Heterogeneous catalytic deuteration

The most commonly used catalysts are palladized charcoal or calcium carbonate and platinum oxide. For better isotopic purity, the use of platinum oxide may be preferred for certain olefins since the substrate undergoes fewer side reactions while being chemisorbed on the platinum surface as compared to palladium.Suitable solvents are cyclohexane, ethyl acetate, tetrahydrofuran, dioxane or acetic acid-OD with platinum oxide. 

The saturation of linear dienones provides a means of labeling even more remote sites. Labeling of the C-2 and C-3 positions of androstan-7- 

A large number of examples have been reported in the literature for the deuteration or tritiation of double bonds and for the hydrogenation of enones labeled at an allylic position. The following list of substrates, in addition to those already discussed, provides a representative cross section  

A solution of 4,4-dimethyl-5a-androst-l-en-3-one (128, 14 mg) in cyclohexane (3 ml) is stirred in a microhydrogenation apparatus in the presence of 10 % palladium-on-charcoal (15 mg) at atmospheric pressure and room temperature. The uptake of one eq of deuterium (1.15 ml) is complete in about 1 min and no more deuterium is consumed. After 5 min the catalyst is removed by filtration, and the solvent evaporated under reduced pressure. The resulting l , 2 -d2-4,4-dimethyl-5a-androstan-3-one (129, 13 mg, 93%), mp 120-122°, exhibits 87% isotopic purity and 13% d species.  

When the l , 2( -d2-labeled product (129) is subjected to alkaline equilibration to back exchange the 2i -label (for experimental conditions see section IT-B), the crystalline l -di-4,4-dimethyl-5a-androstan-3-one (130) exhibits 6% do and 94% d isotopic composition.  

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Within the last few years there has been a growing tendency to draw the analogy between the chemistry of organometallic complexes and that of hydrocarbons adsorbed on metal atoms in surfaces. For example, it is postulated that the isotopic exchan between deuterium and alkanes, catalysed by thin films of nickel, palladium or rhodium, occurs via an equilibrium between a- and 7r-bonded metal-organic species [109, 110, 111, 112]. i-Bonded olefins are postulated in Zie er catalysis, see above, in the catalytic deuteration of aromatic complexes [113], and in a variety of isomerization reactions [112, 114, US]. Other intermediate metal-carbon systems which have been proposed in heterogeneous catalytic systems include carbenes , e.g. M= H2 [109], Ji-enyl systems [114] and of course the metal-carbonyl M—CO and bridging carbonyl M- O—M. 

Thus, the combination of increased acidity and increased solvent power can be exploited for cleaner acid-catalysed reactions of organic compounds, for example Friedel-Crafts alkylation, [31] or hydrolysis of esters [9,28]. The acidity of high-temperature D20 can be used to deuterate organic compounds but the process is more efficient with an additional heterogeneous acid catalyst [32]. A related application, but not a catalytic one, exploits the acidity of SC-H2O for the in situ generation of H2 from metallic Zn the H2 can then be used for the selective reduction of organic nitro-compounds [29,30], see Figure 9.1-2. 

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