Mechanistic Investigations
and Secondary Coordination
Sphere Effects in the Hydration of Nitriles with [Ru(η<sup>6</sup>‑arene)Cl<sub>2</sub>PR<sub>3</sub>] Complexes
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Abstract
The mechanism of the nitrile-to-amide hydration reaction
using
[Ru(η<sup>6</sup>-arene)Cl<sub>2</sub>(PR<sub>3</sub>)] complexes
as catalysts was investigated (η<sup>6</sup>-arene = C<sub>6</sub>H<sub>6</sub>, <i>p</i>-cymene, C<sub>6</sub>Me<sub>6</sub>; R = NMe<sub>2</sub>, OMe, OEt, Et, iPr). Experiments showed that
the mechanism involves the following general sequence of reactions:
substitution of a chloride ligand by the nitrile substrate, intermolecular
nucleophilic attack by water to form an amidate intermediate, and
dissociation of the resulting amide. The effects of secondary coordination
sphere interactions on the rates and yields of the hydration reaction
were investigated. Ligands that are capable of acting as hydrogen
bond acceptors with the entering water molecule result in faster rates
and higher yields than non-hydrogen-bonding ligands. The faster rates
are attributable to the H-bonding-facilitated deprotonation of the
water as the oxygen of the water bonds to the coordinated nitrile.
DFT calculations on the proposed H-bonding intermediates support this
interpretation. Most homogeneous catalysts will not hydrate cyanohydrins
because of the equilibrium amounts of cyanide that are present in
solutions of cyanohydrins; the cyanide poisons the catalyst. Because
of its increased catalytic reactivity due to secondary coordination
sphere effects, the [Ru(η<sup>6</sup>-arene)Cl<sub>2</sub>(P(NMe<sub>2</sub>)<sub>3</sub>)] catalyst gives significant yields of cyanohydrin
hydration products with glycolonitrile, lactonitrile, acetone cyanohydrin,
and mandelonitrile. A Taft plot showed that an increase in the steric
bulk of the nitrile results in a decrease in the hydration rate, and
a Hammett plot showed that electron-withdrawing groups facilitate
nitrile hydration. The decrease in rate as the size of the cyanohydrin
increases is likely due to both increased steric bulk and to the addition
of electron-donating groups on the nitrile. The [Ru(η<sup>6</sup>-arene)Cl<sub>2</sub>(PR<sub>3</sub>)] catalysts are initially less
susceptible to cyanide poisoning than other homogeneous nitrile hydration
catalysts because [Ru(η<sup>6</sup>-<i>p</i>-cymene)(CN)(Cl)(P(NMe<sub>2</sub>)<sub>3</sub>)] forms in the presence of cyanide. The electron-withdrawing
cyanide ligand facilitates nucleophilic attack of water on a coordinated
nitrile in this molecule