Alkyne coordination

The authors proposed the conventional nucleophilic attack by an SH group to the triple bond of the alkyne coordinating to Pd(II) [path (a) in Scheme 7-3]. 

Compounds (L)AuC=CR can appear as ligands in the coordination sphere of transition metals. The interaction may be fluxional with metal-metal contacts M-Au and the alkyne coordinated side-on (if, dihapto) to the gold atom. Typical examples are (cp)(CO)(NO)W[Ph3PAuC=C Bu 90 and l, c3(CO)9[R3PAuC=CtBu], with R = Ph, Pr, for which several isomers have been observed in solution.91... 

On the other hand, the involvement of vinyl cationic species in the reaction cannot be ruled out in some cases, as shown in Scheme 4. In this context, it was found that the reaction of 3-butyn-2-one with mesitylene can occur without Pd(OAc)2, clearly indicating the involvement of vinyl cations generated from alkynes and H+ in this reaction.47 1 The yield difference in the presence and in the absence of Pd(OAc)2 may be explained by the competition between [Pd(n)02CF3]+ and vinyl cationic species in the electrophilic substitution of aromatic G-H bonds. Recent kinetic isotope experiments suggest a mechanism involving alkyne coordination to Pd(n) followed by electrophilic aromatic substitution.476... 

Orbital interaction diagram and EHT calculations show that the 1,2 intramolecular shift of hydrogen is symmetry disfavored [10]. In presence of a transition metal fragment to which the alkyne coordinates, the activation energy is considerably lower. This has been attributed to the tendency of H to shift as a proton rather than as a hydride. 

In contrast to the spontaneous reaction, the catalyzed process seldom gives the 1,2,3-trisubstituted benzene isomer from an acetylene RC CH. The chief product is usually the 1,2,4-isomer, with lesser amounts of the 1,3,5-isomer also generally obtained, but little if any of the 1,2,3-isomer. The mechanism of the catalyzed reaction to form benzenes1083 is believed to go through a species 117 in which two molecules of alkyne coordinate with the... 

As with alkenes, alkynes coordinate to platinum in both the zerovalent and divalen oxidation states. Since the chemistry of platinum alkyne complexes is less extensive than that c alkene complexes, this section is not subdivided into complexes of the two types. The divalen compounds of platinum will be covered first, followed by the complexes of platinum in a forms zerovalent oxidation state. 

A theoretical approach addresses the question of alkynes bonded to PtL2 fragments in both parallel and perpendicular geometries. With each mode of alkyne coordination there is required a different coordination geometry at platinum. The authors use the isolobal analogy to calculate the electronic structures of complexes, and propose several unknown complexes to be stable.842... 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

Reaction of the phosphaalkyne complex Pt(dppe)(f/2-P=CtBu) with either Fe2(CO)9 or Fe3(CO)12 affords high yields of the red cluster Fe2Pt-(A<3-( /2--L)-P=OBu)(CO)6(dppe) (55), which contains a novel /i3-(f/2-1) phosphaalkyne ligand (140,140a). This bonding mode is similar to the alkyne coordination observed in some alkyne clusters, such as Fe3(/i3-(f/2- )-PhC=CPh)(CO)9 (141), and the low value of J(195Pt-31P) = 128 Hz indicates that the P atom of the phosphaalkyne is not coordinated to the Pt... 

Alkenes and alkynes coordinate to transition metals and undergo a variety of reactions, and are very important substrates for transition metal-catalysed reactions. Their reactions with halides, cyclization via carbene complexes, hydrogenation, and oxidative reactions with Pd(II) are treated in Sections 3.2, 8.2, 10.1 and 11.1, respectively. The many other reactions of alkenes and alkynes as main reactants are treated in this chapter. 

Alkyne Coordination, Dimerization, and Scission on a Tungsten—Triiridium Cluster... 

Alkynes, coordination, dimerization, and scission on tungsten-triiridium cluster core, 127-134 Alloys... 

S-Allyloxy tellurides also underwent similar radical 5-exo cyclizations catalyzed by 7 mol% of Ni(acac)2 and 2 equiv. of Et2Zn [113]. The reaction proceeded with high r/.v-selectivity in 56% yield. In contrast to the reactions of 5-hexenyl iodides shown above, 5-exo cyclization reactions of 5-hexynyl iodides were proposed to proceed by a two-electron pathway consisting of alkyne coordination/oxidative addition/intramolecular carbonickelation and reductive elimination, resulting in alkylidenecyclopentanes [114]. 

Figure 6.34 Binuclear alkyne If simple mononuclear alkyne coordination does not complete an...

The structural distortion of alkynes coordinated to hexacarbonyldi-cobalt units has been exploited in the synthesis of metal complexed cyclo[ 18-]carbon and cyc/o[24]carbon (see Scheme 17).182-184 One-pot reaction of the hexatriyne 44 with [Co2(CO)8] followed by dppm and then [Bu4N] F affords complex 45. The dppm imparts the extra stability necessary for the dicobalt unit to withstand treatment with [Bu4N]+F to remove the Si( Pr)3 groups. Oxidative coupling under high dilution conditions affords the cyclo-C18 (46) and cyclo-C24 (47) derivatives. 

The study of polynuclear cobalt-alkyne clusters has provided insight into the coordination, protection, and activation of the carbon-carbon triple bond. It is possible to draw analogies between alkynes coordinated to molecular species and those to surfaces. 

There are few reports of reactions between alkynes and trinuclear clusters of metals other than iron, ruthenium, or osmium. Some rhodium, platinum, and mixed-metal clusters undergo metal-metal bond rupture in reactions with alkynes (54-56), while in other cases the alkyne coordinates to the trinuclear unit without causing any major changes in framework geometry (56-59), as illustrated in Eq. (3). 

The addition of a phosphine group to the organic fragment has been studied in some detail in compounds with cluster-bound vinyl ligands. The zwitterionic adducts which are formed can then undergo nucleophilic addition reactions (411, 461, 462). A reaction of this type also occurs with amine-substituted alkynes coordinated to osmium and ruthenium complexes (117). 

There seems no reason why any of the mechanisms discussed in Sections 3.4-3.6 cannot function in the conversion of alkynes to alkenes. The alkene route of hydrogenation is frequently encountered because alkynes complex more strongly to transition metals than alkenes and their complexes are formed preferentially in competition with the oxidative addition of dihydrogen. Internal alkynes coordinate to bis(arylimino)acenaphthene complexes of palladium and the fricoordinate species activate molecular hydrogen. Transfer of both atoms of hydrogen forms... 

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