Enyne metallacyclopentene

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. 

Metallacyclopentenes, in enyne carbometallation, 10, 324 Metallacyclopropanes, with Ti(IV), 4, 359 Metalladiboranes, with Groups 8 and 9, 3, 157—158 2-Metalla-l,3-dichalcogena-[3]ferrocennophanes, Rh complexes, electrochemistry, 7, 149 Metallaoxiranes, preparation, 4, 917 Metallasilazanes, preparation and characteristics, 3, 450 Metallasiloxanes, preparation and characteristics, 3, 458 Metallate(III) compounds, isolated, preparation, 4, 751 ortho-Metallated complexes... 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene undergo transition metal-catalysed addition. The intermolecular reaction is initiated by 7i-complexation of the two double bonds, followed by cyclization to form the metallacyclopentane 12. This is called oxidative cyclization. The oxidative cyclization of the a,co-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative cyclization of the a,co-enyne 15 affords the metallacyclopentene 16. Formation of the five-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be understood by the formation of the metallacyclopropene or metallacyclopropane 17. Then the insertion of alkyne or alkene to the three-membered ring 17 produces the metallacyclopentadiene or metallacyclopentane 18. 

Insertion of CO to the metallacyclopentenes 197 and 198 formed from enynes and metal complexes offers a useful synthetic route to the cyclopentenone derivatives 199 and 200. This [2+2+1] cycloaddition mediated by Co2(CO)8 is called the Pauson-Khand reaction [80], Both inter- and intramolecular versions are known. 

A ruthenium based catalytic system was developed by Trost and coworkers and used for the intermolecular Alder-ene reaction of unactivated alkynes and alkenes [30]. In initial attempts to develop an intramolecular version it was found that CpRu(COD)Cl catalyzed 1,6-enyne cycloisomerizations only if the olefins were monosubstituted. They recently discovered that if the cationic ruthenium catalyst CpRu(CH3CN)3+PF6 is used the reaction can tolerate 1,2-di- or tri-substituted alkenes and enables the cycloisomerization of 1,6- and 1,7-enynes [31]. The formation of metallacyclopentene and a /3-hydride elimination mechanism was proposed and the cycloisomerization product was formed in favor of the 1,4-diene. A... 

Cyanosilanes can be isocyanide sources since a tautomeric equilibrium exists between cyanosilanes and the corresponding isocyanides. The equilibrium largely favors the cyano tautomer. The use of such dilute isocyanide donors realizes efficient Ti(n)- and Ni(0)-catalyzed cyclizations of enynes to iminocyclopentenes via metallacyclopentene intermediates (Scheme 19).266,266 266b Treatment of zirconacyclopentanes and -pentenes with Me3SiCN provides zirconocene-imine complexes, which serve for carbon-carbon bond formation with various unsaturated bonds.267... 

Metallacyclopentenes typically undergo CO insertion to form the corresponding cyclopentenones. This principle was exploited by Negishi et al. [14] to generate zirconocene-type intermediates 2 from enynes and convert them into the PK products 3 (Scheme 2). 

When an enyne is reductively coupled in an intramolecular fashion with Zr or Ti, the metallacyclopentene that is obtained initially may be carbonylated to the corresponding bicyclic cyclopentenone with CO. The reaction proceeds by insertion of CO and reductive elimination. In Ti-promoted cyclization, in fact, under certain conditions the Ti(II) fragment produced by reductive elimination can promote reductive coupling of another enyne, providing a basis for a variant of this reaction that is catalytic in Ti. 

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