Catalytic Nitrile Hydration with Ru(η6-p-cymene)Cl2(PR2R′] Complexes: Secondary Coordination Sphere Effects with Phosphine Oxide and Phosphinite Ligands

The rates of nitrile hydration reactions were investigated using [Ru(eta(6)-p-cymene)Cl-2(PR2R')] complexes as homogeneous catalysts, where PR2R' = PMe2(CH2P(O)Me-2), PMe2(CH2CH2P(O)Me-2), PPh2(CH2P(O)Ph-2), PPh2(CH2CH2P(O)Ph-2), PMe2OH, P(OEt)(2)OH. These catalysts were studied because the rate of the nitrile-to-amide,, hydration reacfion was hypothesized to be affected by the position of the io., hydrogen bond accepting group in the secondary coordination sphere of the catalyst. Experiments showed that, the rate of nitrile hydration was fastest when using [Ru(eta(6)-p-cymene)Cl2PMe2OH]: i.e., the catalyst with the hydrogen bond accepting group capable of forming the most stable ring in the transition state of the rate-limiting step. This catalyst is also active at pH 3.5. and at low temperatures-conditions where alpha-hydroxynitriles (cyanohydrins produce less cyanide, a known poison for organometallic nitrile hydration catalysts. The [Ru(eta(6)-p-cymene)Cl2PMe2OH] catalyst completely converts the cyanohydrins glycolonitrile, and, lactonitrile to their corresponding alpha-hydroxyamides faster than previously investigated Catalysts. [Ru(eta(6)-p-cymene)Cl2PMe2OH] is not, however, a good catalyst for acetone cyanohydrin hydration, because it is susceptible to cyanide poisoning. Protecting the -OH group of acetone cyanohydrin was shown to be an effective way to prevent cyanide poisoning, resulting in quantitative hydration of acetone cyanohydrin acetate.