Alkenes, reductive

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

This section contains dehydrogenations to form alkenes and unsaturated ketones, esters and amides. It also includes the conversion of aromatic rings to alkenes. Reduction of aryls to dienes is found in Section 377 (Alkene-Alkene). Hydrogenation of aryls to alkanes and dehydrogenations to form aryls are included in Section 74 (Alkyls from Alkenes). 

The most common reaction conditions for alkene reductions use excess tri-fluoroacetic acid and triethylsilane either neat202 204 or in an inert solvent such as nitrobenzene,134 2-nitropropane,205 carbon tetrachloride,206 chloroform,207 or dichloromethane.127,164 Reaction temperatures from —78° to well over 100° are reported. Ambient or ice-bath temperatures are most commonly used, but variations of these conditions abound. 

Scheme 20.16 Alkene reduction with dioxane (39) as hydride donor and a Wilkinson-type catalyst (18).

Far more ruthenium-complex-catalyzed enantioselective hydrogenation has been directed towards ketone reduction rather than alkene reduction. Recent studies carried out on the mechanism of C=C hydrogenation has been rather limited. 

Olefin inversion (c/. 7, 338). Trifluoroacetyl chloride reacts with 1,2-dialkyl epoxides in DMF stereospeciflcally by trans opening to give u/c-chlorohydrin trifluoroacetates. These products are reduced stcrcospccifically by Nal to alkenes with. ryn-elimination to give inverted alkenes. Reductions with zinc arc less selective. Inversion of olefins is also possible by addition of NCS in CFjCOOH (actual reagent is trifluoroacetyl hypochlorite) followed by reduction with Nal. 

Useful for homogeneous reduction of alkenes. As a consequence of the reagent bulk, it is understandable that the reactivity of alkene reduction is dependent on substitution the less-substituted alkenes react faster. Also, reduction occurs from the less-hindered face in a cis-stereochemistry. Many other functional groups are tolerated by conditions. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

Alkene Reduction potential/V Diene potential/V Reduction... 

The generation of stereogenic centers by asymmetric reduction of carbon-carbon double-bonds is a current topic in chemoenzymatic synthesis. Though enzymes of the old yellow enzyme (OYE) family were identified to perform alkene reduction and were characterized some years ago [133-135], applications of enoate reductases in natural product syntheses are still rare. Thus, potential applications are also shown in this chapter. With an increasing number of new enoate reductases, such as YqjM reductase from B. subtilis, more and more possible targets for biotransformations can be found. 

Mechanism of the Electron-deficient Alkene Reduction with Sml2... 

Many nucleophiles, such as water, alcohols, and carboxylates, are compatible with the Pd(II) complex and can attack the complcxed alkene from the side opposite the palladium. The attack of the nucleophile is regioselective for the more substituted position. This parallels attack on bromoni-um ions but is probably governed by the need for the bulky palladium to be in the less hindered position. The resulting Pd(II) <7-alkyl species decomposes by p-hydride elimination to reveal the substituted alkene. Reductive elimination of a proton and the leaving group, usually chloride, leads to palladium (0). The weakness of this reaction is that the catalytic cycle is not complete Pd(II) not Pd(0) is needed to complex the next alkene. 

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