Stereoselective Reactions

Stereoselective Reactions are reactions where one stereoisomer is formed preferentially over others from the same reactant.

Definition of Stereoselective Reactions:

• A stereoselective reaction is a chemical reaction in which a single reactant can produce two or more stereoisomeric products, but one is formed preferentially over the others.
  • The preference arises due to factors like steric hindrance, electronic effects, or transition state stability.
  • The reaction does not require the reactant to be stereoisomeric.

Types of Stereoselectivity:

1. Enantioselective Reaction: Preferential formation of one enantiomer over the other.
2. Diastereoselective Reaction: Preferential formation of one diastereomer over another.

Key Characteristics:

• Selectivity comes from how the reaction pathway favors one stereoisomer.
• Multiple stereoisomers are possible, but the reaction yields one as the major product.

Examples of Stereoselective Reactions:

1. Hydroboration–Oxidation of Alkenes

   • Adds H and OH from the same face of the double bond (syn addition).
   • Results in a specific stereoisomer of an alcohol.
   • Stereoselective, but not stereospecific (could apply to achiral alkenes).

2. Catalytic Hydrogenation of Alkenes

   • H₂ adds syn (from the same face).
   • In cyclic alkenes, this often results in cis-products.
   • The face of attack is controlled by steric accessibility.

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