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R
2
NH
R
2
N-Ar
cat. Cu(OAc)
2
Tempo (1.1 equiv.)
+
ArB(OH)
2
or
or
Et
3
N or Pyridine
Air
ArOH
ArO-Ar
Scheme 5.50
Copper-catalyzed cross-coupling reactions using TEMPO as oxidant
O
t-
Bu
no Tempo: 15 %
6 equiv. Tempo: 99 %
(OC)
3
Co
Co(CO)
3
H
RT, 3d
Toluene
t-
Bu
H
H
H
Scheme 5.51
TEMPO-catalyzed Pauson- Khan reaction
An acceleration of the Pauson - Khand reaction was found by using TEMPO, especially with sterically
demanding alkynes (Scheme 5.51). DFT calculations supported the hypothesis that TEMPO promotes the
decarbonylation of the cobalt complex through single-electron activation of the cobalt - carbon bonds, thus
providing a low energy, radical pathway.
237
For many years, TEMPO has also been used as ligand for transition metals and main group elements.
It has been shown that Ti - O bonds in titanium nitroxide complexes are weaker than those of titanium
alkoxides, and they can be cleaved at moderate temperature (60
◦
C) to generate titanium(III) complexes
and aminoxyl radicals (Scheme 5.52). This result and DFT calculations have demonstrated also that the
strength of these Ti - O bonds can be tuned by ancillary ligand modifications on titanium.
238
Recently, “transition metal free” oxidative homocoupling reactions of aryl, alkenyl, and alkynyl based
Grignard reagents have been reported using TEMPO as the stoichiometric oxidant.
239
The method appears
to be versatile and very efficient. Reaction of TEMPO with Grignard reagents usually affords the cor-
responding alkoxyamines, as illustrated in Scheme 5.53. However, due to the destabilized character of
aryl, alkenyl, and alkynyl radicals, the reaction outcome is different, and this is presumably due to the
non-generation of free aryl, alkenyl, and alkynyl radical intermediates.
The oxidative rhodium-catalyzed coupling reaction of arenes and heteroarenes with various aryl-
boronic acids via direct C-H arylation and using TEMPO as the oxidant has been reported recently
(Scheme 5.54).
240
In the proposed mechanism, it is assumed that TEMPO acts as an oxidant (two
Cp
Cl
Ti
Cp
Cl
Cp
∆
Ti
Cp
O
III
O
Ti
Cl
Cp
O
N
+
O
N
Cp
O
N
Scheme 5.52
Thermal homolysis of Ti- O bonds derived from titanium-TEMPO complex and subsequent ring
opening mediated by the Cp
2
Ti(III)Cl complex followed by TEMPO trapping
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