Hydrogenations of Alkenes

The type of reaction that takes place in these examples is an addition reaction. The product that results from the addition of hydrogen to an alkene is an alkane. Alkanes have only 

Fats and oils (Section 23.2) are glyceryl esters of carboxylic acids with long carbon chains, called fatty acids. Fatty acids are saturated (no double bonds), monounsatu-rated (one double bond), or polyunsaturated (more than one double bond). Oils typically contain a higher proportion of 

A product used in baking that contains oils and mono- and diacylglycerols that are partially hydrogenated. 

There have been several attempts to raise the activity of homogeneous catalysts by use of solvents of low coordinating power. [Ir(PMePH2)2(COD)] [Pp6] is very active in dichloromethane at Reduction of the (dicarbollide)Rh 

There are rather few water-soluble hydrogenation catalysts. A number of rhodium complexes derived from ligands such as (PH2PCH2CH2)2NC0C6H4S03 are active in this way. Catalysts have been developed for hydrogenation of ketonescarboxylic acids/ esters/ nitriles/ and nitro compounds. 

One of the most widely studied examples of photocatalysis by organometallic complexes is the photocatalytic hydrogenation of dienes by Group VI transition metal carbonyls M(CO 6 (M = Cr, Mo, W). One of the earliest discoveries was the finding that Cr(CO)6 is a good catalyst for the light-induced hydrogenation of 2,3-dimethylbutadiene and 1,3-cyclohexadiene to 2,3-dimethyl-2-butene and cyclohexene respectively  

No other isomers are obtained in the reaction. Further work with other dienes has verified the generality of this selective 1,4-hydrogenation. The dienes that are reactive have the ability to readily achieve the s-cis conformation, which is the preferred geometry for coordination to the metal center  

There is also evidence for the intermediacy of alkene chromium carbonyl complexes in alkene hydrogenation. An example are the intermediacy of tranj,rr my-2,4-hexadiene chromium tetracarbonyl in the photocatalytic hydrogenation of tronj,tra/tf-2,4-hexadiene. Substituted chromium tricarbonyl complexes that have ligands that can be replaced by alkenes also act as homogeneous 

By the use of time-resolved infrared spectroscopy, the reaction of Cr(CO)4 with dienes has been studied. This tetracarbonyl species can be photogenerated by the photolysis of Cr(CO)6 in the gas phase at 248 nm. The addition of a diene such as 1,3-pentadiene to this photogenerated Cr(CO)4 results in the formation of a highly excited complex Cr(CO)4( / -diene) that can collisionally relax to form Cr(CO)4(//-Miene), or relax via double bond dissociation to form Cr(CO)4( -diene). This latter / -diene complex can then rearrange at a longer time scale first-order process to give Cr(CO)4(i7 -diene). In each case Cr(CO)4( -diene) is the final product of the relaxation process.  

Transition metal complexes that act as hydrogenation catalysts are usually 

Mechanism. The generally accepted mechanism for the hydrogenation of double bonds over heterogeneous catalysts was first proposed by Horiuti and Polanyi,50,51 and was later supported by results of deuteration experiments. It assumes that both hydrogen and alkene are bound to the catalyst surface. The hydrogen molecule undergoes dissociative adsorption [Eq. (11.1)], while the alkene adsorbs associa-tively [Eq. (11.2)]. Addition of hydrogen to the double bond occurs in a stepwise manner [Eqs (11.3) and (11.4)]  

While the last step [Eq. (11.4)] is virtually irreversible under hydrogenation conditions, both the adsorption of alkene [Eq. (11.2)] and the formation of alkyl intermediate (half-hydrogenated state) [Eq. (11.3)] are reversible. The reversibility of these steps accounts for the isomerization of alkenes accompanying hydrogenation (see Section 4.3.2). Isomerizations, either double-bond migration or cis-trans isomerization, may not be observable, unless the isomer is less reactive, or the isomerization results in other structural changes in the molecule, such as racemization. 

Further studies led to the suggestion of other types of surface species, such as the ir-adsorbed intermediate (1) and dissociatively adsorbed alkenes [a-vinyl (2), a-allyl (3) and Jt-allyl (4)]  

Metals differ in their ability to catalyze isomerization. Both the relative rate of transformation of the individual isomers and the initial isomer distribution vary with the metal. The order of decreasing activity of platinum metals in catalyzing the isomerization of alkenes was found to be Pd - Rh, Ru, Pt Os Ir.53 Platinum is generally the preferable catalyst if isomerization is to be avoided. The most active Raney nickel preparations rival palladium in their activity of isomerization. 

