Chemistry Reference
In-Depth Information
Figure 6.15 Single and double Lewis acid activation in phosphate hydrolysis.
two substitutionally inert Co(
III
) centers [71-73]. The second-order rate constant for
hydroxide-catalyzed transesterification of
24
is 4.3
10
2
M
-1
s
-1
at 25
8
C. By compar-
ison, the second-order rate constant for hydroxide-catalyzed transesterification of the
free diester in solution is 9.8
10
-4
M
-1
s
-1
at 25
8
C [74]. Remarkably, double Lewis acid
activation in the model system provides a ca. 4
10
5
-fold rate enhancement for cleav-
ing the diester, fairly independent of the leaving group basicity. The rate acceleration
due to double Lewis acid activation in
24
is significantly greater than the square of the
rate acceleration due to single Lewis acid activation for phosphate diester cleavage
(50
2
). Thus, there appears to be cooperativity between the two metal centers in double
Lewis acid activation for phosphate diester transesterification. The rate acceleration
may in part be due to strain in the O-P-O bond - the X-ray structure reveals that
the bond angle (117.4
8
) is considerably larger than the tetrahedral value (109.5
8
)
and close to what it should be in the trigonal bipyramidal transition state (120
8
).
In addition to the remarkable effects on reactivity, there is significant cooperativity
between the two metal centers in the dinuclear Co(
III
) complex (
24
) for coordinating
phosphate diesters. The equilibrium constant for monodentate coordination of di-
methyl phosphate to a mononuclear Co(
III
) complex (Figure 6.5) is only about
2.8
M
-1
[43], but is
>
330
M
-1
for bridging dimethyl phosphate to the dinuclear Co(
III
)
center in
24
[75].
Although Co(
III
) complexes are very useful for quantifying the effects of different
types of activation, it is generally difficult to obtain catalytic turnover because they are
Figure 6.16 Quantifying double Lewis acid activation.
