Chemistry Reference
In-Depth Information
N
F
N
F
F
F
I
I
p
-
8f'
p
-
9f'
254 nm
420 nm
254 nm
N
3
N
N
254 nm
Ar, Ne, 4K
-N
2
254 nm
F
F
F
Ar, 4K
35 K
F
F
F
I
I
p
-
6f
p
-
5f
p
-
7f
254 nm
420 nm
N
N
254 nm
F
F
F
F
p
-
8f
I
p
-
9f
I
as minor product upon
320-400 nm
SCHEME 9.10.
Photochemistry of
p-7f
.
activation barriers results in a larger preference of one of the isomers. Since the
activation barrier for the ring closure is lower away from the substituent, azirines
p
-
8
0
are preferred and because the ketenimines
p
-
with no fluorine substituent attached to
the cumulene double bond are lower in energy they are preferred upon UV irradiation.
For
p
-
9
, the activation barrier for the ring closure toward the side with the
ortho
fluorine substituent is 4.7 kcal/mol lower in energy, and the ketenimine with the
cumulene bond at the opposite side of the two adjacent fluorine substituent is
14.5 kcal/mol more stable (Fig. 9.4). Both isomers are preferred.
UV irradiation of 2-iodo-3,4,5,6-tetrafluorphenyl azide
o
-
5f
7b
yields triplet nitrene
o
-
5b
. The nitrene rearrangement can produce two isomers of azirine and ketenimine,
depending on the direction of the ring closure reaction toward or away from the bulky
iodine substituent. The rules for the selectivity of the rearrangement reactions
derived for the 4-iodo derivatives can also be applied for
o
-
8b
0
is preferred
5b
.
o
-
upon visible light irradiation (
420 nm) and has the lower barrier for the ring
closure reaction of the nitrene away from the iodine atom. The corresponding
ketenimine
o
-
l¼
9b
0
is almost exclusively formed upon UV irradiation of the nitrene.
9b
0
also shows to be 8 kcal/mol more stable than its isomer
o
-
o
-
9b
(Fig. 9.5). Under
these conditions, no nitreno radical
o
-
6b
can be detected with IR spectroscopy. EPR
