Heterogeneous catalysis alkynes

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 ... 

Two reviews deal with the relation between homogeneous and heterogeneous catalysis. One of these reviews is of a general physical and theoretical nature, while the other is concerned with specific classes of compounds — alkenes, alkynes, and fats — and concentrates on their hydrogenation and isomerization. Homogeneous catalysis by ruthenium complexes has been reviewed. The application of molecular orbital symmetry rules, to organic as well as to organometallic reaction mechanisms, has been discussed, and a set of rules similar to, but simpler to apply than, the Woodward-Hoffmann rules has been described. ... 

Examples above are related to heterogeneous catalysis. For homogeneous organome-taUic catalysis, many reactions are Unear therefore, visualization of them through graphs does not present any difficulties. Somewhat more complicated mechanisms, eg, oxidative carbonylation of alkynes [4] require presence of non-Hnear steps ... 

In the case of alkynes obtaining the data on the stereoselectivity of the insertion process is straightforward since Z/ -geometry of the double C=C can be easily determined in intermediate complexes and in the products (cf. Schemes 3 and 4). For example, the intermediate complexes dealing with metal-chalcogen bonds transformations were isolated and structurally characterized [48, 49], as well as the insertion pathway was characterized by theoretical calculations for homogeneous catalysis with molecular complexes [50] and for heterogeneous catalysis with nanoparticles [51]. The topic is discussed in detail for various heteroatoms and metal complexes in the other chapters of this book [20-28]. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

Reactions of soluble metal complexes, whose mechanisms of catalysis appear to be reasonably well known, can serve as a guide to the main reaction paths followed on heterogeneous catalysts. Mononuclear complexes catalyze syn addition of H2 to alkynes to yield initially only cis isomers, as in equation (25). 5 More recently, Muetterties and coworkers showed that the dinuclear rhodium hydride complex ( yi-H)Rh[P(OPr )3]2 2 (38) converts alkynes to trans isomers as initial products (equation 26). The alkyne addition compound (39) was isolated its structure shows the vinyl group bonded to one rhodium atom by a a-bond and to the other by a ir-bond, while the substituents on the vinyl group are trans to one another. This structure resembles ones hypothesized earlier to explain the formation of trans isomers and alkanes. Hydrogenations of alkynes which are catalyzed by the dinuclear rhodium hydride are much slower than the hydrogenation of an alkene catalyzed by the dinuclear tetrahydride (40), which is formed rapidly from (38) in the presence of H2 (equation 11). ... 

Metathesis of Alkynes. - Alkyne metathesis has been studied far less than has olefin metathesis. Mortreux and co-workers °° demonstrated that the metathesis of alkynes occurs at ambient temperatures with photochemical activation on a Mo(CO)6-3ClPhOH catalyst. The reaction proceeds via a two-step mechanism involving activation of the Mo(CO)6 by the alkyne and subsequent catalysis by the phenol. Heterogenization of the transition metal and... 

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