Chemistry Reference
In-Depth Information
matrices and it was less intense than that of
38
presumably because the C
N band in
38
coupled with the adjacent C
O group, producing a band with higher intensity and
vibrational frequency.
64
The C
N stretching of triplet methyl nitrene has been
reported at 1040 cm
1
, whereas the C
N stretching of tetrafluoropyridine-4-nitrene
has been observed at 1272 cm
1
.
47-49,78
5.5.1.3 Electron Spin Resonance Spectroscopy.
The direct detection of triplet
alkyl nitrene
36
in solution was further supported by electron spin resonance
spectroscopy.
64
In frozen solvent glasses, the electron spin resonance spectra of
36
8400 G, which is typically observed for a triplet nitrene
intermediate.
29-31,48,55,79,80
The zero-field splitting parameters for nitrene
showed a peak at
36
were
1.64 cm
1
and
0.004 cm
1
. These values are
calculated as follows:
j
D
/hc
j¼
j
E
/hc
j¼
values of 1.607 and 1.616 cm
1
measured for
n
-propyl and
n
-
octyl nitrenes, respectively.
48,79
similar to the
j
D
/hc
j
5.5.2 Solution Reactivity of Triplet Alkyl Nitrene
The reactivity of alkyl nitrenes in solution demonstrated that these intermediates are
highly stabilized because they do not abstract H-atoms or react with the solvent, but
rather decay through dimerization.
64
For example, Singh et al. showed that photoly-
sis of
35
yields
39
as the major product and smaller amounts of
40
and propio-
phenone (Scheme 5.14).
64
The authors proposed that
39
results from dimerization of
two triplet alkyl nitrenes
, which rearranges with the loss of a water
molecule (Scheme 5.15). This is similar to what has been observed for triplet aryl
nitrenes, which primarily dimerize to yield diaryl azo dimers.
81,82
Singh et al.
theorized that the formation of propiophenone proceeds from the secondary photol-
ysis of
36
to form
41
41
to yield a propiophenone radical that abstracts a hydrogen atom from the
solvent, whereas formation of
40
is attributed to the propiophenone radical that is
intercepted by nitrene
36
. In comparison, photolysis of
42
yielded only
43
because
the MeO-Ph groups mask direct absorption of the N
N chromophore in
44
and,
therefore, formation of methoxy-propiophenone (Scheme 5.16).
It was not possible to isolate the azo dimers
41
and
44
from solutions before they
reacted and formed
39
and
43
, but an analogous azo dimer
45
was characterized in
the solid-state (Scheme 5.17).
61
Solid-state photolysis of
46
selectively yielded
cis
-
, as shown by IR, Raman, and
13
C-NMR spectroscopy along with molecular
45
Ph
O
O
hν
O
O
N
Ph
Ph
N
3
Ph
N
Ph
H
Ph
O
35 39
Propiophenone
40
Product ratio 51 11 38
SCHEME 5.14.
