Homogeneous catalytic deuteration

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)  

In benzene solution one of the triphenylphosphine ligands in (131) is replaced by a solvent molecule to give intermediate (132). The latter can add a mole of deuterium leading to (133) or can equilibrate with (134) in the presence of an olefin. There is some evidence, however, that in the presence of alcohol and oxygen the dissociation step (131 - 132) is inhibited and the displacement of the triphenylphosphine by the solvent in forming (133) occurs only in the presence of hydrogen (or deuterium).  

The metal-bound deuteriums in (133) react readily with olefins which enter 

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.  

Isolated tetrasubstituted double bonds do not react under these conditions and the saturation of trisubstituted double bonds is extremely slow, thus limiting the general utility of the method. This difference in reactivity is used to advantage for the selective deuteration of the -double bond in androsta-l,4-diene-3,17-dione (138). In homogeneous solution, saturation usually occurs from the a-side and consequently the deuterium labels are in 

---

Homogeneous catalytic deuteration of various unsaturated 5a-spirostane derivatives is an excellent method for the preparation of side chain labeled analogs. Thus, saturation of the double bonds at positions 20(21), 23 and 24 provided the corresponding deuterated compounds (144), (145) and (146) in high isotopic purity. The preparation of (146) is a rare example of the saturation of an isolated trisubstituted double bond in the steroid field. 

Study of the homogeneous catalytic hydrogenation of BT to DHBT in the presence of [Rh(Cp )(MeCN)3] utilized deuteration experiments and detailed NMR smdies to elucidate a catalytic cycle [147]. The main cycle starts with the t -coordination of BT to a rhodium polyhydrido species, followed by hydride transfer, hydrogenation, finally generating DHBT. 

The photochemically produced PtCPEt fragment, stabilized as the cis- and trans-PtH (PEt- )2 complexes, has proved (54,55,56) to be an efficient and long lived homogeneous catalyst for H2/D2 exchange (Equation 17), deuteration of acetone or acetontrile (Equations 18 and 19), decomposition of formic acid (Equation 20), and hydrolysis of acetonitrile (Equation 21). Because of the catalytic promise... 

---