Metallacyclopentadienes reactions with alkynes

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Scheme 4-25 summarizes the most common metal-promoted /nfe/molecular cyclocoupling reactions of various species with alkynes. The most prominent organic products include arenes, cyclooctatetraenes, cyclohexadienes (with olefins), pyridines (with nitriles), cyclopen-tenones (with olefin ( CO the Pauson-Khand reaction [98]), pyrones (with CO2), and five-membered heterocycles (with X = S, Se) common organometallic products include cyclobutadiene complexes, cyclopentadienone complexes, and metallacyclopentadienes. 

Metallacyclopentadiene is a common intermediate in the [2 -b 2 -b 2] cycloaddition of alkynes. The oxidative cyclization of alkynes to a metal center gives metaUacyclopen-tadiene. Various metallacyclopentadiene complexes are formed by this reaction. It is generally accepted that there are two mechanisms for the reaction of metallacyclopentadiene with alkynes to give benzenes a Diels-Alder type of mechanism to give l-metallabicyclo[2.2.0]hepta-3,6-diene intermediate and an insertion mechanism to give metallacycloheptatriene (Scheme 5.23). 

An intriguing annulation has been observed upon treatment of a Zr(II)-borata-benzene complex with an alkyne (Scheme 23).38 This reaction is believed to proceed through generation of the normal metallacyclopentadiene intermediate, followed by migration of the Zr—C bond to the electrophilic boron, and then formation of the C—C bond. Both the starting Zr(II) complex and the annulation product have been crystallographically characterized. 

Pyridine formation, by the reaction of a metallacyclopentadiene with a nitrile, has been extensively investigated in the case of cobalt [If]. When pyridine derivatives are prepared from two different alkynes and a nitrile, specific substituents are needed for the selective coupling reactions. In most cases, a mixture of two isomers (91 and 92) is obtained, the formation of which can be rationalized as shown in Eq. 2.61 [If,27a,44]. 

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-... 

Photolysis of dicyclopentadienyltitanacyclobutanes in the presence of alkynes gives cyclopropanes and the corresponding metallacyclopentadienes. Similar results are obtained with ethene and phosphanes. With substituted metallacyclobutanes 14, partial loss of stereochemical information depending on the reaction time is observed (15/16 >5 1 to 3 1). Therefore, reversible homolytical cleavage to form a diradical intermediate (with isomerization of the starting material) has been suggested. 

Shibata et al. reported that, in the presence of an iridium catalyst, the carbonylative alkyne—alkyne coupling reaction of the diyne 102 with carbon monoxide gave the tetrahydrofuran-fused cyclopen-tadienone 103 (Scheme 35).114 The rhodium-catalyzed alkyne—alkyne coupling reaction of 102 with the isocyanide 104 produced the iminocyclopentadiene 105 (Scheme 36).114b These reactions proceed through formation of the metallacyclopentadiene intermediate 106, which undergoes insertion either of CO or of the isocyanide 104. 

Likewise when two alkyne molecules coordinate to a transition metal such as Co(I) with subsequent coupling of the C-C bond, oxidative cyclization takes place to give a metallacyclopentadiene. Further reaction of another alkyne molecule with the metallacyclopentadiene followed by reductive elimination liberates benzene derivatives. Thus cyclotrimerization of three alkyne molecules catalyzed by a cobalt complex [40,41] can be performed. If a nitrile is used as the second component, pyridine derivatives are obtained catalytically as shown in Scheme 1.13 [42]. The catalytic cyclotrimerization and cyclodimerization of alkynes and conjugated enynes have found extensive applications in synthesis of complex cyclic compounds such as steroid derivatives [43]. 

There is some material of relevance in a review on niobium and tantalum coordination chemistry. 1 The thermal reactions of the ions M" " (M = Ta, Nb, NbO, NbN) with arenes in the gas phase results in cleavage of the arenes to form metallacyclopentadiene ion complexes via retro-alkyne cyclotrimerization mechanisms. ... 

A f/oro-mthenacyclopentadiene complex 55 was obtained by the reaction of 7 with linear alkanes (Equation (18))." During this reaction, six C-H bonds of alkane were successively cleaved. Such multiple C-H bond cleavage was rationalized by the cooperative action of the neighboring ruthenium centers of the ttimetallic framework, which promotes the incorporation of alkane into the reaction field of the cluster. Such closo-typc metallacyclopentadiene complexes have been intensively studied using carbonyl clusters, and often prepared by the coupling reaction between the two alkyne ligands of the bis(/r3-7 ( )-alkyne) complex, so-called violet isomer.The /i3-sulfido complex 9 obtained by the reaction of 7 with thiophenol also reacts with alkane to yield a /ts-alkylidyne complex. ... 

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