Chemistry Reference
In-Depth Information
Scheme 16.6
Switching of a hetero-Diels-Alder reaction.
Scheme 16.7
Sonolysis of iron
pentacarbonyl and trapping reactions.
O
O
OH
OCH
3
OCH
3
+
(OC)
5
Cr
CH
3
CH
3
H
Ac
2
O, Et
3
N, THF, reflux, 5 h
2%
C
6
H
6
, Et
3
N, 3 h,
Scheme 16.
8
Enhanced formation of
b
-lactones by sonolysis of metal
carbenes.
34%
cation, or the removal of dissolved gases is made
easier by sonication. In fact, when the reaction was
stirred mechanically after careful oxygen exclusion,
the [4 + 2] cycloadduct was obtained in good yields
(see Scheme 16.6).
Both yields and
endo/exo
selectivities in the
Diels-Alder reactions of cyclopentadiene with
methyl vinyl ketone in halogenated solvents are
enhanced largely by sonication [40]. However, these
results are presumably due to the catalytic effect of
Lewis acids (TiCl
4
, TiBr
4
) generated in situ by the
interaction of halogen radicals, resulting from
solvent pyrolysis, with the titanium horn.
As noted before, the sonolysis of metal complexes
and organometallics should give rise to coordina-
tively unsaturated transient species; mechanistic
switching also can occur. Well-documented exam-
ples were reported by Suslick and his associates
in the sonolysis of iron pentacarbonyl, leading to
cluster formation; remarkably, photo- and thermo-
lysis produce different products [41]. The selective
formation of the cluster Fe
3
(CO)
12
is increased
by decreasing the cavitation energy, i.e., increasing
the solvent vapour pressure. The coordinated inter-
mediate species, which are generated in the cavita-
tion bubble and in the superheated liquid shell
surrounding it, can be trapped by external ligands
such as alkenes, and reaction rates are about 10
5
higher than the silent process (see Scheme 16.7)
[42].
Condensation of chromium alkyl(alkoxy)carbene
complexes with propargylic alcohols gives rise to
functionalised b-lactones [43]. Sonication provides
a more effective procedure, particularly in terms of
accessing the less heavily substituted b-lactones,
for which the non-irradiated reaction gives poor
yields (see Scheme 16.8). This can be attributed to
the rapid reaction with the less-stable alkyl carbene
