Chemistry Reference
In-Depth Information
of activation (27.1 kcal/mol) calculated previously
18
at CBS-QB3 level. Moreover,
the DFT calculations at the B3LYP level are in quantitative agreement with the data
from higher level of calculations and experimental values.
Thus, it could be concluded that calculations predict in agreement with exper-
imental results
6,31,44
that the Curtius rearrangement of carbonyl azides (R-CON
3
,
R
alkyl, aryl) is a concerted process. However, it should be noted that results of the
MP2 calculations
57,61
disagree with those of DFTand higher level calculations
45,46,56
and predict that acetyl (
¼
) azides will undergo stepwise Curtius
rearrangement.
61
Note that MP2 barrier for benzoyl azide rearrangement
(47.8 kcal/mol)
61
is significantly higher than experimental activation energy in
hexane (31.6 kcal/mol).
31,44
Therefore, the MP2 method is not well suited for
analysis of the Curtius rearrangement.
As noted earlier, photolysis of carbonyl azides in contrast to their thermolysis
produces carbonyl nitrenes along with isocyanates (products of the photo-Curtius
rearrangement). Generation of the nitrenes manifests itself by formation of the
bimolecular trapping products, typical of the reactions of singlet carbonyl nitrenes
(Scheme 12.2).
6,18,24,25,34,41,42,47-55
The mechanism of photochemical transforma-
tions of carbonyl azides has long been debated.
6,24-26,41,42,50-55,62-67
Four features of the photochemical decomposition of carbonyl azides are note-
worthy. First, trapping the nitrenes has no influence on the yield of isocyanates
produced upon photolysis of carbonyl azides. For example, the yields of the
isocyanates produced upon photolysis of benzoyl azide (R
1d
) and benzoyl (
1a
) and its
para
-methoxy,
para
-chloro, and
meta
-fluoro derivatives were found to be in the
range 40-50% both in inert solvents and in solvents that intercept acyl
nitrenes.
42,43,48,49
As mentioned previously, the yield of isocyanate
¼
Ph,
1a
2b
produced
1b
upon photolysis of pivaloyl azide (
) in an inert solvent methylene chloride (40%) is
the same as in cyclohexene (41%), although photolysis in cyclohexane leads also to a
45% yield of aziridine adduct
(Scheme 12.2).
42,43
Similar results were obtained
4b
for 2-naphthoyl azide (
7
).
51
Irradiation of
7
in cyclohexane at room temperature
produces isocyanate
(8
,
45%),
N
-cyclohexyl-2-naphthamide (
9
)—the product of 2-
naphthoyl nitrene insertion (
50%), and a trace (
<
1%) of 2-naphthamide (
10
)
(Scheme 12.4).
51
Irradiation of
7
in cyclohexane solution containing cyclohexene gives isocyanate
8
, insertion product
9
, and aziridine
11
(Scheme 12.5). As the concentration of
cyclohexene is increased, the yield of
9
decreases, that of
11
increases, but the yield
of isocyanate
8
remains unchanged. Thus, carbonyl nitrenes (R
CO
N) do not
N
C
rearrange to isocyanates (R
O) at a rate that is competitive with their
O
O
N
C
O
N
3
h
v
C
6
H
12
NC
6
H
11
+
H
7
8
9
) in cyclohexane.
51
SCHEME 12.4.
Photolysis of 2-naphthoyl azide (
7
