Alkyne trimerization

Scheme 9.34 Intramolecular iron-catalyzed alkyne trimerization.

Alternatively, with known metathesis catalysts such as (PCy3)2Cl2Ru=CHPh, a metal carbene mechanism of alkyne trimerization has been proposed.141... 

When compounds (7) were heated with alkyne in excess, two types of complexes, both involving alkyne coupling, are formed. A compound with the stoichiometry Co2(CO)4(C4R2CO)2, formed mainly from terminal alkynes having one bulky substituent R, represents derivatives of Co2(CO)g where two CO groups at either metal are replaced by a cyclopentadienone ligand. This compound type represents one of the many instances where alkynes combine with CO in the presence of a transition metal fragment to yield mostly cyclopentadienones, often complexed to the metal this cycloaddition reaction is similar to the Pauson-Khand scheme except for the use of an alkyne in place on an alkene (see also Section 5.1.4 and Scheme 26). The reaction eventually proceeds further to liberate an arene. Thus, from the use of t-BuC=CH, the alkyne trimerization product 1,2,4-tri-f-Bu-benzene was isolated. 

Cationic sandwich complexes of the type CpCo(arene) + were first prepared by hydride abstraction from cyclohexadi-enyl cations (Section 7.1). They are accessible in broader variation from the reaction of CpCoX half-sandwich complexes with arene in the presence of AICI3. Their electrochemical reductions to the corresponding 19-electron monocations and to 20-electron neutral complexes have been studied. The stability of electron-rich sandwich complexes increases with increasing alkyl substitution in either ring despite the more negative redox potential mass spectrometry studies of bond dissociation energies of (arene)Co+ complexes corroborate these results. However, neutral sandwich complexes are not very stable in the polar solvents necessary for the reduction of mono- or dications and have been isolated only from alkyne trimerization with CpCo precursors in nonpolar solvents (Section 5.1.4). 

A number of other mechanistic sequences are thought to occur in alkyne trimerizations mediated by other metal systems. Catalysts derived from Co2(CO)s follow a characteristic sequence of steps that involves exclusively dinuclear complexes (Scheme 26). The nature of the final (alkyne)3C02(CO)4 intermediate, a so-called flyover complex, is supported by both X-ray crystal structure data as well as its involvement in forming other products besides benzenes. These dinuclear cobalt systems are note-... 

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." ... 

G. M. Whitesides and W. J. Ehmann, 7. Am. Chem. Soc., 1969, 91, 3800 see also G. A. Ville, K. P. C. Vollhardt and M. J. Winter, Organometallics, 1984, 3, 1177 for further studies along these lines, and J. R. Strickler, P. A. Wexler and D. E. Wigley, Organometallics, 1988, 7, 2067 for studies of relevant model systems associated with alkyne trimerizations catalyzed by early transition metals. Note that although cyclobutadiene-like structures are not involved in most alkyne trimerization mechanisms, metal-complexed cyclobutadienes are fairly common side products in alkyne trimerization reactions. As their yields are usually low, their formation does not present practical problems in arene synthesis. 

X-Ray crystallographic investigations have been carried out on representatives of all the important metallacycloalkenes. The first key product of alkyne trimerization is a metallacyclopropene, obtained by oxidative addi-... 

Like their metal acycloalkane analogs, the chemical behavior of the unsaturated species is quite different from the corresponding heterocycles without a metal atom. Metallacycloalkenes appear also as reactive intermediates, e.g., in alkyne trimerization or cotrimerization with heterosystems or in alkyne metathesis. Some important examples are given in the following sections. 

Enynes in which three or four atoms separate the double and triple bonds cyclize upon complexation to Co2(CO)g and subsequent heating to give bicyclic enones (equation 52). With the exception of slightly elevated temperatures the conditions required are no different than those of the stoichiometric procedure described earlier for reactive substrates in intermolecular Pauson-Khand reactions. The intramolecular cycloaddition cannot in general be carried out under catalytic conditions. Hex-l-en-5-yne, which would give a four-membered ring upon intramolecular cycloaddition, instead undergoes alkyne trimerization exclusively.3 The most extensively studied systems are those derived from hept-l-en-6-yne, as the products, bicyclo[3.3.0]oct-l-en-3-ones, are useful in the synthesis of numerous cyclopentane-based polycy-clics. 

Aldol condensation, 111, 249 Aldoximes, 41, 42, 383 Alkali acetylides, 166 Alkamines, 261 Alkenyl bromides, 254 Alkyl azides, 403, 404 9-Alkylbicyclo[3.3.1JnonanoI-9, 31 Alkyl bromides, 254 Alkyl fluorides, 136 Alkyl halides, 133, 378 5-Alkyloxazolidones, 122 Alkylsulfenes, 103 Alkynes, 141, 248, 249 Alkynes, trimerization, 23 Allene, 388 AHenes, 274-276, 465 Allenyl ethers, 337 Allyl alcohol, 415 Allyl alcohols, 4)5 Allylailenes, 52 Allylamines, 326 Allyl bromide, 324 Allylcarbinyl chloride, 463 Allyl chloride, 109 Allyl ethers, 158... 

A synthetically important breakthrough in the application of transition metal catalyzed alkyne trimerization was made in the selective cocyclotrimerization of diynes with silylated alkynes using dicarbonyl(t/5-cyclopentadienyl)cobalt as precatalvst. The silylated alkynes do not show any tendency to undergo self-trimerization thus, depending on the nature of the chain between the two alkyne units, benzo-fuscd carboeyclic or heterocyclic rings are formed3,31 34... 

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