Deuteration with homogeneous catalysts

This reaction sequence proceeds by cis addition of deuterium and the reduction products usually exhibit high isotopic purity. For example, 5a-cholest-2-ene (136), which is known to give a product of very unsatisfactory isotopic purity when deuterated with heterogeneous catalysts (see section V-A), gives 2<, 3 -d2-5a-cholestane (137) with better than 95% isotopic purity in homogeneous solution. ... 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

Reduction of the A" -double bond with the rhodium complex is a very slow reaction, but it has been accomplished in 17)S-hydroxyandrost-4-en-3-one (140)d The product, 4a, 5a-d2-androstan-17j3-ol-3-one (141), is a further example of the preferential a-side deuteration in homogeneous solution as contrasted with the )S-face attack with heterogeneous catalysts. [For a more convenient preparation of compound (141) see section V-C.]... 

Deformytation. In a study of the homogeneous deuteration of ot-hydroxy acids with this catalyst, Calf and Gamatt found that benzilic acid undergoes deformyla-... 

Deuterium exchange. Garnett et al. have found that rhodium, in addition to platinum and iridium (3, 134), catalyzes deuterium exchange. RhCla functions as a homogeneous catalyst, whereas RhCls reduced with aqueous NaBH4 functions as a heterogeneous catalyst. Rhodium has several advantages, one of which is that deuteration of saturated hydrocarbons e.g., cyclohexane) is possible at reasonable rates. 

This step has been shown to be stereospecific in the case of hydrogenation (deuteration) catalyzed by dicyclopentadienylmolybdenumdihydride (dideuteride) (62) with cis addition of the metal hydride to the double bond (70). In these systems there is spectral evidence for the initial formation of cr- and 7r-electron donor-acceptor complexes between the catalyst and substrate prior to 7r-complex formation and n-a- rearrangement (70, 71). This catalyst (62) has also been used for the selective homogeneous hydrogenation of 1,4- or 1,3-dienes to monoenes, for example, cyclo-pentadiene (63). The reactions are run at elevated temperature (72). 

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