Chemistry Reference
In-Depth Information
O
*
-N
2
R
N
O
O
+
+
N
N
hv
N
N
-
-
?
R
N
R
N
-N
2
N C O
N
FIGURE 2.13.
Two possible channels in the photochemistry of a carbonyl azide.
isocyanates, which has been detected as final products in most of the photochemical
studies (Fig. 2.13). It was first reported by Tiemann
155
and Stieglitz
156
that
isocyanate formation occurs via Curtius rearrangement (Tiemann proposed it as a
Hoffmann rearrangement) with the intermediacy of the singlet carbonylnitrene. Later
studies debated the issue as to whether the singlet carbonylnitrene generates the
isocyanate or whether an excited state of the azide produces the isocyanate in a
concerted manner without involving the nitrene. Moreover, unlike alkyl
10,31,37,157
or
arylnitrenes
31,37,50
where the ground state is the triplet state, as was discussed earlier,
carbonylnitrenes have singlet ground states. At first, Schuster and coworkers
26,27
pointed out this distinct nature of carbonylnitrenes which was then confirmed by
high-level quantum chemical calculations by Gritsan and Pritchina
158
and further
supported by calculations of several groups.
28-30
Furthermore, Gritsan and Pritchina
proposed that the interaction between the electron-rich oxygen of the carbonyl group
can interact with the electron-poor nitrene center and can aid the nitrene to have a
singlet (presumably zwitterionic) ground state.
Lwowski and coworkers
159,160
demonstrated that both thermolysis and photolysis
of pivaloyl azide (
t
BuC(
O)N
3
) leads to isocyanate formation, and without involv-
ing the nitrene; therefore, they concluded that there are two possible channels for the
excited states of carbonyl azides. They found that the usage of nitrene traps in the
reaction did not affect the yield of isocyanate product. The finding was further
supported by experiments of Autrey and Schuster who found that the interplay
between the
-naphthyl
isocyanate; hence, there are possibly two channels on the excited state surface of the
azide.
Our calculations on the singlet acetylnitrene and the ground state of acetyl azide
showed an energy barrier of 27.3 and 27.1 kcal/mol, respectively, to generate methyl
isocyanate using CBS-QB3 computations.
16
Both of these energy barriers are similar
and within the error bar of the calculations; however, one can imagine a smaller
energy barrier for the concerted process on the excited state potential energy surface
of the carbonyl azide. Computational work on the ground states of carbonyl azides
also revealed that the conformation of the azide linkage relative to the carbonyl
moiety also plays an important role. The
cis
conformer was concluded to go through
a concerted process and produce the isocyanate while avoiding the singlet nitrene;
however, the
trans
conformer goes through a stepwise process and produces the
nitrene first.
16
Nevertheless, the above-mentioned results are not in exact agreement
b
-naphthoylnitrene products does not affect the amount of
b
