Alkenes Group VIII metal

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... 

Metal catalysed decomposition of diazocarbonyl compounds in the presence of alkenes provides a facile and powerful means of constructing electrophilic cyclopropanes. The cyclopropanation process can proceed intermolecularly or intramolecularly. Early work on the topic of intramolecular cyclopropanation (mainly using diazoketones as precursors) has been surveyed31. With the discovery of powerful group VIII metal catalysts, in particular the rhodium(II) derivatives, metal catalysed cyclopropanation of diazocarbonyls is currently the most fertile area in cyclopropyl chemistry. In this section, we will review the efficiency and versatility of the various catalysts employed in the cyclopropanation of diazocarbonyls. Cyclopropanations have been organized according to the types of diazocarbonyl precursors. Emphasis is placed on recent examples. 

Hydroperoxo species intervene as reactive species in the Group VIII metal-catalyzed oxidation of alkenes by 02 or H202. 

A large number of Group VIII metal-dioxygen complexes catalyze the oxidation of phosphine to phosphine oxide or isocyanides to isocyanates by molecular oxygen.6,12,56,140,141 146,184 However, their use as catalysts for the oxidation of alkenes generally leads to the same products as those obtained from free radical chain autoxidations.184,196-198... 

With the notable exception of rhodium, Group VIII metal-peroxo complexes are generally reluctant to react with simple alkenes by nonradical pathways. However, such an oxygen transfer has been shown to occur in the reaction of 180-labeled [(AsPh3)4Rh02]+C104" with terminal alkenes under 02-free, anhydrous conditions, producing lsO-labeled methyl ketone (equation 52).131... 

Lowering the temperature, -10 to -30 C, has improved the selectivity of the Lindlar catalyst when used in the presence of quinoline dissolved in pentane, hexane or hexane/THF. However, raising the temperature increases the rate of desorption of the alkene, which appears to account largely for the resulting increase in the selectivity of the heavier Group VIII metals for the hydrogenation of the simple al-kynes. ... 

The reaction of D2 with simple alkynes on Group VIII metals indicates that the reaction is more complicated than is implied by equation (25). Which of these complications should be assigned to the readsorption and subsequent reactions of the first-formed alkene or to other transformations of the adsorbed intermediates is unsettled. - - ... 

Stable five-membered dioxametallacyclic adducts are obtained from the reaction of Group VIII metal-peroxo complexes (Rh, Ir, Pd, R) with nucleophilic ketones and electrophilic alkenes or ketones. The adducts of R02(PPh3)2 with acetone and l,l-dicyano-2-methylpropene have been characterized by X-ray crystallography. The reaction of (Ph3P)2R02 with nucleophilic ketones involves, as the major pathway, the precoordination of the carbonyl compound to the vacant axial site of platinum prior to intramolecular 1,3-dipolar cycloaddition (equation 42). ... 

The catalytic hydrocarbonylation and hydrocarboxylation of olefins, alkynes, and other TT-bonded compounds are reactions of important industrial potential.Various transition metal complexes, such as palladium, rhodium, ruthenium, or nickel complexes, have widely been used in combination with phosphines and other types of ligands as catalysts in most carbonylation reactions. The reactions of alkenes, alkynes, and other related substrates with carbon monoxide in the presence of group VIII metals and a source of proton affords various carboxylic acids or carboxylic acid derivatives.f f f f f While many metals have successfully been employed as catalysts in these reactions, they often lead to mixtures of products under drastic experimental conditions.f i f f f In the last twenty years, palladium complexes are the most frequently and successfully used catalysts for regio-, stereo-, and enantioselective hydrocarbonylation and hydrocarboxylation reactions.f ... 

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. ... 

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

It therefore appears that coordination of alkene and dioxygen on the same metal center is a necessary but not sufficient condition for achieving an oxygen transfer reaction from Group VIII dioxygen complexes. Suitable positions for both alkene and 02 are also required, probably coplanar and adjacent in the coordination sphere of the metal. 

The hydrocarboxylation reaction of alkenes and alkynes is one which utilizes carbon monoxide to produce carboxylic acid derivatives. The source of hydrogen is a protic solvent (equation 35) dihydrogen is not usually added to the reaction. There are a number of variations to this reaction, since the solvent can be water, alcohols, amines, acids, etc. The catalysts can be Group VIII-X transition metals, but cobalt, rhodium, nickel, palladium and platinum have found the most use. 

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