Metallacycles metallacyclopentadiene

Terminal acetylenes and Ru3(CO)j2 yield complexes of the type [57] (9,190, 336), whereas internal acetylenes form either complexes [56] or acetylene-substituted RU4 complexes (229). Alternatively, two acetylene moieties are incorporated with formation of metallacyclopentadienes (229), a class of compounds more familiar in osmium cluster chemistry (cf. Chapter 3.4.). Instead of two acetylene molecules, one molecule of an arylbutadiene may be the precursor of the metallacycle (382). 

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). 

Unsaturated metallacycles have also had a fascinating history. Often these are readily formed by the coupling of alkyne units and may be involved in the catalytic cyclization of alkynes. There are numerous examples of structurally characterized metallacyclopentadienes, and even some examples of metal-lacycloheptatrienes and 4-benzene complexes (77). More recently metal-lacyclobutadienes have been implicated as the active intermediates in alkyne... 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

Alkyne complex 37 is prepared by intramolecular hydrogen abstraction followed by methane elimination from the methyl metallocene compound 36 in the presence of a donor ligand (Scheme 6.8). 37 reacts with alkenes, alkynes, carbonyl compounds, and nitriles to give 5-membered metallacycles [89, 116-117]. In the absence of any donor ligand, metallacyclopentadiene 38 is formed. Complex 37 is also isolated by trapping 30 with an alkyne [89]. 

Recently the oxidative coupling of olefins and alkynes, not only with late transition metal complexes, but also with low valent early transition metal complexes to give metallacyclopentane or metallacyclopentadiene complexes is attracting increasing attention. For example, titanium(II) and zirconium(II) complexes react with olefins and form metallacycles. Various stoichiometric processes to produce useful compounds that are otherwise unavailable can be produced by exploit-... 

On the basis of the fact that metallacycles such as metallacyclopentadiene derivatives have unique and versatile reactivity,incorporation of these units into the backbone of the polymers are quite attractive to realize mainchain reactive materials (Fig. 2). Besides, these materials are potentially attractive as novel functional materials if the unique geometry of the organometallic systems reflects the character of the polymers and/or each metal center included in the polymer chains interacts by appropriate macromolecular design. 

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