Metallacyclopentene

Metallacyclopentanes, 3,4-dimethylene-synthesis, 1, 669 Metallacyclopentan-2-ones synthesis, 1, 669 Metallacyclopentenes synthesis, 1, 670 Metallafluorenes synthesis, 1, 671 Metallaindanes synthesis, 1, 670 Metallaindenes synthesis, 1, 670, 671 Metallaxanthenes synthesis, 1, 671 Metalloporphyrins anions, 4, 398 demetallation, 4, 389... 

The presence of a cyclopropyl moiety in the carbene complexes makes them useful for synthesis. The cyclopropylcarbene complexes 95 undergo a cycloaddition reaction with alkynes to give the cyclopentenones 96 [51]. The reaction course is explained as being metallacyclopentene fragmentation. (Scheme 34)... 

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. 

In order to prepare very clean unsymmetrical zirconacyclopentadienes, the use of ethene is a prerequisite [14] (Eq. 2.4). An excess of ethene stabilizes the intermediates such as zirconacydopentane 5a and zirconacyclopentene 4. Such a transformation from a metallacyclopentane to a metallacyclopentene was first demonstrated by Erker in the case of the hafnium analogues [15]. 

An allenylaldehyde can be transformed efficiently into an a-methylene-y-butyro-lactone by a ruthenium-catalyzed carbonylative cycloaddition process (Scheme 16.34) [37]. The reaction mechanism may involve a metallacyclopentene, which undergoes insertion of CO and reductive elimination leading to the product. 

When trimethylsilyl substituted diyne 607 was reacted with methyl vinyl ketone, the reaction proceeded with complete regioselectivity and without aromatization to afford 608 with 56% yield (equation 174). The regioselectivity observed was considered to result from a metallacyclopentene intermediate which was built up of the nickel atom, the double bond of methyl vinyl ketone and the less substituted triple bond of 607. 

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. 

The proposed mechanism of the ring cleavage reaction of HCI (and other protic acids) with cyclopropyl carbynyl complexes involves addition of HQ across the carbyne triple bond to give a carbene complex as key intermediate. In the absence of a carbonyl ligand this is followed by ring expansion to a metallacyclopentene complex, /J-hydrogen elimination and reductive elimination to the diene complex (equation 111)164. 

Mechanistic studies are consistent with photochemical electron transfer from the carbyne complex to chloroform followed by H atom abstraction. Ring expansion then occurs to give a metallacyclopentene, which undergoes carbonyl insertion. Finally, reductive elimination yields the cyclopentenone complex that slowly releases the free enone (equation 119)158. 

The reactions of Ru(CO)4(C2H4) with activated alkynes have been shown in some cases to yield metallacyclopentanes as well as metallacyclopentenes and metal-lacyclopentadienes, depending on the nature of the activating group (CF3 vs. C02Et)62 (Scheme 20). 

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. 

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). 

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. 

As mentioned in the introduction [lc], no selectivity was observed in early dimerization experiments of 1. But when other partners were offered, the corresponding crossdimerizations were quite selective. Probably methylene metallacyclopentenes 2 [4], which could be isolated, are intermediates that then react with the other partners. Generally, the related 1,3-dienes are less reactive than 1 with its reactive allenic double-bond and do not react in a similar manner [4a]. Rh-catalyzed [4+1] cycloadditions with CO as a second reaction partner led to alkylidene cyclopentenones 3 and 4 [4, 5], while in Pd-catalyzed reactions where 1 was generated in situ and a base was present, only 4 [6] was formed. When Pt(0) was used instead of Rh(I) in the carbonylation reaction, both in the presence of the (R,R)-DuPHOS-ligand, opposite enantiomers of 3 were obtained [5b], This observation still needs a precise explanation. [Fe(CO)5]-mediated reactions of diallenes form dialkylidene cyclopentenones 7 (Scheme 2, here 10 mol-% of catalyst are needed) [7],... 

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... 

The (s-cA-butadiene) Group 4 metallocenes adopt a a2,7i-type structure. The actual strength of the n-bonding component and, hence, the metallacyclopentene character of the complexes depends very much on the substitution pattern of the diene ligand6 and it is also strongly influenced by the nature of the bent metallocene unit. These various influences were recently analyzed for some ansa-metallocene/ 1,3-diene combinations by means of computational chemistry,83,84 and the results were compared with the dynamic features (AGfw of the ring-flip inversion process, solid... 

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... 

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