Felkin Ahn Model and Chelation Model (40 mins) | Lecture 234/354 | CSIR (Council of Scientific & Industrial Research) Chemical Sciences

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Organic ChemistryPrinciples of StereochemistryDiastereoselectivity

Felkin ahn model and Chelation model

• A chemical compound can differ from other compounds by having different elements or by attaching those elements in different orders.
• But even if two compounds have the exact same elements attached in the same order, there can still be differences between them. The differences are in the 3D orientation of the bonds.
• These types of differences between two compounds are called stereoisomers. Two types of stereoisomers are enantiomers and diastereomers. Enantiomers are when two compounds are non-superimposable mirror images, like our hands. Diastereomers are when two compounds have different 3D orientations but are not mirror images of each other.
• The Felkin-Anh model, also known as the Felkin-Anh rule or the Felkin control model, is a concept in organic chemistry related to the stereochemistry of reactions. It is used to predict the stereochemical outcome of nucleophilic additions to trigonal planar carbonyl compounds (such as ketones and aldehydes)
• The model was proposed by Hugh Felkin and John Anh in the 1970s.
• According to the Felkin-Anh model, the stereochemistry of the addition reaction depends on the size of the nucleophile and the transition state leading to the reaction. The model states that when a nucleophile adds to a trigonal planar carbonyl compound, the nucleophile prefers to attack from the least hindered side of the carbonyl, leading to the formation of the thermodynamically more stable product.
• In simpler terms, if a nucleophile adds to a carbonyl compound, it is more likely to approach from the side where there are fewer steric hindrances, leading to the formation of the major product.
• it՚s important to note that while the Felkin-Anh model provides useful predictions for many reactions, it is not a strict rule and may not apply to all cases.
• Cram՚s Rule, formulated by American chemist Donald J. Cram, is a principle in stereochemistry that describes the preferred orientation of attacking nucleophiles in certain chemical reactions, particularly those involving highly strained organic molecules.
• Cram՚s Rule is essential in predicting the stereochemistry of addition reactions to cyclic compounds, especially those with substituents that cause steric hindrance. The nucleophile or reagent tends to attack from a position anti-periplanar to the leaving group. This anti-arrangement maximizes the distance between the incoming nucleophile and the departing group, reducing steric repulsion.
• One classic example of Cram՚s Rule is found in the addition reactions of nucleophiles to bridged bicyclic compounds, where the orientation of attack is highly influenced by the bridgehead stereochemistry.
• it՚s important to note that while Cram՚s Rule is a useful guideline, there are exceptions and variations based on specific reaction conditions and substrates. Cram՚s Rule is particularly relevant in reactions involving cyclic compounds, where the stereochemistry of the products is influenced significantly by the orientation of attack due to the ring strain.