Sonochemical switching

A cornerstone in this evolution is the discovery of Sonochemical Switching , which established that sonication induces a specific reactivity. A classification of the sonochemical reactions was then proposed. These aspects constitute What ultrasonic waves can do. How and Why are only partially understood and are discussed case by case in the following descriptions. 

In a paper published in 1984,3 Ando et al provided unambiguous evidence that sonication is not (only) a more or less complicated agitation method. The original system used for the demonstration consisted of a suspension of alumina-supported potassium cyanide in toluenic benzyl bromide (Fig. 1). 

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There have been examples of sonochemical switching in homogeneous reactions. The decomposition of lead tetraacetate in acetic acid in the presence of styrene at 50 °C generates a small quantity of diacetate via an ionic mechanism. Under otherwise identical conditions sonication of the mixture gives 1-phenylpropyl acetate predominantly through an intermediate methyl radical which adds to the double bond (Scheme 3.8) [55,56]. These results are in accord with the proposition that radical processes are favoured by sonication. 

Another example of sonochemical switching is found in the Kornblum-Russell reaction (Scheme 3.9). 4-Nitrobenzyl bromide reacts with 2-lithio-2-nitro-propane via a predominantly polar mechanism to give, as a final product, 4-nitrobenzaldehyde [57]. An alternative SET pathway exists in this reaction leading to the formation of a dinitro compound. Sonication changes the normal course of the reaction and gives... 

Lead(IV) in acidic media has been found to promote oxidative addition of Cl-, CF3CO2-, AcO-, MeS03- and CIO4- to cyclohexene, 1-hexene and styrene433. Sonochemical switching from ionic to radical pathway in the reactions of styrene and fraws-/l-methylstyrene with (AcO Pb has been observed434. 

Ultrasound is defined as sound waves with frequencies in the range from 20 kHz to several megahertz. The term sonochemistry is used for the application of ultrasound to reaction systems with the aim to change reaction pathways. This so-called sonochemical switching was first described by Ando et al. [185] (Scheme 15.14). 

Ando, T., Sumi, S., Kawate, T. et al. (1984) Sonochemical switching of reaction pathways in solid-liquid two-phase reactions. J. Chem. Soc., Chem. Commun., 439-40. 

Ultrasound can have a profound effect on any chemical reactions taking place on a surface. One of the earliest reported examples of this was the sonochemical switching of a reaction pathway from a Friedel-Crafts catalytic reaction to one involving nucleophilic displacement. Ando et al. reported the efficient preparation of benzyl cyanide from benzyl bromide (70%) by direct reaction of the benzyl bromide with KCN in toluene in the presence of activated alumina and ultrasound [26]. This is an interesting reaction in that it is representative of a whole range of supported reactions which are enhanced by ultrasound. Without ultrasound the reaction follows a different course and the product formed at the same temperature and under otherwise identical conditions, is a mixture of the 2- and 4-benzyltoluenes produced as a result of a Friedel-Crafts reaction catalysed by the alumina (Scheme 10.11). 

An example of sonochemical switching was found with the sonolysis of iron pentacarbonyl (Fig. 23).Pyrolysis and photolysis form pyrophoric iron and diiron nonacarbonyl respectively, in contrast to sonolysis, which gives triiron dodecacarbonyl, a compound not easily accessible by other methods. A pyrolytic... 

Of greater interest is the sonolysis of lead tetraacetate (LTA) in the presence of styrene. i In this reaction, several products are formed, 1-phenylpropanol acetate via a radical pathway, and gem- and vie- diacetates via polar mechanisms (Fig. 27). 2 Conventionally, the free radical process is initiated photochemically. The cleavage of LTA to an acetoxyl radical is followed by decarboxylation and the resulting methyl radical adds to the C=C double bond. As expected, the radical process is favored by sonication. These reactions can be considered as an example of sonochemical switching. The mechanism is still unclear, and LTA was found to be stable when sonicated in the absence of styrene.53... 

Figure 27 - Sonochemical switching in the reaction of lead tetraacetate with styrene...

A similar conclusion can be drawn from an unexplained case of sonochemical switching (Fig. 29). The addition of primary aromatic amines to methyl pyruvate produces first the tautomeric imine-enamine condensation product. This step seems to be sonication independent. Under stirring at room temperature, the condensation goes on with the condensation of the enamine with the pyruvic ester to yield a lactone. In contrast, sonication under the same conditions... 

Sonochemical switching was observed in the Komblum-Russell reaction (Fig. 33). 2-Lithio-2-nitropropane is an ambident anion which reacts under stirring with 4-nitrobenzyl bromide at the oxygen atom via a predominant polar mechanism giving 4-nitrobenzaldehyde as the final product. In contrast, alkylation at the carbon atom occurs via a less favored SET pathway and provides a dinitro compound. 

Sonochemical switching was observed in the oxidation of primary alcohols with concentrated nitric acid (Fig. 36). Stirring of a solution of n-octanol and 60% nitric acid at room temperature produces a slow esterification reaction providing the nitrate quantitatively.Under sonication, the mixture immediately turns yellow green and gives octanoic acid in 100% yield. 

