Chemistry Reference
In-Depth Information
TABLE 10.1. Optimized Geometries for Singlet and Triplet State Nitrenium Ions and
their Associated S-T Gaps
N
R
1
R
2
α
a
(
)
(Triplet/Singlet)
DE
ST
(kcal/mol)
a
R
1
R
2
Entry
Method
Reference
10
1 H H
þ
33.6
152.82/108.38 B3LYP/6-311
þþ
G
2 e H
þ
20.8
b
CISD/6-311G
5
151.1/112.8
þ
22.5
b
CISD/6-311G
5
3 e e
150.4/120.5
CISD/6-31G
5
þ
4.3
4
-(CH
2
)
4
-
121.4/109.4
CISD/6-31G
5
þ
9.6
5
-(CH
2
)
5
-
135.7/121.6
B3LYP/6-311
þþ
G
10
6 N H
þ
27.5
180.0/120.4
QCISD(T)/6-311
þþ
G
10
7 l H
1.2
133.7/108.7
G
10
8 F H
4.69
125.7/105.1
QCISD(T)/6-311
þþ
11
9 F F
57.3
124.8/107.6
MP4/6-311G
QCISD(T)/6-311
þþ
G
12
10
F
Cl
25.6
129.6/111/5
11
11
Cl
Cl
19.8
137.0/117.3
MP4/6-311G
HF
(6-31G
(13)
13
12
NH
2
H
32.4
-
HF
(6-31G
(13)
13
13
NH
2
CHO
38.8
-
HF
(6-31G
(13)
13
14
OH
H
21.8
-
HF
(6-31G
(13)
13
15
OH
CHO
12.0
-
14
16
Ph
H
18.8
-
BPW91/cc-pVDZ
PNP
c
14
17
H
16.8
-
BPW91/cc-pVDZ
PMP
d
14
18
H
26.7
-
BPW91/cc-pVDZ
Bdtbp
e
15
19
Bdtbp
þ
7.3
180/141
BPW91/cc-pVDZ
16
20
3,5-(NH
2
)
2
Ph H
þ
7.7
111.0/111.7
UB3LYP/6-31G(
d
,
p
)
a
Positive values of
E
st
indicate a T
1
ground state.
b
Singlet is a saddle point; rearrangement to iminium ion is spontaneous on
ab initio
hypersurface.
c
PNP
¼
4-nitrophenyl.
d
PMP
¼
4-methoxyphenyl.
e
Bdtbp
¼bis
-(2,6-di-
tert
-butyl)phenyl.
D
but the case of CN, which has significant
-donor/
heteroatom-substituted nitrenium ions shown in Table 10.1 are predicted to be lower-
energy ground-state singlets with high N
s
-withdrawing effects, the
p
character (entries 9-16). Glover,
using MNDO-level computations, has shown that while there is a significant barrier
to rotation (29.7 kcal/mol) about the N
X
p
O bond in the
N
-hydroxy-
N
-formyl nitre-
nium ion (entry 16), reflecting the importance of the stabilizing effect of the oxygen
lone pair, little or no rotational barrier exists for the N
C bond indicating that the
acyl substituent plays a minor role in the stability of this alkoxy nitrenium ion.
17
Such is the electronic stabilization imparted by the presence of
two
-donating amine
ligands, that nitrenium ions of this type, exemplified by triazolium and benzotria-
zolium cations
p
(Scheme 10.1),
18
form stable salts and are significantly
less reactive that the corresponding bis-amino-substituted carbenes discovered by
8
and
9
