Selective Hydrogenation of Isolated Double Bonds

No hydrazine hydrate was used in the final stage of the catalyst preparation. 

4-Vinylcyclohexene (29) has been selectively hydrogenated to 4-ethylcyclohexene (30) in high yields of 97 and 98% over P-2 Ni19 and Nic,20 respectively, in ethanol at 25°C and 1 atm H2. Both the nickel catalysts are known to be of low isomerization activity and sensitive to the structure of substrates. The same selective hydrogenation was also achieved over a nickel catalyst in the presence of ammonia, which minimized the isomerization to a more highly substituted double bond.72 Similarly, over P-2 Ni, 

The preparation of high-purity A9,10-octalin from 2-naphthol 

5-methylenenorbornene (31) was readily hydrogenated to give 2-methylenenorbor-nane (32) in 96% yield. The hydrogenation of cw/o-dicyclopentadiene (33) has been carried out on a 3-mol (400-g) scale without difficulty to yield 5,6-dihydro-iw/o-di- 

The selective hydrogenation of 1,5-cyclooctadiene (1,5-COD) and 1,5,9-cyclodode-catriene (1,5,9-CDT), cyclic oligomers of 1,3-butadiene, to the corresponding monoenes has been the subject of considerable interest, since the hydrogenation may constitute one of the steps leading to the synthesis of C8 and C12 lactams, dicarboxylic acids, and their derivatives. 

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The selectivity in the hydrogenation of isolated double bonds depends on the type of substitution of the unsaturated carbon atoms, as in the reaction in equation 55144. 

The a-tocotrienol has a number of isolated double bonds on the alkyl chain and also has the double bonds of the aromatic ring. The isolated double bonds are easier to reduce by catalytic hydrogenation as the aromatic ring would prefer to retain its resonance energy from aromaticity. Thus, low-pressure hydrogenation and the use of simple catalysts like Pt will selectively reduce the isolated double bonds of a-tocotrienol to a-tocopherol. 

Selective reduction of both double bonds in a,jS-unsaturated carbonyl compounds that also contain an isolated olefin is possible with appropriate catalyst and reaction conditions . Chlorotris(biphenylphosphine)rhodium hydrogenates the isolated double bond. Thus, carvone, 13, is reduced to dihydrocarvone, 14, with careful measure of the uptake of Hj ... 

The selective diene hydrogenation of monoterpenes such as myrcene, which contain both isolated monoene and diene moieties, forms a particular challenge [84]. The catalyst [RhH(CO)(PPh3)3] (60) has been reported to perform remarkably well for such hydrogenation reactions, and the diene moiety was shown to be selectively reduced to the monoene, while the isolated double bond remained unaffected under the reaction conditions used (Scheme 14.20). The rates of reaction expressed as average TOF (determined at ca. 80% conversion) ranged from ca. 640 (in benzene, 20 atm H2 at 100 °C) to 7600 mol mol 1 h 1 (in cyclohexane, 20 atm H2 at 80 °C). The hydrogenation in benzene solution resulted in... 

In the case of molecules which have both conjugated and isolated double bonds, the selectivity of the hydrogenation depends on the catalysts and on the nature of the substituents of the unsaturated compound (equations 57 and 58)145,146. 

Hydrogenation of aromatic rings will not be described in detail in this review since it is mostly concerned with isolated double bonds. This is however an area of active research, as recently demonstrated by the use of catalytic niobium complexes for the selective hydrogenation of aryl phosphines (Scheme 3)56. 

Regioselectivity in hydrogenating isolated double bonds is governed mainly by the same structural effects which determine the relative competitive reactivities of monoenes. Thus terminal double bonds are reduced in preference to nonterminal and the less alkyl-substituted to the more. Strain in the double bond alters this preference, for example 5-methylenebicyclo[2.2.1]hept-2-ene is hydrogenated to 2-methyl-enebicyclo[2.2.1 Iheptane. - Nickel and palladium catalysts are usually most selective. 

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