Tritiolyses

Analog erhalt man nach Tritiolyse mit Essigsaure-O-T trans-1 -Tritio-hexen-(l)4. 

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... 

Although the presence or absence of base is often unimportant for iodo-vs-chloro selectivity, it is often crucial for bromo-vs-chloro selectivity. Selective tritiodebromination in the presence of chloro substituents was observed in the tritiolyses of 127 ( 2 EtOH/ THF, 10% Pd/C) and of 128 ( H2, DMF, 10% Pd/C) in the absence of added base, giving monotritiated dichloro products. However, when the reactions were performed with added triethylamine, the carbon-chlorine bonds also underwent tritiolysis. Similarly, with ethanol as solvent and no added base, treatment of 129 ( Hj, EtOH, 10% Pd/C) gave only tritiodebromination and the chloro substituent was preserved the effect (if any) of added base was not reported in this case. An alternative approach recommended the use of Pd/BaS04 in the absence of a base to achieve high bromo-vs-chloro selectivity for a series of compounds including 130 and 131 ( H2, 5% Pd/BaS04, EtOH or DMF). ... 

While there are numerous reports of successful bromo-vs-chloro selectivity in tritiolyses, selective tritiodeiodination in the presence of bromine seem to be more difficult and less promising. So far only the Raney nickel- promoted tritiodeiodination of 4-bromo-2-iodo-6-trifluoromethylaniline 11361 has been published ( H2, Raney nickel, NaOH) ° (Figure 4.41). 

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. ... 

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

Figure 4.42 Deuteriolyses and tritiolyses in the presence of other reducible functions...

Figure 4.46 Selective deuteriodehalogenations catalyzed by Pd(PPhj)4 4.2.2 Tritiolyses of Benzylic N- and O-Functions...

Palladium catalysts are well known to cleave N- and 0-benzyl moieties this can either be an opportunity or a serious side reaction in palladium-catalyzed tritiolyses of halo compounds. Again, the presence of an appropriate base may enhance the desired selectivity. In the case of 159 (X = Br), tritiodebromination over 5% Pd/C in ethanol in the absence of added base produced significant amounts of debenzylated bromo compound 160. whereas reduction in pyridine provided exclusively 159 (X = H) in excellent yield and at high specific activity °. Analogous results were obtained with the compound 134 above. In contrast, conditions giving satisfactory selectivity between tritiodebromination of 161 could not be found and the more reactive iodo analog 162 had to be resorted to (6kPa H2, Pd(OH)2, EtOH,... 

Applied to 5-(l,3-dithiacyclopent-2-yl)uracil 11681 and 3/3-acetoxy-7-spiro(l, 3 -dithia-cyclopent-2 -yl)cholest-5-ene (170), Raney nickel tritiolyses furnished [5-methyl- H]-thymine 71691 and [7- H]cholesteryl acetate 71711 in moderate yields with specific activities of 11.6 Ci/mmol and 8.1 Ci/mmol, respectively. ... 

Figure 4.60 Palladium(O) modification of sodium borotritide for tritiolyses of aryl and alkyl halides...

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