According to the Horiuti-Polanyi mechanism, isomerization requires the participation of hydrogen. The first addition step, formation of the half-hydrogenated state [Eq. (11.3)], cannot take place without hydrogen. Numerous investigations have supported the role of hydrogen in these so-called hydroisomerizations. 

More recent examples of experiments with sterically congested molecules are Mylroie and Stenberg s hydrogenation of the sterically hindered substituted tryptycenes,60 and hydrogenation of the double bond in tetraethyl bicy-clo[2.2.2]oct-7-ene-2-syn,3-syn,5-syn,6-syn-tetracarboxylate (4) which occurs over 5% Pd/C in ethyl acetate at room temperature and under 1 atm of hydrogen in 48 hours.61 

On the other hand, severe congestion and a lower temperature (20°C) prevent the hydrogenation of the propylidene double bond (tetrasubstituted but 

In the following section, we discuss the basic mechanism of homogeneous hydrogenation by Wilkinson s catalyst, RhCl(PPh3)3. Many other complexes of rhodium as well as complexes of other metals such as ruthenium, platinum, lutetium, etc. have also been used as homogeneous, laboratory-scale, hydrogenation catalysts. The mechanisms in all these cases may differ substantially. 

The basic mechanism of hydrogenation is shown by the catalytic cycle in Fig. 7.3. This cycle is simplified, and some reactions are not shown. Intermediate 7.9 is a 14-electron complex (see Section 2.1). Phosphine dissociation of Wilkinson s complex leads to its formation. Conversion of 7.9 to 7.10 is a simple oxidative addition of H2 to the former. Coordination by the alkene, for example, 1-butene, generates 7.11. Subsequent insertion of the alkene into the metal-hydrogen bond gives the metal alkyl species 7.12. The latter undergoes reductive elimination of butane and regenerates 7.9. 

Note that although the conversion of 7.11 to 7.12 assumes anti-Markovnikov addition, the Markovnikov product also gives butane. Conversion of 7.9 to 7.11 could also take place by prior coordination of alkene followed by the oxidative addition of dihydrogen. Indeed this parallel pathway for the formation of 7.11 does operate. Like the equilibrium shown between RhClL3, 7.9, and the dimer [RhClL L, there is an equilibrium between 7.9 and the alkene coordinated complex RhCl(alkene)L2. 

It was mentioned in Section 3.3.4 that alkenes react with H2 on the surface of elemental Pd or elemental Pt to form alkanes. Similar hydrogenations occasionally also can be accomplished using Raney nickel as a catalyst. Raney nickel is prepared from a 1 1 Ni/Al alloy and aqueous KOH. 

In 2001, Knowles and Noyori s pioneering achievements in this field were honored with the award of the Nobel Prize in Chemistry. 

The asymmetric hydrogenation of nitrogen-free acrylic acids or acrylic acid esters proceeds in a similar enantioselective fashion. For these substrates Rh- and Ru-catalysts are used with the same frequency. 

You will find that the absolute configuration of the newly formed stereocenter will change in said asymmetric hydrogenations upon alteration of one (no matter which) of the following variables (1) to (3)  

For enantioselectivity to occur with homogeneous hydrogenations, the unsaturated substrate must bind to the catalytic center in such a way that a complex with well-defined stereostructure is formed. Accordingly, a highly enantioselective hydrogenation is assured—at least in most cases—if the substrate forms two bonds to the metal. The substrate is -bonded to the metal via the C=C double bond that is to be hydrogenated. It is also (7-bonded to the metal via a heteroatom that is close enough to this C=C double bond. 

The relationship between reactants and products in addition reactions can be illustrated by the hydrogenation of alkenes to yield alkanes. Hydrogenation is the addition of H2 to a multiple bond. An example is the reaction of hydrogen with ethylene to form ethane. 

The uncatalyzed addition of hydrogen to an alkene, although exothermic, is very slow. The rate of hydrogenation increases dramatically, however, in the presence of certain finely divided metal catalysts. Platinum is the hydrogenation catalyst most often used palladium, nickel, and rhodium are also effective. Metal-catalyzed addition of 

The French chemist Paul Sabatier received the 1912 Nobel Prize in Chemistry for his discovery that finely divided nickel is an effective hydrogenation catalyst. 

What three alkenes yield 2-methylbutane on catalytic hydrogenation  

The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents (or, indeed, in any solvent). Two phases, the solution and the metal, are present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase are called heterogeneous reactions. 

