Hydrogenation I Selectivity

Heterocyclic and Carbocyclic Aromatics 0 X = C, N.O Rh Room temp, 2-4 atm Raney Ni 100-120X, 100 atm Ru 150 C. lOOatm  

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. 

Even though esters are relatively difficult to hydrogenate it is possible to selectively hydrogenate an ester group in the presence of a double bond using a 

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Figure 4 Stabilized bromine antimicrobials are produced by eosinophils, a type of mammalian white blood cell. Bacteria are captured by phagocytosis and contained intracellularly within vesicles called phagosomes. Granules release cationic surfactants, lytic enzymes, and eosinophil peroxidase into the phagosome in a process known as degranulation. Eosinophil peroxidase, an enzyme that is structurally similar to the bromoperoxidases found in seaweed (Figure I), selectively catalyzes oxidation of bromide to hypobromite by reducing hydrogen peroxide to water. The hypobromite immediately reacts with nitrogenous stabilizers such as aminoethanesulfonic acid (taurine) to form more effective and less toxic antimicrobial agents.

An elegant four-enzyme cascade process was described by Nakajima et al. [28] for the deracemization of an a-amino acid (Scheme 6.13). It involved amine oxidase-catalyzed, (i )-selective oxidation of the amino acid to afford the ammonium salt of the a-keto acid and the unreacted (S)-enantiomer of the substrate. The keto acid then undergoes reductive amination, catalyzed by leucine dehydrogenase, to afford the (S)-amino acid. NADH cofactor regeneration is achieved with formate/FDH. The overall process affords the (S)-enantiomer in 95% yield and 99% e.e. from racemic starting material, formate and molecular oxygen, and the help of three enzymes in concert. A fourth enzyme, catalase, is added to decompose the hydrogen peroxide formed in the first step which otherwise would have a detrimental effect on the enzymes. 

Ethoxypenta-l, 3-diene underwent the Diels-Alder reaction withp-benzoquinone to give the cis-fused diketone I. Selective catalytic hydrogenation (Ni) of I, followed by reduction with lithium aluminum... 

It is clear that all three types of selectivity are relevant to catalytic hydrogenation reactions and from a consideration of the reaction scheme for alkyne hydrogenation (Fig. 4), it can be deduced that all three factors may be operative simultaneously. Clearly, the selectivity for the formation of the alkene relative to alkane will depend upon a number of factors. If both the alkene and the alkane are formed during one residence of the parent molecule on the surface, the selectivity will depend upon the relative values of k, and k2 (Type II selectivity) and upon the ratio kjk4 (Type II selectivity). Since both of these depend upon the specific properties of the catalyst, they have been termed the mechanistic selectivity factor [38], Once the alkene is produced, the system contains another potential adsorbate and Type I selectivity must be taken into account. It... 

Perbenzoic acid fcrf-butjl ester (51) is the source — under copper I) bromide catalysis —of a benzoate anion (52) and radical 53. Radical 53 subsequently abstracts a hydrogen atom selective ) from the 11 -position of 55 in a homolytic bond cleavage to give a buta diene system with opening of the cyclopropane ring.Jn... 

Table I. Selectivity in oxidations using Fe /zeolites and hydrogen peroxide (22)...

CDTECH Isobutylene Raffinate I Selective hydrogenation of butadiene and hydroisomerization of butene-1 to butene-2 to produce isobutylene 1 1994... 

Type I selectivity is that occurring when two simultaneous reactions are taking place.33 An example of this type is the hydrogenation of the C4 acetylenes... 

The selective hydrogenation of one alkene in the presence of another is an example of Type I selectivity (Chapter 5). The olefin that is hydrogenated faster will be favored for saturation. However, with this type of selectivity the faster reaction is more influenced by diffusion limitations, so it is best to use a relatively unreactive catalyst. It is particularly important that the migration of the organic substrates to the catalyst is beyond mass transport limitations so the catalyst will have equal access to both alkenes. As the reaction proceeds, the amount of the more readily adsorbed species decreases so reaction selectivity may also decrease. 

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