Investigation of the Reactivity of Pt Phosphinito and Molybdocene Nitrile Hydration Catalysts With Cyanohydrins

Aldehyde- and ketone-derived cyanohydrins were reacted with the nitrile hydration catalysts [PtCl(PR2OH){(PR2O)(2)H}] (1) and Cp2Mo(OH)(OH2)(+) (2) under a variety of hydration reaction conditions. In general, the cyanohydrins were hydrated to the amides rather slowly using these catalysts, but no subsequent hydrolysis of the amide products occurred. Catalyst 2 was much less reactive than catalyst 1, showing at best trace amounts of amide product. Product inhibition-, substrate inhibition-, and cyanide poisoning-tests demonstrated that coordination of cyanide, generated by dehydrocyanation of the cyanohydrins, is responsible for the generally low catalytic activity of I and 2 with cyanohydrin substrates. Addition of KCN to reaction mixtures of acetonitrile and I gave a linear plot of rate versus cyanide concentration, indicating that binding of cyanide to the catalysts is irreversible. Density functional theory (DFT) calculations showed that, for the hydration reaction catalyzed by 2, the formation of most intermediates and the overall reaction itself are energetically more favorable for lactonitrile (a cyanohydrin) than for 3-hydroxypropionitrile (not a cyanohydrin). From this result, it is concluded that, from an electronic standpoint, there is no intrinsic reason for the lack of reactivity observed for cyanohydrins, a result consistent with the finding that the slow hydration reactivity is caused by cyanide poisoning. In addition, DFT calculations showed that, for nitriles in general (not necessarily cyanohydrins), product inhibition occurs because coordination of the amide product to the metal center is stabilized by isomerization to the more strongly bonded iminol tautomer.