Acceptor-substituted enynes 1.6- reduction

In this chapter, nucleophilic l,n-additions (n = 4, 6, 8,. ..) to acceptor-substituted dienes, enynes and polyenes are presented2. Addition reactions which obviously proceed via non-nucleophilic pathways (e.g. catalytic reductions, electrophilic or radical additions3), as well as 1,2-additions to the acceptor group, are not covered. 

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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