Ultrasound perfluoroalkylation

Under sonication conditions, the reaction of perfluoroalkyl bromides or iodides with zinc can be used to effect a variety of functionalization reactions [39, 40, 41, 42] (equation 30) Interestingly, the ultrasound promoted asymmetric induction with the perfluoroalkyl group on the asymmetric carbon was achieved by the reaction of perfluoroalkyl halides with optically active enamines in the presence of zinc powder and a catalytic amount of dichlorobisftc-cyclopenta-dienyl)titanium [42] (equation 31)... 

Despite the lower reactivity of solvated perfluoroalkylzinc reagents, perfluoroalkyl iodides undergo synthetically useful zinc-mediated reactions under Barbier conditions which often employ ultrasound and co-catalysts. Under these conditions, the zinc reagents are not well characterized and radical intermediates and SET mechanisms are proposed in some cases. Representative examples are presented below and include the ultrasound-promoted, zinc-mediated perfluoroalkylation of various substrates as reported by Ishikawa and coworkers (Scheme 10)123 126. Yields of carbinols could be improved by use of Ti(II) co-catalyst. Ultrasound promoted the coupling of perfluoroalkyl iodides with vinyl and allyl halides in the presence of Pd co-catalysts. 

Iron (23-25). Ultrasound-promoted reactions were also applied to the syntheses and rate enhancements of fluorinated compounds such as perfluoroalkyl iodides in the presence of zinc and carbon dioxide to give perfluoroalkanoic acids (26,27). 

The direct carbojgdation of perfluoroalkyl iodides has been reported to afford RFCO2H in the presence of a Zn-Cu couple [8] or that of zinc and ultrasounds [9]. To our knowledge, no reports on the carbon dioxide fixation into perfluoroalkyl olefins have been yet described. 

Perfluoroalkylzinc iodides, generated in situ from the iodides and zinc powder under ultrasound irradiation, couple smoothly with alkenyl bromides in the presence of Pd(PPha)4 as catalyst, linear and branched perfluoroalkyl groups being coupled with retention of the ( )-alkene stereochemistry (equation 49). ... 

The use of ultrasounds to aid the extraction is another possibility. Phthalates (Abb et al. 2009) and perfluorinated alkanoic acid (PEA) (Kato et al. 2009) determinations have been reported. Ethyl acetate (phthalates) or formic acid with methanol (perfluoroalkyl chemicals) was added to the dust sample, and extraction was performed in an ultrasonic bath at room temperature for 10 min. In the case of phthalates an enrichment step was not carried out to avoid the risk of contamination. Pyrethroids and their metabolites can also been extracted from indoor dust samples by adding methylene chloride followed by sonication for 10 min (Starr et al. 2008). Another option is the combination of Soxhlet extraction with an ultrasonic treatment (Schecter et al. 2009). 

Kitazume and Ishikawa have published a number of communications on the regiospecific zinc mediated perfluoroalkylation of a variety of substrates [148-151]. A compilation of these results appeared in 1985 [152] (Scheme 71). Secondary alcohols are available in moderate yield by reaction of perfluoro-alkyl halides (RpX) with aldehydes as the result of a modified Barbier-type reaction [148-152]. However, reaction with ketones gave low yields of the expected tertiary alcohols, although the yields could be doubled by reaction in the presence of a bis[7i-cyclopentadienyl]titanium(II) catalyst, which was prepared in situ by reduction of the dichloride with zinc in the presence of ultrasound. 

Ultrasound can give useful improvements in the yield, reaction time, and convenience of organometallic reactions, and these facets of the technique are demonstrated well in its application to the Reformatsky reaction. Yields are improved by up to almost 100 % over the conventional method, and are nerally superior even to those of the trimethyl borate method. Reaction times are reduced by a factor of 5-10, and the need to prepare zinc powder by the active-metal reduction of anhydrous zinc chloride is eliminated. This technique has also been applied to the zinc-mediated dimerization of a,a -dibromo -xylene, although only low yields are obtained, and to the Pd-catalysed zinc coupling of allyl or aryl halides with perfluoroalkyl iodides. ... 

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