posted on 2016-02-21, 18:12authored byElkin Tílvez, María I. Menéndez, Ramón López
Nitrile hydration to amides catalyzed by [Cp2Mo(OH)(OH2)]+ has been theoretically investigated
by using
acrylonitrile as a model and performing density functional theory
calculations (B3LYP), both in the gas phase and in water solution.
In both media, our results confirm the experimental belief that, among
four plausible proposals, the intramolecular nucleophilic mechanism
is the most favored for this kind of process. A hydrogen migration
from oxygen to nitrogen atoms is the rate-limiting step in the gas
phase. In the continuum solvation model the most significant energy
barriers become larger than in the gas phase due to the relatively
large solvation of the [Cp2Mo(OH)]+ complex,
which is taken as reference to measure such barriers. However, the
inclusion of explicit water molecules in the hydrogen migration between
oxygen and nitrogen atoms notably stabilizes this step; thus the attack
of the catalyst hydroxide to the nitrile becomes the rate-limiting
step in water solution with a Gibbs energy barrier of 33.8 kcal/mol,
in agreement with the slow reaction rate experimentally observed.
The replacement of acrylonitrile by lactonitrile, isobutyronitrile,
acetonitrile, propionitrile, and 3-hydroxypropionitrile also gives
rise to rate-determining Gibbs energy barriers in water solution consistent
with experimental trends for the effect of electron-withdrawing substituents
and for the effect of enlargement of the backbone of the nitrile,
thus corroborating the reaction mechanism found for the title hydration
process investigated.