Sonolysis halides

Another example is the influence of ultrasonic sound treatment. In chlorinated or bromi-nated solvents it leads to extreme rate accelerations and higher selectivities (Table 6)84. This observation was explained by the formation of hydrogen halide from the sonolysis of the solvent molecules, followed by protonation of the dienophiles and ordinary acid catalysis. Nevertheless, although there are quite a few aspects of the Diels-Alder reaction which are not totally understood, the general mechanisms leading to selectivities and catalysis are clear. 

From the sonolysis of haloforms, either pure or in an organic solution, hydrogen halides are formed in amounts sufficient to induce several reactions (pp. 68 and 99). In non-volatile, viscous solutions, for instance ethylene glycol, lO M solutions of chloromethanes are totally decomposed in times as short as 10 min. 

Figure 24 - Sonolysis of dinuclear complexes to metal carbonyl halides...

Applications of the selective sonolysis of transition metal complexes are not numerous despite the obvious synthetic potential (Fig. 25). The palladium catalyzed addition-oxidation of halides to olefins (the Heck reaction) in its intramolecular version is improved by sonication. The sonochemical reaction can be conducted at lower temperatures than the silent one, and the usual deiodinated by-products are formed in smaller amounts (see p. 343). ... 

However, it seems that these effects are due to a solvent sonolysis, producing hydrogen halides (p. 60), the actual catalyst of the cycloaddition. The conclusion of the authors was the absence of a direct sonochemical effect. This question will be discussed below. 

Other workers have reported that prolonged sonolysis of organolithiums derived from organic halides [94], chlorosilanes and chlorostannanes [95] results in Wurtz-type coupling. Yields are moderate and the reactions are of little synthetic interest. However, coupling of dichlorosilanes and stannanes produces a novel route to the cyclic polysilanes (9) and (10) (Scheme 37). The product obtained is determined by the steric bulk of the alkyl groups and only low levels of contamination by other silanes is observed [95]. Silylene intermediates did not appear to be involved. However, Boudjouk et al. [101] later reported formation of the tetramesityl silylene (11) which had previously been made by photolysis of (Mes)2Si(TMS)2 [102] (Scheme 38). 

The well-documented effects of ultrasound on heterogeneous systems have been exploited to produce beneficial increases in the rates of a number of reactions that would more commonly be executed under homogeneous conditions. For instance, the reduction of aryl halides with lithium aluminium hydride occurs extremely slowly in THF solution. However, Han and Boud-jouk have reported that the reaction can be carried out in a fraction of the time if the reaction mixture is sonicated in dimethoxyethane (DME) solution [6]. Sonolysis of the heterogeneous reaction mixture gives high yields (70-99 %) of the reduced product. The effect was particularly marked in the case of deactivated aryl halides for example, p-bromotoluene gave a 97 % yield of toluene in 5 h. In contrast, the yield of the stirred reaction was a modest 21 % after 24 h at room temperature in THF (Scheme 112). 

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