Structural Differentiation in Asymmetric Reactions 159 



result to be expected will depend on the symmetry or asymmetry of E. 

 If E is symmetrical, the products Vila and VII6 are optical antipodes 



\ 



VI i 



b 



I 



I 



l 

 ■ CM 



I 



I 



I 



c 

 VI b 





E-a'- 



Vlla 



b 



I 

 I 

 I 

 ■ » 

 I 

 I 

 I 



c 

 VII b 



a"-E 



and hence possess equal stability. Similarly, any approach of E to 

 a' can be matched by a mirror-image approach to a" possessing equal 

 probability. Hence no differentiation of a' and a" is possible. If E 

 is asymmetric, the products Vila and VIIfc> are no longer optical antip- 

 odes but diastereoisomers and hence would be expected to possess 

 different stabilities. Even if the direction of motion of E towards a' 

 is the mirror image of that of E towards a", the corresponding situa- 

 tions during any stage of such an approach (e.g., Via and VIb) are 

 related like diastereoisomers and not like mirror images and hence can 

 be expected to possess different probabilities. The final proportion 

 of Vila and of Vllb may be determined by the probability of a success- 



\ 



o— © 



Villa 



•V 



/ 



o — © 



VHIb 



ful collision between E and a' or a" (kinetic control) , or by the relative 

 stabilities (or solubilities) of the products (thermodynamic control) 

 if the process is reversible. In either case the ratio of reaction products 

 would be expected to be different from unity. If the reaction does not 



