Pericyclic Reactions: Electrocyclic Reaction
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1 Pericyclic Reaction
2 Pericyclic Reactions: Electrocyclic Reaction 1. Electrocyclic ring closing 2. Electrocyclic ring opening Electrocyclic ring closing reaction is characterized by a. The formation of a ring from an open chain conjugated system b. Via a new σ-bond at both end of the conjugated π-component c. with a reduction in the length of the conjugated system Electrocyclic ring opening reaction has opposite characteristics of Electrocyclic ring closing reaction 4 π-electrocyclization 6 π 8 π
3 Pericyclic Reactions: Electrocyclic Reaction cis-5,6-dimethyl- 1,3-cyclohexadiene trans-5,6-dimethyl- 1,3-cyclohexadiene How to describe this electrocyclic reaction? What is the main difference between cycloaddition and electrocyclic reaction?
4 Pericyclic Reactions: Sigmatropic Reaction Sigmatropic reaction is a. A unimolecular isomerization reaction b. That involves movement of a σ-bond from one position to another in the conjugated π-component c. with a concomitant movement of π-bond [3,3] sigmatropic rearrangement
5 Pericyclic Reactions: Sigmatropic Reaction How to describe a given sigmatropic reaction? First, mentally break the σ-bond in the reactant and give a number 1 label to the each atom that were attached by the bond. Then look at the new σ-bond in the product. Count the number of atoms in each of the fragments that connect the broken σ- bond and the new σ-bond. The two numbers are put in brackets with the smaller number stated first. [2,3] sigmatropic rearrangement, generally lowest number comes first
![Pericyclic Reactions: Sigmatropic Reaction [1,5] sigmatropic rearrangement [3,3]](/docs-images/76/74256008/images/6-0.jpg "sigmatropic rearrangement What kind of sigmatropic reaction is it?")
6 Pericyclic Reactions: Sigmatropic Reaction [1,5] sigmatropic rearrangement [3,3] sigmatropic rearrangement What kind of sigmatropic reaction is it? [1,5] sigmatropic reaction
7 Pericyclic Reactions: Sigmatropic Reaction Main difference between cycloaddition and sigmatropic rearrangement Cycloaddition Involves two components Sigmatropic Involves one components Difference between electrocyclic reaction and sigmatropic rearrangement Electrocyclic Involves one components π-bond breakes; σ-bond formed Sigmatropic Involves one components σ-bond breaks and σ-bond formed What type of pericyclic reactions are these? Electrocyclic reaction Sigmatropic reaction
8 Pericyclic Reactions: Group Transfer Reaction Two component reaction but no cycle is formed; not a cycloaddition As it is two component reaction, so distinguished from electrocyclic and sigmatropic reaction A group or atom is transferred from one π-system to another π-system Group transfer reaction is a pericyclic reaction where an atom or group is transferred from one component to another
9 e. Pericyclic Reactions
10 Pericyclic Reactions: Reactivity and Selectivity Why only one isomer? Why [2+2] cycloaddition requires photochemical condition? cis-5,6-dimethyl- 1,3-cyclohexadiene trans-5,6-dimethyl- 1,3-cyclohexadiene Why stereoselectivity changes with changes in reaction condition?
11 Pericyclic Reactions: Frontier Molecular Orbital (FMO) Theory Molecular Orbital: Atomic orbitals of atoms can combine and overlap to give more complex Orbitals that is called molecular orbital two atomic orbitals that results in outof-phase interaction of orbitals is called Antibonding MO MO having same energy as the isolated interacting atomic orbitals Nonbonding MO Two orbitals on adjacent atoms that forms in-phase interaction between the atoms is called Bonding MO
12 Pericyclic Reactions: Frontier Molecular Orbital (FMO) Theory What is /are Frontier Molecular Orbital? Tow molecules interact with each other means two molecular orbital interacts Molecular orbitals that interacts are HOMO and LUMO HOMO and LUMO are called Frontier Molecular Orbitals What does the Frontier Molecular Orbital theory says? HOMO of one molecule interact with LUMO of another Why HOMO-HOMO or LUMO-LUMO interaction are not feasible?
13 Pericyclic Reactions: π-molecular Orbital Pericyclic reaction involves the interaction between pi-molecular orbital of one or more component Antibonding interaction (-1) ψ 2 LUMO Bonding interaction (+1) ψ 1 HOMO
14 Pericyclic Reactions: π-molecular Orbital Provide the molecular orbitals of allyl anion
15 1. Find out no of p-atomic orbital (say n) in the conjugated system (only p-oribtal not the no of carbon or no of electron) 2. Total number of π-molecular orbital will be the same as the number of p-atomic orbital 3. If n p is even, n BMO = n p /2 BMO: bonding molecular orbital n AMO = n p /2 AMO: anti-bonding molecular orbital n NMO = 0 NMO: non-bonding molecular orbital 4. If n p is odd, n BMO = (n p -1)/2 n AMO = (n p -1)/2 n NMO = 1 Pericyclic Reactions: π-molecular Orbital 5. If the wavefunction of π-molecular orbital are symbolized as ψ n (n = 1,2,3,4.), the number of node of ψ n will be (n-1). 6. Nodes are arranged symmetrically with respect to the center of π-electron system 7. Electrons in each MO are placed according to Aufbau Principle, Hund's rule and Pauli exclusion principle
16 Pericyclic Reactions: π-molecular Orbital Draw the molecular orbitals of 1,3-butadiene
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