page_title: Asymmetric induction
section_title: Reagent control: addition of chiral allylmetals to achiral aldehydes
hierarchical_section_title: Asymmetric induction / Reagent control: addition of chiral allylmetals to achiral aldehydes
caption_attribution_description: English: Fig. 2: Example of chiral allylmetals used: (a) allylboron, (b) allyltitanium, and (c) allyl silicon
context_page_description: In stereochemistry, asymmetric induction 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. Asymmetric induction is a key element in asymmetric synthesis.
  Asymmetric induction was introduced by Hermann Emil Fischer based on his work on carbohydrates. 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.
context_section_description: In organic synthesis, reagent control is an approach to selectively forming one stereoisomer out of many, the stereoselectivity is determined by the structure and chirality of the reagent used. When chiral allylmetals are used for nucleophilic addition reaction to achiral aldehydes, the chirality of the newly generated alcohol carbon is determined by the chirality of the allymetal reagents (Figure 1). The chirality of the allymetals usually comes from the asymmetric ligands used. The metals in the allylmetal reagents include boron, tin, titanium, silicon, etc.
  Various chiral ligands have been developed to prepare chiral allylmetals for the reaction with aldehydes. H. C. Brown was the first to report the chiral allylboron reagents for asymmetric allylation reactions with aldehydes. The chiral allylboron reagents were synthesized from the natural product (+)-a-pinene in two steps. The TADDOL ligands developed by Dieter Seebach has been used to prepare chiral allyltitanium compounds for asymmetric allylation with aldehydes. Jim Leighton has developed chiral allysilicon compounds in which the release of ring strain facilitated the stereoselective allylation reaction, 95% to 98% enatiomeric excess could be achieved for a range of achiral aldehydes.
caption: Fig. 2: Example of chiral allylmetals used: (a) allylboron, (b) allyltitanium, and (c) allyl silicon
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