Astatine studies with

Meyer and Rossler 101) showed that the overall yields for wet and dry extractive procedures are comparable, being approximately 60% for Some workers have found that yields vary somewhat, due to adsorption of evaporated At onto vessel walls, and to the possibility of the retention of astatine within the target due to the formation of nonvolatile compounds 11). However, the dry evaporation method is more applicable to studies with high-activity targets it is rapid and lends itself to further development within the scope of remote handling techniques. Aspects of both extraction approaches have been discussed widely 2, 7, 33, 89,101,116,120,160). 

Therapeutic Studies with Astatine and Its Labeled Compounds... 

C. Chemical and Radiochemical Synthesis and Studies with Astatine-211 and Astatine-209... 

All isotopes of element 85, astatine, are intensely radioactive with very short half-lives (p. 795). As a consequence weighable amounts of the element or its compounds cannot be prepared and no bulk properties are known. The chemistry of the element must, of necessity, be studied by tracer techniques on extremely dilute solutions, and this introduces the risk of experimental errors and the consequent possibility of erroneous... 

Astatine is a radioactive element that occurs in nature in uranium and thorium ores, but only to a minute extent. Samples are made by bombarding bismuth with a particles in a cyclotron, which accelerates the particles to a very high speed. Astatine isotopes do not exist long enough for its properties to be studied, but it is thought from spectroscopic measurements to have properties similar to those of iodine. 

These studies show that radon can be classified as a metalloid element, together with boron, silicon, germanium, arsenic, antimony, tellurium, polonium, and astatine. Like these elements, radon lies on the diagonal of the Periodic Table between the true metals and nonmetals (Figure 5) and exhibits some of the characteristics of both (Stein, 1985). 

Numerous nuclear reactions have been employed to produce astatine. Three of these are particularly suited for routine preparation of the relatively long-lived isotopes with mass numbers 209, 210, and 211. The most frequently used is the ° Bi(a,xn) At (a = 1-4) reaction, in which bismuth 44, 74,120) or bismuth oxide (7,125) is bombarded by 21-to 40-MeV a-particles. The ° Bi(He, xn) At reaction can also be used to produce isotopes of astatine 152), the nuclear excitation functions (62) favor a predominant yield of ° At and °At. The routine preparation of astatine is most conveniently carried out through the ° Bi(a,xn) At nuclear reactions, from which a limited spectrum of astatine nuclides may be derived. The excitation functions for these nuclear reactions have been studied extensively (78, 89, 120). The... 

Only limited information about the electrochemical properties of astatine is available. The formation of a singly positively charged cation, At+, in aqueous and other solvent environments has been observed. A study in a nitric acid environment investigated the adsorption into ion exchange materials of this cation [206]. In a similar study in aqueous perchloric acid, the mobility of At+ was found to change with pH and the presence of a strongly water bound species, At(H20)2", with a pA of ca. 1.5 was proposed [207]. 

In 1940 D. R. Corson, K. R. Mackenzie, and E. Segre at the University of California bombarded bismuth with alpha particles (26, 27). Preliminary tracer studies indicated that they had obtained element 85, which appeared to possess metallic properties. The pressure of war work prevented a continuation of these studies at the time. After the war, the investigators resumed their work, and in 1947 proposed the name astatine, symbol At, for their element. The name comes from the Greek word for unstable, since this element is the only halogen without stable isotopes (28). The longest lived isotope is At210 with a half-life of 8.3 hours and a very high activity. 

An important conclusion drawn from the last two studies is that, with the help of trialkylstannyl derivatives, astatine can easily be incorporated into compounds with an aromatic ring or with an olefinic bond. This enables the method to be applicable for labelling a wide variety of biologically active species, drugs or proteins. 

The properties of organic astatine compounds applicable for biomedical studies have been investigated with special emphasis on the stability of the C—At bond under changing temperature and pH conditions, in the presence of oxidizing or reducing agents as well as—in some cases—in vivo. 

The most important transmutations by a particles are of the (a,p) and (a,n) types. The (a,p) processes (for example, Na23(a,p)Mg26) are common with targets of low atomic weights (Z > 25) as has been seen, these were the first artificial transmutations to be studied. The (a,n) reactions (and the closely related reactions in which two, three, or more neutrons are ejected by a particles of high energy) are of considerable interest in connection with the synthesis of the transuranium elements and of astatine (element 85). The following are typical and important examples ... 

The combination of triphenylphosphine with esters of trihaloacetic acids provides a reagent system for the stereo- and regio-selective conversion of alcohols into alkyl halides.The bromine-triphenylphosphine adduct has been used at low temperatures (-50 C in dichloromethane) for the removal of the tetrahydropyranyl protecting group from tetrahydropyranyl ethers derived from secondary and tertiary alcohols.The reactions of tertiary phosphines (and other trivalent phosphorus compounds) with iodine in aprotic solvents have received further study, a range of species being identified.The first reported study of the reactions of trivalent phosphorus compounds with monopositive astatine has led to the identification of stable complexes with triphenylphosphine, trioctylphosphine, and triethylphosphite. 

---