Chemistry Reference
In-Depth Information
perfluoro-substituted aryl nitrenes.
13-15
It is well known that alkyl-, aryl-, and vinyl
nitrenes have the triplet ground state.
13-15
The situation with acyl nitrenes is more
complicated and ambiguous and was fully understood only recently.
16-19
Acyl nitrenes are commonly generated by decomposition of acyl azides,
1,6,13-15
although other precursors, such as
N
-
p
-nitrobenzenesulfonoxyurethan,
20,21
or cor-
responding nitrile oxides,
22
N
-aroyl ylides,
23
iminosulfuranes,
24
and sulfili-
mines,
25,26
are known. Acyl azides have been applied as photoresists in
lithography,
27
in material science for functionalization of carbon nanotubes
28
and
fullerenes,
29
and in organic synthesis.
6,30-32
Schuster proposed to apply the aroyl
azides in photoaffinity labeling of biological systems.
33
This proposition was
inspired by a report that benzoyl nitrene inserts stereospecifically into unactivated
carbon
hydrogen bonds.
34
This unique behavior makes this acyl nitrene a suitable
photolabeling agent.
Synthesis of acyl azides has been recently reviewed
35
and will not be covered in
this chapter. This review is limited to the acyl nitrenes commonly encountered in
organic chemistry. Among acyl nitrenes, the carbonyl nitrenes (RCON, R
alkyl
and aryl) have attracted the most attention in the literature.
6,13-15
Properties of
nitrene esters (ROCON, R
¼
alkyl and aryl) were also studied in some details.
6,13-15
Moreover, there has been a significant progress in understanding the nature of the
carbonyl nitrenes and nitrene esters.
13-19,25,26
The photochemistry of corresponding
azides was also revealed using matrix isolation, time-resolved spectroscopic tech-
niques, and modern quantum chemistry.
16-19,36,37
Therefore, carbonyl nitrenes and
nitrene esters are mainly discussed in this chapter. In addition, properties of sulfonyl-
(RSO
2
N) as well as phosphinyl (R
2
PON) and phosphoryl ((RO)
2
PON) nitrenes will
be also considered. The most attention will be paid to the direct observations of acyl
nitrenes and identification of their reactivity.
¼
12.2 GENERATION AND PROPERTIES OF CARBONYL NITRENES
Thermolysis and photolysis of acyl azides are commonly applied to generate acyl
nitrenes. Unfortunately, in most cases generation of acyl nitrenes upon decompo-
sition of acyl azides is accompanied by formation of isocyanates.
6,13-15
For
example, the thermal rearrangement of carbonyl azides (Curtius rearrangement)
produces isocyanates in quantitative yields. The Curtius rearrangement (Scheme
12.1)
38,39
of carbonyl azides was identified some time ago and has been studied in
detail.
6,13-15,31,40-46
Neither carbonyl nitrene insertion or addition products, nor
amides were isolated in the Curtius rearrangement of carbonyl azides (
).
31,44
1
O
Δ
RC
RN C
O + N
2
R=alkyl, aryl
1
2
N
3
SCHEME 12.1.
Curtius rearrangement of carbonyl azides.
6
