Level 2 – Case 21
142
Table 21. 2
U obs (r)
Substituent constants
Rh 2 (Oac) 4
Rh 2 (Ooct) 4
Rh 2 (tfa) 4
V
+
–1.29(–0.99)
–1.31(–0.99)
–1.46(–0.99)
V
–1.72(–0.94)
–1.69(–0.92)
–1.94(–0.94)
V
–
–1.11(–0.91)
–1.07(–0.87)
–1.23(–0.89)
r = correlation coefficient.
D Di is sc cu us ss si io on n
The experimental fact is that the decomposition of diazo compounds 1 is accelerated by electron-donating substituents in the aromatic ring, independently of the
catalyst employed. This is in full agreement with the Hammett correlation study
that gives negative values of the reaction constants U obs in all cases studied (Table
21.2).
Can we then conclude that each of the two key reaction steps is favored by electron-donating groups? Not necessarily. To make an accurate analysis of the effect
of the substituents upon a reaction mechanism it is essential to consider each step
of the reaction independently.
Step 1 involves the complexation of 1 with the Rh(II) catalyst and step 2 (the
rate-determining step) is the generation of the rhodium carbene intermediate 4 by
decomposition of complex 3. We should point out that step 3, the reaction of the
carbene 4 with triethylsilane, is a fast step that has no influence on the current
t
study.
Starting with the Hammett equation for the rhodium(II)-catalyzed decomposition of D-diazo esters 1, the general form of the Hammett equation in this case is:
log (k obs(X)
k k
/ k obs(H)
k k
) = U obs V
(21.2)
Considering that k obs
k k = K k 2
k k [catalyst] (Eq. 21.1), the Hammett equation could be
expressed in the following way:
log (k obs(X)
k k
/ k obs(H)
k k
) = log [(K ( ( (X)
K K k 2(X)
k k / K (H)
K K k 2(H)
k k ] = U obs V
(21.3)
Equation 21.3 can also be written as:
log (k obs(X)
k k
/ k obs(H)
k k
) = log [(K ( ( (X)
K K / K (H)
K K ] + log [k 2(H)
k k / k 2(X)
k k ] = U obs V
(21.4)
or as:
log (k obs(X)
k k
/ k obs(H)
k k
) = U V V and U U U
(21.5)
Therefore, the measured U values (U obs ) are the sum for the two individual steps U 1
and U 2 .
Next, we will discuss if the two steps have the same electronic requirements.
142
Table 21. 2
U obs (r)
Substituent constants
Rh 2 (Oac) 4
Rh 2 (Ooct) 4
Rh 2 (tfa) 4
V
+
–1.29(–0.99)
–1.31(–0.99)
–1.46(–0.99)
V
–1.72(–0.94)
–1.69(–0.92)
–1.94(–0.94)
V
–
–1.11(–0.91)
–1.07(–0.87)
–1.23(–0.89)
r = correlation coefficient.
D Di is sc cu us ss si io on n
The experimental fact is that the decomposition of diazo compounds 1 is accelerated by electron-donating substituents in the aromatic ring, independently of the
catalyst employed. This is in full agreement with the Hammett correlation study
that gives negative values of the reaction constants U obs in all cases studied (Table
21.2).
Can we then conclude that each of the two key reaction steps is favored by electron-donating groups? Not necessarily. To make an accurate analysis of the effect
of the substituents upon a reaction mechanism it is essential to consider each step
of the reaction independently.
Step 1 involves the complexation of 1 with the Rh(II) catalyst and step 2 (the
rate-determining step) is the generation of the rhodium carbene intermediate 4 by
decomposition of complex 3. We should point out that step 3, the reaction of the
carbene 4 with triethylsilane, is a fast step that has no influence on the current
t
study.
Starting with the Hammett equation for the rhodium(II)-catalyzed decomposition of D-diazo esters 1, the general form of the Hammett equation in this case is:
log (k obs(X)
k k
/ k obs(H)
k k
) = U obs V
(21.2)
Considering that k obs
k k = K k 2
k k [catalyst] (Eq. 21.1), the Hammett equation could be
expressed in the following way:
log (k obs(X)
k k
/ k obs(H)
k k
) = log [(K ( ( (X)
K K k 2(X)
k k / K (H)
K K k 2(H)
k k ] = U obs V
(21.3)
Equation 21.3 can also be written as:
log (k obs(X)
k k
/ k obs(H)
k k
) = log [(K ( ( (X)
K K / K (H)
K K ] + log [k 2(H)
k k / k 2(X)
k k ] = U obs V
(21.4)
or as:
log (k obs(X)
k k
/ k obs(H)
k k
) = U V V and U U U
(21.5)
Therefore, the measured U values (U obs ) are the sum for the two individual steps U 1
and U 2 .
Next, we will discuss if the two steps have the same electronic requirements.