Level 2 – Case 21
144
We can use a similar reasoning to discuss the good correlation of the reaction
Hammett plots with the substituent constant V
+ . We should remember that a good
correlation with V
+ suggests that a positive charge (or partial positive charge) is
being developed in the transition state of the reaction. In this case, in step 1 the
partial negative charge that was placed in the carbon next to the aromatic ring in
r
1
disappears when the substrate proceeds to complex with the catalyst to form 3. In
this situation we could speculate that a better correlation with the substituent constant V
– rather than with
–
V or V
+ should be observed. On the other hand, a better
correlation with V
+ than with V should be obtained during step 2, as a partial positive charge develops in the transition state. Considering that the observed effect is
an excellent correlation with V
+ it is reasonable to think that the electronic effects
in the decomposition of diazocompounds 1 are more significant during the second
(rate-limiting) step. In other words, U 2 dominates over U 1 .
Finally, we should make a little comment about the information that could be
obtained from the magnitude of U in the reaction we have been discussing. The
magnitude of U expresses the sensitivity of a reaction to the electronic effects of
the substituents. As shown in Table 21.2 the magnitude of U obs is moderate in all
cases, (ranging from 1.29 to 1.46) which indicates that the rhodium(II)-catalyzed
decomposition of compounds 1 is not very sensitive to the introduction of electron-donating substituents. This fact could be interpreted considering that either
the positive charge buildup in the carbene 4 is small, or that the N 2 extrusion step
has a relatively earlier (reactant-like) transition state. In other words, the transition
state of the slow step of the reaction resembles more diazocarbene 3 than the carbene complex 4.
In Summary
The Hammett correlation study on the Rh(II)-catalyzed decomposition of diazo esters 1 indicates that the reaction is accelerated by the presence of electrondonating groups in the aromatic ring. The excellent correlation with V
+ suggest
that the second step of the reaction (formation of the Rh(II) carbene intermediate
4) is more sensitive to electronic effects than the initial pre-equilibrium.
A Ad dd di it ti io on na al l C Co om mm me en nt ts s
An example of a stepwise mechanism in which the sign of U is opposite for each
individual step is the acid-catalyzed hydrolysis of benzoates (U obs = +0.14) which
shows a negative U 1 in the initial protonation step and a positive U 2 during the nucleophilic attack of water.
This problem is based on the work by Qu Z, Shi W, Wang J (2001) J. Org.
Chem. 66:8139-8144.
S Su ub bj je ec ct ts s o of f R Re ev vi is si io on n
Hammett equation. Hammett parameters.
144
We can use a similar reasoning to discuss the good correlation of the reaction
Hammett plots with the substituent constant V
+ . We should remember that a good
correlation with V
+ suggests that a positive charge (or partial positive charge) is
being developed in the transition state of the reaction. In this case, in step 1 the
partial negative charge that was placed in the carbon next to the aromatic ring in
r
1
disappears when the substrate proceeds to complex with the catalyst to form 3. In
this situation we could speculate that a better correlation with the substituent constant V
– rather than with
–
V or V
+ should be observed. On the other hand, a better
correlation with V
+ than with V should be obtained during step 2, as a partial positive charge develops in the transition state. Considering that the observed effect is
an excellent correlation with V
+ it is reasonable to think that the electronic effects
in the decomposition of diazocompounds 1 are more significant during the second
(rate-limiting) step. In other words, U 2 dominates over U 1 .
Finally, we should make a little comment about the information that could be
obtained from the magnitude of U in the reaction we have been discussing. The
magnitude of U expresses the sensitivity of a reaction to the electronic effects of
the substituents. As shown in Table 21.2 the magnitude of U obs is moderate in all
cases, (ranging from 1.29 to 1.46) which indicates that the rhodium(II)-catalyzed
decomposition of compounds 1 is not very sensitive to the introduction of electron-donating substituents. This fact could be interpreted considering that either
the positive charge buildup in the carbene 4 is small, or that the N 2 extrusion step
has a relatively earlier (reactant-like) transition state. In other words, the transition
state of the slow step of the reaction resembles more diazocarbene 3 than the carbene complex 4.
In Summary
The Hammett correlation study on the Rh(II)-catalyzed decomposition of diazo esters 1 indicates that the reaction is accelerated by the presence of electrondonating groups in the aromatic ring. The excellent correlation with V
+ suggest
that the second step of the reaction (formation of the Rh(II) carbene intermediate
4) is more sensitive to electronic effects than the initial pre-equilibrium.
A Ad dd di it ti io on na al l C Co om mm me en nt ts s
An example of a stepwise mechanism in which the sign of U is opposite for each
individual step is the acid-catalyzed hydrolysis of benzoates (U obs = +0.14) which
shows a negative U 1 in the initial protonation step and a positive U 2 during the nucleophilic attack of water.
This problem is based on the work by Qu Z, Shi W, Wang J (2001) J. Org.
Chem. 66:8139-8144.
S Su ub bj je ec ct ts s o of f R Re ev vi is si io on n
Hammett equation. Hammett parameters.