Reduction reaction alkenes

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

Polyarenes, reduction reactions, 49 Polyenes, alkene to alkane reductions, 41-45... 

Rhodium(II) acetate complexes of formula [Rh2(OAc)4] have been used as hydrogenation catalysts [20, 21]. The reaction seems to proceed only at one of the rhodium atoms of the dimeric species [20]. Protonated solutions of the dimeric acetate complex in the presence of stabilizing ligands have been reported as effective catalysts for the reduction of alkenes and alkynes [21]. 

Transition metals can display selectivities for either carbonyls or olefins (Table 20.3). RuCl2(PPh3)3 (24) catalyzes reduction of the C-C double bond function in the presence of a ketone function (Table 20.3, entries 1-3). With this catalyst, reaction rates of the reduction of alkenes are usually higher than for ketones. This is also the case with various iridium catalysts (entries 6-14) and a ruthenium catalyst (entry 15). One of the few transition-metal catalysts that shows good selectivity towards the ketone or aldehyde function is the nickel catalyst (entries 4 and 5). Many other catalysts have never been tested for their selectivity for one particular functional group. 

Burk et al. showed the enantioselective hydrogenation of a broad range of N-acylhydrazones 146 to occur readily with [Et-DuPhos Rh(COD)]OTf [14]. The reaction was found to be extremely chemoselective, with little or no reduction of alkenes, alkynes, ketones, aldehydes, esters, nitriles, imines, carbon-halogen, or nitro groups occurring. Excellent enantioselectivities were achieved (88-97% ee) at reasonable rates (TOF up to 500 h ) under very mild conditions (4 bar H2, 20°C). The products from these reactions could be easily converted into chiral amines or a-amino acids by cleavage of the N-N bond with samarium diiodide. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

Brunner, Leitner and others have reported the enantioselective transfer hydrogenation of alpha-, beta-unsaturated alkenes of the acrylate type [50]. The catalysts are usually rhodium phosphine-based and the reductant is formic acid or salts. The rates of reduction of alkenes using rhodium and iridium diamine complexes is modest [87]. An example of this reaction is shown in Figure 35.8. Williams has shown the transfer hydrogenation of alkenes such as indene and styrene using IPA [88]. 

The balance between biocatalytic and other, organometallic-based, methodology is heavily biased in favour of the latter section when considering reduction reactions of importance in synthetic organic chemistry. Two areas will be described to illustrate the point, namely the reduction of carbonyl groups and the reduction of alkenes, not least since these points of focus complement experimental work featured later in the book. 

Very few enzyme-catalysed reactions involving the reduction of alkenes have achieved any degree of recognition in synthetic organic chemistry. Indeed the only transformation of note involves the reduction of a, (3-unsaturated aldehydes and ketones. For example, bakers yeast reduction of (Z)-2-bromo-3-phenylprop-2-enal yields (S)-2-bromo-3-phenylpropanol in practically quantitative yield (99 % ee) when a resin is employed to control substrate concen-tration[50]. Similarly (Z)-3-bromo-4-phenylbut-3-en-2-one yields 2(5), 3(,S)-3-bromo-4-phenylbutan-2-ol (80% yield, >95% ee)[51]. Carbon-carbon double bond reductases can be isolated one such enzyme from bakers yeast catalyses the reduction of enones of the type Ar—CH = C(CH3)—COCH3 to the corresponding (S)-ketones in almost quantitative yields and very high enantiomeric excesses[52]. 

In order to place later chapters in proper context, the first chapter offers a comprehensive overview of industrially important catalysts for oxidation and reduction reactions. Chapters 2 and 3 describe the preparation of chiral materials by way of the asymmetric reduction of alkenes and ketones respectively. These two areas have enjoyed a significant amount of attention in recent years. Optically active amines can be prepared by imine reduction using chiral catalysts, as featured in Chapter 4, which also discloses a novel reductive amination protocol. 

Addition of hydrogen atoms in the presence of a metal catalyst to double or triple bonds is known as hydrogenation or catalytic hydrogenation. Alkenes and alkynes are reduced to alkanes by the treatment with H2 over a finely divided metal catalyst such as platinum (Pt—C), palladium (Pd—C) or Raney nickel (Ni). The platinum catalyst is also frequently used in the form of Pt02, which is known as Adams s catalyst. The catalytic hydrogenation reaction is a reduction reaction. 

Oxymercuration-reduction of alkenes preparation of alcohols Addition of water to alkenes by oxymercuration-reduction produces alcohols via Markovnikov addition. This addition is similar to the acid-catalysed addition of water. Oxymercuration is regiospecific and auft -stereospecific. In the addition reaction, Hg(OAc) bonds to the less substituted carbon, and the OH to the more substituted carbon of the double bond. For example, propene reacts with mercuric acetate in the presence of an aqueous THF to give a hydroxy-mercurial compound, followed by reduction with sodium borohydride (NaBH4) to yield 2-propanol. 

Addition of alcohol to alkenes hy alkoxymercuration-reduction produces ethers via Markovnikov addition. This addition is similar to the acid-catalysed addition of an alcohol. For example, propene reacts with mercuric acetate in aqueous THF, followed hy reduction with NaBFl4, to yield methyl propyl ether. The second step is known as demercuration, where Flg(OAc) is removed hy NaBH4. Therefore, this reaction is also called alkoxymercura-tion-demercuration. The reaction mechanism is exactly the same as the oxymercuration-reduction of alkenes. 

Nonstoichiometric metal oxides are effective catalysts for a variety of oxidation-reduction reactions (as might be expected) since the variable valence of the constituent ions enables the oxide to act as a sort of electron bank. Nonstoichiometric metal oxides resemble metals in that they can also catalyze hydrogenation and alkene isomerization reactions. However, on zinc oxide, for instance, these two processes are independent, whereas hydrogen must be present for isomerization to occur on metals. 

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... 

ALKENES AND ALKYNES II. OXIDATION AND REDUCTION REACTIONS. ACIDITY OF ALKYNES... 

Alkenes and Alkynes II. Oxidation and Reduction Reactions. Acidity of Alkynes... 

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