The Structural Basis for the 



Differentiation of Identical Groups 



in Asymmetric Reactions 



HANS H1RSCHMANN 



Classical organic chemistry defined asymmetric syntheses x as proc- 

 esses which convert symmetrical molecules into optically active ones 

 by the intermediate use of asymmetric agents, provided the methods 

 employed take no recourse to processes of resolution. Numerous ex- 

 amples have demonstrated the reality of the phenomenon. The asym- 

 metric agent used to bring about this change can either be an "asym- 

 metric form" of energy such as circularly polarized light or an asym- 

 metric form of matter such as an asymmetric molecule. The success 

 of the operation occasioned little inquiry into its structural require- 

 ments since the essential features appeared to be trivial ones, i.e., the 

 symmetry of the starting compound and the asymmetry of the product. 



Quite a different situation arose, however, with the discovery of 

 closely related phenomena which could not be detected by testing the 

 product for optical activity. If one of the substituents a in compound 

 I is converted preferentially to a substituent d which is different from 

 the three others, the product II is optically active. On the other hand 



b b b 



i i i 



i i i 



i i i 



' i i 



i i i 



I I i 



c c c 



II I Til 



if the reaction involves the selective conversion of one group a into 

 a group b identical with one already present, the product (III) has 

 a plane of symmetry and hence is optically inactive. However, if one 

 of the groups to be substituted is isotopically labeled, it is possible 

 to demonstrate the asymmetry of the process in either case. 



The difference between these two types of sterically selective reac- 



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