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

Abstract

The rates of nitrile hydration reactions were investigated using [Ru(η 6 - p -cymene)Cl 2 (PR 2 R′)] complexes as homogeneous catalysts, where PR 2 R′ = PMe 2 (CH 2 P(O)Me 2 ), PMe 2 (CH 2 CH 2 P(O)Me 2 ), PPh 2 (CH 2 P(O)Ph 2 ), PPh 2 (CH 2 CH 2 P(O)Ph 2 ), PMe 2 OH, P(OEt) 2 OH. These catalysts were studied because the rate of the nitrile-to-amide hydration reaction was hypothesized to be affected by the position of the 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(η 6 - p -cymene)Cl 2 PMe 2 OH]: 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 α-hydroxynitriles (cyanohydrins) produce less cyanide, a known poison for organometallic nitrile hydration catalysts. The [Ru(η 6 - p -cymene)Cl 2 PMe 2 OH] catalyst completely converts the cyanohydrins glycolonitrile and lactonitrile to their corresponding α-hydroxyamides faster than previously investigated catalysts. [Ru(η 6 - p -cymene)Cl 2 PMe 2 OH] 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.