Chemistry Reference
In-Depth Information
rearrangement of singlet nitrene which must surmount a 8.8 kcal/mol barrier to
cyclize and eventually form dehydroazepine.
24
Our product studies were in good
agreement with recently reported calculations by Karney and Borden.
15
They
indicated that in the transition states for cyclization, an
ortho
-fluoro substituent
interacts in a sterically repulsive manner with the attacking nitrogen favoring
cyclization away from the substituent. They also indicated that in the case of
o
,
o
0
-difluorosubstituted systems, such as
1
PFPhN and 2,6-difluorophenyl nitrene, this
repulsion leads to a considerably higher barrier to ring expansion than in
1
PhN. They
mentioned that even though computational results indicated substantial steric effect,
electronic effects undoubtedly play a role as well.
In 2001, Platz et al.
22
studied a series of FPhN
3
by LFP techniques. At ambient
temperature
1
PhN decays by cyclization to BA. Barriers to cyclization were
calculated for
1
FPhN and were found to be higher than the barrier for
1
PhN. To
understand this substituent effect, the atomic charges for the different centers were
calculated. Fluorine substitution makes the adjacent carbon very positively charged
(
0.48). In the transformation of
o
-fluorophenyl nitrene, there is an increase in
positive charge at the hipso carbon bearing the nitrogen. The increased activation
barrier to cyclization for
o
-fluorophenyl nitrene relative to
p
-fluorophenyl nitrene is
due to a greater amount of positive-positive charge interaction between the
ortho
-
and hipso carbons in the transition state. Platz
12c
indicated that the
ortho
-difluoro
effect is due to a combination of steric influence and the build-up of positive charge
on adjacent carbon atoms.
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11.4.2 Reactions in Aniline
To investigate the reactivity of
1
FPhN with a less reactive amine, we studied the
photochemistry of several FPhN
3
in aniline.
32
In the case of a phenyl azide with
one fluorine, a mixture of products was obtained. For example, photolysis of
2-fluorophenyl azide in aniline gave
N
-(4-aminophenyl)-2-fluoroaniline in 8% yield,
and 2-fluoroazobenzene in 10% yield and 2-(phenylamino)-3-fluoro-3
H
-azepine
in 15% yield.
Irradiation of 2,6-difluorophenyl azide in chloroform containing aniline gave
2,6-difluoroazobenzene in 50% yield. Similar results were obtained with
PFPhN
3
. Only in the photolysis of 2,6-difluoro and PFPhN
3
,wherethetwo
ortho
positions are occupied by fluorine atoms, the ring expansion was inhibited
and no AZ-type products were observed (Scheme 11.14). Singlet 2,6-difluor-
ophenyl nitrene was efficiently trapped by aniline to produce an aromatic
hydrazine that eventually gave an asymmetric fluoroazobenzene on oxidation.
The formation of asymmetric fluoroazocompounds has been reported in the
thermolysis of PFPhN
3
in aniline,
18
although this reaction was not observed with
the parent phenyl azide PhN
3
. The difference in reactivity between these two aryl
azides could be attributed to the much greater electrophilicity of singlet penta-
fluorophenyl nitrene
1
PFPhN, which enables it to be trapped more efficiently by
an aromatic amine.
