Coordinated alkynes, displacement

In contrast to the inertness of bisalkynebisdithiocarbamate complexes, alkyne displacement from bisalkyne cyclopentadienyl derivatives is common. An extensive series of cationic [CpMo(CO)L(RC=CR)][BF4] complexes has been prepared from [CpMo(CO)(RC=CR)2][BF4] reagents by substitution of one of the coordinated alkynes [Eq. (19)] (72). Reaction of the carbonyl reagent with phosphines occurs smoothly at room temperature in methylene chloride to form monoalkyne products in high yields... 

Several studies of the kinetics and effects of structure on reactivity lend support to a mechanism of oxidative coupling of the type first proposed by Bohlmann and coworkers The rate is second order with respect to Cu(ii) and alkyne, and varies inversely with [H+] . This is interpreted in terms of rapid steps involving displacement of a solvent molecule or other ligand from the coordination sphere of Cu(n) by an alkyne molecule, followed by acid dissociation of the coordinated alkyne to give an acetylide complex. In the rate-determining step, copper(ii) is reduced and simultaneously the alkynyl groups are coupled. These steps are summarized in equations (6), (7) and (8), where L represents a ligand—solvent, for... 

Recent additions to the family of alkene complexes are fullerene derivatives such as Rh(CO)(q -Cgo)(H)(PPh3)2 Pd(q -Cgo)(PPh3)2 (Figure 23.17b) and (q -Cp)2Ti(q2-Cgo). The Cgo cage (see Section 13.4) functions as a polyene with localized C=C bonds, and in Cgo Pt(PEt3)2 6, six C=C bonds (remote from one another) in the Cgo cage have undergone addition. Reaction 23.73 illustrates CgQ-for-ethene substitution (the 16-electron centre is retained), and reaction 23.74 shows addition to Vaska s compound (a 16- to 18-electron conversion). Equation 23.75 shows the formation of the first fullerene complex of titanium, by fullerene displacement of a coordinated alkyne. 

In the reductive coupling part of the reaction, one alkyne displaces 1-butene from Zr(II) to give a new tt complex then a second alkyne coordinates to the Zr(IV) metallacyclopropene and inserts into the C—Zr bond to give a Zr(IV) metallacyclopentadiene. 

The relative ease of the cyclization step from A to C may also be linked to the nucleophilic or coordinative ability of the heteroatom bound to the metal. The reaction of 7 with diphenylacetylene (Ph2C2) leads to the seven-membered derivatives 68 and 69 after prior isolation of the monoinsertion product 24, treatment with a silver salt, followed by the usual thermolytic conditions. This is another rare example of an intramolecular formation of a C-S bond within the coordination sphere of a transition metal and a novel, albeit limited to one alkyne, route to the rare family of dibenzo[bd] thiepins. With the closely related 8, which differs from 7 only by the tertiary amine unit in the metallacyclic framework instead of a thioether function, a carbocyclic product 71 is obtained (see under carbocycle reactions, next section). The formation of the seven-membered S-heterocycles is attributed to the good coordinative ability of the thioether group in 7. The S-atom remains close to the vinylic carbon function before the cyclization. With the poorly coordinating, readily displaced amine function in 8, the N-atom is detached from the metal and ultimately affords a spirocyclic product (see Scheme 18). 

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

Many examples of complexes containing enynyl ligands are known from reactions of 1-alkynes with various metal complexes two coordination sites are necessary for this reaction. Displacement of the ligand often results in a catalytic cycle of head-to-head dimerization of the alkyne. The protonation may occur by solvent, e.g., MeOH, in other cases acid is required, e.g., CF3CO2H. Coupling... 

These reactions can be used to prepare a novel series of complexes where cyclic alkynes can be stabilized by coordination to platinum(O).831,832 The compounds are feasible because coordination of a triple bond to platinum causes a distortion of the alkyne from linearity by displacement of the alkynic substituents back away from the platinum. Also these methods can be used to prepare platinum(O) alkyne complexes with substituents other than triphenylphosphine.833-836... 

Five-coordinate d4 alkyne derivatives are rare. Addition of acetylene gas to Mo(CNBu )4(SBu )2 in toluene at 30°C forms Mo(HC=CH)(CNBu )2-(SBu )2 as a single acetylene displaces two bulky isonitrile ligands. Slightly higher temperatures (50°C) are required to promote formation of... 

Complexes containing coordinated tertiary phosphines or arsines of the type [Rh2(PF3)4L2(alkyne)] and [Rh2(PF3)2L2(alkyne)] (L = mono-dentate, L = bidentate ligand) are readily obtained by displacement of PF3 from [Rh2(PF3)6(alkyne)] under mild conditions (method D). There are also reports of displacement of CO by PF3 from an alkyne metal carbonyl complex (method E) e.g., 113). 

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