The Development of Some Stereochemical Concepts

Explanations of the stereochemistry of hydrogenation have been dominated by ideas concerning the manner in which a given unsaturated compound may best be fitted onto a planar surface from which hydrogen is abstracted. Cis addition is readily understood in such terms. 

The formation of meso-dimethylsuccinic acid from dimethylmaleic acid and the racemic mixture from dimethylfumaric acid implies that both hydrogen atoms add to the same side of the unsaturated molecule 11). Bourguel (12) also noted that disubstituted acetylenes yielded initially cis-ethylenes but that trans isomers were formed if the hydrogenations were protracted. 

Hadler 26) employed conformational analysis to explain the difference in the proportion of cholestane to coprostane derivatives resulting from the reduction of A and d steroids. He suggested that the hydrogenation process involved the formation of a quasi-ring structure between the unsaturated carbon atoms and two hydrogens originally dissolved in the metal, a mechanism which is similar to one proposed by Beeck (27) and by Jenkins and Rideal 28). He assumed, in effect, that the saturated struc- 

The concept that the stereochemistry of reduction of an unsaturated hydrocarbon is determined at the adsorption stage of the reaction is evident in each of the above accounts, and others could be cited. However, the development of techniques which permit the identification of different product-controlling reactions directs one to consider the stereochemical consequences of various postulated reaction sequences and this will be discussed in Section IV. 

Eischens and Pliskin have interpreted the infrared spectra of ethylene chemisorbed on nickel dispersed on silica 32). When introduced to a surface previously exposed to hydrogen, ethylene gave rise to absorption bands which correspond to the C—H stretching frequencies of a saturated hydrocarbon (3.4-3.5 p) and a deformation associated with a methylene group (6.9 p). A weak band at 3.3 p was attributed to an ole-finic C—H. Treatment of the chemisorbed ethylene with hydrogen caused the spectrum to change to one which was interpreted as due to an adsorbed ethyl radical. Apparently in the presence of hydrogen most of 

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