Chiral and Achiral Pathways of Degenerate Reactions

The most common and important case are the narcissistic reactions. Under this type the degenerate transformations are classified [81] in which the structure of the products is viewed as a reflection of the structure of the reactants in a mirror 

An example of special significance for stereochemistry is given by a conceivable enantiotopomerization of methane, which is accompanied by an inversion of the bond configuration at the tetrahedral carbon atom [82,83]  

Similarly to sulfuranes XXII, in this case a transition state with planar 04 structure is theoretically admissible, however, direct calculations (see review listed under Ref. [83]) have shown that such a structure does not correspond to the saddle point on the PES, and the reaction develops along one of two possible enantiomeric paths which include transition states of a symmetry lower than 04 and without a mirror plane. 

Generally, a choice between the symmetrical (achiral) and the asymmetrical (chiral) routes of the reaction in question is dictated by the number of independent geometry parameters, antisymmetrical relative to the mirror plane and adequately defining the reaction coordinate, as well as by the degree of their interrelation [81]. In case the reaction coordinate is described by the change of a single parameter, for example, of the pyramidalization angle a in the ammonia inversion XXV, the reaction path takes its course via the achiral transition state 

The calculation of the surface featured in Fig. 1.15 has been done by the simplest EHMO method. But even a more rigorous treatment [20,83] gives a similar energy sequence in the Td, C4, and D4 structures. The situation changes when all symmetry constraints (two symmetry planes and equivalence of all C—H bond lengths) are removed and a complete geometry optimization is carried out. In such a case, the structure C4, reduces its symmetry to the C2, form (see Fig. 1.16) which is associated with the true saddle point on the PES. (For a more detailed analysis of this problem see Ref. [20].) 

---