Chemistry Reference
In-Depth Information
Scheme 16.9
Hydroxystannation of
olefins induced by sonication.
complexes, thereby avoiding the possibility of
decomposition.
Results of a chemical activation induced by ultra-
sound have been reported recently by Nakamura
et
al
. in the initiation of radical chain reactions with tin
radicals [44]. When an aerated solution of R
3
SnH
and an olefin is sonicated at low temperatures
(0-10°C),
hydroxystannation
of the double bond
occurs and not the conventional hydrostannation
achieved under silent conditions (see Scheme 16.9).
This point evidences the differences between radical
sonochemistry and the classical free-radical chem-
istry. The result was interpreted on the basis of
the generation of tin and peroxy radicals in the
region of hot cavities, which then undergo synthetic
reactions in the bulk liquid phase. These findings
also enable the sonochemical synthesis of alkyl
hydroperoxides by aerobic reductive oxygenation of
alkyl halides [45], and the aerobic catalytic conver-
sion of alkyl halides into alcohols by trialkyltin
halides [46].
It is obvious that in order to make predictions, the
chemistry induced by cavitation will depend on the
lifetimes of primary radicals compared with the life-
time of bubbles [47]. According to theory, ultrasonic
frequency will influence the time taken for the col-
lapse of a cavitation bubble, although most synthetic
studies do not take into account this important para-
meter. At high frequencies of, say, 500 kHz the col-
lapse occurs in 4 ¥ 10
-7
s, a period shorter than the
lifetime of most radicals. These species, after the col-
lapse, will migrate into the liquid phase and interact
with other species or substrates. At a frequency of
20 kHz, however, the bubble collapse occurs in
~10
-5
s and this is a long enough period for
•
OH rad-
icals to be able to undergo recombination reactions
to give hydrogen peroxide, superoxides, excited
water molecules or other reactions in which dis-
solved gases also may participate. This suggests that
'primary' sonochemistry will be observed only at
high frequencies, whereas the chemistry of low fre-
quencies will be a consequence of sequential trans-
formations of radicals.
To test the above considerations, the sono-
oxidation of 2,2,6,6-tetramethylpiperidin-4-one was
investigated at 520 and 20 kHz and the formation of
its stable nitoxide was monitored by election spin
resonance [15]. The reaction requires the presence
of
•
OH and either molecular oxygen or superoxide
radical anion. A higher rate for nitroxide formation
was observed at 520 kHz with the oxygen saturated
solution, but no formation occurred under argon.
In contrast, the same experiment run at 20 kHz
proceeded slowly under oxygen but at a higher
rate under argon. Because oxygen is necessary for
this transformation, it must be produced at low
frequency under argon by reaction pathways involv-
ing
•
OH recombination (see Scheme 16.1).
It is noteworthy to mention a homogeneous
process involving C
60
solutions, because the sono-
chemistry of fullerenes is still underexploited [48].
The ultrasonic irradiation (at 20 kHz) of a solution of
C
60
in decahydronaphthalene results in the forma-
tion of C
60
H
2
[49]. Owing to its low vapour pressure
at room temperature, no C
60
will be inside the cavi-
tation bubbles, although it will experience secondary
reactions in the liquid phase. The dihydrofullerene
results from the reaction with atomic hydrogen
generated by sonolysis of the solvent. Although
there are already many studies on hydrogenated
fullerenes, it is remarkable that sonication does not
produce more highly hydrogenated derivatives.
Moreover, continued sonication results in the disap-
pearance of both C
60
and C
60
H
2
from the solution,
