Asymmetric induction
Asymmetric induction is the preferential formation of one enantiomer or diastereomer over the other in a chemical reaction as a result of an optically active functional group in the substrate , in a reagent , an auxiliary or a catalyst .
Asymmetric induction is the basic principle of asymmetric synthesis .
The concept was originally introduced by Emil Fischer based on observations made during the synthesis of higher sugars .
A distinction is made between the following cases:
Educt -controlled asymmetric induction
One or more chiral centers that are covalently close to the actual reaction center induce a diastereomeric excess in the reaction . Examples of substrate-controlled reactions are the classic sugar syntheses by Emil Fischer .
Auxiliary- controlled asymmetric induction
The chiral information is covalently incorporated in a preceding step and removed in a subsequent step. An auxiliary is required in stoichiometric amounts. An established example of auxiliary-controlled asymmetric induction is the use of Enders reagent (SAMP / RAMP hydrazone method).
Reagent- controlled asymmetric induction
A chiral reagent induces an asymmetric conversion. A chiral reagent is required in stoichiometric amounts.
Catalytic asymmetric induction
The chiral information is only induced during the reaction, in the transition state, by a catalyst with one or more chiral ligands. As a rule, catalytic asymmetric reactions are the most economical processes, since the catalyst is usually only required in sub-stoichiometric amounts. Prominent examples of a catalytic asymmetric reaction are Sharpless epoxidation , asymmetric dihydroxylation and asymmetric hydrogenation.
A special case of catalytic asymmetric reactions are enzymatic reactions.
Several, sometimes contradicting, models and theories have been set up ( Donald J. Cram , Hugh Felkin , David A. Evans ) and developed, in particular to understand and predict 1,2-asymmetric inductions in nucleophilic additions to carbonyls .
Individual evidence
- ↑ DJ Cram, FAA Elhafez: Studies in Stereochemistry. X. The rule of “Steric Control of Asymmetric Induction” in the Syntheses of Acyclic Systems. In: Journal of the American Chemical Society . tape 74 , 1952, pp. 5828–5835 , doi : 10.1021 / ja01143a007 .
- ↑ M. Chérest, H. Felkin, N. Prudent: Torsional strain involving partial bonds. The stereochemistry of the lithium aluminum hydride reduction of some simple open-chain ketones. tape 9 , 1968, p. 2199-2204 , doi : 10.1016 / S0040-4039 (00) 89719-1 .
- ↑ DA Evans, JL Duffya, MJ Darta: 1,3-Asymmetric induction in the aldol addition reactions of methyl ketone enolates and enolsilanes to β-substituted aldehydes. A model for chirality transfer. tape 35 , 1994, pp. 8537-8540 , doi : 10.1016 / S0040-4039 (00) 78430-9 .