Alkenes reactions with metal carbonyl clusters

Reactions of alkenes with metal carbonyl clusters may give simple substitution products such as Os3(CO)n(ri -C2H4) or may involve oxidative addition of one or more C—H bonds. Reaction 23.76 illustrates the reaction of Ru3(CO)i2 with RHC=CH2 (R = alkyl) to give isomers of H2Ru3(CO)9(RCCH) in which the organic ligand acts as a 4-electron donor (one tt- and two tr-interactions). 

Although the reactions between alkynes or alkenes and metal clusters are the main source of alkyne-substituted complexes, there are other reagents which can produce similar products. Two such reagents are tetraphenylcyclopentadienone, which in the reaction with Ru3(CO)i2 produces Ru3(CO)10(PhCCPh) (167), and dimethyl-vinylarsine, which has been made to react with several carbonyl clusters [Eq. (8)] (168, 169). In the reaction of M3(CO)12 (M = Ru, Os) with a number of tertiary phosphines and aromatic alcohols, an oxidative addition takes place and benzyne-triosmium compounds are obtained (170-176). The fact that Os3(CO)uPEt3 can be converted into an alkyne compound (177) suggests that the conversion goes through substituted intermediates. Carbene derivatives of clusters have also... 

Alkene hydrogenation with transition metal carbonyl clusters in ILs has been studied using high-pressure NMR by Dyson and Laurenczy [65]. The clusters showed increased stability in the IL and reactions were up to 3.6-fold faster than in conventional solvents. 

The reactivity of metal-carbonyl clusters with unsaturated hydrocarbons gives an idea of what may happen on surfaces in heterogeneous catalysis, although the conditions and pathways are different, because surfaces of heterogeneous catalysts are not saturated with ligands whereas clusters are (see Chap. 20). Many bond activations have been shown (CO, PPhs, alkenes, alkynes, etc.), a simple one being the reaction of ethylene leading to double C-H activation. Such bond activations are common and rather facile because of the proximity of the metals in the clusters ... 

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

Polyhydrido clusters are a very interesting class of compounds from the viewpoint of reaction chemistry because they easily generate vacant coordination sites upon heating or treatment with a hydrogen acceptor such as an alkene. Electron density at the metal centers of the cyclopentadienyl polyhydrido cluster is higher than that of the carbonyl cluster and the reactivities of these clusters are different from each other. 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

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