Hydration metal catalysis

Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

Hydration of nitriles providing carboxamides is usually carried out m strongly basic or acidic aqueous media - these reactions require rather bars conditions and suffer from incomplete selectivity to the desired amide product. A few papers in the literature deal with the possibihty of transition metal catalysis of this reaction [28-30]. According to a recent report [30], acetonitrile can be hydrated into acetamide with water-soluble rhodium(I) complexes (such as the one obtained from [ RhCl(COD) 2] and TPPTS) under reasonably mild conditions with unprecedently high rate... 

Both acid and metal catalysis are usually required to accomplish hydration of alkynes to yield carbonyl compounds.34 The addition is usually regioselective, allowing for conversion of terminal alkynes to ketones. Hydration of acetylene to produce acetaldehyde used to be an industrially significant process but was replaced by the Wacker synthesis. 

The direct hydration of a C—C triple bond has been recently reviewed.302,303 Hydration is possible under acid or metal catalysis, as well as by photohydration. Hydration under acid catalysis is generally done with sulfuric acid, although other acid systems such as phosphoric acid/boron trifluoride have been reported.304 It is established that acid catalysis occurs by a vinyl cationic intermediate which reacts with... 

With the help of metal catalysis amidation takes place at room temperature under apparently neutral conditions. Manganese dioxide in large quantities, or, even better, manganese dioxide on silica gel, is well suited to this conversion. Even compounds with functional groups labile to manganese dioxide, for example benzylic groups, are suited for the hydration. With pentacarlwnylmanganese bromide, nitrile hydration takes place under PTC conditions. ... 

An important question to consider is where does metal catalysis takes place in multiphase systems In bulk oils systems, the hydrophilic metals would be oriented in the air-oil interface to catalyse lipid oxidation (Figure 10.7). In emulsions and liposomes, the metals would be in solution in the aqueous phases and oriented in either the oil-water or phospholipid-water interfaces, where they may have an affinity for the hydrated layer around the droplets (Figure 10.4). 

Anti-Markovnikov addition of H2O to olefins is of enormous importance in view of the production of linear alcohols directly from alkenes. It is a general phenomenon, however, that reaction of water with olefins and alkynes, as in the previous example, gives products of Markovnikov addition. The first anti-Markovnikov hydration of terminal alkynes (Scheme 42) with transition metal catalysis was reported in 1998 (227). A series of aliphatic and aromatic alkynes... 

Positionalisomeri tion occurs most often duting partial hydrogenation of unsaturated fatty acids it also occurs ia strongly basic or acidic solution and by catalysis with metal hydrides or organometaUic carbonyl complexes. Concentrated sulfuric or 70% perchloric acid treatment of oleic acid at 85°C produces y-stearolactone from a series of double-bond isomerizations, hydration, and dehydration steps (57). 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

Since Bruce s pioneering work in the area of ruthenium vinylidene chemistry (1), it has been well known that isomerization of a terminal alkyne to a vinylidene on a metal center is not only favorable but also effects a reversal in the reactivity of the carbon atoms. However, hydration catalysis was not possible, because alkyl migration from a proposed acyl intermediate led to an... 

In the case of the bis phosphine derivatives of the group 7b metals, the lability of the oxygen atoms was so markedly retarded by substitution that it was necessary to enhance their reactivity by means of base catalysis (15, 14), a process having mechanistic features common wi Tx general base catalysis of the hydration of ketones (eq. 6) (15). 

Judging from these findings, the mechanism of Lewis acid catalysis in water (for example, aldol reactions of aldehydes with silyl enol ethers) can be assumed to be as follows. When metal compounds are added to water, the metals dissodate and hydration occurs immediatdy. At this stage, the intramolecular and intermolecular exchange reactions of water molecules frequently occur. If an aldehyde exists in the system, there is a chance that it will coordinate to the metal cations instead of the water molecules and the aldehyde is then activated. A silyl enol ether attacks this adivated aldehyde to produce the aldol adduct. According to this mechanism, it is expected that many Lewis acid-catalyzed reactions should be successful in aqueous solutions. Although the precise activity as Lewis acids in aqueous media cannot be predicted quantitatively... 

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