Nitriles, catalytic hydrogenation alkenes

In the synthesis we should not wish to make 21 as it would cyclise and, in any case, we d rather reduce nitrile, nitro and alkene all in the same step by catalytic hydrogenation. The very simple method used for the conjugate addition is possible only because of the slow aldol reaction of the hindered aldehyde 24. The aldol 25, also called a Henry reaction, needs a separate dehydration step but the three functional groups in 26 are reduced in one step in good yield.7... 

Catalytic hydrogenation is commonly used for the reduction of alkenes, alkynes, aromatic hydrocarbons, and aromatic heterocycles, carbonyl derivatives, nitriles, and nitro compounds. The reaction with alkenes proceeds on the surface of a heterogeneous metal catalyst, via cleavage of diatomic hydrogen and adsorption... 

In previous sections, hydride reagents such as NaBH4 or LiAlH4 reduce ketones or aldehydes and most acid derivatives to the corresponding alcohol. LiAlH4 reduces amides to amines and also reduces nitriles to amines. Catalytic hydrogenation reduces alkenes and alkynes, as well as ketones, aldehydes, acid chlorides, and nitriles. Acid chlorides may be reduced to aldehydes, and nitriles are also reduced to amines by catalytic hydrogenation. 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives. 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives 56, 57, 104, 105, 106, 107, 108, 109. Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures 59, 60, 61, 62, 63, 64, 65, 85, 104, 106, 107, 108,109,110,113,114,118. 

Catalytic hydrogenation of nitriles also gives primary amines (Fig. 18.49). Notice that only primary amines can be formed this way. Alkenes are more reactive toward typical hydrogenation conditions than are nitriles. 

Many other functional groups are also reactive under conditions of catalytic hydrogenation. The reduction of nitro compounds to amines, for example, usually proceeds very rapidly. Ketones, aldehydes, and esters can all be reduced to alcohols, but in most cases these reactions are slower than alkene reductions. For most synthetic applications, the hydride transfer reagents to be discussed in Section 5.2 are used for reduction of carbonyl groups. Amides and nitriles can be reduced to amines. Hydrogenation of amides requires extreme conditions and is seldom used in synthesis, but reduction of nitriles is quite useful. Scheme 5.3 gives a summary of the approximate conditions for catalytic reduction of some common functional groups. 

Although hydrogenation of A-benzylideneaniline in the presence of 11 afforded the corresponding product (eq. 1 in Scheme 11), the a,(3-unsaturated ketone was converted into a mixture of unsaturated and saturated alcohols in the 42 56 ratio (eq. 2 in Scheme 11). Several substrates (nitrile derivatives, epoxides, esters, internal alkynes, and terminal alkenes), which are shown in Fig. 4, are not hydrogenated in this catalytic system. 

Ring fused products can be elaborated from isoxazolines (80S757). Several nitrocyclo-alkenes (516) were prepared and reacted with phenyl isocyanate to generate the intermediate nitrile oxides which underwent internal cycloaddition to afford the tricyclic isoxazolines (517). Cleavage of the N—O bond by hydrogenation in the presence of a catalytic amount of Raney nickel and subsequent hydrolysis afforded the /3-ketol (518 Scheme 113). 

The cis-1,2-addition of M-X bonds to unsaturated A=B bonds and its reverse, the -elimination of X from M-B-A-X, are fundamental elementary steps of catalytic reactions such as hydrogenation, hydroformylation, oligomerization, polymerization, hydrosilation, hydrocyanation, or alkene isomerization processes, as well as the Heck reaction. Most of the reactions described in the literature involve M-H or M-C bonds, and alkenes or alkynes. Besides them there are processes where the unsaturated substrate is different from alkene or alkyne This includes CO2, CS2, aldehydes and ketones, imine, or nitrile. Also, there are processes involving M-Si, M-Sn, M-B, M-N, M-P, or M-M bonds. The insertion of alkenes into M-carbene bonds is not essentially different in their intimate mechanism, but it is not discussed in this chapter. 

The same team has also described the selective hydrogenation of cis-2-pentenenitrile with surfactant-stabiUzed ammonium perfluorotetradecanoate bimetallic Pd-Ru nanopartides prepared via in situ reduction of their simple salts in reverse micelles in SCCO2 [22]. The optimized ratio Pd Ru nanopartide (1 1) shows the highest activity for the hydrogenation of functionalised alkene under mild conditions. No hydrogenation of the terminal nitrile of the molecule in amine was observed and, finally, this fluorinated micelle-hosted bimetallic catalyst gives relevant activity and selectivity in the supercritical fluid without deactivation for at least three catalytic cycles. 

Insertion, to be studied in detail in Chapter 7, is particularly important because it allows us to form a metal alkyl from an alkene and a metal hydride. We see in Chapter 9 how this sequence occurs in an extensive series of catalytic transformations of alkenes, such as hydrogenation with H2 to give alkanes, hydroformylation with H2 and CO to give aldehydes, and hydrocyanation with HCN to give nitriles. Such catalytic reactions are among the most important apphcations of organometaUic chemistry. 

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