Chemistry Reference
In-Depth Information
(Figs. 12.1 and 12.17). For both azides, the
syn
conformers decompose by concerted
migration of methyl or methoxy groups along with nitrogen extrusion. In turn, the
anti
conformers undergo dissociation giving singlet nitrenes and molecular nitrogen
(Figs. 12.1 and 12.17).
18
For azide
1d
, the free energy of activation of the concerted
reaction is significantly lower than that of the dissociation (Fig. 12.1). Carbomethoxy
azide
, on the other hand, does have a preference for stepwise rearrangement via
the free nitrene.
18
This is due to the profound destabilization by the oxygen atom
of the transition state for the concerted Curtius rearrangement of azidoformates
(Figs. 12.1 and 12.17).
On the contrary, the oxygen atom stabilizes the transition state of the nitrene
Curtius rearrangement and accelerates the isomerization of methoxycarbonyl nitrene
1
48b
G
#
49b
(Fig. 12.17,
D
¼
14.1 kcal/mol at the CBS-QB3 level) relative to acetyl
nitrene
1
3d
G
#
(Fig. 12.17,
D
¼
21.7 kcal/mol). The rate constant of Curtius
rearrangement of
1
49b
at ambient temperature is predicted to be on the order of
10
2
s
1
. Nevertheless, the rearrangement of nitrenoformates does not compete
with their trapping by solvents.
As demonstrated in Section 12.1, carbonyl nitrenes (R-CON, R
4
aryl, alkyl) have
singlet ground state with the geometry and electronic structure of cyclic oxazirene.
The only exception was found to be trifluoroacetyl nitrene
¼
47
, which most likely has
a triplet ground state.
26
In contrast to carbonyl nitrenes, the carbethoxy nitrene
49a
has a triplet ground
1.603 cm
1
,|
E
|
state. This was demonstrated by ESR spectroscopy (|
D
|
¼
¼
0.0215
cm
1
).
77
A similar ESR spectrum (|
D
|
1.65 cm
1
,|
E
|
0.024 cm
1
) was recorded
¼
¼
for the triplet (4-acetylphenoxy)carbonyl nitrene.
51,52
Recently, the photochemistry of carbethoxy azide
48a
was studied by LFP
266 nm) in Freon-113 (CF
2
ClCFCl
2
) at room temperature.
125
The formation
of at least two intermediates, namely, triplet nitrene
3
49a
l
ex
¼
(
l
max
¼
t
¼
m
(
400 nm,
1.5
s)
51
l
max
¼
t
¼
m
and ethoxycarbonyl
s), was observed
(Scheme 12.22). The rate constants of reactions of
3
49a
with tetramethylethylene
(TME, stepwise aziridine formation,
k
radical
(
333 nm,
0.4
10
8
M
1
s
1
) and triethylsilane (TES,
¼
3
10
6
M
1
s
1
) were measured.
125
Both TME and TES quench also the yield of triplet nitrene
3
49a
H atom abstraction,
k
¼
due to their
reactions with the singlet nitrene
1
49a
. Stern-Volmer treatment of the data yields
(
1
49a
k
Q
t
)
¼
10 and 0.4, respectively. Methanol hardly quenches the yield of
3
(
1
49a
49a
, except at very high concentration (
k
Q
t
)
0.1), but does not shorten
the lifetime of
3
49a
.
It is well known on the basis of product studies,
6,119-123,126
that singlet carboe-
thoxy nitrene
1
49a
will both add to alkenes such as TME to form aziridine and insert
EtO
EtO
EtO
h
v
+
O
O
O
3
N
N
3
3
49a
51
48a
SCHEME 12.22.
Laser flash photolysis of carbethoxy azide
48a
in Freon-113 at ambient
temperature.
125
