Stereoselektivitou

Stereoselektivitou is the term used to describe the tendency of a chemical reaction to favor the formation of one stereoisomer over others. In practice, it captures the selectivity for chiral configurations or arrangements around stereogenic elements, such as stereocenters or double bonds. The concept is typically discussed under the broader idea of stereoselectivity, including enantioselectivity and diastereoselectivity.

Enantioselectivity refers to preferential formation of one enantiomer, yielding an excess of one mirror image. Diastereoselectivity describes the preference for one member of a diastereomeric set. Both phenomena are quantified using enantiomeric excess (ee) and, for diastereomers, diastereomeric ratio (dr).

Measurement: ee expresses the difference in amount between the two enantiomers, while dr expresses the ratio between diastereomers. Regioselectivity is related but concerns the site of bond formation rather than stereochemistry.

Factors and control: Stereoselectivity arises from the reaction mechanism, with kinetic control favoring the fastest-forming isomer and thermodynamic control favoring the most stable one. Practical control is achieved with chiral catalysts or reagents (enzymes, organocatalysts, metal complexes with chiral ligands), substrate design, and reaction conditions such as solvent, temperature, and concentration. Methods include asymmetric synthesis, kinetic resolution, and the use of chiral auxiliaries.

Applications: High stereoselectivity is essential in the synthesis of pharmaceuticals, agrochemicals, and natural products, as well as in materials science. Classic examples are enantioselective hydrogenation, epoxidation, and carbon–carbon bond-forming reactions guided by chiral catalysts.