From the empirical systematization of sonochemistry, it is sufficient to remember that, when the possibility exists, radical pathways are privileged at the expense of polar pathways, and sonochemical switching can result in a number of cases. In heterogeneous systems, reactions which follow ionic mechanisms are still sensitive to the mechanical effects of sonication. This "false sonochemistry" can, in principle, also be observed when efficient mixing techniques are applied, but the so-called simple mechanical effects are strongly dependent on geometrical factors and prove to be much more complicated than expected. 

Lead tetraacetate (LTA) reacts with olefins via several mechanisms.23 The first one implies a methyl radical intermediate generated by the decomposition of LTA and results in the formation of a monoacetate after addition of the methyl group. Polar mechanisms give the gem- and pfc-diacetate. Sonochemical enhancement of the first process was predicted by Ando et ah, who indeed found an example of sonochemical switching when the reaction was performed on styrenes (Fig. 3). 24.25... 

An important work in this type of reactions was carried out by Ando et ah, who studied the effect of sonication on the substitution of halides in various compounds by several anions, acetate, thiocyanate, and cyanide supported on alumina, silica gel, or celite, /i46 nd discovered the principle of sonochemical switching. 

With these examples, the notion of sonochemical switching was introduced, which means that for the first time it was evidenced that the interaction of ultrasound with a reactive system is able to change the nature of the reaction products, not only to increase a rate or a yield. An application of this reaction was mentioned in a patent. It seems that a change in the mechanisms was not examined. The observation that electron acceptors such as nitroaromatics in catalytic amounts increase the yield of cyanide at the expense of the diphenyl-methane product was not exploited in this respect. The simultaneous occurrence of the destruction of acidic catalytic sites and the enhancement of a non-polar mechanism would constitute an interesting general interpretation. 

The reaction of alkoxytrimethylsilanes with benzenediazonium tetrafluoroborate constitutes a case of sonochemical switching (Fig. 30). Thermally, fluorobenzene is the major product, but under sonication, phenyl ethers are formed. The authors did not investigate a possible mechanistic change. However, the solvent used in the experiments. Freon 113, is an electron carrier (Ch. 2, p. 66) and aryl-diazonium salts generate radicals upon sonolysis. ... 

When stirred in toluene under oxygen with solid potassium hydroxide and PEGMe, 4-nitrotoluene couples to the bibenzyl and styrene products. This coupling does not proceed by a direct reaction of the 4-nitrobenzyl radicals. Sonication increases the conversion rate and yield and more importantly, leads to a different chemical evolution with the formation of 4-nitrobenzoic acid. This sonochemical switching cannot be fully interpreted in the absence of accurate kinetic measurements. Oxidation of the side chain in alkyl aromatics was studied... 

Sonication is generally beneficial to reductive processes of unsaturated bonds by various metals. The mechanism of these reactions is not always simple, as illustrated in the dissolving metal reductions of the carbonyl group. In any case, however, the first step is a sonication-dependent electron transfer. Though in a majority of cases the qualitative outcome in this family of reactions is not modified by sonication, sonochemical switching was found in one case. 

A sonochemical switching was found in the reaction of o-allyl benzamides with lithium (Fig. 20).When stirred, the reaction provides a-naphthol in high yield, but sonication drives the system towards methylindanone. This switching is especially important when the nitrogen atom bears bulky substituents. 

High-power, low-frequency waves are often associated with better mechanical treatment and less importantly with chemical effects. This rule, however, suffers exceptions, and sonochemical switchings can be observed, even in solutions, under low-frequency irradiation. A heterogeneous reaction with a metal was also reported to be improved by the use of a high-frequency irradiation. ... 

Some reactions are simply accelerated by application of acoustic waves to a reaction mixture no change in the nature of the products formed is associated with the sonochemical activation. Other reactions, in sharp contrast, display "sonochemical switching", which in a number of instances is a symptom of a class 1 reaction the application of ultrasonic waves may completely change the proportion of products or even cause the appearance of new compounds. 

Three cases of Type la activations illustrate a class of reactions expected to give positive results. The first one is provided by SrnI or ETC processes. Figure 1 shows the chain mechanism of the reaction of lithium nitronate with 4-nitro-benzyl bromide established by Komblum and Russell. This reaction was expected to display sonochemical switching, which was indeed foimd. The mechanism suggests that the sonochemical activation should find its origin either in creating species 1 or 2 (no direct entry to 3 seems plausible). The creation of 1 within a cavitation bubble could result either from high-pressure-promoted electron transfer (activation volumes for some electron transfer reactions may be found in Ref. 9) or local conditions at the interface between the cavitation bubbles and the bxilk solution (Qi. 1). The creation of radical 2 could result from a direct sonolysis of the benzylic C-Br bond (p. 86) but... 

Ultrasound is known to enhance the reaction rate, thus minimizing the duration of a reaction. A large number of published examples, which highlight this observation, are shown in Appendix 2.1. Apart from this, it is known to induce specific reactivity, known as "sonochemical switching." Ando et al. (1984) reported that benzyl bromide, on treatment with alumina impregnated with potassium cyanide, yielded benzyl cyanide on sonication, while, without sonication, on heating the reaction mixture yielded diphenylmethanes (see Fig. 2.2). This work was the first experimental evidence that ultrasonic irradiation induces a particular... 

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