Transcript Carbocation Analogues: Nitrenium ions?

Carbocation Analogues: Nitrenium ions?
Co n sider th e f o llo win g data:
Cl
Cl
NHR
N
Ca(OCl)2
NHR
NHR
+
R
Cl
( I)
( I I)
P o ssible M ech anist ic P ath way s:
+
NR
R
NR
NR
+
Cl
N
-1
- Cl
+
( I) + ( II )
NR
+1
- Cl
NR
NR
Evidence to support the existence of
nitrenium ions
S tu dy of re arran ge me n t ru n wi th para-s u bstitu te d N-ch loroan i li n e s:
Cl
Z
N
R
where Z = CH3 , F, H, Cl, and CN
Observation:
T he rearrangement is associat ed with
n eagative rh ovalue indicative of
a reaction favored by electron donat ing benzene ring subst it uent s.
Conclusion:
+
Z
N
R int ermediates take part in t he rearrangement .
Carbocation Analogues
Oxenium ions?
Observations:
OCH3
OH
+
O
OH
OCH3
+
C6 H5 OCH3

N+
O
BF4
-
O2 N
NO2
(ortho & para)
NO2
(ortho & para)
O
C6 H 5 C N
NO2

C6 H 5
C
OH
O
O
C6 H 5
ONH2
N
+
+
+
NO2
NO2
NO2
(35%)
(28%)
(13%)
NO2
Relevant Facts (I)
Evidence against a free radical pathway
O2 N
O
C
O
O
O
C
O
C6 H5 OCH3 
O2 N
O
O
+
O
O
-CO2
O2 N
C
-CO2
C
O
O
O
COOH
OH
O2 N
O
C6 H5 OCH3
C
+
+
OCH3
OCH3
Under t he above condit ions:
+
N
O
BF4
NO2
O2 N
O
-
OCH3
O2 N
+
OCH3
OH
Th e se produ cts are not produ ce d from
radi cal i n te rme diates un de r i de n tical
reaction con di tion s .
Relevant Facts (II)
Evidence against a concerted pathway
+
CH3 O
CH3 O
-H
H
CH3 O
+
+
O2 N
O N
BF4
O2 N
-
O2 N
O
R
ether product
or
+
CH3 O
CH3 O
H
H
-H
+
CH3 O
OH
+
O2 N
O
O N
BF4
-
O2 N
R
O
O2 N
biphenyl product
In a concerted pathway, t he pyridine ring subst it uent would be expected t o affect the leaving ability
of the pyridine group and thereby have an impact on the rat io of et her:biphenyl products.
Observation: For R = 4-met hyl, 4-methoxy, 4-phenyl, and 2-hydroxy, a constant ratio of ortho t o
para diphenyl et hers (28%:72%) was obt ained. A const ant rat io of ort ho to para biphenyl products
(75%:25%) was obt ained.
C on clu s ion: Th e e vi den ce s u gge s ts th at attack i s in de pe n de n t of th e pyridi ne ri n g
R-su bs ti tu e n t, a fact con trary to wh at on e wou ld expe ct for a con ce rte d me ch an i s m.
A plausible explanation involving the
intermediacy of Oxenium ions
+
O
N O
+
BF4
NO2
-
+
loss of pyridine
via t hermolysis
NO2
OCH3
CH3 O
O
O
NO2
-H
+
NO2
OCH3
C6 H5 OCH3
+
O
(plus ortho isomer)
oxenium ion
+
H
O2 N
H
H
O
-H
+
NO2
O2 N
O2 N
OCH3
OCH3
OH
H
O
A plausible explanation involving the
intermediacy of Oxenium ions cont’d.
H
+
O
+
O
O H
abstract s 2H
NO2
singlet oxenium ion
NO2
t riplet oxenium ion
+
O2 N
OH
+
NH BF4
-
NO2
N
O2 N
O2 N
O+
C
O2 N
O
N
O
+
N
C
+
C
O2 N
N
O
H
OH
N
C
+
-H
NO2
O
O2 N
N
O2 N
+
O
C
N
+
C
OH
H
O2 N
NO2
O
N
OH
+
H
O2 N
O
C
O
+
N
C
H
O
H2 O
NO2
NO2
O
O2 N
ONH2 + O2 N
O
C