Asymmetric induction
Asymmetric induction is the preferential formation of one enantiomer or diastereoisomer over the respective other in a chemical reaction as a result of optically active functional group in the substrate, a reagent, an auxiliary or catalyst.
Asymmetric induction is the basic principle of asymmetric synthesis.
Originally, the concept of Emil Fischer was introduced, based on observations in the synthesis of higher sugars.
The following cases are distinguished:
Reactant - controlled asymmetric induction
One or more chiral centers, which are covalently attached in close proximity to the actual reaction center, induce a diastereomeric excess in the reaction. Examples of substrate-controlled reactions are the classic sugar synthesis by Emil Fischer.
Auxiliary - controlled asymmetric induction
The chiral information is covalently incorporated in a preceding stage, and removed in a subsequent step. An auxiliary is required in stoichiometric amounts. An established example of auxiliar - controlled asymmetric induction is the use of Enders reagent ( SAMP-/RAMP-Hydrazonmethode ).
Reagent - controlled asymmetric induction
A chiral reagent induces an asymmetric implementation. A chiral reagent is required in stoichiometric amounts.
Catalytic asymmetric induction
The chiral information is induced only during the reaction, in the transient state by means of a catalyst containing one or more chiral ligands. In general catalytic asymmetric reaction is the most economical process, since the catalyst is usually required only in sub- stoichiometric amounts. Prominent examples of a catalytic asymmetric reaction are the Sharpless epoxidation, the Asymmetric dihydroxylation and asymmetric hydrogenation.
A particular case of catalytic asymmetric reactions are enzymatic reactions.
Several, partly contradictory, models and theories have been proposed ( Donald J. Cram, Hugh Felkin, David A. Evans ) and developed in order to understand, especially 1,2- asymmetric induction in nucleophilic additions to carbonyls and predict.