Chemistry Reference
In-Depth Information
N
N
N
N
N
o
-
6a
o
-
6a'
m
-
6a
p
-
6a
p
-
6a'
SCHEME 9.3.
Nitreno radicals
6
with a
s
radical center in the
ortho
,
meta
, and
para
positions of the phenyl ring.
calculated.
9
For
o
-
6a
0
can be
formulated that suggest that these triradicals also have some carbene (cyclohex-
adienylidene) character. In contrast, for
m
-
6a
0
and
p
-
6a
and
p
-
6a
quinoid resonance structures,
o
-
no such resonance structure is possible.
Since both phenyl nitrenes and cyclohexadienylidenes have robust triplet ground
states, a quartet state for
o
-
6a
is energetically favorable since in this case
local high-spin triplet configurations at the formal nitrene and carbene centers are
maintained.
A synthetic route to nitreno radicals starts from iodo-phenyl azides
6a
and
p
-
6a
that on
photolysis in low-temperature matrices in a first-step split off N
2
to give the
corresponding phenyl nitrene
10
and subsequently split off an iodine atom to give
the nitreno radical.
11-15
However, phenyl nitrenes are photolabile
16
7
and easily
rearrange to azirines
10
and ketenimines,
17
and the cleavage of the C
I bond to
form the additional radical center has to compete with these rearrangements.
Thus, photolysis of matrix-isolated (argon, 10 K) phenyl azide
7
produces mixtures
of phenyl nitrene
5
, azirine
8
, and ketenimine
9
in photostationary equilibria
is therefore generally low.
18
Fluorine
(Scheme 9.4), and the yield of nitrene
5
substituents
in the
ortho
positions of
5
decrease the tendency of
these
N
3
N
N
7
S-5
8
N
N
9
T-5
SCHEME 9.4.
Photochemistry of phenyl azide
7
.
