Acceptor-substituted enynes mechanism

In contrast to nucleophilic addition reactions to activated dienes (Sect. 4.2.1), the mechanism of 1,6-cuprate additions to acceptor-substituted enynes is quite well understood, largely thanks to kinetic and NMR spectroscopic investigations [3oj. 

Scheme 4.4. Proposed mechanism for the 1,6-addition of organocuprates to acceptor-substituted enynes.

In contrast to nucleophilic addition reactions to activated dienes, the mechanism of 1,6-cuprate additions to acceptor-substituted enynes is quite well understood, the main tools being kinetic and NMR spectroscopic investigations. C-NMR spectroscopic studies have revealed that these transformations proceed via jr-complexes with an interaction between the jr-system of the C=C double bond and the nucleophiUc copper atom (a soft-soft interaction in terms of the HSAB principle), as well as a second interaction between the hard lithium ion of the cuprate and the hard carbonyl oxygen atom (Scheme 4) q of C-labeled substrates has confirmed that the cuprate does... 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

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