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Asymmetric induction
Asymmetric induction (also enantioinduction) in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment.〔IUPAC Gold Book definition (Link )〕 Asymmetric induction is a key element in asymmetric synthesis.
Asymmetric induction was introduced by Hermann Emil Fischer based on his work on carbohydrates.〔''Asymmetric Synthesis of Natural Products'', Ari Koskinen ISBN 0-471-93848-3〕 Several types of induction exist.
Internal asymmetric induction makes use of a chiral center bound to the reactive center through a covalent bond and remains so during the reaction. The starting material is often derived from chiral pool synthesis. In relayed asymmetric induction the chiral information is introduced in a separate step and removed again in a separate chemical reaction. Special synthons are called chiral auxiliaries. In external asymmetric induction chiral information is introduced in the transition state through a catalyst of chiral ligand. This method of asymmetric synthesis is economically most desirable.
==Carbonyl 1,2 asymmetric induction==
Several models exist to describe chiral induction at carbonyl carbons during nucleophilic additions. These models are based on a combination of steric and electronic considerations and are often in conflict with each other. Models have been devised by Cram (1952), Cornforth (1959), Felkin (1969) and others.
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