Complexes of alkynes

Some platinum complexes in which the alkyne lies perpendicular to the plane defined by P PtMe have been prepared as follows  

Here the alkyne is attacked by nucleophiles, like the alkenes in Fp(alkene) or the alkynes in the analogous [CpFe(CO)(PPhj)(RC=CR)] .  

The preparation of the complexes just cited is given below. They undergo some unusual reactions with nucleophiles. 

There are also complexes in which an alkyne forms a bridge between two metal atoms. To a first approximation, one of the 7r-systems interacts with the first metal atom and the other with the second (Fig. 7.11). The cobalt compounds are prepared in high yield by reacting alkynes with Co2(CO)g. The cobalt can be removed cleanly by mild oxidation. This provides a useful procedure for protecting an alkyne during organic synthesis. 

Alkyne molecules are often linked together in their reactions with transition metal compounds. An early example of this, discovered by Niewland in 1931, was the linear dimerization of ethyne to but-l-en-3-yne, catalysed by copper(I) chloride. The product was for many years an intermediate in the manufacture of chloroprene for synthetic rubber. 

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Hexacarbonyldicobalt complexes of alkynes have served as substrates in a variety of olefin metathesis reactions. There are several reasons for complex-ing an alkyne functionality prior to the metathesis step [ 125] (a) the alkyne may chelate the ruthenium center, leading to inhibition of the catalytically active species [125d] (b) the alkyne may participate in the metathesis reaction, giving undesired enyne metathesis products [125f] (c) the linear structure of the alkyne may prevent cyclization reactions due to steric reasons [125a-d] and (d) the hexacarbonylcobalt moiety can be used for further transformations [125c,f]. 

NbBrs, and NbCls-Pl Sn evidently proceeds via cyclotrimerization of diynes, which most probably involve cyclic carbometallation, details are not very clear.246 2463 Related reactions of Ta and Mo complexes were also investigated in this study. Formation of tantallacyclopropenes by complexation of alkynes with Ta complexes has also been reported247 (Scheme 51). In addition to the Ta-catalyzed polymerization of diynes mentioned above, Ta-catalyzed or -promoted cyclotrimerization reactions of alkynes to produce benzene derivatives, a Ta-promoted ethylene... 

The structures of platinum(O) complexes of alkynes are fully summarized in Chapter 39 of Comprehensive Organometallic Chemistry and need not be covered here. 

The carbonyl Co2(CO)6 forms stable 7i-complexes of alkynes 0/2 complexes). Four effects on alkyne reactivity are expected from this coordination (i) protection of the triple bond (ii) stabilization of the carbonium ion on the a-carbon (or propargylic cations (iii) syntheses of common and medium-size cycloalkynes and (iv) steric effects. 

Alkyne hexacarbonyidicobalt complexes. The complexes of alkynes with CoifCO) react with 2,5-dihydrofuranes to give 3-oxabicytlol3.3.0]-7-octene-6-ones in high yield (equation I) when the reaction is conducted undci a CO atmosphere. 

In the presence of [Cp2Mo2(CO)4], 3 > [Ni2(COD)2( JL-ri2-RC R)],223 = and [Ni4(RNC)4( X3-V-RC=CR)3], alkynes are converted to ds-alkenes. The alkenes once formed no longer bind to the complex and hydrogenation does not proceed further to give alkanes or even isomerized alkenes. Cocondensation of lanthanoid metal atoms with internal alkynes generates lanthanoid complexes of alkynes, which are potential catalysts for hydrogenation. [ Sm(l-hexyne) n] or [ Er(3-hexyne) n] catalyzes hydrogenation of hex-3-yne to ds-hex-3-ene (97% cis) at room temperature and atmospheric pressure of H2. ... 

In fact, a wide variety of tt complexes Cp2Zr(X=Y), including complexes of alkynes, cycloalkynes, arynes, imines, and thioaldehydes, can be generated by /3-hydride abstraction from complexes of the type Cp2Zr(Me)X-Y-H. These complexes can in turn be made by hydrozirconation of X=Y with Cp2Zr(H)Cl followed by addition of MeLi, or by addition of X-YH to Cp2Zr(Me)Cl. 

There are several examples known where methylenecyclopropanes have been employed as cosubstrates in reactions of the Khand—Pauson type. Generally, this cobalt-mediated reaction is performed with hexacarbonyldicobalt complexes of alkynes, preferably strained alkenes such as norbornene or cyclobutene, and carbon monoxide to produce cyclopent-2-enones. Stoichiometric amounts of cobalt (in the form of the alkyne complexes) are normally needed. Methylenecyclopropane (1) can be employed as the strained alkene and reacts with a variety of alkynes to yield spiro-fused products. Silica gel or zeolites function as promoters of the reaction, giving rise to improved yields. ... 

Decomplexation. Dicobalt hexacarbonyl complexes of alkynes are efficiently decomposed at room temperature with NaSMe in DMF. 

Most of the metal complexes of alkynes discussed so far are stabilized by proximity of an sp hybrid orbital on the alkyne carbon the alternative approach to metal complex formation is donation of the triple-bond electrons to the vacant orbitals of a metal. This yields a vast range of complexes, many of which are stable and can be used in synthesis, but is some cases the alkyne can undergo rearrangements which are not observed in the free organic species. 

Some of the modifications for the Nicholas reaction include the fixation of a propargylic substrate onto a solid phase,application of montmorillonite K-10 as acid to generate the propargylic cation, complexation of alkyne with a ruthenium complex, and application of tetrabutylammonium fluoride for decomplexation. ... 

Nickel complexes of alkynes are involved in many important catal)rtic transformations. A fundamental transformation of nickel(0)-alkyne complexes that forms the basis of a number of usefiil stoichiometric and catal)rtic reactions is the oxidative cyclization of one alkyne and a second unsaturated imit to form a five-membered metallacyclopentene 47 (Scheme 29). If the second unsaturated unit is also an aUqme, linear oligomerizations or cydooligomerizations result The catal5rtic tetramerization of acetylene to octatetra-ene was discovered more than 50 years ago by Reppe,l l and an excellent review on the historical development of this area has appeared.Pl The complexities of the mechanistic... 

Advantage can be taken of several effects of the cobalt complexation of alkynes (Scheme 7.23). This is most often by using the cation stabilization effect to form the substrate for the Pauson-Khand reaction. The dicobalt complex of vinyl acetylene 7.82 underwent a Friedel-Crafts like acylation reaction, via a cobalt-stabilized cation 7.83." The cation was then trapped by an added nucleophile, methanol. Reduction of the ketone 7.84, followed by an intramolecular Pauson-Khand reaction yielded the tricyclic compound 7.85. 

The complexation of alkynes by the fragment Co2(CO)6, achieved by reaction with [Co2(CO)g], is often used to protect the alkynes, a process that is very useful in organic synthesis (see Chap. 21.1). These complexes are clusters, and they react in acidic medium to give the famous Seyferth cluster ... 

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