Cyclopentane 1,2-divinyl

A Ni(0)-catalyzed l,oo-hydrosilylation across the two dienyl moieties of 1,3,8,10-undecatetraene 9 proceeds regioselectively and stereoselectively and provides vie-trans-divinyl cyclopentane products 10 in modest yield (Eq. 3) [13]. The reaction shows an interesting stereoselectivity with respect to the substituent geometry both of the vinyl groups of 10a and 10b are stereoisomeric to each other, and one of the two double bonds is cis and the other is trans. 

Scheme 5 Cope equilibria in 1,2-divinyl-cyclopentanes and -cyclohexanes...

Electrocyclic reactions are not limited to neutral polyenes. The cyclization of a pentadienyl cation to a cyclopentenyl cation offers a useful entry to five-membered carbocycUc compounds. One such reaction is the Nazarov cyclization of divinyl ketones. Protonation or Lewis acid complexation of the oxygen atom of the carbonyl group of a divinyl ketone generates a pentadienyl cation. This cation undergoes electrocyclization to give an allyl cation within a cyclopentane ring. The allyl cation can lose a proton or be trapped, for example by a nucleophile. Proton loss occurs to give the thermodynamically more stable alkene and subsequent keto-enol tautomerism leads to the typical Nazarov product, a cyclopentenone (3.220). 

The rate of the reaction is influenced by the energy of the starting materials, and strained compounds react at much lower temperatures than do unstrained molecules. The 1,2-divinyl derivatives of cyclopropane, cyclobutane, and cyclopentane illustrate this point. c -Divinylcyclopropane rearranges to qrcloheptadiene... 

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