2.4 Sonochemical Approaches in Organic Synthesis
33
Fig. 2.18 Selective oxidations of carbohydrate derivatives under ultrasonic activation
ture prior to the addition of sucrose. Moreover, the results were dependent on the
chosen frequency and output power (Fig. 2.18). This paper also reported the heterogeneous oligomerization of glucose with hydrophobic alcohols, whose acceleration
was attributed to enhanced sonomixing. The same group underlined the role that frequency effects play during the oxidation of other glucosides using TEMPO-mediated
oxidation in the presence of NaOCl/NaBr (Brochette-Lemoine et al. 1999). The catalytic cycle under ultrasound produces the nitrosonium ion as the active oxidizing
species, making the presence of a stoichiometric amount of NaBr unnecessary, and
thus reducing toxicity. More recently, a selective oxidation of d-glucose to gluconic
acid under mild and economic conditions (H 2 O 2 , FeSO 4 , room temperature), with
ultrasound (20 kHz, 0.25 W/mL), has been reported (Rinsant et al. 2014). Almost
quantitative yields were obtained in 15 min at 25 °C. This transformation is a case of
a sono-Fenton reaction, where ultrasound favours the in situ generation of hydroxyl
radicals (HO), via reaction between H 2 O 2 and Fe(II) ions. As proof of the mechanism, a radical scavenger (t-butyl alcohol), was used to quench the reaction, which
then yielded less than 1% of gluconic acid.
The above-mentioned oxidations are usually performed with sonic horns, which
can deliver energy at known frequencies and high power intensities. These aspects
and their reproducibility may be problematic when using ultrasonic baths (regrettably some past papers on carbohydrate sonochemistry do not mention the working
frequency of the baths). Nevertheless, the relatively low acoustic energy of ultrasonic baths is not necessarily a major drawback in many syntheses and often allows
sensitive reagents and enzymes to be used. Unsuccessful protocols should however
be redesigned for use with probes. An example of the benefits of ultrasonic baths can
33
Fig. 2.18 Selective oxidations of carbohydrate derivatives under ultrasonic activation
ture prior to the addition of sucrose. Moreover, the results were dependent on the
chosen frequency and output power (Fig. 2.18). This paper also reported the heterogeneous oligomerization of glucose with hydrophobic alcohols, whose acceleration
was attributed to enhanced sonomixing. The same group underlined the role that frequency effects play during the oxidation of other glucosides using TEMPO-mediated
oxidation in the presence of NaOCl/NaBr (Brochette-Lemoine et al. 1999). The catalytic cycle under ultrasound produces the nitrosonium ion as the active oxidizing
species, making the presence of a stoichiometric amount of NaBr unnecessary, and
thus reducing toxicity. More recently, a selective oxidation of d-glucose to gluconic
acid under mild and economic conditions (H 2 O 2 , FeSO 4 , room temperature), with
ultrasound (20 kHz, 0.25 W/mL), has been reported (Rinsant et al. 2014). Almost
quantitative yields were obtained in 15 min at 25 °C. This transformation is a case of
a sono-Fenton reaction, where ultrasound favours the in situ generation of hydroxyl
radicals (HO), via reaction between H 2 O 2 and Fe(II) ions. As proof of the mechanism, a radical scavenger (t-butyl alcohol), was used to quench the reaction, which
then yielded less than 1% of gluconic acid.
The above-mentioned oxidations are usually performed with sonic horns, which
can deliver energy at known frequencies and high power intensities. These aspects
and their reproducibility may be problematic when using ultrasonic baths (regrettably some past papers on carbohydrate sonochemistry do not mention the working
frequency of the baths). Nevertheless, the relatively low acoustic energy of ultrasonic baths is not necessarily a major drawback in many syntheses and often allows
sensitive reagents and enzymes to be used. Unsuccessful protocols should however
be redesigned for use with probes. An example of the benefits of ultrasonic baths can