, respectively. Hydrogenation of the three C=C bonds of both α- and γ-tocotrienyl acetate led to α- and γ-tocopheryl acetate with very high diastereoselectivity. The same catalysts were successfully applied in the hydrogenation of trisubstituted alkenes with a carboxylic ester or a keto group in the γ position. This reaction was used as a key step in a highly enantioselective synthesis of the pheromone of the caddisfly Hesperophylax occidentalis. The hydrogenation of a structurally analogous allylic alcohol also gave high enantioselectivities.
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