Carbon tetrachloride sonolysis

Dauben et al. found that the CCI3 radical produced by sonolysis of carbon tetrachloride can be used in a decarboxylation-halogenation sequence (Scheme 3.5) [43]. Sonication of a thiohydroxamic ester at 33 °C for 10 - 50 min in carbon tetrachloride leads to the corresponding chloride in high yield. In the presence of bromotrichloromethane or iodoform, bromides and iodides are formed in yields > 80 %. This reaction can be successfully applied to primary, secondary, or tertiary esters and offers an interesting variant to the usual Hunsdiecker procedure. 

This reaction was reinvestigated recently in carbon tetrachloride solution at 20 kHz.57 Sonolysis of the solvent was not detected and, if present, does not induce any isomerization. Of the several alkyl bromides tried, f-butyl bromide is the most efficient, a result which is explained by its higher volatility and the lower dissociation energy of the C-Br bond. It was concluded that the rate limiting step is actually the sonolysis of the carbon-halogen bond inside the bubble.58... 

Similarly, the sonolysis of carbon tetrachloride in methanol gives hydrogen chloride used for the fast and selective deprotection of silyl ethers (Fig. 18). A vinylic methoxy group is kept intact under these conditions. Conventional methods require more sophisticated reagents to obtain an equivalent selectivity. Work-up is simplified, and a simple evaporation of the solvent provides the desired products sufficiently pure for most further uses. ... 

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