Olefins Nazarov cyclization

The majority of yc/n-dichIorocycIopropane substrates examined in this study provided the desired a-chlorocyclopentenones as a result of sequential electrocyclic ring opening and Nazarov cyclization. In general, those substrates lacking additional substitution on the cyclopropane moiety provided products 75 selectively as a result of regioselective elimination to deliver the more electron-rich olefin. The mechanism for this transformation is believed to involve disrotatory halocyclopropane ring opening... 

Nazarov cyclization (in presence of Ets SiH) and the interrupted Nazarov reaction (in presence of pendant olefin, eq 60). ... 

While the product of this reaction somewhat resembles a Nazarov cyclization, it is thought that the reaction proceeds via a different mechanistic pathway and depends on the nature of the palladium catalyst. Activation of the olefin by palladium, followed by cyclization gives palladium enolate 41. When Pd(OAc)2 is used as the catalyst, P-hydride elimination of 41 produces cyclopentenone 42, and molecular oxygen reoxidizes the palladium. On the other hand, when PdCl2(MeCN)2 is used as a catalyst, hydrolysis generates HCl which results in rapid protonation of 41, giving cyclopentenone 43 as the product. 

Leitich J, Heise I, Rust J, Schaffner K. The photo-Nazarov cyclization of l-cyclohexenyl(phenyl)methanone revisited-trapping of the 2-oxyallyl intermediates by olefins. Eur. J. Org. Chem. 2001 2719-2726. 

In general, steric hindrance at the P-carbon atom slows down the reaction. Tetra-substituted alkenes cyclize slower than tri-substituted ones, which react slower than di-substituted olefins. One could suggest that steric interactions diminish the stability of the s-tmnsis-tram conformer. We should keep in mind, however, that EDG or EWG groups also have an influence on the ease by which a Nazarov reaction proceeds. 

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