Double-Bond Hydrogenation

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Normally, the hydrogenation of a readily hydrogenated double bond occurs over palladium-on-charcoal in ethanol at room temperature and atmospheric pressure. The more difficultly reduced olefins require elevated reaction temperatures and/or pressures for the reaction to proceed at a reasonable rate. The saturation of an 8(14)-double bond is virtually impossible under normal hydrogenation conditions. In order to remove unsaturation at this position it is necessary to first isomerize the double bond to the readily hydrogenated 14 position by treatment with dry hydrogen chloride in chloro-form. ° ... 

FIGURE 2.1 Classical Horiuti-Polanyi half-hydrogenated state mechanism for hydrogenation, double bond migration, cis-trans isomerization, and deuterium exchange. 

One very fast and reliable method for the reduction of double bonds is that of transfer hydrogenation with diimine (Scheme 20.30). Under the influence of traces of copper ion and oxygen from air, hydrazine is rapidly transformed into diimine. This compound is able to hydrogenate double bonds with great success under the formation of nitrogen [120],... 

The sulfonyl group in sulfones resists catalytic hydrogenation. Double bonds in a, -unsaturated sulfones are reduced by hydrogenation over palladium on charcoal (yield 94%) [686, 687] or over Raney nickel (yield 62%) without the sulfonyl group being affected [686]. In p-thiopyrone-1,1-dioxide both double bonds were reduced with zinc in acetic acid but the keto group and the sulfonyl group survived [655]. Raney nickel may desulfurize sulfones to hydrocarbons [673]. 

The initial rate kinetics for hydrogenation, double bond migration and ci —trans isomerisation are shown in Table 10. The activation energies for isomerisation and hydrogenation are shown in Table 11. 

No. of trans double bonds formed No. of hydrogenated double bonds... 

In the case of multiply unsaturated carbonyl compounds, regioselectivity is also sensitive to the nature of the catalyst, to reaction conditions, and to the structure and degree of substitution of the hydrogenated double bonds. For example, hydrogenation of 3,5-heptadien-2-one over nickel on alumina or nickel on zinc oxide occurs mainly at the y,8-double bond. But if the catalyst is modified by the addition of lead or cadmium, reduction occurs mainly at the a,p-double bond (Scheme 24). 

This hydrocarboration method is a valuable tool in industrial and laboratory synthesis, since it allows introduction of the one-carbon unit of carbon monoxide into unsaturated substrates and construction of new carbon skeletons with aldehyde functions or derivatives thereof formed by reduction, oxidation, condensation and other conversions. Hydroformylation, mainly catalyzed by cobalt, rhodium, or platinum complexes is an unsymmetrical 1,2-addition leading to linear and branched products if terminal olefins are used as the substrate. Since linear products are normally the industrial products wanted54, considerable efforts have concentrated on the control of regiochemistry. Other problems of the hydroformylation method arise from side reactions such as hydrogenation, double bond migration, and subsequent reactions of the products (e.g., condensation, reduction, dccarbonylation)54. 

Another microorganism with the ability to oxidize a wide range of secondary alcohols is also able to hydrogenate double bonds. This microorganism belongs to the genus Corynebacteri-um42. When allylic alcohols were used as substrates the saturated ketone was produced. 

Dialkyl A-cyanoimidodithiocarbonates, NCN=C(SR)SMe or (NCN—C(SR)S)2CH2, were electrochemically reduced at a vitreous C cathode in DMF. An irreversible one-electron process giving a radical anion followed by a fast C—S bond cleavage into a thiolate and alkyl radical was suggested (Scheme 30). Electroreduction of carbon-carbon double bonds becomes easier with increasing number of cyano substituents. Three polarographic waves were observed for l,2-dicyano-l,2-dialkylthio-ethylenes in DMF solution, the first at ca -1.0 V (v SCE). Controlled potential electrolysis in MeOH/HOAc/NaOAc solution at a Hg pool cathode produced the hydrogenated double-bond derivatives (NC(SR)CHCH(SR)CN). 

Another industrially exploitable feature of these alloy catalysts is their restrictive hydrogenation of acetylenic bonds to olefinic bonds. A number of studies using rapidly cooled Pd77Ge23 (Carturan et al., 1982 Boitiaux et al., 1983 Molnar et al., 1986) clearly indicate remarkably high selectivities for the semi-hydrogenated double bond product, almost 98%. 

In addition to the stereoisomerism during catalytic hydrogenation, double bond migration is also observed. Show how esters of c/s-9-octadecenoic acid are converted to cis- and frans-S-octadecenoic esters under these conditions. 

---