Astatination nucleophilic

Nucleophilic astatination of halobenzenes CgHjX (X = Cl, Br, I) in homogeneous mixtures with -C4H9NH2, (C2H5)2NH, and (C2Hj>3N at 210° C has led to the formation of astatobenzene with radiochemical yields of 75-90% 143, 144). A two-step process has been postulated [Eqs. (10) and (11)], with the latter reaction [Eq. (11)] as the ratedetermining step ... 

Astatination of the diazonium salt of 4-amino-methylene blue led to only 10% yield of 4-astato-methylene blue (VI). Alternatively, 4-astato-methylene blue was rapidly synthesized by heterogeneous nucleophilic isotopic exchange in the presence of 18-crown-6 ether at 80° C, with 4-iodo-methylene blue. The product was separated and identified by TLC yields ranged from 50 to 65% 41). Additionally, 2-astato-8-iodo-methylene blue (VII) has similarly been prepared in 60% yield from 2,8-diiodo-methylene blue 41). 

Astatination by means of nucleophilic halogen exchange, occasionally with the help of catalysts, and electrophilic replacement via demetalation seem to have become the preferred techniques. Short synthesis and separation time together with the possibility of carrier-free preparation of labelled compounds are especially important factors, bearing in mind the short half-life of astatine isotopes and the requirement of high specific activity for chemical and biomedical investigations. 

---