Chemistry Reference
In-Depth Information
D
D
D
D
70
°
C
O
+
+
N
O
N
D
D
80 %
20 %
Scheme 5.37
First step in the reaction of TEMPO with cyclohexene
at 70
◦
C demonstrated that both pathways occur, the more important being the allylic hydrogen atom
abstraction/addition process (80 %).
188
This work was undertaken to sort out the different mechanisms
reported in the literature concerning the reaction of nitroxide with cyclohexene (Scheme 5.37).
Under photoirradiation, the surprising hydrogen atom abstraction by TEMPO from acetonitrile and
toluene was reported with nearly quantitative yield.
189
Because the O-H bond energy in TEMPO hydrox-
ylamine is known, TEMPO has been also used to confirm the Bond Dissociation Energy (BDE) of O-H
bonds in non-heme iron and manganese complexes as model of metalloproteins.
190
5.6.5 Cross-coupling reactions
Nitroxides have often been used as radical scavenging agents, due to their fast reaction with other free rad-
icals, such as carbon- and nitrogen-centered radicals
191 - 194
and protein radicals.
195,196
Due to their efficient
radical trapping behavior, nitroxides have been used for many years as stabilizers and inhibitors in plastics
and rubber manufacture, as regulators in controlled/living radical polymerization, and as antioxidants in
the biomedical field.
197,198
Absolute rate constants for the cross-coupling of various nitroxides with several carbon-centered radicals
have been determined and are in the range 5
10
9
M
−
1
s
−
1
at room temperature, the nature of
the trapped radicals and of the nitroxide affecting mainly the rate value.
199
As a result, trapping of carbon-
centered radicals with nitroxides is an important and effective method for the synthesis of alkoxyamines.
When stabilized carbon-centered radicals are formed, the alkoxyamine O-C bond undergoes facile and
clean thermal homolysis (Scheme 5.31).
Recently, it has been reported that photoinduced homolysis of alkoxyamines is possible, opening up
possible applications of alkoxyamine free radical initiation at low temperature.
200
Indeed, the photolysis of
an alkoxyamine bearing a quinoline antenna proceeds via intramolecular energy transfer from the quinoline
moiety leading to the homolysis of the C-O alkoxyamine bond.
10
7
×
to 2
×
5.6.6 Nitroxides in synthetic sequences
The use of nitroxides in organic synthesis has been more and more important in the last decade, as
illustrated by the growing number of review articles
201 - 204
and topic chapters.
205
Nitroxides have often
been used for the oxidation of primary and secondary alcohols as well as sulfides to the corresponding
aldehydes and ketones, and sulfoxides respectively. A chapter of the topic is devoted to this topic.
5.6.6.1 Trapping of carbon-centered radicals
In various radical initiation methodology, radical carbon - carbon bond formation has been successfully
combined with subsequent nitroxide trapping reactions,
206 - 212
allowing the introduction of a functional
group after removal of the alkoxyamine. For instance, Scheme 5.38 describes the reaction of an aryl iodide
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