Chemistry Reference
In-Depth Information
p
K
a
and is the dominant term at neutral pH for
all esters.
154
This term is consistent with uncatalyzed heterolysis of the N
The
k
B
term predominates at pH
O bond
of the neutral ester to generate a heteroaryl nitrenium ion. Products consistent with
generation of a nitrenium ion are detected under these conditions, although ester
hydrolysis to form the hydroxamic acid is also part of the
k
B
rate term for less
reactive substrates.
150,151,154
The rate law is more complicated than that for reactive
esters of carbocyclic hydroxylamines or hydroxamic acids that show pH-independent
decomposition over a wide pH range.
18,19,27
The carbocyclic esters do not have basic
sites that can be protonated under mild pH conditions. For all cases examined only the
neutral form of the heterocyclic substrate undergoes N
O bond heterolysis.
154
were characterized by their reactions with N
3
,2
0
-deoxygua-
nosine (dG) and the aqueous solvent.
147-152
The log (
k
az
/
k
s
) ratios derived from azide
clock studies or by direct measurement are provided in Table 4.2 with some
comparisons to carbocyclic ions. The more selective ions
The ions
103a-l
103d-g
exhibit reactivity
TABLE 4.2. Selectivity Data for Heterocyclic Nitrenium Ions and Their Conjugate
Bases
a
log(
k
phos
b
/
k
s
)
log(
k
phos
a
/
k
s
)
Ion or Conjugate Base
log(
k
az
/
k
s
)
1.00
b
103a
2.48
b
103b
1.90
b
103c
6.36
c
103d
2.72
1.76
6.71
c
103e
2.74
1.60
4.72
c
103f
2.64
1.30
5.08
c
103g
2.72
0.85
4.54
d
103h
3.08
d
110h
4.65
d
103i
4.36
e
103l
5.83
f
103m
4.33
f
110m
5.74
f
103n
3.93
g
113
3.41
h
116
5.07
i
64g
3.45
j
a
Conditions: 5% CH
3
CN-H
2
O,
m¼
0.5 (NaClO
4
),
T¼
20
C, unless otherwise indicated.
b
Ref. 147.
c
Ref. 149.
d
Ref. 148.
e
Ref. 152.
f
64n
Ref. 153: 20 vol% CH
3
CN,
m¼
0.1.
g
Ref. 155: 20 vol% CH
3
CN,
m¼
0.1.
h
Ref. 156.
i
Ref. 21.
j
Refs. 19,21.
