Zinc(II)/Ketoxime System as a Simple and Efficient Catalyst for Hydrolysis of Organonitriles

The hydrolysis of sterically hindered and unhindered alkyl nitriles, and also of benzyl and phenyl nitriles RCN (R = Me, CH₂Cl, Et, n-Pr, i-Pr, n-Bu, t-Bu, p-MeOC₆H₄CH₂, Ph), to carboxamides is catalyzed by a novel system of superior simplicity consisting of cheap, widely commercially available, and rather environmentally friendly compounds, that is, a ZnX₂/ketoxime combination, but it does not proceed at all with either the zinc salt or the ketoxime taken alone. The nature of the anion X^- in the zinc salt (X = NO₃, Cl, CF₃SO₃) or of the ketoxime (Me₂CNOH, C₄H₈CNOH, C₅H₁₀CNOH) does not affect strongly the catalytic properties of the system, but the best results were obtained so far with a Zn(NO₃)₂·6H₂O/2-propanone oxime molar ratio of 1:4; turnover numbers are typically above ca. 100 but reach as high as 1000 for p-MeOC₆H₄CH₂C(O)NH₂. The previously unknown structures of the two carboxamide products n-BuC(O)NH₂ and p-MeOC₆H₄CH₂C(O)NH₂ were determined by X-ray diffraction studies. The complexes [ZnX₂(R₂CNOH)₂] (X = Cl, R₂ = 2Me, C₄H₈, C₅H₁₀; X = NO₃, R = C₄H₈), prepared by heating the appropriate zinc salts with 2 equiv of the ketoxime in acetone and characterized by C, H, N analyses, FAB-MS, ¹H and ¹³C{¹H} NMR spectroscopies, and also X-ray crystallography (for X = Cl, R₂ = 2Me; X = NO₃, R = C₄H₈), proved to be catalyst precursors in the conversions because the activity of these species is high only in the presence of 2 equiv of the ketoxime.