Alkenes, viii activated

There have also been significant advances in the imido chemistry of ruthenium and osmium. A variety of imido complexes in oxidation states +8 to +6 have been reported. Notably, osmium (VIII) imido complexes are active intermediates in osmium-catalyzed asymmetric aminohydroxyl-ations of alkenes. Ruthenium(VI) imido complexes with porphyrin ligands can effect stoichiometric and catalytic aziridination of alkenes. With chiral porphyrins, asymmetric aziridination of alkenes has also been achieved. Some of these imido species may also serve as models for biological processes. An imido species has been postulated as an intermediate in the nitrite reductase cycle. " ... 

In Fig. 1.6 a simplified mechanism for as -dihyroxylation of alkenes and ketohydroxylation of R CH=CHR by RuCl3/Oxone /aq. Na(HC03)/Et0Ac-CH3CN is shown. The cA-dihydroxylation route involves (3 + 2) cycloaddition of RuO to the alkene giving a Ru(VI) ester (1) which is oxidised by (HSOj) to the Ru(VIII) ester (2). Reversible nucleophilic addition of water to (2) gives the diol R CH(OH) CH(OH)R (3). Ketohydroxylation ensues when the activated Ru(VIII) ester... 

Moreover, Fiirstner shotved that tvhereas complex VIII was inactive for RCM reaction, its PCy3 analog IX was, in contrast, very active in a variety of RCM reactions. The latter is now commercially available and currently used in alkene metathesis (see Section 8.3 for further applications). 

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

Alkenes. Most Group VIII metals, metal salts, and complexes may be used as catalyst in hydrosilylation of alkenes. Platinum and its derivatives show the highest activity. Rhodium, nickel, and palladium complexes, although less active, may exhibit unique selectivities. The addition is exothermic and it is usually performed without a solvent. Transition-metal complexes with chiral ligands may be employed in asymmetric hydrosilylation 406,422... 

The black solid (VIII) is converted into the green fulvalene titanocene (V) at 110°C, and also reacts with H2, N, and alkenes 42), as do the active metastable forms of titanocene (30). With hydrogen, (VIII) yields a green-gray precipitate, formulated as [(CsHsMCsH TU] H2, from toluene solution. The infrared spectrum and deuteration studies show this solid to contain a Ti-H bond, probably with the hydrogen in a bridging position, as either... 

The coordinatively unsaturated cluster [H20s3(CO),o] is found to be cat-alytically active for hydrogenation of alkynes or isomerization of alkenes in solution. Hydridotriosmium and -ruthenium carbonyl clusters bound to a variety of oxides are summarized in Table VIII. The relative activities for... 

The basic mechanism of the Heck reaction (as shown below) of aryl or alkenyl halides or triflates involves initial oxidative addition of a pal-ladium(O) species to afford a a-arylpalladium(II) complex III. The order of reactivity for the oxidative addition step is I > OTf > Br > Cl. Coordination of an alkene IV and subsequent carbon-carbon bond formation by syn addition provide a a-alkylpalladium(II) intermediate VI, which readily undergoes 3-hydride elimination to release the product VIII. A base is required for conversion of the hydridopalla-dium(II) complex IX to the active palladium(O) catalyst I to complete the catalytic cycle. 

The rhodium (O) complex, (VIII), reacts with an alkyne to form a 1 1 addition complex which is catalytically active for the hydrogenation of alkynes and (weakly) alkenes. Complexes of types (III), but with chloro-ligands, and (VII, X = Cl) also form complexes with acetylenes, which can be subsequently hydrogenated. A series of complexes of structure (VH) with or... 

VIII.2.2 Palladium-Promoted Alkene-Arene Coupling via C—H Activation... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

The rates of olefin hydrogenation and isomerization by Group VIII metal-phosphine complexes are increased by the presence of hydroperoxides and/or oxygen. A similar rate enhancement is observed in the hydroformylation of alkenes catalysed by [RhCl(CO)(PPh3)2]. The addition of small amounts of cyclohexenyl hydroperoxide is considered to effect the unusual transformation of [RhCl(CO)(PPh3)2] to cw-[RhCl(CO)2(PPh3)], which appears to be a very active alkene hydroformylation and isomerization catalyst. Asymmetric induction in hydroformylation reactions has been achieved. ... 

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