Metallacyclopentadiene complexes from alkynes

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 wide variety of homogeneous and heterogeneous catalysts are available for alkyne cyclotrimerization. As a result, numerous mechanistic pathways have been established for the different versions of this process, each characteristic of the metals involved in the system. The most common involves the intermediacy of metallacyclopentadienes, derived as already shown from any number of metal fragments and two alkynes. Upon opening a vacant coordination site, these systems may readily complex a third alkyne, which may insert to give a transient metallacycloheptatriene from which the benzene product is ultimately released via reductive elimination of the metal (Scheme 24). ... 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

Next, 1-butene is displaced from Zr(II) by an alkyne to give a new 77 complex then, the other alkyne coordinates to the Zr(IV) metallacyclopropene and inserts into the C-Zr bond to give a Zr(IV) metallacyclopentadiene. 

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]. 

A reasonable mechanism would involve formation of a bis-alkyne complex, followed by oxidative cy-clization to give a metallacyclopentadiene (Scheme 11.11). Coordination of a second alkyne, followed by insertion and reductive elimination from a metallacycloheptatriene, would yield the trimeric product. An analogous mechanism may be drawn for the formation of cyclooctatetraene, but with an additional alkyne coordination-insertion sequence prior to reductive elimination, a process that seems to be favoured by employing a nickel catalyst with fewer ligands. 

Heterobimetallic complexes involving group 4 metallocene and Ni(0)-cycloolefin moieties have been prepared from Ni-alkyne complexes. For instance, reacting the butadiyne-Ni(O) complex Ni(r7 -Ph-G=G-G=G-Ph)(PPh3)2, 10, with MGp2 precursors (M = Ti and Zr) leads to the species 11, in which metallacyclopentadienes are coordinated in ry -fashion to the Ni center (Equation (2)). It is important to note that these complexes are stable with Ph substituents only. 

Compared with symmetrical metallacyclopentadienes, unsymmetrical met-allacyclopentadienes from two different alkynes are not always easily prepared. Several methods have been reported for preparation of unsymmetrical zir-conacyclopentadienes in which the key step is the addition of a second alkyne to zirconocene-alkyne complexes generated in situ [20]. Formation of homocoupling products from the same alkyne is often the biggest problem (Eq. 21). 

Six-membered heterocycles were obtained from two different alkynes and other unsaturated organic substrates involving C=0 and C=N moieties. The Reppe-type cyclotrimerization can be also applied for preparation of pyridine derivatives when one of the alkynes is replaced by a nitrile. The pyridine formation from two alkynes and a nitrile with Co complexes was originated by Wakatsuki and Yamazaki [77]. Although this method is effective, there is a critical problem for the selective intermolecular coupling of two different alkynes with a nitrile. As shown in Eq. 67, two isomers of pyridine derivatives are formed when a metallacyclopentadiene reacts with a nitrile, due to the two possible orientations of the nitrile in its coupling with the unsymmetrically substituted metallacyclopentadienes. 

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