2.4 Sonochemical Approaches in Organic Synthesis
21
gies for difficult reactions (e.g. C–H activation, cross-dehydrogenative couplings in
aqueous media); (3) cross-couplings of unactivated partners in water, where the H 2 O
molecule is a competing nucleophile; and (4) direct transformations of renewable
biomass (especially lignins for the production of fuels and aromatic compounds),
and abundant feedstocks (CO 2 and CH 4 ), into high-value chemicals (Li et al. 2015;
Li 2017). These processes can be appropriately managed using enabling and intrinsically sustainable methods, such as microwaves, ultrasound, electrocells and photoreactors. The future of chemical synthesis—even synthesis on demand—will depend
on machines (or micromachines) in continuous flow, which can reduce the risk of
hazards and allow multistep and complex settings to be realized (Fitzpatrick et al.
2016).
As expected, the field of sonochemistry has already focused its attention on the
above-mentioned targets and interested readers are referred to a number of comprehensive works that deal with the subject in detail (Luche 1998; Cella and Stefani 2009;
Cintas et al. 2011; Mojtahedi and Abaee 2011; Shingare and Shingate 2011; Chatel
and MacFarlane 2014; Tagliapietra et al. 2015; Chatel 2017; Domini et al. 2017;
Lupacchini et al. 2017). This volume will look at two specific synthetic applications,
namely reactivity in neoteric media, ionic liquids in particular (Chap. 3), and biomass
valorization (Chap. 4), in later chapters, although some overlap with this chapter is
inevitable. We shall now concentrate on some synthetic situations where sonication
shows distinctive or improved results over the non-irradiated procedure. Illustrative
examples have been restricted to coupling reactions that lead to carbon–carbon and
carbon–heteroatom bond formation and include heterocyclic assembly via one-pot
and multicomponent reactions, as well as oxidations and reductions, which all show
high levels of atom economy and mass efficiency, making them suitable for combinatorial and parallel syntheses in the pharmaceutical industry. Transformations in aqueous media are noticeable as sonication has proven to be advantageous by favouring
mixing and influencing hydrophobic interactions. Finally, before concluding, a short
section will be devoted to sono-electro-organic synthesis. Sono-electrochemistry was
actually one of the first combinations with ultrasound leading to various successful
applications (Pollet and Hihn 2011). This subdiscipline deserves a monograph on
its own and, obviously, it cannot be treated here in detail. We shall highlight a few
references that illustrate various synthetic applications of current interest.
2.4.1 Catalytic and Non-catalytic Couplings
The rapid micromixing and thermal conditions caused by sonication-pulse-driven
bubble collapse are sufficient to promote numerous catalyst-free organic reactions
(Banerjee 2017a). In fact, ultrasound alone can replace the action of phase-transfer
catalysts by efficiently disrupting the organic–water interface in two-phase reactions.
A sonochemical variation of ring-closing metathesis (RCM), a powerful strategy in synthetic chemistry, can be advantageously performed in water at ambient
temperature without surfactants and organic co-solvents (Gułajski et al. 2008). The
21
gies for difficult reactions (e.g. C–H activation, cross-dehydrogenative couplings in
aqueous media); (3) cross-couplings of unactivated partners in water, where the H 2 O
molecule is a competing nucleophile; and (4) direct transformations of renewable
biomass (especially lignins for the production of fuels and aromatic compounds),
and abundant feedstocks (CO 2 and CH 4 ), into high-value chemicals (Li et al. 2015;
Li 2017). These processes can be appropriately managed using enabling and intrinsically sustainable methods, such as microwaves, ultrasound, electrocells and photoreactors. The future of chemical synthesis—even synthesis on demand—will depend
on machines (or micromachines) in continuous flow, which can reduce the risk of
hazards and allow multistep and complex settings to be realized (Fitzpatrick et al.
2016).
As expected, the field of sonochemistry has already focused its attention on the
above-mentioned targets and interested readers are referred to a number of comprehensive works that deal with the subject in detail (Luche 1998; Cella and Stefani 2009;
Cintas et al. 2011; Mojtahedi and Abaee 2011; Shingare and Shingate 2011; Chatel
and MacFarlane 2014; Tagliapietra et al. 2015; Chatel 2017; Domini et al. 2017;
Lupacchini et al. 2017). This volume will look at two specific synthetic applications,
namely reactivity in neoteric media, ionic liquids in particular (Chap. 3), and biomass
valorization (Chap. 4), in later chapters, although some overlap with this chapter is
inevitable. We shall now concentrate on some synthetic situations where sonication
shows distinctive or improved results over the non-irradiated procedure. Illustrative
examples have been restricted to coupling reactions that lead to carbon–carbon and
carbon–heteroatom bond formation and include heterocyclic assembly via one-pot
and multicomponent reactions, as well as oxidations and reductions, which all show
high levels of atom economy and mass efficiency, making them suitable for combinatorial and parallel syntheses in the pharmaceutical industry. Transformations in aqueous media are noticeable as sonication has proven to be advantageous by favouring
mixing and influencing hydrophobic interactions. Finally, before concluding, a short
section will be devoted to sono-electro-organic synthesis. Sono-electrochemistry was
actually one of the first combinations with ultrasound leading to various successful
applications (Pollet and Hihn 2011). This subdiscipline deserves a monograph on
its own and, obviously, it cannot be treated here in detail. We shall highlight a few
references that illustrate various synthetic applications of current interest.
2.4.1 Catalytic and Non-catalytic Couplings
The rapid micromixing and thermal conditions caused by sonication-pulse-driven
bubble collapse are sufficient to promote numerous catalyst-free organic reactions
(Banerjee 2017a). In fact, ultrasound alone can replace the action of phase-transfer
catalysts by efficiently disrupting the organic–water interface in two-phase reactions.
A sonochemical variation of ring-closing metathesis (RCM), a powerful strategy in synthetic chemistry, can be advantageously performed in water at ambient
temperature without surfactants and organic co-solvents (Gułajski et al. 2008). The