Stereoselectivity

Stereoselectivity is the preferential formation of one stereoisomer over other possible stereoisomers in a chemical reaction. It describes the degree to which reaction conditions favor a particular three-dimensional product.

Transition state: A high-energy arrangement of atoms at the peak of a reaction pathway, where bonds are partly broken or formed. Different transition-state energies can make one stereoisomer-forming pathway faster.
Stereoisomer: One of a set of molecules with the same connectivity but different spatial arrangements of atoms. Stereoselectivity compares the formation of stereoisomers with unchanged atom-to-atom connectivity.
Asymmetric catalysis: Catalysis that preferentially produces one enantiomer or diastereomer through a chiral catalyst. Chiral catalysts are designed to impose stereoselectivity on otherwise competing pathways.
Enantioselectivity: The preferential formation of one enantiomer over its mirror-image counterpart in a reaction. It is the stereoselectivity specifically concerned with competing enantiomers.
Curtin–Hammett principle: A principle relating product ratios to transition-state energies when rapidly interconverting reactants form products irreversibly. It explains why the most abundant reactive conformation need not yield the major stereoisomer.
Enantiomer: One of two stereoisomers that are non-superimposable mirror images of each other. Preference between enantiomers is the defining outcome measured as enantioselectivity.
Sharpless epoxidation: A titanium-catalyzed reaction that converts allylic alcohols into enantioenriched epoxy alcohols using a chiral tartrate ligand. It is a landmark example of catalyst-controlled enantioselective oxygen transfer.
Diastereoselectivity: The preferential formation of one diastereomer over other diastereomers in a reaction. It is the stereoselectivity specifically concerned with products that are not mirror images.
Steric hindrance: The slowing or redirection of a chemical process caused by atoms or groups occupying space near a reactive site. Crowding can block one approach to a reaction center and favor another stereochemical outcome.
Diastereomer: A stereoisomer that is not a mirror image of another stereoisomer with the same connectivity. Preference among diastereomeric products is measured as diastereoselectivity.

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