Introduction-to-Pericyclic-Reactions.pptx

Introduction-to-Pericyclic-Reactions.pptx

1. Introduction to Pericyclic Reactions Pericyclic reactions are a class of organic reactions that involve the concerted cyclic reorganization of electrons within a molecule. They are characterized by the formation of a cyclic transition state, which is often highly symmetrical. by Deepak Musmade Assistant Professor Department of Pharmaceutical Chemistry Sanjivani College of Pharmaceutical Education and Research, Kopargaon

2. Definition and Characteristics 1 Concerted Mechanism The reaction occurs in a single step, without any intermediates. All bond breaking and bond forming events happen simultaneously. 2 Cyclic Transition State The transition state involves the formation of a closed ring of atoms, where the electrons flow in a cyclic manner. 3 Stereospecificity Pericyclic reactions are highly stereospecific, meaning that the stereochemistry of the reactants determines the stereochemistry of the products. 4 Thermal or Photochemical Pericyclic reactions can be induced by heat (thermal) or light (photochemical), which affects the electron configuration and the reaction pathway.

3. Types of Pericyclic Reactions Electrocyclic Reactions The formation or breaking of a sigma bond within a conjugated system, leading to the formation of a cyclic compound. Cycloaddition Reactions The reaction of two unsaturated molecules to form a cyclic compound by the formation of two new sigma bonds. Sigmatropic Rearrangements The migration of a sigma bond within a molecule, involving the movement of a group of atoms from one position to another. Cheletropic Reactions The addition of a molecule to a multiple bond, forming a new ring system through the migration of a pi bond.

4. Electrocyclic Reactions Conrotatory The terminal groups rotate in the same direction, either clockwise or counterclockwise. Disrotatory The terminal groups rotate in opposite directions, one clockwise and the other counterclockwise. Stereochemical Outcome The stereochemistry of the product is determined by the type of rotation, conrotatory or disrotatory.

5. Cycloaddition Reactions Type Description Diels-Alder The reaction of a diene with a dienophile to form a six-membered ring. [2+2] Cycloaddition The reaction of two alkenes to form a four-membered ring. [4+2] Cycloaddition A variation of the Diels-Alder reaction, involving a four-pi system and a two-pi system.

6. Sigmatropic Rearrangements [1,5]-Sigmatropic Rearrangement The migration of a sigma bond over five atoms, resulting in a shift of a substituent along the pi system. [3,3]-Sigmatropic Rearrangement The migration of a sigma bond over six atoms, involving the rearrangement of a conjugated system. [1,3]-Sigmatropic Rearrangement The migration of a sigma bond over four atoms, leading to the transposition of a substituent on a conjugated system.

7. Cheletropic Reactions 1 Step 1 The addition of a molecule to a multiple bond. 2 Step 2 The migration of a pi bond to form a new ring system. 3 Step 3 The formation of a cyclic compound with a new sigma bond.

8. Pericyclic Reaction Mechanisms Frontier Molecular Orbital Theory This theory explains the reaction mechanism based on the interaction of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the reactants. Woodward-Hoffmann Rules These rules predict the stereochemistry and the allowed or forbidden nature of pericyclic reactions based on the symmetry of the molecular orbitals. Conservation of Orbital Symmetry This principle states that pericyclic reactions proceed smoothly when the symmetry of the molecular orbitals is conserved throughout the reaction.

9. Stereochemistry of Pericyclic Reactions Cis Alkenes In electrocyclic reactions, cis alkenes usually produce cis products, while trans alkenes produce trans products. Trans Alkenes The stereochemistry of the product is determined by the rotation of the terminal groups, which can be either conrotatory or disrotatory.

10. Applications of Pericyclic Reactions Pericyclic reactions are crucial in organic synthesis, playing a vital role in the synthesis of complex molecules, natural products, and pharmaceuticals. They allow for the efficient formation of cyclic systems with specific stereochemistry.