Absolute Asymmetric Synthesis and Asymmetric Catalysis 1 1 1 



Chemical models of enzymes are to be found, not only among micro- 

 heterogeneous catalysts, but also among heterogeneous ones. The adsorption 

 of the reacting substances on the surface of the catalyst is fundamental to any 

 heterogeneous-catalytic process. It is, therefore, necessary to distinguish between 

 cases in which the catalyst itself is dissymmetrical and those in which the 

 structure of the catalyst is symmetrical but it is carried on a dissymmetrical 

 carrier. It might be expected that, in both cases, asymmetric adsorption would 

 lead to asymmetric catalysis. 



When the (C2H5)2N group has been introduced into the cellulose of cotton 

 fibres they are transformed into active catalysts which, like the alkaloids, can 

 simulate the asymmetric activity of carboxylase in breaking down camphor- 

 carboxyhc acid, and that of oxynitrilase in catalysing the synthesis of the ( — )- 

 nitrile of mandehc acid. A catalyst of this sort was obtained in 1932 by Bredig & 

 Gerstner [54]. Although these authors regard the example they give as the first 

 case of heterogeneous asymmetric catalysis, namely that in which the optically 

 selective activity of the catalyst itself is the determining factor rather than that 

 of the carrier, nevertheless, the mechanism of the dissymmetric action of a 

 catalyst of this sort is, in fact, dependent on asymmetric adsorption on the 

 optically active cellulose with ordinary, optically non-specific catalysis. 



The mechanism of action is the same in the case of the catalyst palladium on 

 silk fibroin (Akabori [55]) in reactions of asymmetric reduction of C ^z N bonds. 

 In support of this explanation we have the facts of asymmetric adsorption on 

 cellulose, which has had extensive appUcation recently in the chromatographic 

 resolution of racemic amino acids on paper [31, 32]. 



Attempts by Ghosh [56] to bring about asymmetric catalysis on asymmetric, 

 dichroic, thin layers of metals (silver, platinum and palladium), which have a 

 high optical rotation, were imsuccessful. We therefore have, for the present, no 

 evidence for the existence of dissymmetric catalysts which are not associated 

 with a dissymmetric carrier. 



More successful experiments in the field of the optically selective activity of 

 catalysts have been done with optically non-specific catalysts carried on a dis- 

 synmietric carrier. 



Schwab and his colleagues [57] used, as their catalyst, copper, platinum or 

 nickel, deposited in a thin layer on optically active crystals of quartz. They 

 achieved optically selective decomposition of racemic butan-2-ol at 400-500 °C. 

 In this reaction the optical antipodes of the racemate are dehydrated at unequal 

 rates and an excess of one optical antipode gradually accumulates in the re- 

 maining alcohol : 



CH 

 H' 



M 



^H ^ 



-CH, 

 ^H 



These effects have been successfully reproduced in the dehydration of other 

 alcohols, namely menthol and methylethylbutylcarbinol [58]. 



Terent'ev & Klabunovskii [59] have made a detailed study of the dehydration 

 and dehydrogenation of butan-2-ol using various metal-quartz catalysts and 



