Chemistry Reference
In-Depth Information
N
*
N
*
NN
*
N
N
37
37'
38
* =
15
N or
14
N
FVT
N
N
N
N
3
N
N
N
N
N
N
37
38
39A
NuH
39T
or H
2
O
N
NH
N
N
HN
NH
Nu
Nu
O
42
41
40
SCHEME 8.8.
absorb strongly in the 2000 cm
1
range in the IR and are stable in some cases up to
38
40
C.
27
Several examples of ring expansion of hetaryl nitrenes are described later.
The photolysis of tetrazolo[1,5-
a
]pyridines/2-azidopyridines
in solution in
the presence of amines, alcohols, or water as trapping agents is a useful method
for the synthesis of 1,3-diazepines and 1,3-diazepin-2-ones
39
(Scheme 8.8).
28a
An analogous ring expansion of 4-azidouracil affords a triazepine-2,4-dione
nucleoside.
28b
40
42
-
8.2.1 Azirenes
Bicyclic intermediates, (aza)bicyclo[4.1.0]hepta-2,4,6-trienes, for example
above
(Scheme 8.2), are theorized to be fleeting intermediates occupying shallow energy
minima between the nitrenes and the seven-membered rings. Several azirenes have
been observed by IR spectroscopy in matrix photolyses (Schemes 8.10-8.13), but the
parent compound
6
has not. Ultrafast spectroscopy in solution has indicated that
there are two routes to the seven-membered ring intermediates.
3
As aforementioned,
vibrationally hot singlet nitrenes may rearrange directly to azacycloheptatetraenes
on a
ps
timescale.
3
In contrast, ring expansion of relaxed nitrenes is believed to take
place via the azirenes
43
on a
ns
timescale (Scheme 8.9).
Fluorine atoms induce a partial positive charge on the carbon atoms towhich they are
attached.
10
There is also a development of positive charge at the
ipso
carbon in the
transition state for cyclization of a nitrene to an azirene. The repulsion between these
positivechargesraises thebarrier towardazireneformation(e.g.,
43
45!46
,Scheme8.10),
