Chemistry Reference
In-Depth Information
3
(
π
,
π
)
3
(
n
,
π
)
3
(
n
,
π
)
3
(
π
,
π
)
1
n
2
1
n
2
1
n
2
3
(
π
,
π
)
FIGURE 6.12.
Qualitative state energy diagram illustrating the effect of
meta p
donors on the
electronic states of phenyl nitrenium ion.
p
)
intermediate cases where the ground state is singlet but the lowest triplet has (
p
,
p
) triplet state is lowest in
configuration. Finally, with the best
p
-donors, the (
p
,
p
) triplet. These compu-
tational predictions currently await experimental confirmation.
p
) singlet followed by the (
n
,
energy, followed by the (
n
,
6.6 ALKYLARYL NITRENIUM IONS
The stabilization derived from conjugation to an aromatic ring also serves to reduce
the propensity for rearrangements from an alkyl substituent on the same nitrogen.
For example,
N
-methyl-
N
-phenyl nitrenium ion
was examined computationally
(Fig. 6.13).
37
The effect of the alkyl group is to slightly shift the energy gap to favor
the triplet (
55
E
st
¼
14.1 kcal/mol by BPW91/cc-PVDXZ). This can be understood
to occur as a result of a steric effect: the methyl group favors a larger C
D
C bond
angle, which destabilizes the singlet. The same study identified a barrier of
21.5 kcal/mol for H atom migration to form
N
.
An experimental study carried out on the same species showed data consistent
with the prediction of a nontrivial barrier to rearrangement.
38
While the nitrenium
ion itself could not be detected using nanosecond LFP, product trapping studies
56
CH
3
+
N
Caic 21.5 kcal/mol
Expt: ~10
8
s
-1
H
56
CH
3
+
N
-1
)
Nuc (MeOH, Cl
55
Nuc
H
CH
3
CH
3
N
Nuc
N
H
57
+isomer
FIGURE 6.13.
Rearrangement versus nucleophilic addition in
N
-methyl-
N
-phenyl nitrenium
ions.
