Hydrogenation of polyenes

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.

The selective partial hydrogenation of polyenes is interesting from both preparative and commercial points of view. Success depends on the nature of the polyene as well as on a careful choice of catalyst and conditions. 

The complexes derived from platinum complexes and SnCh exhibit catalytic activity for hydrogenation under mild conditions. These complexes have also been used for alkene hydroformylation. The above catalyst system and [PtCl2(PPh3)2] assist hydrogenation of polyenes to monoalkenes, but isomerization of double bonds has been observed. The anionic cluster complex [ Pt3(CO)6 5] has been used as a catalyst for hydrogenation of cyclohexene. 

Hermans S, Raja R, Thomas JM, Johnson BFG, Sankar G, Gleeson D (2001) Solvent-free low temperature selective hydrogenation of polyenes using a bimetallic nanoparticle Ru-Sn catalyst. Angew Chem Int Ed Engl 40 1211... 

Selective hydrogenation of polyenes to monoenes is possible with homogeneous catalysts. Dichlorodicarbonylbis(triphenylphosphine)ruthenium(II) catalyzed the selective reduction of 1,5,9-cyclododecatriene to cyclododecene, as well as dichloro-tris(triphenylphosphine)ruthenium(II) ... 

It has also been found that highly active catalysts can be prepared in situ by reducing a platinum, palladium, or rhodium salt with sodium borohydride in the presence of a carbon support. Similar reductions of nickel salts produce colloidal nickel-boron, which is highly selective toward olefins of different structural types. The normal order of reactivity—i.e., terminal > disubstituted > trisubstituted—is observed, but the reactivity spread is sufficiently large that selective hydrogenation of polyenes is possible, as illustrated by a step in a synthesis of the natural product... 

Identification of the hydride-platinum species, [HPt(SnCl3)4], suggests that the hydrogenation in this salt medium proceeds by a conventional olefin insertion mechanism (see Figure 14). The large excess of ligand in the molten salt probably accounts for the selective hydrogenation of polyenes (e.g., eq. 31). 

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

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

The olefinic C=C double bond is easy to reduce, under mild conditions, with most of the hydrogenation catalysts, with noble metals, with different forms of nickel as heterogeneous catalysts, with Rh, Pt, Co complexes and with Ziegler catalysts as homogeneous catalysts. In the hydrogenation of dienes and polyenes the selectivity is the most important issue, i.e. how can one double bond be saturated with retention of the other(s). When high selectivity is required, homogeneous catalysts are used. Nevertheless, as known, their separation from the reaction mixture is a difficult task. 

In the industrial processes for the hydrogenation of dienes and polyenes, heterogeneous catalysts are used in most cases, although their selectivity is not perfect. 

For the hydrogenation of dienes and polyenes the most frequently applied catalysts are heterogeneous metal catalysts. Their advantage is the high reaction rate, good selectivity under optimized conditions and the easy separation. 

The liquid-phase hydrogenation of dienes and polyenes is also an extensively studied topic. The behavior of metals in such reactions is similar to that in the gas-phase reactions, i.e., Pd is the most selective catalyst. 

In most cases during the hydrogenation of dienes and polyenes there is an easily observed decrease in the rate of hydrogenation after the uptake of one mole of hydrogen. When this decrease is not easily detectable, it is worthwhile to stop the reaction after... 

Raney-nickel was found to be selective in the hydrogenation of cyclopentadiene and cyclohexadiene and of their methyl and ethyl derivatives at 0-40 °C and 2-5 bar pressure137,138. The skeletal nickel proved to be selective in the semihydrogenation of conjugated polyenic compounds (equation 52)139. 

The aim of the reduction of polymeric compounds is the complete saturation of the substrate, which is different from the usual practice of the hydrogenation of dienes and polyenes, where the incomplete reduction, i.e. a selective reduction, is the main goal. 

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