Chemistry Reference
In-Depth Information
O
C
O
C
CC
O
C
O
C
*
-N
2
1
R
1
N
R
N
h
v
1
3
4
R
R
N
3
N
3
-N
2
1
NCO
R
2
R=alkyl, aryl
SCHEME 12.2.
Photolysis of carbonyl azides.
6
For Curtius rearrangement of aliphatic carbonyl azides (R
¼
alkyl), the rate
constants are of the order of 10
3
-10
2
min
1
in the temperature range from 25
aryl), in the range 50-80
C.
31,44
The
ortho
-substituted benzoyl azides decompose much faster (in the 25-45
C tempera-
ture range).
31,44
The activation energy of the Curtius rearrangement of benzoyl azide
(
to 45
C; for aromatic carbonyl azides (R
¼
Ph) and a series of its
meta
- and
para
-substituted analogues is in the range
25-29 kcal/mol in toluene. The activation energy of rearrangement of benzoyl
azide depends slightly on the solvent nature and decreases from 31.6 kcal/mol
in hexane to 25.4 kcal/mol in aniline and acetic acid.
31,44
In turn, photolysis of carbonyl azides gives rise to two types of reactions. The
photo-Curtius rearrangement proceeds to form isocyanates. In addition, bimolecular
trapping products, typical of the reactions of singlet carbonyl nitrenes, are also
observed (Scheme 12.2).
6,18,24,25,34,41,42,47-55
It has been shown that the yield of isocyanates (
1a
,R
¼
2
), formed upon photolysis of a
series of carbonyl azides (
alkyl, aryl), remains constant in the
presence and in the absence of a nitrene traps.
40,43,48,49
For example, the yield of
isocyanate produced on photolysis of pivaloyl azide (R
1
,R
CO
N
3
,R
¼
¼
tert
-Butyl,
1b
) in methyl-
ene chloride (an inert solvent) is 40%.
42,43
Photolysis of
1b
in cyclohexene leads to a
45% yield of aziridine adduct
4b
and a 41% yield of isocyanate
2b
(Scheme
12.2).
42,43
Trapping the nitrenes (
3
) does not depress the yield of isocyanates (
2
).
Hence, isocyanate
cannot be derived from the nitrene in its lowest electronic state,
but instead the isocyanate must be formed from the excited state of the azide (or at
least from the electronically or thermally excited nitrene). Therefore, it was
concluded that the loss of nitrogen and migration of R are concerted processes in
the Curtius rearrangement of carbonyl azides.
6,44
This conclusion has been recently
supported by quantum chemical calculations.
18,45,46,56,57
It should be noted that first theoretical study of the thermal and photochemical
decomposition and Curtius rearrangement of acyl azides has been performed by
Rauk and Alewood.
58
To study the properties of formyl azide (1c, R
2
H) and
products of its decomposition, Rauk and Alewood
58
employed the SCF calculations
augmented with CI. The STO-3G and extended (9s5p/4s contracted to 4s2p/2s) basis
sets were used. They
58
also attempted to calculate the potential energy surface (PES)
for the decomposition of 1c in the ground and low-lying excited states. As these
calculations were performed at relatively low level of theory, at least by current
standards, most results do not agree with the modern view on the mechanism of
Curtius and photo-Curtius rearrangement.
13-19,36,37
¼
