Alkenes, viii hydrogenation reactions

The hydrogenation of unsaturated aldehydes IV can be a complex transformation, as depicted in Scheme 2. Although the desired reactions are normally either the formation of allylic alcohol V, or saturated aldehyde VI, by 1,2 addition of hydrogen across the functional group, 1,4-addition across the conjugated functions can provide the enol, VII. Over-hydrogenation can result either in further saturation or, for allylic alcohols, hydrogenolysis to the alkene VIII (which can, in turn, be further saturated). 

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. 

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. 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

Many different co-oxidants can be used in conjunction with osmium tetroxide for the catalytic dihydroxylation reaction. The most popular is A -methylmorpholine N-oxide (NMO) the use of NMO with less than one equivalent of osmium tetroxide is often referred to as the Upjohn conditions. Other oxidants, such as [K3Fe(CN)6], tert-hutyl hydroperoxide, hydrogen peroxide or bleach are effective. In these reactions, the intermediate osmate ester is oxidized to an osmium(VIII) species that is then hydrolysed with regeneration of osmium tetroxide to continue the cycle. For example, less than 1 mol% of osmium tetroxide is needed for the dihydroxylation of the alkene 74 (5.80). 

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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