Catalytic tritiolyses

In contrast to tritiations (reductions in which tritium is added to unsaturated substrates) tritiolyses are reductive reactions in which tritium replaces another atom or group. The most common class of tritiolyses is replacement of halogen (tritiodehalogenations), but some use 

The most active catalysts for many tritiolyses are palladium metal (freshly prepared from PdO) or palladium supported on charcoal, barium sulfate or aluminum oxide, but palladium catalysts are sometimes associated with side reactions such as O- and /V-debenzylations, isomerizations and labeling of certain sites by catalytic exchange (see Chapter 3, Section 3.3). For example, in the case of [ H]papaverine (10 and the [ H]pargyline precursor 

As with tritiations of carbon-carbon multiple bonds, catalytic tritiodehalogenations are favored by polar and protic solvents, such as methanol, ethanol, and water. On the other hand, catalyst-mediated /H exchange between solvent and tritium gas may give rise to a 

Tritiodehalogenation of appropriately chlorinated, brominated or iodinated precursors over PdO or Pd/CaC03 has emerged as a main source for commercially available tritiated nucleosides and nucleotides of high specific activities such as [2- H] adenosine (20-25 

Ci/mmol), [8- H]adenosine (20-30 Ci/mmol), [5- H]cytidine (10-30 Ci/mmol), 2 -deoxy [5- H]cytidine (10-30Ci/mmol), 2 -deoxy[5- H]uridine, (5-15 Ci/mol), [8- H]guanosine (10-30 Ci/mmol), [methyl- HJthymidine (25-30 Ci/mmol), and [5- H]uridme . In some cases simultaneous incorporation of tritium into remote positions by H/H exchange processes has been reported. For example, incorporation exclusively into adenine-C2 has recently been accomplished during the tritiodehalogenation of a bromoadenosine-5 -triphosphate in phosphate buffer pH 8.6 in the presence of PdO as a catalyst . 

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Figure 4.41 lodo-bromo and chloro-fluoro selectivity in catalytic tritiolyses... 

Selectivity between chloro and fluoro in tritiolyses is not a problem because of the inertness of the latter halogen to catalytic reduction under mild conditions. A simple example is (45)-8-chloro-6-fluorospiro[chroman-4,4 -imidazohdine]-2, 5 -dione 11371. reported to have been tritiodechlorinated ( H2, 10% Pd/C, 15 equiv. EtaN, EtOH r.t., 18 h) to give the corresponding 6-fluoro[8- H]derivative at a specific activity of about lOCi/mmol. ... 

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