Direct hydrogenation, alkene

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

In addition to the use of hydrogen directly, hydrogen generated from CO and water (water-gas shift reaction) is also very effective in hydrogenating activated alkenes under basic conditions (Eq. 10.3).5... 

When the reactant is cyclohexene, in the first step of Scheme 26, the direct hydrogen abstraction for the allylic oxidation (path 1) competes with the electron transfer (from the alkene to the M-oxo complex) for the epoxidation (path 2). Because the manganese complex is more readily reduced than the chromium... 

In spite of the success of asymmetric iridium catalysts for the direct hydrogenation of alkenes, there has been very limited research into the use of alternative hydrogen donors. Carreira and coworkers have reported an enantioselective reduction of nitroalkenes in water using formic acid and the iridium aqua complex 69 [66]. For example, the reduction of nitroalkene 70 led to the formation of the product 71 in good yield and enantioselectivity (Scheme 17). The use of other aryl substrates afforded similar levels of enantioselectivity. 

But notice that because this alcohol does not have any hydrogens on its j8 carbon, it cannot dehydrate directly. Any alkenes that are formed must arise by rearrangement processes. Consider, for example, migration of either of the two equivalent methyl groups at C-2. 

Cyclic Alkenes. The directing effect of alcohol substituents is perhaps most dramatically demonstrated with cyclic alkenes in which products are formed via hydrogenation from the more hindered face (Table 1). Entries 1-3 serve to illustrate that the directing alcohol may reside in either the allylic, homoallylic, or bis(homoallylic) position relative to the alkene undergoing hydrogenation, while synthetically useful levels of selectivity are retained. Entry 4 is included to illustrate the dramatic steric congestion which can be overcome in directed hydrogenation... 

Applications in Total Synthesis. Two recent examples of directed hydrogenations employing (1) in the total synthesis of complex molecules are illustrated. In eq I a simultaneous diastereoselective reduction of the trisuhstituted alkene and the a,p-unsaturated ester afforded the illustrated advanced intermediate in the asymmetric total synthesis of ionomycin. In addition, a two-directional application has been utilized in an asymmetric synthesis FK-506 (eq 2). ... 

As with cyclic alkenes, nonhydroxylic directing groups can be used in the directed hydrogenation of acyclic alkenes. The selective reductions of 3-substituted itaconate esters illustrate the directing capacity of esters (Table 5). It appears that the presence of coordinating allylic substituents can effect the level of selectivity. ... 

Excellent diastereoselectivity has been observed in the directed hydrogenation of cyclic alkenic alcohols (19) and (21).In all cases, trans products are formed predominantly. 

Using equimolar amounts of palladium(II) salts, Tt-allylpalladium complexes can be formed directly from alkenes with predominant abstraction of hydrogen at the higher substituted allylic position. 

Hydrogen cyanide may, however, be added directly to alkenes e.g., butadiene gives adiponitrile (for Nylon) in the presence of palladium or zerovalent nickel phosphite catalysts25 which operate by oxidative-addition and transfer reactions (Chapter 24). 

Complexes of the type [Ir(bzn)(cod)(L)](C104) (bzn = benzonitrile L = tricyclohexylphosphine, neomenthyldiphenylphosphine) catalyze the homogeneous hydrogenation of tetrasubstituted prochiral amido alkenes R R C=CR N. Under very mild conditions, the catalysis occurs for N = NHCOR, R = COjMe R = R = Me, R" = Me, Ph R = Me, R- = Ph, R = Me, Ph R = Ph, R = Me, R = Ph. [Ir(cod)(PCy3)(py)](PFg) serves as a catalyst in the hydroxyl-directed hydrogenation of cyclic and acyclic alkenic alcohols, wherein the reaction shows diastereoselectivity dependent on catalyst substrate stoichiometry. Park el have noted that the iridium(I)... 

Such reactions of coordinated ligands are important in the organometallic chemistry of transition elements they may proceed in the opposite direction, an alkene hydride being formed from an alkyl. A trick widely adopted to prevent this reverse reaction is to replace the p hydrogens by alkyl groups or the P carbon by a silicon (with alkyl groups attached). 

These are believed to bind to the metal via the amide carbonyl just as we saw happen in directed hydrogenation. This improves the rigidity of the alkene-catalyst complex, which in turn increases the chiral discrimination of the system. As in directed hydrogenation, a 12e catalyst fragment, such as that formed from the Schrock-Osbom catalyst, is required. 

During the mechanistic studies of estrogen biosynthesis, selective oxidation of androstene-3,17,19-trione (n) to the corresponding carboxylic acid was found to proceed by iron porphyrin complexes (Scheme 14A) [253]. On the basis of substituent effect on the benzaldehyde oxidation and kinetic isotope effect, direct hydrogen abstraction mechanism has been proposed [254]. The relative reactivity of aldehydes and alkenes is as follows cyclooctene, styrene > aldehyde, terminal alkene > a, 3-unsaturated ketone. 

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