Aldehyde- and ketone-derived cyanohydrins were reacted with the nitrile hydration catalysts [PtCl(PR(2)OH){(PR(2)O)(2)H}] (1) and Cp(2)Mo(OH)(OH(2))(+) (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 1 and 2 with cyanohydrin substrates. Addition of KCN to reaction mixtures of acetonitrile and 1 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.

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The synthesis, characterization, and reactivity of the new water-soluble ansa-molybdocene catalyst [{C2Me4(C5H4)2}Mo(OH)(OH2)][OTs] (3) and the related hydroxo-bridged dimer [{C2Me4(C5H4)2}Mo(μ-OH)]2[OTs]2 (5) are described. The effect of the ethylene bridge on the metallocene structure was evaluated by comparing the crystal structures of {C2Me4(C5H4)2}MoH2 (2) and 5 to those of the non-ansa analogues. The ethylene bridge changed the bite angles of the metallocene fragment by only a few degrees in both ansa structures. To probe the electronic consequences of the tetramethylethylene bridge, the {C2Me4(C5H4)2}Mo(CO)H (4) complex was prepared. On the basis of the ν(C⋮O) stretching frequencies, the ansa ligand C2Me4(C5H4)2 was found to be electron-withdrawing relative to two η5-C5H5 ligands. The reactivity of 3 in nitrile hydration, phosphate ester hydrolysis, and carboxylic acid ester hydrolysis was explored, and the rate constants for these transformations were compared to rate constants obtained using the Cp2Mo(OH)(OH2)+ and Cp‘2Mo(OH)(OH2)+ catalysts. In all cases, the Cp2Mo(OH)(OH2)+ catalyst, having intermediate electron density, had the largest rate constants. The reactivity trends for the three catalysts are explained by the relative electrophilicities of the Mo centers. If electron-donating cyclopentadienyl ligands are employed, the reactivity of the bound substrate is decreased relative to Cp and the rate is decreased. Conversely, if electron-withdrawing Cp cyclopentadienyl ligands are employed, the reactivity of the bound hydroxo nucleophile is decreased and the rate is decreased. In the case of the Cp2Mo(OH)(OH2)+ complex, these two opposing trends converge, and optimal activity is observed.

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