Allyl compounds hydrogenation

Stereochemical features in the oxidative addition and the elimination of /3-hydrogen of cyclic and acyclic alkenes are different. The insertion (palladation) is syn addition. The syn addition (carbopalladation) of R—Pd—X to an acyclic alkene is followed by the syn elimination of 3-hydrogen to give the trans-a ksne 6, because free rotation of 5 is possible with the acyclic alkene. On the other hand, no rotation of the intermediate 7 is possible with a cyclic alkene and the syn elimination of /3-hydrogen gives the allylic compound 8 rather than a substituted alkene. 

Reduction of vinylic and allylic compounds without hydrogenolysis may present a problem. The ratio of olefin saturation to hydrogenolysis depends importantly on catalyst, temperature, solvent, and pH. In both classes of compounds, hydrogenolysis is favored by polar solvents, acid, and elevated temperatures hydrogenation, by nonpolar solvents and low temperatures. 

For allyl acetate a significant deuterium isotope effect supports the hydrogen abstraction mechanism (Scheme 6,31).183 Allyl compounds with weaker CTT-X bonds (113 X=SR, S02R, Bi etc.) may also give chain transfer by an addition-fragmentation mechanism (Section 6.2.3). 

In some cases it is necessary to hydrogenate allylic compounds to saturated molecules without hydrogenolysis. It was found that nickel boride is a good catalyst for this purpose.46... 

A reaction of an allylic compound containing a hydrogen to an alkene function is an ene reaction. Thus a reaction between propene and ethylene is an example of an ene reaction. 

C=C-CH Allyl compounds (olefins with allylic hydrogen atoms), including most alkenes... 

The isomerisation of allyl and propargylic alcohols involves the 1,3-shift of an oxygen atom rather than a hydrogen atom. Isomerisation of allyl alcohols can be catalysed by a variety of metal oxo complexes and in this instance the reaction does not involve metal carbon bonds as we will see. One could imagine that allylic metal species can participate in isomerisation of allylic compounds, but for the alcohols themselves this is not an easy reaction. In chapter 13 reactions of allyl acetates and the like, which are more prone to... 

Deuteration of one end of the allyl moiety in these compounds removes the equivalence of the two positions and in place of the single line for the terminal position, two separate absorptions should appear (W). One, in the normal decoupled spectrum, is a singlet for the hydrogen substituted carbon, and the other a weak quintet for the deuterated end which would be difficult to observe. These two signals would bracket the normal singlet. If a mixture of deuterated and undeuterated allyl compound is used, therefore, two easily observable peaks should appear, one in the normal position, the other shifted. In the spectra of allyllithium and allylsodium the line from the deuterated compound appeared I and 11 Hz upfield respectively, at 0°C, of the normal lines. The potassium compound only showed a somewhat broadened line. At -80°C the separation for allyllithium was 22 Hz. 

Ene reaction (Hydro-allyl addition)-A reaction of an allylic compound with an olefin which resembles both a cycloaddition and a [1, 5]-sigmatropic shift of hydrogen. In this reaction an alkene having an allylic hydrogen atom reacts thermally with a dienophile (enophile) C = C, C = 0, N = N, N = 0, C = S... 

This method is particularly effective with cyclic substrates, and the combined effects of intramolecular and intermolecular asymmetric induction give up to 76 1 (kf/ks) differentiation between enantiomers of a cyclic allylic alcohol. This kinetic resolution provides a practical method to resolve 4-hydroxy-2-cyclopentenone, a readily available but sensitive compound. Hydrogenation of the racemic compound at 4 atm H2 proceeds with kf/ks =11, and, at 68% conversion, gives the slow-reacting R enantiomer in 98% ee. The alcoholic product is readily convertible to its crystalline, enantiomerically pure fert-butyldimethylsilyl ether, an important building block in the three-component coupling synthesis of prostaglandins (67). 

Allylsilanes are known as interesting and useful reagents for organic synthesis.85-88 Hurd first reported the direct reaction of elemental silicon with allyl chloride in the presence of copper catalyst to give a mixture of allylchlorosilanes and polymeric materials.15 Among the volatile compounds, allyltrichlorosilane, formed by the 1 1 1 reaction of allyl chloride, hydrogen chloride and elemental silicon, was observed as the major product instead of the normally expected diallyldichlorosilane. Since diallyldichlorosilane thermally polymerizes at temperatures above 130°C, the product mixture polymerized during the reaction or upon distillation. 

This hypothesis is supported by Lin s studies of the reactions of the Cg to C13 1,2-cycloalkadienes with HRh(PPh3)4 and ClRh(PPh3)3, complexes classified by Collman et al. as monohydride and dihydride hydrogenation catalysts, respectively. The monohydride complex unites with allenes to form TT-allyl compounds which have characteristic NMR spectra in solution. 1,2-Cyclononadiene yields almost the same proportion of cis- and trani-cyclononene with either Pd/Al203 or HRh(PPh3)4 as catalyst little of the trans isomer is formed when ClRh(PPh3)3 is used. The X-ray crystal structure of the complex formed from the reaction of ClRh(PPh3)3 with 1,2-cyclononadiene shows that only one double bond of the diene is coordinated with rhodium. ... 

The highly reactive silyl ttiflates and silyl bis(triflates) are obtained by reaction of the corresponding aminosilanes with triflic acid in diethylether as shown in Schemes 1 and 2 [12]. The substitution patterns at the silicon atoms are variable. Functional substituted silanes, e.g., vinyl, allyl, or hydrogen derivatives can also be obtained. (Diethylamino)diphenylsilyllithium is formed from (diethylamino)diphenylchlorosilane with lithium in THF. Yields of both reactions are high (90-95%). The products can be used for following reaction without further purification. Silyl triflates and (aminosilyl)lithium compounds react to give the amino-substituted trisilanes la - Ic (Scheme 1). 

Ene reaction The addition of a reactive dieneophile to an allylic compound that has a hydrogen in the 3 position. 

It is interesting that at variance with these conclusions, Volta et. al. (38, 39), studying oriented samples of MoO, obtained from intercalation compounds of MoCl - graphite, concluded that selective oxidation of propylene to acrolein or isobutene to niethacrolein is mainly catalysed by the (100) side faces, whereas the (010) basal race is responsible for total oxidation. It should be remembered, however, that selective oxidation is a multistep process, consisting of the consecutive abstractions of hydrogen atoms and insertion of oxygen atoms. As already described, experiments with allyl compounds (15) showed... 

Acetylenic carbinols, which are manufactured from aldehydes or ketones and acetylenes, can be partially hydrogenated, with the exception of propargyl alcohol itself. Hydrogenolysis is a minor side reaction unless the hydrogenation is completed to the alkane. Nickel boride (PI or P2) catalysts achieve the saturation of propargyl and allylic compounds in the presence of double bonds without hydrogenolysis, such as in 2 an intermediate in the total synthesis of the sesquiterpene sesquicarene ... 

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