Hydrogenation monosubstituted olefins

Both intra- and (more commonly) inr nnolecular Heck reactions have been reported. Note that organic halides with -hydrogen atoms cannot be used because they would form olefins at Pd. Eleetron-poor, monosubstituted olefins are more reactive than electron-rich, disubstituted or cyclic olefins. Many functional groups are compatible with Heck conditions [16], providing a means of synthesizing carbo- and heterocyelie eompounds (e.q. (4), C-C-double bond isomerization ineluded) [21] comprising natural products [22],... 

Reduction of C=C and C C. Berkowitz and Rylander" report that hydrogenation in the presence of 5% ruthenium on Norit selectively reduces monosubstituted olefins in the presence of di- and trisubstituted olefins. Thus, in mixtures, (1) is CH3 CH, CH3... 

Quite a considerable number of papers deal with the effect of structure of olefinic substrates on their reactivity in the catalytic hydrogenation 65). Lebedev 66) attempted a generalization of the problem. His conclusion that the rate of hydrogenation of olefins decreases in the order monosubstituted - symmetric disubstituted - asymmetric disubstituted - trisubstituted tet-rasubstituted ethylene derivatives is called the Lebedev rule. Campbell 67) supplemented it by demonstrating that the rate of hydrogenation decreases with the number and size of substituents on carbon atoms of the double bond, cis isomers are usually hydrogenated more quickly than trans isomers, and olefins containing the terminal double bond are more reactive than those with the double bond inside the chain. 

The chemoselectivity of the catalyst for alkynes over alkenes is of interest. Excellent discrimination is achieved in substrates containing an alkyne paired with a hindered olefin (Eqs. 27, 33) [38]. When offered a monosubstituted olefin (Eq. 34) the catalyst is less selective, producing mixtures of alkyl- and vinylsi-lanes. As previously noted for the hydrogenation of dienes, the addition of a group allylic to the alkene sterically shields the double bond and can electroni-... 

In hydroformylation, the optical purity of the recovered olefin is higher for monosubstituted ethylenes in hydrogenation it is higher for 1,1-disubstituted ethylenes. 

For monosubstituted ethylenes chiral products arise only when the hydrogen of the catalyst binds to the unsubstituted unsaturated carbon atom of the substrate. Since this product is, in general, the minor one, from the enantiomeric excess no prediction can be made of the face of the substrate preferentially attacked but only the face of the substituted unsaturated carbon atom preferentially formylated to form the chiral product can be established. Indications about all 4 transition states can be obtained using either labelled 2-(2H]-olefins or carrying out a deuterio-formylation instead of a hydroformylation (see Sect. 2.1.5.). 

Because ruthenium catalysts are relatively unreactive for alkene hydrogenation and they are poor for double-bond isomerization, these catalysts are particularly effective for the selective hydrogenation of monosubstituted alkenes in the presence of di- and tri-substituted olefins at ambient temperature under 2-3 atmospheres of hydrogen (Eqn. 15.28). Water in the reaction medium... 

To test the selectivity of the bimetallic system, monosubstituted acetylenes (Table I) and disubstituted acetylenes (Table II) were hydrogenated with the use of the silica supported palladium-copper catalysts. The yields of the olefins at 100% conversion of the acetylene are given. However, in all cases with longer hydrogenation times it was possible to end up with the fully saturated product. 

The silica supported palladium-copper catalysts are selective in the hydrogenation of monosubstituted acetylenes giving high yields of either olefins or saturated hydrocarbons, depending on the reaction time. In addition, the catalytic system shows reasonable selectivity towards cis-olefins in the hydrogenation of disubstituted acetylenes. 

The chemical shift changes brought about by substituents bonded to the various positions of the cyclic alkenes are often dramatic, particularly on the position-2 olefinic hydrogen. A series of monosubstituted compounds is listed below. 

Selective hydrogenation is apparently best realized for dialkyl-la,b,2°3,298 and diaryl-acetylenes,286 and for compounds in which the triple bond is endocyclic,299 since in such cases further hydrogenation of the resulting olefins is markedly slower for instance, cyclodecene is obtained in 96% yield from cyclodecyne.296 The ethylene derivatives formed from monosubstituted... 

Hydridoarene clusters of Rh and Ru are moderately active catalysts of hydrogenation of simple olefins [20]. Conversely, benzene and monosubstituted benzenes can be efficiently hydrogenated in aqueous biphasic systems with hydridoareneruthe-nium cluster catalysts, such as [Ru3(p2-H)2(p2 OH) (pj-O) (ri -C5Hg)(ri "-C5Me5)2] [21]. 

Aromatic compounds like benzene undergo a highly characteristic reaction called electrophilic substitution. For example, halogens, such as chlorine and bromine, instead of simply adding to the formal double bonds as if it were an olefin (i.e. electrophilic addition in which both halogen atoms add to the double bond), displace one of the hydrogen atoms to give a monosubstituted aryl halide... 

In general, radical hydrosilylation of alkenes cannot be conducted with tri-alkylsilanes, which is due to a rather strong Si—H bond in the latter. However, the hydrosilylation of carbon-carbon multiple bonds with modified silanes such as tris(trimethylsilyl)silane has been successfully used in radical hydrosilylation (16). The reversible addition of tris(trimethylsilyl)silyl [(TMSlsSi] radical to the C=C bonds is due to the ability of this radical to isomerize alkenes. The hydrosilylation of monosubstituted and gem-disubstituted olefins are efficient processes and have been shown to proceed with high regioselectivity for both electron-rich and electron-poor olefins (140). Walton and Studer presented the results of the radical hydrosilylation with silylated cyclohexadienes as radical initiators (141). The bisvinylic methylene group acts as the hydrogen donor in these reactions. H-transfer leads to a cyclohexadienyl radical (2) that subsequently rearranges to provide er -butyldimethylsilyl radical and arene (3) (see Scheme 20) (141). 

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