Pericyclic Reactions Part 4: Electrocyclizations (Conrotatory/Disrotatory and Nazarov Cyclizations)

The video tutorial explores various paracyclic reactions, including 4+2 cycloadditions, sigmatropic shifts, and electrocyclizations. It discusses the stereochemical rules governing these reactions, using frontier orbital theory. The tutorial also covers 4-electron cyclizations, highlighting their challenges due to anti-aromatic character and ring strain. A detailed explanation of cascade reactions, including a double electrocyclization Diels-Alder cascade, is provided, emphasizing stereoselectivity. Finally, the Nazarov cyclization is introduced, showcasing its mechanism and synthetic utility.

What is the main characteristic of a 4+2 cycloaddition reaction?

• Rearrangement of a single sigma bond

• Formation of a cyclic compound with two new sigma bonds

• Breaking of a pi bond

• Formation of a single sigma bond

In a six-electron electrocyclization, what is the result of disrotatory rotation?

• A single pi bond is formed

• No cyclization occurs

• Substituents end up cis to each other

• Substituents end up trans to each other

Why are four-electron cyclizations less common?

• They have an anti-aromatic transition state

• They require high temperatures

• They are irreversible

• They produce unstable products

What is the significance of the conrotatory mechanism in a four-electron cyclization?

• It leads to the formation of a cis product

• It results in a trans product

• It prevents cyclization

• It forms a five-membered ring

What is the outcome of a double electrocyclization Diels-Alder cascade?

• Formation of a linear chain

• Formation of a single four-membered ring

• Formation of a complex polycyclic structure

• Formation of a single six-membered ring

What role do strong acids play in the Nazarov cyclization?

• They deprotonate the carbonyl

• They form a pi bond

• They break a sigma bond

• They protonate the carbonyl

What is a key advantage of using electrocyclizations in synthesis?

• They are always reversible

• They produce rings with high stereo selectivity

• They require no catalysts

• They only form six-membered rings