Metallacyclopentenes, formation

Chelation of the alkene and the alkyne moieties to a metal species generally results in the formation of a metallacyclopentene that can undergo three kinds of transformations. 

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. 

Zhang has proposed a mechanism for the rhodium-catalyzed Alder-ene reaction based on rhodium-catalyzed [4-1-2], [5-i-2], and Pauson-Khand reactions, which invoke the initial formation of a metallacyclopentene as the key intermediate (Scheme 8.1) [21]. Initially, the rhodium(I) species coordinates to the alkyne and olefin moieties forming intermediate I. This intermediate then undergoes an oxidative cycHzation forming the metallacyclopentene II, followed by a y9-hydride elimination to give the appending olefin shown in intermediate III. Finally, intermediate III undergoes reductive elimination to afford the 1,4-diene IV. 

The formation of a metallacyclopentene, which is presumed to be the key intermediate in the preceding examples, prompted the question of whether a Ci unit, such as CO or an isocyanide, could be inserted into this intermediate and thereby allow for an [m-i- -i-l] carbocyclization reaction (Scheme 11.2). These types of reactions wiU be discussed in detail in this chapter. 

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. 

Many cyclization reactions via formation of metallacycles from alkynes and alkenes are known. Formally these reactions can be considered as oxidative cyclization (coupling) involving oxidation of the central metals. Although confusing, they are also called the reductive cyclization, because alkynes and alkenes are reduced to alkenes and alkanes by the metallacycle formation. Three basic patterns for the intermolecular oxidative coupling to give the metallacyclopentane 94, metallacyclopentene 95 and metallacyclopentadiene 96 are known. (For simplicity only ethylene and acetylene are used. The reaction can be extended to substituted alkenes and alkynes too). Formation of these metallacycles is not a one-step process, and is understood by initial formation of an tj2 complex, or metallacyclopropene 99, followed by insertion of the alkyne or alkene to generate the metallacycles 94-96, 100 and 101-103 (Scheme 7.1). 

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... 

The mechanism of this transformation is unclear at the present time, but two possibilities are pictured below. In the first (Fig. 5), loss of a CO ligand and binding of the acetylene initially provides the T -alkyne complex 17. Subsequent loss of a second equivalent of CO allows for coordination of the alkene to give 17a. Insertion of the olefin into the titanium-carbon bond of the alkyne complex produces metallacyclopentene 18. The insertion of CO generates acyl complex 19 which, upon reductive elimination, yields the observed cyclopentenone product. A second plausible mechanism (Fig. 6) involves initial formation of metal-... 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

Initial oxidative coupling of the ligated Rh complex with both the alkyne and alkene gave the metallacyclopentene A, followed by olefin insertion to form metallacycloheptene B. Tricyclic compound 430 was obtained by reductive elimination of Rh from B when is not hydrogen. In contrast, when R is hydrogen, a 1,3-hydride shift with concomitant ring opening takes place to afford metallacycle D. Subsequent reductive elimination of Rh resulted in the formation of bicyclic compound 431. 

The mechanism proposed involves formation of a metallacyclopentene 11.156, followed by strain-driven ring opening of the cyclopropane to form a metallacyclooctadiene 11.157 (Scheme 11.52). This ring expansion is then followed by reductive elimination. 

The formation of metallacyclopentenes is very general and occurs with almost all the coordinatively unsaturated transition metals and with great variety of unsaturated partners. The main steps of the catalytic cycle are described in... 

The two general mechanisms that have been proposed for the rhodium-catalyzed [5-1-2] cycloaddition are depicted in Scheme 20.10. One would proceed through initial formation of a metallacyclohexene followed by alkyne insertion and then reductive elimination. A second would involve initial formation of a metallacyclopentene followed by cleavage of the cyclopropane (ring expansion) and then reductive elimination [25]. 

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