Chemistry Reference
In-Depth Information
O
P
+
N
N
-
MeO
N
MeO
TS3
O
P
MeO
N
MeO
O
P
+
75.8
N
TS2
N
-
MeO
N
OMe
3
81d
T
S1
3
99d + N
2
0.1
3
99d
+ N
2
-6.4
75.4
3
O
P
3
O
P
-45.5
39.1
MeO
OMe
N
3
OMe
MeO
N
3
79d + N
2
O
P
3
O
P
3
N
OMe
MeO
OMe
MeO
N
FIGURE 12.24.
Reaction pathways for concerted and stepwise rearrangement of the triplet
dimethylphosphoryl azide
3
81d
and triplet state of the nitrene
3
79d
G
#
(
D
G
and
D
in
kcal/mol).
169
Source
: Reprinted with permission from Ref. 169. Copyright 2007 ACS Publications.
80c
90c
Figure 12.24. Unlike the rearrangement of phosphinyl azide
to amidate
, the
81d
99d
rearrangement of phosphoryl azide
in the singlet state is
endothermic by 28.7 kcal/mol. The free energy of activation is also slightly larger
(47.0 kcal/mol) for the Curtius-like rearrangement of
81d
. The
D
to amidate
G
#
value for the
release of nitrogen to form the singlet nitrene is 41.7 kcal/mol, which is 5.3 kcal/mol
lower in energy than the Curtius-like rearrangement barrier. The lower barrier for the
extrusion of nitrogen will favor the formation of phosphoryl nitrene over
rearrangement to the phosphonamidate during pyrolysis. Thus, the predicted thermal
reactions of phosphoryl azides yield products similar to those arising from photoly-
sis, namely, products derived from the reactions of corresponding nitrenes.
As aforementioned, the Curtius-like rearrangement of the singlet phosphoryl
nitrene
1
79d
proceeds through the formation of cyclic intermediate
1
79d
0
undergoing
further the 1,2-migration. The rate limiting step involves breaking the weak
interaction between nitrogen and the phosphoryl oxygen, accompanied by the
formation of an interaction between the nitrogen and the methoxy oxygen.
The
G
#
D
value of 18.0 kcal/mol
in the rate-limiting step for the Curtius-like
rearrangement of
1
79d
does not allow the Curtius-like rearrangement to compete
with the bimolecular reactions of this nitrene. The Curtius-like rearrangement of
triplet nitrene
3
79d
has very large free energy of activation (75.4 kcal/mol;
Fig. 12.24) and does not compete with the bimolecular reactions either. Thus,
