Stereoselectivity of Hydrogenation

Catalytic hydrogenation of the following compound gives a mixture of cis- and trans-l-tcrt-butyl-4-methylcyclohexanes in a 7 1 ratio. Explain why. 

Draw the structures of the two products obtained by hydrogenation of the following unsaturated bicyclic compound. Will they be obtained in equal amounts  

Predict the stereochemistry of the product obtained by catalytic hydrogenation of cholesterol. The six-membered rings are all in the chair conformation, and the methyl groups at the bridgehead positions are axial. (Refer to Section 4.19 to determine the shape of the steroid.) 

---

Scheme 16.2 illustrates the catalytic mechanism proposed by Muetterties and coworkers [13]. Salient features of this mechanism are the coordination of benzene in the -fashion, to give a transient Col I( 4-C, iH, i)(PR3)2 complex, and the intramolecular hydride transfer to form the allylic intermediate Co(//3-Ctl l7) (PR3)2. Hydrogen addition would give an 4-1,3-cyclohexadiene complex that ultimately releases cyclohexane via H2 addition/hydride migration steps. Complete cis stereoselectivity of hydrogen addition was demonstrated by replacing H2 with D2. 

Therefore we used 4-androsten-3,17-dione 1 and 5a-androstan-3,17-dione 2 as model substrates to investigate the chemo- regio- and stereoselectivity of hydrogen transfer from different secondary alcohols, 2-propanol, 2-octanol, cyclohexanol, 1-phenyl-ethanol and diphenylmethanol in the presence of CU/AI2O3. In particular, hydrogenation of 1 allowed to determine the selectivity towards 5p isomers, whereas the percent of axial alcohol was derived from the hydrogenation of 2. These results can be compared with those obtained with the same catalyst in the presence of molecular hydrogen. 

Stereoselectivity of Hydrogen-Atom Transfer at Anomeric Radicals. 103... 

A comparison of the proportion of the saturated stereoisomers, formed under the same conditions (Pt02, AcOH) from the isomeric xylenes and the derived dimethylcyclohexenes, leads to the proposal that the reaction proceeds through the desorbed cyclohexenes. Low concentrations of the intermediate cyclohexenes were detected later. The effect of the metal on the stereoselectivity of hydrogenating o-xylene follows closely the effect of the metal on the hydrogenation of 1,2- and 1,6-dimethylcyclo-hexene. The highest selectivity for the conversion of o-xylene to the cis isomer is given by iridium... 

The facial stereoselectivity of hydrogenation is affected by the presence of polar functional groups that can influence the mode of adsorption to the catalyst surface. For... 

However, the relation between stereoselectivity of hydrogenation and solvents polarity is not always so clear as shown by the hydrogenation of P-octalone 9 to trans decalone-3 iO and cis-decalone-3 U. (6). 

It is also important to note that several factors influence both the stereoselectivity of hydrogen exchange and enolate formation in base-promoted reactions. Houk, Ando and co-workers found that differing conju-gative stabilization by CH p-orbital overlap does not directly influence stereoselectivity.205 Steric effects only dominate is exceptionally crowded transition structures, but torsional strain involving vicinal bonds contributes significantly to the stereoselectivity of all cases studied. 

Selective catalytic hydrogenation of the 6,7-double bond of 17/3-acetoxy-7-methylandrosta-4,6-dien-3-one was achieved with Pd-C-PhCH20H and gave the 7/8-methyl dihydro-compound. Added FeCls has been reported to improve the selectivity of reduction of a,/S-enones in metal-ammonia reactions, thereby improving the yield of the saturated ketones. Similar improvements were observed in the lithium-ethylamine reductions at -78 C when a substantial excess of lithium was used and t-butyl alcohol was the proton source. The influence of solvent and added nitrogenous bases on the stereoselectivity of hydrogenation of A - and A -3-oxo-steroids with Pd catalysts has been studied, and the stereoselectivity of Pd-catalysed hydrogenation of various A -7-oxo-steroids has been reported to be unaffected by substituents at C-3 or C-17. 

Looking for the influence of reaction conditions on the stereoselectivity of hydrogenations, one will of course find numerous examples reporting changing isomer ratios on variations of catalysts and solvents [39]. 

However, this route does not explain the stereoselectivity of hydrogen migration observed in the benzannulation of monodeuterated -60 and Z-60 enynes (Scheme 14.22) [14b], since a 1,5-hydride shift should not exhibit stereochemical preference in planar cyclic cumulene 54. 

SCHEME 14 2 Deuterium labeling studies on stereoselectivity of hydrogen migration. 

---