Diene hydrogenation

Copper-nickel alloy films similarly deposited at high substrate temperatures and annealed in either hydrogen or deuterium were used to study the hydrogenation of buta-1,3-diene (119) and the exchange of cyclopentane with deuterium (120). Rates of buta-1,3-diene hydrogenation as a function of alloy composition resemble the pattern for butene-1 hy-... 

The complexes Cr(CO)3L, with L = phenanthrene, naphthalene, or anthracene, are more active for diene hydrogenation than with L = substituted benzenes (see also Section VIII), and this is attributed to an easier displacement of the arene by the diene substrate, the phenanthrene type being asymmetrically bonded, having two longer and more readily cleaved chromium-carbon bonds (198, 199). 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

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... 

Finally, other rhodium catalysts for the selective diene hydrogenation worth mentioning include [RhCl(PPh3)3] (64), [RhCl(nbd)]2 (65), and the catalytic sys-... 

Besides the catalysts mentioned in the introductory part of this topic, other catalytic systems were used successfully in diene hydrogenation. An example is NiH(PPh3)(AlCLt), which hydrogenated 1,4-cyclohexadiene to cyclohexene in toluene at 40 °C28. 

Catalysts differ in their ability to promote double-bond migration and cis-trans isomerization, in their thermodynamic and mechanistic selectivities in diene hydrogenation and in their tendencies to catalyze 1,2-, 3,4-, or 1,4-addition34. 

In order to increase the selectivity in diene hydrogenation, low-temperature basic additives and the use of less polar solvents may help. In special cases, treatment of the catalysts with the salts of heavy metals (Zn, Cd, Pb) can be the method used to modify the activity and selectivity53. Rh and Ir catalysts could be selectively poisoned with CO-containing hydrogen, in order to saturate 1,3-butadiene to 1-butene without isomerization54. 

The H2 + l)2 equilibration reaction was much faster than the diene hydrogenation, so that the rate-limiting step is not the hydrogen dissociation. The Pt behaves as a bifunctional... 

Bond Thompson mechanistic proposal, 46 bridges, 6 bromide, 185 brookite, 56, 68 Bmst method, 93 Bronsted acidic site, 77 Brpnsted acid sites, 144 buta-1,3-diene hydrogenation, 283... 

Fig. 34. Mechanisms for buta-1 2-diene hydrogenation operative over Types A and B nickel catalysts [207]. (Numbers refer to species in Fig. 33.)...

The transfer of hydrogen to the alkene also occurs by a radical process,55,50 and not by the insertion reaction which is much more common in homogeneous hydrogenation. These kinetic studies were carried out using cinnamic acid as alkene. The observation of alkenyl and allyl species in diene hydrogenation suggests that a different mechanism of hydrogen transfer may... 

Modification of the [Co(CN)5]3 catalyst by addition of diamines has been reported to lead to higher catalytic activity in diene hydrogenation and a predominant 1,2-addition of hydrogen. Such systems are thought to contain the species [CoH(CN)3(diamine)]. Ethylenediamine, 2,2 -bipyridyl and 1,10-phenanthroline were the diamines used.57 58... 

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