Hydrogenation of monoenes

Hydrogenation of monoenes, 123 Hydrogenation of ring A aromatic steroids, 138... 

The regioselectivity exhibited in the hydrogenation of monoenes also holds for the hydrogenation of dienes specifically, the terminal double bond in nonconju-gated dienes is usually reduced selectively 111... 

We found previously (10) that in a natural mixture of mono-, di-, and triunsaturated fatty esters the hydrogenation of monoenes with Fe(CO)s was minor. Therefore, competitive hydrogenation studies were carried out with an equal mixture of methyl oleate and linoleate. Diene hydrogenation in such a mixture was indeed dominant (Figure 2). At O.IM initial concentration of Fe(CO)s the formation of stearate was a minor reaction the diene-Fe(CO)3 complex reached a maximum of 4% and remained constant. On the other hand, at 0.5M Fe(CO)5, stearate formation became a more important reaction diene-Fe(CO)3 reached a maximum of 7% and decreased during the course of hydrogenation. Free conjugated dienes were minor products. 

In the light of such catalytic properties of R/SiC>2, some of the rare earth introduced, especially up to the fraction of 0-15 wt.%, is preferentially bound to Si02 to form =Si-0-R-NH2 species, and the rest seems to deposit in active metallic form on SiC>2 with high dispersion. The properties of the latter species largely reflect the hydrogenation of monoenes and alkynes. 

Table VI. Catalytic Hydrogenation of Monoenes with PtCL(P03)2 ...

A. Andreetta, F. Conti, and G. F. Ferrari Selective homogeneous hydrogenation of dienes and polyenes to monoenes, pp. 204-295. 

A m-RuCl2(CO)2(PPh3)2 complex, which catalyzes hydrogenation of dienes and monoenes, becomes useful in the presence of added phosphine for selective hydrogenation of 1,5,9-cyclododecatriene to cyclododecene the catalyst is a HRuCl(CO)2(PPh3)2 hydride that operates via steps analogous to those of reactions (9)-(11) (144, 145). 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

Borohydride reduction of NiCl2 in dimethylformamide or dimethyl-acetamide leads to very active catalysts, thought to be homogeneous, for hydrogenation of monoolefins, unsaturated fats, cyclic dienes to monoenes, and saturated aldehydes and ketones (165, 538, 539). Cobaltous chloride systems have also been used (540). 

To do so, one can take the enthalpy of formation of n -hexane from Pedley, and with the phase independence assumptions in Reference 7, employ the enthalpies of hydrogenation of 1-hexene and 1,5-hexadiene from References 11 and 12 respectively. Alternatively13, one can forget about the first quantity altogether and simply take the difference of the enthalpies of hydrogenation of the diene and twice that of the monoene. This reaction is endothermic by 1.1 1.8 kJ mol-1, a value statistically indistinguishable from the absence of any interolefin interaction in the diene. Relatedly, for the isomeric 1,4-hexadienes 14 and 15, equation 8 may be used. 

Again, one may take the difference of the enthalpies of hydrogenation of the diene and the sum of those for the two monoenes. Doing this separately for 14 and 15, we find the reaction enthalpies for the Z- and -dienes are —1.9 1.2 and —1.8 1.1 kJmol-1. These values are effectively zero. A stabilizing—or destabilizing—interaction was not expected for nonconjugated acyclic dienes and none was found. 

Scheme 4.14 Pathway for the selective hydrogenation of polyenes to monoenes catalyzed by PtX2(QPh3)2/SnCI2 Q= P or As, X=halogen or pseudo-halogen.

Nickel is frequently used in industrial homogeneous catalysis. Many carbon-carbon bond-formation reactions can be carried out with high selectivity when catalyzed by organonickel complexes. Such reactions include linear and cyclic oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions [1], Many of the complexes that are active catalysts for oligomerization and isomerization reactions are supposed also to be active as hydrogenation catalysts. 

Selectivity in the hydrogenation reaction of dienes to monoenes can be achieved by two types of catalytic system (i) those which are completely inert with respect to the hydrogenation of the resulting monoenes and (ii) those for which the selectivity is due to the discrimination based on thermodynamic and/ or kinetic factors that suppress the rate of formation of the saturated hydrocarbon. The latter approach is the most common way of achieving selectivity for these hydrogenations. 

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