Chemistry Reference
In-Depth Information
Scheme 16.3
The Kornblum-Russell
reaction under mechanical stirring and
sonication.
Scheme 16.4
Effects of stirring and sonication on the composite pathway of the reaction of lead tetraacetate with styrene.
Scheme 16.5
The Mn(III)-induced generation of malonate
radicals.
tional collapse are estimated to be lower than those
employed in high-pressure chemistry. Anyway, these
results are difficult to interpret because most exper-
iments were partially heterogeneous.
An alternative explanation could be based on the
existence of a SET mechanism, which would be
favoured by sonication, involving radical cation
species [38]. The hetero-Diels-Alder cycloaddition
of a monoazadiene with quinoline-1,4-dione gives
the [3 + 2] furan adduct as the major product
under mechanical stirring, whereas sonication leads
to the expected, though non-spontaneous, [4 + 2]
cycloadduct [39]. Probably, ultrasound promotes an
electron transfer from the diene, leading to its radical
under ultrasonic irradiation, despite being very sen-
sitive to heat and pressure. Some exceptions to this
rule were found by Snyder and his group in the
preparation of quinonoid natural products [37]. The
application of pressures of 10 kbar produces equiva-
lent results, in terms of yields and selectivities, to
those obtained during ultrasonic irradiation, even
with a simple cleaning bath. Accordingly, the pres-
sure effects of cavitation might be invoked in this
case, although the pressures reached in the cavita-
