Chemistry Reference
In-Depth Information
O
+
-
P
N
N
C
CH
3
98
SCHEME 12.46.
Structure of ylide
98
.
Distribution of products in the reactions of singlet phosphoryl nitrene
1
79a
with
2,3-dimethylbutane and 2,2,3-trimethylbutane and their mixture has been carefully
analyzed,
162,163
and the rate constants of reactions and intersystem crossing were
deduced.
162
The rate constant of intersystem crossing for nitrene
79a
was indirectly
10
8
s
1
in perfluorohexane.
162
Later,
164
the triplet nitrene
3
79b
estimated to be 2
was directly detected in an EPA glass at 77 K and
at room temperature in the LFP experiments. Photolysis of
in an EPA glass at
77 K led to the formation of a species with electronic absorption band at 336 nm and
the EPR spectrum characteristic for triplet nitrenes (
81b
1.5408 cm
1
j
D
/
hc
j¼
and
0.00739 cm
1
). Thus, this EPR and absorption spectra were assigned to
the triplet nitrene
3
79b
j
E
/
hc
j¼
.
164
The transient absorption with a maximum at 345 nm
attributed to the triplet nitrene
3
79b
was detected upon LFP of
81b
in cyclohexane,
ethanol, methanol, and ethanol-water mixture. The lifetime of
3
79b
was measured to
be 3.8
0.6
m
s in deoxygenated ethanol, 3.5
0.6
m
s in ethanol saturated with
s in deoxygenated ethanol-d
6
. The lifetime of
3
79b
oxygen and 4.1
0.8
m
was
slightly reduced in 60% ethanol/40% water mixture (2.3
s). Similar results
were obtained in methanol. In all the solvents under study, the transient signal
appeared within the rise time of the laser (
0.8
m
5 ns).
164
in acetonitrile produced similar transient absorption at 340 nm, but the
signal did not decay within 50
m
s. This transient absorption was assigned to the long-
lived ylide
LFP of
81b
formed in the fast reaction of singlet nitrene
1
79b
98
with acetonitrile
(Scheme 12.46).
164
In the recent LFP study,
165
Gohar and Platz reproduced the results of Maloney and
coworkers.
164
The transient absorption with maximum at about 340 was detected and
assigned to the triplet nitrene
3
79b
. Kinetics of the
3
79b
decay was measured in
chloroform, cyclohexane, 1,2-dichloroethane, dichloromethane, ethyl acetate, 1,1,2-
trichlorotrifluoroethane, and tetrahydrofuran. The triplet nitrene lifetime (
) was
measured to be several tens of microseconds in these solvents. The magnitude of 1/
t
t
was linearly dependent on the concentration of diphenylphosphoryl azide
81b
10
6
M
1
s
1
)inCH
2
Cl
2
, the extrapolated lifetime of
3
79b
(
k
in the absence
of precursor is 500ms in this solvent at ambient temperature. It was proposed that the
relatively long lifetime (several tens of microseconds) of
3
79b
¼
1.2
in the aforementioned
solvents is controlled by the reaction of
3
79b
with precursor
81b
. The one exception
is methanol as solvent for which the triplet lifetime is only 6
s.
As Maloney and coworkers,
164
Platz and Gohar
165
assigned the transient absorp-
tion detected in acetonitrile to ylide
m
98
. Indeed, in contrast to other solvents, the
