What is the felkin-Ahn model?

What is the felkin-Ahn model?

Felkin-Ahn model: In Felkin-Ahn model, a nucleophile comes from the least hindered side. The best way to do Felkin-Ahn model is to draw a newmen projection. Then have the nucleophile attack from the smallest group. Here, the model shows that the nucleophile prefers to attack from the least hindered side.

What is the crams rule?

Cram’s rule: As stated by Donald Cram of UCLA in 1952, “In certain non-catalytic reactions that diastereomer will predominate, which could be formed by the approach of the entering group from the least hindered side when the rotational conformation of the C-C bond is such that the double bond is flanked by the two …

What is felkin Ahn model in stereochemistry?

The polar Felkin–Anh model is applied in the scenario where X is an electronegative group. The polar Felkin–Anh model postulates that the observed stereochemistry arises due to hyperconjugative stabilization arising from the anti-periplanar interaction between the C-X antibonding σ* orbital and the forming bond.

What is cieplak model?

In organic chemistry, the Cieplak effect is a predictive model to rationalize why nucleophiles preferentially add to one face of a carbonyl over another. Proposed by Andrzej Stanislaw Cieplak in 1980, it predicts anomalous results that other models of the time, such as the Cram and Felkin–Anh models, cannot justify.

What is meant by asymmetric synthesis?

asymmetric synthesis, any chemical reaction that affects the structural symmetry in the molecules of a compound, converting the compound into unequal proportions of compounds that differ in the dissymmetry of their structures at the affected centre.

How do chiral auxiliaries work?

A chiral auxiliary is a stereogenic group or unit that is temporarily incorporated into an organic compound in order to control the stereochemical outcome of the synthesis. The chirality present in the auxiliary can bias the stereoselectivity of one or more subsequent reactions.

What is asymmetric synthesis in stereochemistry?

Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. Put more simply: it is the synthesis of a compound by a method that favors the formation of a specific enantiomer or diastereomer. Enantiomers are stereoisomers that have opposite configurations at every chiral center.

Why asymmetric synthesis is important in chemistry?

6 Encapsulated Chiral Complexes. Asymmetric synthesis is a useful method to produce stereoisomeric compounds for pharmaceutical applications because different enantiomers of molecules are known to have different biological activities.

Why are aldehydes more reactive towards nucleophiles than ketones?

Electrically, two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively in ketones than in aldehyde. Hence, Aldehydes are more reactive toward nucleophilic addition reactions than ketones.

What is the difference between nucleophilic substitution and nucleophilic addition?

The substitution reactions are those which involve the displacement of the group or molecules and attachment of the attacking group to that position. While addition reactions do not have any displacement, as the reactant simply adds the attacking species.

What are the disadvantages of the Cram model in organic chemistry?

Felkin argued that the Cram model suffered a major drawback: an eclipsed conformation in the transition state between the carbonyl substituent (the hydrogen atom in aldehydes) and the largest α-carbonyl substituent.

What causes anti-Felkin selectivity in nucleophiles?

A non-chelating electron-withdrawing substituent effect can also result in anti-Felkin selectivity. If a substituent on the α-carbon is sufficiently electron withdrawing, the nucleophile will add anti- relative to the electron withdrawing group, even if the substituent is not the largest of the 3 bonded to the α-carbon.

Can the conformation of reaction products be explained by Cram’s rule?

In his 1952 publication Cram presented a large number of reactions described in the literature for which the conformation of the reaction products could be explained based on this rule and he also described an elaborate experiment ( scheme 1) making his case. The experiments involved two reactions.

Can the Cram-chelate model be extended to predict chelated complex?

According to Reetz, the Cram-chelate model for 1,2-inductions can be extended to predict the chelated complex of a β-alkoxy aldehyde and metal. The nucleophile is seen to attack from the less sterically hindered side and anti- to the substituent R β, leading to the anti- adduct as the major product.