What is Cram chelation model?

What is Cram chelation model?

Chelation model 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 Rβ, leading to the anti-adduct as the major product.

What is cram 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 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 Diastereoselectivity?

A diastereoselective reaction is one in which one diastereomer is formed in preference to another (or in which a subset of all possible diastereomers dominates the product mixture), establishing a preferred relative stereochemistry.

Who discovered aldol reaction?

The aldol reaction, first discovered by Wurtz in 1872,1 is one of the most powerful transformations in organic chemistry. The process unites two carbonyl partners to give β-hydroxyketones with up to two new stereocenters (Scheme 1).

What is the difference between the Enantioselectivity and Diastereoselectivity?

Diastereoselective reactions are very common. In contrast, enantioselective reactions are rare becuase they require special chiral reagents or catalysts. Therefore, the term stereoselective is casually used to mean diastereoselective.

What causes Diastereoselectivity?

Diastereoselective reactions occur when one diastereomer happens more frequently than another in a reaction. This can occur due to two atoms being connected to each other. Or it can occur due to something blocking one side from being attacked.

What is difference between erythro and threo isomers?

Erythro Isomers: In erythro isomers, the two identical substituents are on the same side. Threo isomers: In threo isomers, the two identical substituents are on the opposite sides. In the structure (I), the two hydroxyl groups are on the same side. Hence, they both are threo isomers.

What are threo and erythro isomers give examples?

When two similar groups at non-identical chiral carbons are on the same side of the vertical line of the Fischer projection, the isomer is called erythro isomer, if the groups are on opposite sides, the isomers are called threo isomers.

What is the difference between Felkin’s and Felkin-Nguyen models?

An improvement over Felkin’s model was the Felkin-Nguyen (Felkin-Anh) model, which suggested that the nucleophile would attack the sp2 center of the carbonyl at 950 to 1050 angle relative to the carbonyl bond axis, favoring an attack from the least hindered direction (Figure 1.2.6).

What is Felkin’s experiment with Cram?

Felkin pointed out that the reactive conformations of Cram had severe eclipsing strain between R group on carbonyl center and L group on the – carbon (see 2B in Figure 1.2.2). He progressively increased the bulk of the R group and observed that the diastereomeric excess could swing from erythro 74\% to threo 96\% as the bulk increased (Fig 1.2.4).

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 is meant by Cram’s rule?

The attacking nucleophile would prefer to attack from the side of the small group, resulting in the predominant formation of one diastereomer in the product. This is now known as Cram’s Rule. When the starting material is a pure enantiomer, the product mixture would show predominance of one enantiomer.