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Enantioselective Addition of Secondary Phosphines to Methacrylonitrile:  Catalysis and Mechanism

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posted on 07.12.2005 by Aaron D. Sadow, Antonio Togni
A highly enantioselective intermolecular hydrophosphination reaction is described. The (Pigiphos)nickel(II)-catalyzed reaction of secondary phosphines and methacrylonitrile gives chiral 2-cyanopropylphosphines in good yield and high enantiomeric excess (ee's up to 94%; (R)-(S)-Pigiphos = bis{(R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}cyclohexylphos phine). We propose a mechanism involving coordination of methacrylonitrile to the dicationic nickel catalyst followed by 1,4-addition of the phosphine, and then, rate-determining proton transfer. This mechanism is supported by (a) the experimentally determined rate law (rate = k‘[Ni][methacrylonitrile][t-Bu2PH]), (b) a large primary deuterium isotope effect kH/kD = 4.6(1) for the addition of t-Bu2PH(D) at 28.3 °C in toluene-d8, (c) the isolation and characterization of the species [Ni(κ3-Pigiphos)(κN-methacrylonitrile)]2+, and (d) DFT calculations of model compounds.

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