Cyclopentene annulations, Michael addition

In 1986, we found that alkynyl-A3-iodanes serve as good Michael acceptors toward soft nucleophiles, because of the highly electron-deficient nature of the /3-acetylenic carbon atom. This conjugate addition of nucleophiles constitutes a key step of a highly versatile cyclopentene annulation of alkynyl-A3-iodanes via the tandem Michael-carbene insertion (MCI) reaction [Eq. (103)] [185]. 

Scheme 3-5 Cyclopentene annulations via a tandem Michael addition-carbene insertion reaction [51].

The homoenolates generated from enals by NHC catalysis undergo annulation with acyclic dienones to form cyclopentenes and cyclopentanones (Scheme 6.19). In the present case, the enolate resulting from the Michael addition of NHC-bound homoenolate to dienone either undergoes intramolecular aldol reaction, concomitant with the ejection of catalyst yielding cyclopentanone, or proceeds via the same pathway proposed in Scheme 6.16. to yield cyclopentenes [23]. 

The authors gave the following rationalization to explain the ylide annulation (Scheme 20.25). Triphenylphosphine reacts with bromide 26 to form a phospho-nium salt, which is deprotonated by K2CO3 to generate the corresponding phos-phonium ylide in situ. A Michael addition of the ylide to the electron-deficient olefins, followed by another intramolecular Michael addition of the zwitterionic intermediates, and then P-eHmination of triphenylphosphine completes the catalytic cycle. The formation of the two isomers can be explained by a- or y-attack of yHde to dually activated olefins 28. Of course, the possibility of migration of the double bond in cyclopentene products 30 under the reaction conditions cannot be excluded. 

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