Substitutions leading to carbon-heteroelement bonds

A Finkelstein substitution with practical utility was reported some years ago (Fig. 34).130 An co-bromo fatty acid undergoes halogen exchange with radioactive Na 23i to provide the labelled iodo acid, used as a tracer for the medical exploration of cardiac metabolism. The sonochemical reaction conducted in butanone provides quantitative yields of the expensive and thermally labile product, and the presence of water in the solvent, up to 7%, is tolerated. In comparison, the usual method requires heating at ca. 180 C and anhydrous conditions, and the purity of the product is much less satisfactory. It can be noticed that the unusually high temperature of the sonochemical reaction, close to the solvent boiling point, makes an interpretation hazardous. A second case of a Finkelstein substitution, a mesylate-iodine exchange, was published recently without experimental detail. i 

A sugar with a bromine atom at the anomeric position undergoes a clean substitution by an alcoholic group in the presence of mercuric cyanide. In comparison with the traditional method, the reaction is much faster and stereoselective (Eq. 23). Only the easily purified p-anomers are obtained, while the silent method gives mixtures. 

130 Mertens, I. Vanryckeghem, W. Bossuyt, A. Van der Winkel, P. Vanden Driessche, R. /. Label Comp. Radiopharm.l9S, 21, 843-856. 

The substitution of chlorine by azide (Eq. 24) in 4-pyridyl-chlorodiazirine was studied using isotopically labelled ions.l33 The SrnI mechanism was established from the label content of the final nitrile.i34 In the dark in DMSO solution, the sonicated reaction proceeds much faster than in the absence of ultrasound. The sensitivity to sonication was used to propose the SET mechanism, confirmed in further works. 

The substitution of aromatic halides by a diphenylphosphide ion in liquid ammonia occurs with higher yields under sonication (Eq. 25). The reactions were studied at 25°C under a 9-bar pressure and compared to their analogues under the same conditions of temperature and pressure without sonication. With 4-iodoanisole and 1-halonaphthalene, a substantial increase of the substitution product was noted. In parallel, the relative yield of the undesired dehalogenation reaction was decreased. 

---