Hydrogenation, of a double bond

Finally in our treatment of one group disconnections we ought to consider how to synhiesise Mly saturated hydrocarbons - compormds witli no FG at all These are often made by hydrogenation of a double bond, and so the discoimection can be made anywhere we like ... 

Most problems concerned with olefin hydrogenation involve the competitive hydrogenation of a double bond in the presence of another double bond or other function. There Is usually a way of achieving the desired selectivity. 

The mechanism for the addition of 50 to anthraquinone (Scheme 52), 1,4-diacetylbenzene (Scheme 53), and 1,2-diacetylbenzene (Scheme 54) has been proposed.326 The addition of 50 to benzophenone followed by reaction with Wilkinson s catalyst formally results in the hydrogenation of a double bond (Equation (262)).325 This species also undergoes stereospecific inertion into the vinyl chloride bond of various halogenated alkenes with high yields in most cases (Equations (263)-(267)).329... 

The hydrogenation of a double bond during cathodic reductions generally results from addition of protons to an electrogenerated basic species. It is worth noting that partial and selective hydrogenation can take place according to the nature of the heterocycles and the experimental conditions. 

Metathesis reactions are now widely used in natural product synthesis. Novel retrosynthetic analyses were developed because a carbon-carbon single bond can be formed after hydrogenation of a double bond constructed by metathesis. Although many types of metathesis are now known, the reaction is classified by olefin, enyne, and alkyne metatheses in this chapter. 

An N-debenzylation reaction with concomitant hydrogenation of a double bond in the thiazine ring was observed with 1,2-thiazine nitrogen 107 (see Equation 12) <1994TL2911>. 

Stereoselective hydrogenations. The stereochemistry of the hydrogenation of a double bond catalyzed by this Ir(I) complex is markedly controlled by the presence of a carboxamide group. The effect is attributed to coordination between the CONH group and iridium. Reductions of the same substrates with Pd/C show no stereoselection.2... 

Fig. 9.8 The heat of hydrogenation of a double bond in triquinacene is essentially the same as that of a double bond in dihdrotriquinacene and in tetrahydrotriquinacene, and is about the same as in cyclopentene, indicating that triquinacene is not homoaromatic...

Although the hydrogenation of a double bond in a simple olefin is considerably more favored thermodynamically (Table I) than its hydroformylatioh, the hydrogenation of an olefinic linkage only begins to compete with the hydroformylation when the double bond of the system is conjugated to another unsaturated system. 

The catalytic hydrogenation of a double bond involves the adsorption of the alkene on a metal surface and the transfer of hydrogen from the surface to the double bond. Typical catalysts are finely divided forms of nickel, platinum or palladium, the latter often supported on an inert carrier such as charcoal or barium sulfate. Hydrogenations are carried out in solution, with the hydrogen at atmospheric or higher pressure. The addition of hydrogen is typically cis and from the less-hindered face of the molecule (e.g. the hydrogenation of a-pinene, 3.1). 

The STO procedure can be used to measure the amoxmts of four different types of sites involved in the hydrogenation of a double bond, the direct saturation sites, 5M and Mr, two-step saturation sites, MH, and isomerization sites, M. In addition, there are sites that only adsorb hydrogen but do not take part in alkene hydrogenation. These are labeled M and the number of such sites present is measured by the relationships shown in Eqns. 3.4 and 3.5.5 They are based on the mechanisms shown in Schemes 3.2, 3.4 and 3.5 in which the 5M and Mr sites initially adsorb two hydrogen atoms and all of the other sites only one. 

It is usually easy to affect the selective hydrogenation of a functional group listed near the top of Table 14.1 in the presence of one from the middle or near the bottom of the list. For example, the hydrogenation of a double bond in the presence of a ketone or ester is reasonably straightforward (Eqns. 14.2-3). Selectivity is more difficult to attain, however, when the two functional groups are close to each other in reactivity such as triple bonds and nitro groups or when the desired reaction involves the reduction of a group in preference to one that is listed above it in Table 14.1. 

DIMETHOXY-, ETHYL ESTER, 31,56 2-CARBOXY-, 34, 8 Hydrogenation, of a double bond,... 

Dehydrodiketopiperazines afford another way to control the asymmetric hydrogenation of a double bond leading, after hydrolysis, to fairly enantiomerically pure amino acids. Preferential adsorption from the less bulky side of the near planar ring rationalizes the steric course of the reaction. ... 

Generally, hydrogenation of a double bond involves svn addition of two hydrogen atoms from the metal to the adjacent face of the double bond19. A typical example is the hydrogenation of dimethyl 1,2-cyclohexenedicarboxylate (I) which yields the saturated m-isomer 2 exclusively19-29. 

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