Metallacycles from alkene insertion

In the proposed mechanism, the Rh catalyst selectively reacts with the diyne moiety of 525 to form metallacycle A. Alkene insertion to the Rh-C bond and CO coordination gives metallacycle B. From metallacycle B, CO insertion to the Rh-C bond gives metallacycle C or C , and subsequent reductive elimination affords cycloadduct 526. Reductive elimination of Rh from metallacycle B prior to CO insertion gives the [2+2+2] cycloadduct 527. Kaloko et al. forther expanded the scope of this novel [2+2+2+1] cycloaddition process to cyclohexene-diyne substrates 528, which gave the corresponding tetracyclic products 529 as single diastereomers in high yields (Scheme 2-78). 

A rare example of a ferracycloheptane 108 was obtained as the product of the photochemical reaction of a Petitt s cyclobutadiene iron complex with dimethyl-maleate [Eq. (43)].118 The ferracycloheptane arises from the insertion of a maleate into each of two Fe-C bonds and might therefore be considered a special case of alkene trimerisation (vide infra). The cyclobutene fragment in the final metallacycle remains coordinated to iron, as established crystallographically (Fig. 34). 

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). 

Dissociation of Me3P from the 18-electron complex gives a 16-electron complex. Association of the carbonyl group gives a Ti(II) n complex that can also be described as a Ti(IV) metallaoxirane. Dissociation of the second Me3P, association of the alkene, and migratory insertion into the C2-Ti bond gives a five-membered metallacycle. 

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. 

Extrusion is the reverse reaction of insertion (CN +1, VE +2, ON unchanged). The reaction plays a very important role in ethylene oligomerization according to the insertion/elimination mechanism as the so-called P-H-elimination. Scheme 6.16.6 illustrates this elementary step for the extrusion of a 1-hexene product from a Ni-hexyl complex. The extrusion step is followed by 1-hexene dissociation from the complex (see above) to finally liberate the 1-hexene product. Below, in Scheme 6.16.8, the extrusion step is shown as part of a more complex reaction sequence for the liberation of a 1-alkene product from a chromium metallacyclic intermediate. 

Ni-alkyne bonding consists of contributions from both the 77, 7t- and cr,diyl tautomers. This bonding picture helps visualize the insertion reactions with alkynes, alkenes, and CO that result in the formation of metallacycles. Thanks to such insertion reactions, Ni-alkyne species are active intermediates in a number of catalytic applications such as alkyne oligomerization, carbonylation, and insertion of heterocumulenes such as CS2 and GO2. For example, a recent example of a C02-fixation reaction involved the stoichiometric, alkylative or arylative carboxylation of alkynes to give a,(3- and / ,/ -unsaturated carboxylic acids. Ni(0)-alkyne complexes have also been used as pre-catalysts in the addition of hydrosilanes to alkynes. In most cases, monoalkynes react to give the products of m-addition, whereas diynes produce enynes (1,2-addition), allenes (1,4-addition), or 1,3-butadienes (1,2,3,4-addition). ... 

Zirconocene or titanocene mediated intramolecular cyclization reactions of enynes followed by CO insertion into their corresponding five-membered metallacycles led to the formation of bicyclic cyclopentenones (Eqs.40,41) [37,38]. Intermolecular coupling of alkynes, alkenes, and CO mediated by zirconocene or titanocene affording cyclopentenone derivatives have also been achieved (Eq. 39) [18, 39, 40]. It is noteworthy that, in order to obtain the desired cyclopentenones from the reaction of zirconacyclopentenes with CO, termination of the reaction mixture with I2 is necessary. Alcohols are normally formed if the reaction mixture is treated with aqueous acid. However, in case of titanacy-clopentenes, quenching with 3 N HCl gave cyclopentenones exclusively [18]. 

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