34
2 Efficient Organic Synthesis: What Ultrasound Makes Easier
be found in the selective and reproducible room-temperature oxidation of glucose to
gluconic acid (100% conversion in 15 min), in which the low acoustic energy protects
the active gold catalyst (Au/SiO 2 ), and substrates from decomposition. This same
catalytic system, when applied to cyclohexene oxidation, leads to the corresponding
alcohol in 55% conversion at room temperature, and 68% conversion at 80 °C (Bujak
et al. 2012).
2.4.4 Sono-Electro-Organic Syntheses
In recent years the old field of electrochemistry, dating back to pioneering studies in
physical chemistry at the dawn of the twentieth century, has received considerable
attention as a mild, versatile and sustainable technique in organic synthesis with a
focus on the preparation of fine chemicals and drugs (Yan et al. 2017; Atobe et al.
2018; Mitsudo et al. 2018; Moeller 2018). Both anodic oxidations and cathodic
reductions involve single electron transfer (SET) reactions, which in line with the
rules of true sonochemistry, could be further enhanced by ultrasonic irradiation. Very
often, however, this important point is not evaluated by mechanistic studies. In fact,
most effects should be ascribed to improved mass transfer from solution to electrodes
caused by the more efficient ultrasonic stirring (Pollet and Hihn 2011).
Perhaps the most known electrochemical transformation is the Kolbe reaction,
where the anodic oxidation of alkyl carboxylates releases alkyl radicals that then
dimerize to form a new carbon–carbon bond. In the classical Kolbe electrolysis,
a base is added to generate carboxylate salts serving as substrates and electrolytes.
The aqueous medium complicates the transformation of poorly soluble organic compounds as well as the subsequent workup and product isolation. Compton and his
group developed an aqueous protocol for water-immiscible aliphatic acids through
sonication and emulsion formation (Wadhawan et al. 2001). Unlike homogeneous
reactions, yields in this biphasic system were independent of the electrode material.
The positive role of sonication via emulsification reactions is also evidenced in a
useful allylation reaction performed in an ionic liquid (1-ethyl-3-methylimidazolium
BF 4 ). The nucleophile (allyl trimethylsilane) is scarcely soluble in that medium, but
acoustic emulsification favours its condensation with an iminium electrophile generated anodically (Asami et al. 2008). It should be noted that mechanical stirring
(1500 rpm) was unable to generate a stable emulsion due to the high viscosity of the
ionic liquid, while ultrasonication (20 kHz, 150 W/cm
2 ) triggered an efficient coupling. A sono-electrochemical reaction using Et 3 N-3HF ionic liquid was successfully
applied to a Pummerer-type fluorination of organosulfur compounds having electronwithdrawing groups at the α-carbon in moderate to good yields (Sunaga et al. 2009).
Again, sonication improved both yields and selectivity (mono- vs. di-fluorination)
relative to mechanical stirring (Fig. 2.19). The best selectivity (quantitative monofluorination) was achieved at lower current density (2.5 mA/cm
2 ), although high current
density (50 mA/cm
2 ) still afforded good selection and overall yields.
2 Efficient Organic Synthesis: What Ultrasound Makes Easier
be found in the selective and reproducible room-temperature oxidation of glucose to
gluconic acid (100% conversion in 15 min), in which the low acoustic energy protects
the active gold catalyst (Au/SiO 2 ), and substrates from decomposition. This same
catalytic system, when applied to cyclohexene oxidation, leads to the corresponding
alcohol in 55% conversion at room temperature, and 68% conversion at 80 °C (Bujak
et al. 2012).
2.4.4 Sono-Electro-Organic Syntheses
In recent years the old field of electrochemistry, dating back to pioneering studies in
physical chemistry at the dawn of the twentieth century, has received considerable
attention as a mild, versatile and sustainable technique in organic synthesis with a
focus on the preparation of fine chemicals and drugs (Yan et al. 2017; Atobe et al.
2018; Mitsudo et al. 2018; Moeller 2018). Both anodic oxidations and cathodic
reductions involve single electron transfer (SET) reactions, which in line with the
rules of true sonochemistry, could be further enhanced by ultrasonic irradiation. Very
often, however, this important point is not evaluated by mechanistic studies. In fact,
most effects should be ascribed to improved mass transfer from solution to electrodes
caused by the more efficient ultrasonic stirring (Pollet and Hihn 2011).
Perhaps the most known electrochemical transformation is the Kolbe reaction,
where the anodic oxidation of alkyl carboxylates releases alkyl radicals that then
dimerize to form a new carbon–carbon bond. In the classical Kolbe electrolysis,
a base is added to generate carboxylate salts serving as substrates and electrolytes.
The aqueous medium complicates the transformation of poorly soluble organic compounds as well as the subsequent workup and product isolation. Compton and his
group developed an aqueous protocol for water-immiscible aliphatic acids through
sonication and emulsion formation (Wadhawan et al. 2001). Unlike homogeneous
reactions, yields in this biphasic system were independent of the electrode material.
The positive role of sonication via emulsification reactions is also evidenced in a
useful allylation reaction performed in an ionic liquid (1-ethyl-3-methylimidazolium
BF 4 ). The nucleophile (allyl trimethylsilane) is scarcely soluble in that medium, but
acoustic emulsification favours its condensation with an iminium electrophile generated anodically (Asami et al. 2008). It should be noted that mechanical stirring
(1500 rpm) was unable to generate a stable emulsion due to the high viscosity of the
ionic liquid, while ultrasonication (20 kHz, 150 W/cm
2 ) triggered an efficient coupling. A sono-electrochemical reaction using Et 3 N-3HF ionic liquid was successfully
applied to a Pummerer-type fluorination of organosulfur compounds having electronwithdrawing groups at the α-carbon in moderate to good yields (Sunaga et al. 2009).
Again, sonication improved both yields and selectivity (mono- vs. di-fluorination)
relative to mechanical stirring (Fig. 2.19). The best selectivity (quantitative monofluorination) was achieved at lower current density (2.5 mA/cm
2 ), although high current
density (50 mA/cm
2 ) still afforded good selection and overall yields.