Chemistry Reference
In-Depth Information
O
O
PhCOCH
3
> 310 nm
+
+
EtO
N
3
EtO
NH
2
52a
48a
SCHEME 12.23.
Photosensitized decomposition of carbethoxy azide
48a
in cyclohexene
solution.
126
into an allylic C
H bond to form urethane, insert into an unactivated C
Hor
Si
H bond of methanol.
Assuming that the rate constant of reaction with TME is less that 5
H bond as in TES, and insert into the O
10
9
M
1
s
1
and recalling that
3
49a
is formed within the 10 ns time resolution of the spectrometer,
it was deduced that the lifetime of
1
49a
in Freon-113 at ambient temperature is
between 2 and 10 ns and that the rate constant of intersystem crossing (
k
ISC
)is
about (1-5)
10
8
s
1
.
125
This value of
k
ISC
is 2 orders of magnitude greater
than the corresponding ISC rate constant of singlet phenyl nitrene (3.2
0.3
10
6
s
1
).
14-16,130
Thus, direct irradiation of carbethoxy azide
produces singlet nitrene
1
49a
48a
along with radical
. In inert solvent singlet carbethoxy nitrene undergoes fast
intersystem crossing (
k
ISC
ΒΌ
51
10
8
s
1
) yielding triplet nitrene
3
49a
.
125
On the
1
5
other hand, photolysis of
in cyclohexene and cyclohexane gives mainly product
of singlet nitrene addition (aziridine,
48a
50%) in the former solvent and C
H
50%) in the latter.
126
Thus, in these
solvents trapping of singlet nitrenes competes with its ISC.
Photosensitized decomposition of azide
insertion product (cyclohexyl urethane,
has also been studied.
126
Sensitized
decomposition was obtained when a cyclohexene solution of azide
48a
and
acetophenone was irradiated using 310-410 nm light. The main products were
urethane
48a
in 74% and 3,3
0
-biscyclohexene in 63% yield (Scheme 12.23).
126
Thus, triplet-sensitized decomposition of azidoformate
52a
, in contrast to that of
carbonyl azides, gives products of triplet nitrene reactions. This agrees with the
triplet ground state of carbethoxy nitrene
48a
.
The photochemistry of methoxycarbonyl azide (
49a
) and its deuterated derivative
was studied in inert gas matrices at 4 and 10 K.
127,128
The IR spectrum of the
products shows characteristic bands assigned to methoxyisocyanate (
48b
) and
products of its further phototransformations, namely, formaldehyde and isocyanic
acid (Scheme 12.24).
127
Triplet carbomethoxy nitrene
3
49b
50b
was not detected,
127,128
apparently due to the photochemical transformation of the latter to
50b
.
Recently, Gudmundsdottir and coworkers
129
synthesized a series of azide for-
mates
53a-d
, which have a built-in intramolecular triplet sensitizers (Scheme 12.25),
MeO
MeO
h
v
-N
2
h
v
h
v
O
O
MeO
N
C
O
H
2
C
O + HN
C
O
3
N
N
3
3
49b
48b
50b
SCHEME 12.24.
Photolysis of carbmethoxy azide
48b
in neon matrix at 4 K.
