Chemistry Reference
In-Depth Information
Figure 6.3 Charge build-up on coordinated oxygen.
to the metal-bound hydroxide when compared to the charge buildup on going from the
metal-bound ester to the metal-bound transition state for the hydrolysis reaction
(Figure 6.3). Assuming a linear relationship between energy and charge, it should
be possible to estimate the effect of metal ions on the rate of hydrolysis reactions.
If the net change in charge (on the metal coordinated oxygen) due to the hydrolysis
reaction at the transition state is 50% of that due to the deprotonation reaction, a metal
ion that provides a million-fold increase in the acidity of a water molecule should give a
thousand-fold increase in the rate of hydrolysis. Lewis acid catalysis can be compared
to buffer catalysis. In general acid catalysis, increasing the acidity of the catalyst ten-
fold increases the rate of the catalyzed reaction by a maximum of ten-fold (Brønsted
coefficient
= 1). If the extent of proton transfer at the transition state of the general
acid catalyzed reaction is 50%, increasing the acidity of the catalyst a million-fold
should result in a thousand-fold increase in the rate of the catalyzed reaction [33, 34].
Substitutionally inert Co(
III
) or Ir(
III
) complexes have been used to measure directly
the effect of Lewis acid activation on the hydrolysis of an amide [35-37], a nitrile [38]
and a phosphate triester [39] (Figure 6.4). The pK
a
of the cobalt-bound water molecule
in
5
is 6.6 [40]. Thus the upper limit for the rate-acceleration due to Lewis acid activa-
tion with this metal in the hydrolysis of esters, amides, nitriles and phosphates should
be close to 10
9
-fold. Although the observed rate accelerations for the hydrolysis reac-
a
Figure 6.4 Hydrolysis of Co(
III
) complex bound substrates with hydroxide.
