Chemistry Reference
In-Depth Information
N
3
N
N
254 nm
Ar, Ne, 4 K
-N
2
254 nm
F
F
F
F
F
F
Ar, 4K
35K
I
I
p
-
6c
p
-
5c
p
-
7c
254 nm
254 nm
420 nm
N
F
N
F
254 nm
F
F
420 nm
p
-
9c
I
p
-
8c
I
SCHEME 9.9.
Photochemistry of
p-
7c
.
broadband irradiation), the matrix temperature (between 3 and 15 K), and the matrix
host (neon or argon). UV irradiation of the nitrenes leads to didehydroazepines
(ketenimines)
p
-
with characteristic IR absorptions of the N
C
C structure
vibration between 1840 and 1893 cm
1
, whereas irradiation with visible light
produces bicyclic azirines
p
-
9a-h
with characteristic C
N structure vibrations in
the range between 1680 and 1740 cm
1
. Upon subsequent irradiation of
p-
8b-i
9
with UV
light of
can be isolated in argon
matrix at 4 K. Annealing results in the disappearance of its signals and subsequent
UV irradiation at 4 K in their reappearance.
15
The photochemistry of the symmetrically substituted nitrenes
p-
l¼
254 nm, small amounts of nitreno radicals
p-
7
5b
,
p-
5c
, and
p-
5i
is quite similar (Scheme 9.9 ) with only little variations, as nitreno radical
p-
6i
and
azirine
p-
is
complicated by the possibility of the nitrene nitrogen to rearrange toward or away
from the adjacent fluorine substituent or toward or away from the side with more
fluorine atoms (
p-
8b
could not be isolated in the matrix. The photochemistry of
p-
5d-h
). However, unsymmetrical substitution leads to the preferential
formation of isomers that cannot be easily understood (Scheme 9.10). A complica-
tion for the interpretation of the experimental results is that there is no simple way to
discriminate between true photochemical reactions and hot ground state reactions. In
the latter case, the thermal deactivation of electronically excited states results in
thermally excited molecules, which undergo subsequent thermal rearrangements.
Nitrenes
p
-
5d
5
show quite similar selectivities. Calculations on the B3LYP/6-311G
(
d
,
p
) level of theory show that there is a correlation between the activation barriers for
the ring closure of the nitrenes
p
-
5
and the relative yields of the isomeric azirines
p
-
8
,
9
0
correlatewith their
relative stabilities. Although details of the formation of azirines and ketenimines are
unknown, it is obvious that DFT calculations of the relative stabilities of the isomers
and of the barriers for their formation can be used to predict the distribution of isomers
even in semiquantitative way. A larger difference in the thermal stability or the
8
0
in thematrix. In contrast, the yields of the ketenimines
p
-
p
-
9
,
p
-
