Stereochemistry medium/solvent effects

Deschamps, B., Seyden-Penne, J. Solvent effects on the stereochemistry of the Darzens reaction. III. Condensation of chloroacetonitrile and aromatic aldehydes in a basic medium. Tetrahedron 1971, 27, 3959-3964. 

The mechanistic aspects of nucleophilic substitution reactions were treated in detail in Chapter 5 of Part A. That mechanistic understanding has contributed to the development of nucleophilic substitution reactions as importantl synthetic processes. The SN2 mechanism, because of its predictable stereochemistry and avoidance of carbocation intermediates, is the most desirable substitution process from a synthetic point of view. This section will discuss the role of SN2 reactions in the preparation of several classes of compounds. First, however, the important role that solvent plays in SN2 reactions will be reviewed. The knowledgeable manipulation of solvent and related medium effects has led to significant improvement of many synthetic procedures that proceed by the SN2 mechanism. 

Using the preference rules of phosphorus chemistry, the most stable intermediates would be the same whatever the nature of the solvent and the experimental conditions. So, it is impossible to explain in this way the changes of stereochemistry with medium effects or with the nature of... 

---