Nazarov cyclization reaction

A Nazarov cyclization is an organic reaction converting a divinylketone into a cyclopentenone under the influence of a Brønsted acid such as sulfuric acid or a Lewis acid such aluminum chloride. The key intermediate in this reaction is a hydroxypentadienyl cation and the key reaction step is a thermally allowed electrocyclic ring closure which in accordance with the Woodward-Hoffmann rules as a 4n π system is conrotatory.

The reaction was first reported by I.N. Nazarov in 1949.

Mechanism
The reaction mechanism for this reaction is outlined in scheme 2. Divinylketone 1 is protonated with a proton donor to the pentadienyl cation 2 in which a positive charge is delocalized over 6 positions, a hybrid of 4 mesomers 2a to 2d. An electrocyclic ring closure takes place to the cyclopentenyl cation 3 and after an elimination reaction the proton is released again. The enol 4 rearranges to the ketone tautomer 5.



Scope
In one study the Nazarov cyclization is paired with a Michael reaction. In this tandem reaction (scheme 3) the iridium catalyst is required for both conversions: it acts as the Lewis acid in the Nazarov cyclization and in the next step the nitro group of nitrostyrene first coordinates to iridium in a ligand exchange with the carbonyl ester oxygen atom before the actual Michael addition takes place to the opposite face of the methoxybenzene group.



In an extension any pentadienyl cation regardless of its origin is capable of bringing about Nazarov reaction. In one study this species is obtained from an alkenyl-substituted dihalocyclopropane :