Chemistry Reference
In-Depth Information
Scheme 16.12
The sonochemical
Barbier reaction.
Scheme 16.13
Sonochemical
preparation of metal reagents.
faces, the studies on the Barbier reaction clearly evi-
dence that electron transfer from the metal to the
halide is easier under sonication [56] (see Scheme
16.12).
The Barbier reaction with lithium works very well
and starts almost immediately under sonication,
whereas Barbier and Barbier-Grignard reactions
with magnesium often require long induction
periods. This metal undergoes fractures and surface
erosion before chemical reaction occurs, the role of
ultrasound being the creation of lattice-defect sites
where reaction takes place preferentially [57].
Recently, it has been demonstrated that Grignard
reagent formation involves diffusion steps—
reactions of radicals that diffuse in solution (the D
model)—rather than adsorption steps in which the
radicals remain adsorbed at the magnesium surface
until they react [58]. In other words, the reaction
would be more homogeneous than once thought
and this could explain why sonication activates the
reaction.
A direct consequence of the ultrasound-promoted
electron transfer is the facile preparation of radical
anions and their propagation reactions, as illustrated
here in the preparation of lithium amides. It is note-
worthy to point out the beneficial effect of an elec-
tron carrier like isoprene [59]. Similarly, a series
of useful organometallic reagents such as sodium
phenylselenide, the important hydride [(Ph
3
P)CuH]
and the versatile lanthanide SmI
2
can be prepared
easily by sonication upon addition of sodium/
benzophenone [60] (see Scheme 16.13). Because the
rate-limiting step of these processes is the transfer of
electrons from the metal surface, the set-up of the
reaction is due to the presence of the ketyl radical
anion, a fact that reinforces again the chemical role
of ultrasound. Thus, the sonochemical preparation of
SmI
2
can be conducted starting from samarium metal
and iodine in tetrahydrofuran (THF) using a simple
cleaning bath, which affords a yellow triiodide
within 5 min. Further addition of a catalytic amount
of mercury to the sonicated reaction leads to the
desired reagent in quantitative yield. The overall
process is complete in less than 30 min, which con-
trasts with the classical protocols performed under
an inert atmosphere, in dried solvents and requiring
longer reaction times [61].
Other interesting examples of heterogeneous
sonochemistry do not involve metals but inorganic
solids. Thus, Ando and co-workers reported one of
the first cases of sonochemical switching in the reac-
tion of benzyl bromide, potassium cyanide and
alumina, which when stirred mechanically in
toluene at 50°C gives rise to a mixture of
o
- and
p
-
benzyltoluene in 75% yield. In contrast, irradiation
with ultrasound (45 kHz) of the same reaction
mixture at 50°C yielded also benzyl cyanide in 71%
yield (see Scheme 16.14) [62].
