Chemistry Reference
In-Depth Information
TABLE 6.3. Acyl-, Alkoxy-, and Vinyl-Nitrenium Ions
Barrier
Singlet-
Triplet
Isomer
Energy
Singlet-Triplet
E
st
a
Nitrenium Ion
Isomer
D
þ
1.6
þ
12.9
72.4/
5.3
O
H
O
CH
3
N
CH
3
CH
3
N
H
H
26
þ
4.6
O
N
O
27
4.2
þ
17.7/
þ
47.8
66.6/
þ
0.5
CH
3
N
H
N
28
28.5
þ
28.5/nd
25.7/
þ
2.9
O
CH
3
O
CH
3
CH
3
N
N
H
29
14.1
þ
22.6/nd
31.4/
21.4
O
H
OC N
O
CH
3
30
H
N
OCH
3
Energies (B3LYP/6-31G(
d
,
p
)) are in kilocalories per mole (D. D. Falvey, unpublished results).
a
A negative value implies a ground state singlet.
ions have been incorporated into various synthetically useful reactions, such as
cyclizations via electrophilic aromatic substitution. These calculations confirm that
they are, in fact, distinct singlet state intermediates.
6.5 ARYL NITRENIUM IONS
Nitrenium ions with a single aromatic substituent can be conveniently generated
from photolysis of azides
(Fig. 6.9) under protic condition (e.g., in water or in
88% formic acid). The photolysis forms a singlet nitrene
31
32
, which then abstracts a
.
23,24
While this method appears to
be limited to forming mono-substituted aryl nitrenium ions in aqueous (or strongly
acidic) solution, it is sufficiently convenient that a large number of singlet state aryl
nitrenium ions have been spectroscopically characterized using time-resolved UV-
Vis, and Raman spectroscopy as well as low-temperature matrix methods (Fig. 6.9).
The parent in this series is the phenyl nitrenium, Ph
solvent proton to form a singlet nitrenium ion
33
H
þ
. The aromatic ring,
being electron-donating relative to the nitrenium center, significantly stabilizes the
N
