Preparation of Tritium‐Labeled Compounds by Chemical Synthesis

Abstract

In contrast to isotope exchange methods, which utilize the target compound or a late intermediate as substrate for labeling, chemical synthesis involves transformation of some precursor chemical with concurrent introduction of tritium. Four basic types of transformations are distinguished for discussion each in turn. They are catalytic tritiations (Section 4.1), which usually involve the reduction of a precursor compound using a catalyst and tritium gas; catalytic tritiolyses (Section 4.2), involving the replacement of functional groups by tritium, again using a catalyst and tritium gas; tritide reductions (Section 4.3), utilizing tritiated versions of hydridic reducing reagents; and the use of small tritiated building blocks (Section 4.4) in synthetic operations to construct target molecules.A phenomenon strongly relevant to the catalytic methods discussed in Sections 4.1 and 4.2, and of some relevance also for tritide chemistry (Section 4.3) is solvent exchange. Most catalytic tritiations and tritiolyses are conducted in solution, and a wide variety of solvents have been used, ranging from nonpolar ones to highly polar, hydroxylic media. The properties of the solvent, however, influence not only the course and rate of a heterogeneously catalyzed reaction, but also the extent to which solvent hydrogen may exchange with tritium gas as a side reaction and thereby dilute the isotope introduced into the substrate: solvent exchange. It is commonly recognized that the use of protic solvents for tritiations and tritiolyses increases the risk of obtaining products of less than maximal specific activity, although it is equally clear that such reactions often succeed without dilution of label. The crucial factor in each experiment is the relationship between the rate of the tritium-incorporation reaction and the rate(s) of any tritium-diluting reaction(s). It is documented that the rate of metal-catalyzed exchange between elemental hydrogen and water is significant, especially at low pH 1 , but little quantitative data are available to guide practice. Recently a turnover number of 17.3 h À1 was reported for the exchange between H 2 and 2 H 2 O catalyzed by 10% Pd/C at room temperature and a pressure of 1 atm 2 ; this is rapid enough to provide the basis for an economical method of catalytic deuterium exchange Preparation of Compounds Labeled with Tritium and Carbon-14 Rolf Voges, J. Richard Heys and Thomas Moenius