Hydrogenation of Alkenes and Dienes

The hydrogenation of many different alkenes, dienes, polyenes, and alkynes may be catalyzed by homogeneous complex catalysts. Many of the soluble complexes have the ability to reduce one particular unsaturated group in the presence of other reducible groups. The selectivity rather than their universality makes these catalysts particularly useful in synthetic organic chemistry. With careful choice of catalyst and reaction conditions, remarkable selectivities are attainable. Most of the practically useful catalysts work under ambient conditions. 

Mechanism. Hydrogen must first be activated before it can be added to an unsaturated bond, and generally both hydrogen and the organic substrate must be brought into the coordination sphere of the metal catalyst. The most important property of a homogeneous catalyst is a vacant coordination site, which may be occupied by a readily displaceable ligand. 

A common, quite general route for activation of hydrogen is the oxidative addition by homolytic cleavage to form a dihydride complex24,124,125 [Eq. (11.21)]. It involves an increase of 2 in the formal oxidation state of the metal. In turn, monohydrido complexes can be formed through both homolytic [Eq. (11.22)] and heterolytic [Eq. (11.23)] cleavages of hydrogen 109 

Activation of hydrogen through oxidative addition is best exemplified by the Wilkinson catalyst. The hydrogenation mechanism characteristic of dihydride complexes was originally suggested by Wilkinson2,109 and was later further 

There are many examples where monohydridometal complexes are formed via homolytic or heterolytic hydrogen splitting.24 31 The basic difference between the 

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Hydrogenation of Alkenes and Dienes with Croups V-VII Transition-Metal... 

The photocatalytic hydrogenation of alkenes and dienes by Group 6 metal carbonyls has been investigated in LNG solvents [15]. Photolysis of trans-[M(C0)4(C2H4)2] (M = Cr, Mo, W) in liquid xenon doped with H2 leads to formation of mer-[M(CO)3(C2H4)2(q -H2)] and ds-[M(CO)4(C2H4)(q -H2)]. The q -H2 complexes for M = Cr and Mo are much less stable than those for M = W. The evidence supported -coordination of H2 rather than oxidative addition to give dihy-... 

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12-14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes. 

ILs also allow tuning selectivity in consecutive reactions. Palladium nanoparticles (1-2 nm diameter) immobilized in 1,1,3,3-tetramethylguanidinium lactate (TMGL) supported on molecular sieves, for example, were used as catalyst of Type B for the solvent-free hydrogenation of alkenes and dienes at low temperatures (20-40 °C) [61]. Cyclohexadiene was converted faster than cyclohexene (TOP 65 and 20 mol molp min at 20 °C, respectively), which allowed the selective hydrogenation of cyclohexadiene to cyclohexene (Eq. (10.2)). Note that also the access of hydrogen and small alkenes to a metal surface below a layer of IL (Type C) does not seem to be restricted [55, 73]. 

The kinetics of many hydrogenation reactions over catalysts based on Ni, Pt, Rh, Pd, Ir, and so on as active components were already studied, and much attention has been directed to the selective hydrogenation of alkenes and dienes, for example, of cyclooctadiene (COD) to cyclooctene (COE) [1-6], which is also discussed in this paper as a model reaction. 

Other additions, such as addition of alkyl halides and carbonyl compounds, are discussed in Chapter 5, whereas Chapter 7 covers addition reactions involving carbon monoxide (hydroformylation, carboxylations). Hydrogen addition is discussed in Chapter 11. The nucleophilic addition of organometallics to multiple bonds is of great significance in the anionic polymerization of alkenes and dienes and is treated in Chapter 13. 

Davies MJ, Gilbert BC (1984) Electron spin resonance studies, part 68. Addition versus overall one-electron abstraction in the oxidation of alkenes and dienes by S04, Cl2, and OH in acidic aqueous solution. J Chem Soc Perkin Trans 2 1809-1815 Deeble DJ, Parsons BJ, Johnson GRA (1990) Reactions of the hydrogen atom with azide, bromide and iodide ions in aqueous solution. Radiat Phys Chem 36 487-491 Eibenberger FI, Steenken S, O Neill P, Schulte-Frohlinde D (1978) Pulse radiolysis and E.S.R. studies concerning the reaction of S04" with alcohols and ethers in aqueous solution. J Phys Chem 82 749-750... 

The hydrocyanation of alkenes and dienes has similarly provided an exceptionally useful process for the conversion of simple feedstocks into more complex structures. [Caution Hydrogen cyanide is a highly toxic gas.] The process is best known as a key step in the DuPont adiponitrile process, which involves the dihydrocyanation of 1,3-butadiene (Scheme 3-95). The overall sequence first involves butadiene hydrocyanation to afford a mixture of 3-pentenenitrile and 2-methyl-3-butenenitrile. The unwanted branched isomer 2-methyl-3-butenenitrile is isomerized to 3-pentenenitrile under different conditions, and then 3-pentenenitrile is isomerized to 4-pentenenitrile in a subsequent nickel-catalyzed process in the presence of Lewis acidic additives. Finally, hydrocyanation of the remaining alkene generates the desired product adiponitrile, which serves as a precursor for nylon. A vast number of studies describing the optimization and mechanistic study of this process has appeared, and the interested reader is referred to the many excellent studies describing the details of this process. " ... 

In recent years there has been increased use of the transitory species, diimide, for hydrogenation of alkenes and this has been applied to diene polymer systems (Harwood et a/., 1973 Mango and Lenz, 19731. 

Table 14.1 Heats of Hydrogenation for Some Alkenes and Dienes...

Burgess and coworkers investigated the hydrogenation of the conjugated diene 87 (Scheme 30.1) [48]. Kinetic studies showed that the reaction occurred mostly stepwise via 2,3-diphenyl-l-butene, while only a small part of the diene was converted directly to 2,3-diphenylbutane, without dissociation of the catalyst from the intermediate mono-alkene. The first hydrogenation step was found to proceed with low enantioselectivity, whereas the second step was characterized by strong catalyst and strong substrate control. 

There are other possibilities for selective reduction in the hydrogenation of symmetrically substituted dienes. Raney-nickel afforded 1-alkenes, whereas supported Pd catalysts gave a mixture of 1- and 2-alkenes148. A selective reduction of a terminal double bond was carried out in the presence of an endocyclic double bond, which was trisubstituted149-152. 

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