Chemistry Reference
In-Depth Information
FIGURE 12.22.
Selected bond lengths (A
) and bond angles (in degrees) of singlet dime-
thylphosphoryl nitrene
1
79d
and its isomer
1
79d
0
optimized at the B3LYP/6-31G(
d
) level.
169
lifetime of the intermediate in acetonitrile did not depend on the concentration of
precursor.
The value of 1/
for
3
79b
is linearly dependent on the concentration of (Me
3
Si)
3-
SiH, and the absolute rate constants of hydrogen abstraction are 3.4
t
10
5
and
10
5
M
1
s
1
3.1
in 1,2-dichloroethane and 1,1,2-trifluorotrichloroethane, respec-
tively.
165
In addition, (Me
3
Si)
3
SiH reduces the yield of triplet nitrene
3
79b
by the
trapping of singlet nitrene
1
79b
. Assuming that
1
79b
reacts with (Me
3
Si)
3
SiH with a
10
9
M
1
s
1
rate constant 5
(close to diffusion limit), the lifetime of the singlet
nitrene
3
79b
was estimated as 1 ns in 1,2-dichloroethane and methanol and 4 ns in
1,1,2-trifluorotrichloroethane. It was proposed that singlet phosphoryl nitrenes have
cyclic structure similar to the singlet carbonyl nitrenes and nitrene esters.
165
Indeed, recent calculations of Hadad and coworkers
169
demonstrated (Fig. 12.22)
that singlet dimethylphosphoryl nitrene
1
79d
has a cyclic structure with a weak
O bond is 1.931 A
N
O bond. The length of this N
at the B3LYP/6-31G(
d
) level of
theory.
169
It is only slightly longer than the weak N
O bonds in other closed-shell
singlet acyl nitrenes (Figs. 12.6, 12.18, and 12.19). At all levels of theory employed,
the triplet nitrene
3
79b
was predicted to be the ground state. At the highest level of
theory employed (CBS-QB3),
the singlet-triplet splitting was predicted to be
13.8 kcal/mol.
169
The structure isomeric to singlet nitrene
1
79d
was also optimized (
1
79d
0
).
169
This
singlet species also has a cyclic structure with a week N-OCH
3
bond. However, the
isomer
1
79d
0
is thermodynamically less preferable than nitrene
1
79d
(
D
G
ΒΌ
6.8
kcal/mol; Fig. 12.23).
Hadad and coworkers
169
calculated also the free energies of reaction (
D
G
) and
G
#
) for the Curtius-like rearrangement of phosphoryl azide
activation (
D
81d
as
well as corresponding singlet and triplet nitrenes
1
79d
and
3
79d
. The stationary
points on the free energy surface for the Curtius-like rearrangement of azide
are
shown in Figure 12.23. The corresponding triplet free energy surface is displayed in
81d
