Asymmetric Homogeneous Hydrogenation with Rhodium(I) Complexes of Chiral Phosphines Sir:There has been recent interest in asymmetric hydrogenations catalyzed by soluble rhodium catalysts of the Wilkinson type.1-4 For example, asymmetric hydrogenation of -phenylacrylic acid was accomplished (18) NSF Undergraduate Summer Research Participant, 1969.
Lithium diphenylphosphide has been found to react with p-bromo-and p-iodotoluene, m-bromotoluene, pbromobiphenyl, and p-dibromobenzene producing díphenyl(p-tolyl )phosphine, diphenyl(m-tolyl)phosphine, diphenyl(p-bipheny 1 )phosphine, and p-bis(diphenylphosphino)benzene, respectively, in 70-80% yields. Gasliquid partition chromatography at 240-300°was used to establish the presence of only one phosphine in each of these reactions. The products were isolated as oxides. Addition of phosphide to aryl halide has been found to lead to little or no tertiary aryl phosphine in the reactions of the tolyl halides, but addition of lithium diphenylphosphide to a mixture of aryl halide and lithium chloride was found to lead to tertiary phosphine in fair to good yields. It was concluded that the reaction does not occur by an elimination-addition mechanism nor a simple halogen-metal interchange. A transition state involving two molecules of lithium diphenylphosphide is suggested in which one molecule of lithium diphenylphosphide coordinates with the halide atom while the other displaces it from carbon.
5 ) P S. Skell and I . Starer, J . A m . ChPm. S o r . , 84, 3962 (IR62). (6) W . F Hawthorne, UT. D . E m m o n s , and K . S RIcCallum, rbid , 8 0 , (7) J . 11. Roberts and M. H a i m a n n , i b i d . , 71, 6759 (10.53). 6393 (1958).
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