The most commonly employed substrates are (Z)-a-acetamidocinnamic acid (ACA), its methyl ester (MAC), acetamidoacrylic acid (AAA) and its methyl ester (MAA). Quantitative yields at mild hydrogen pressure (1-3 atm) are usually obtained with these substrates. Values of ee very close to 100% have been reached for dozens ligands, including many P-stereogenic phosphines. The most successful examples of the hydrogenation of a-dehydroamino acid derivatives are listed in Table 7.1. 

Results on hydrogenation of AAA are collected in entries 1-6. Although DiPAMP is a reasonably stereoselective ligand for this substrate (entry 1) the structure of the phosphine has been optimised leading to significantly improved 

Similar trends have been observed in the hydrogenation of ACA (entries 23-36). With this substrate the best enantioseleetivities have been obtained with modified DiPAMP ligands (entry 24), Trichickenfootphos (entry 28), Tang-Phos (entry 30), BIPNOR (entry 34) and MeO-POP (entry 36). 

Many other ot-dehydroamino acids have been successfully hydrogenated with rhodium catalysts containing P-stereogenic ligands. A small selection is given in entries 76-82. Entry 78 shows once more that differences in the 

The most frequently applied substrates are (Z)- and ( )-p-acet-amidocrotonates (MAB). The best results obtained with those substrates and some derivatives using Rh(I)/P-stereogenic phosphines are listed in Table 7.2. 

5 Carbocation Rearrangements in Hydrogen Haiide Addition toAikenes 225 

13 Free-Radicai Addition of Hydrogen Bromide to Aikenes 242 

15 introduction to Organic Chemicai Synthesis Retrosynthetic Anaiysis 245 

Petroleum-derived benzene is the source of the six carbon atoms of adipic acid, an industrial chemical used to make nylon. An alternative process has been developed that uses genetically engineered strains of the bacterium Escherichia coli to convert glucose, a renewable resource obtained from cornstarch, to ds,c/s-muconic acid. Subsequent hydrogenation gives adipic acid. This chapter is about reactions that involve addition to double bonds and begins with hydrogenation. 

Now that we re familiar with the structure and preparation of aikenes, let s look at their chemical reactions, the most characteristic of which is addition to the double bond according to 

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Figure C2.7.2. Catalytic cycle (witliin dashed lines) for tire Wilkinson hydrogenation of alkene [2]. Values of rate and equilibrium constants are given in [2]...

C2.7.6.1 WILKINSON HYDROGENATION OF ALKENES CATALYSED BY A RHODIUM COMPLEX... 

Halpern J, Okamoto T and Zakhariev A 1976 Mechanism of the chlorotris(triphenylphosphine)rhodium(l)-catalyzed hydrogenation of alkenes J. Mol. Catal. 2 65-9... 

FIGURE 6 1 A mechanism for heterogeneous catalysis in the hydrogenation of alkenes... 

Section 6 2 Hydrogenation of alkenes is exothermic Heats of hydrogenation can be... 

Compounds A and B are isomers of molecular formula C9Hi9Br Both yield the same alkene C as the exclusive product of elimination on being treated with potassium tert butoxide in dimethyl sulfoxide Hydrogenation of alkene C gives 2 3 3 4 tetramethylpentane What are the structures of compounds A and B and alkene C2... 

Like the hydrogenation of alkenes hydrogenation of alkynes is a syn addition CIS alkenes are intermediates in the hydrogenation of alkynes to alkanes... 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

Hydrogenation of benzene and other arenes is more difficult than hydrogenation of alkenes and alkynes Two of the more active catalysts are rhodium and platinum and it IS possible to hydrogenate arenes m the presence of these catalysts at room temperature and modest pressure Benzene consumes three molar equivalents of hydrogen to give cyclohexane... 

Grignard shared the prize with Paul Sabatier who as was mentioned in Chapter 6 showed that finely divided nickel could be used to cat alyze the hydrogenation of alkenes... 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Hydrogenation (Section 11.16) Hydrogenation of aromatic rings is somewhat slower than hydrogenation of alkenes, and it is a simple matter to reduce the double bond of an unsaturated side chain in an arene while leaving the ring intact. 

Heterogeneous reaction (Section 6.1) A reaction involving two or more substances present in different phases. Hydrogenation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst. 

Figure 2.15 Cycle for the hydrogenation of alkenes catalysed by RhCl(PPh3)3.

When an oxidation or a reduction could be considered in a previous chapter, this was done. For example, the catalytic hydrogenation of alkenes is a reduction, but it is also an addition to the C=C bond and was treated in Chapter 15. This chapter discusses only those reactions that do not fit into the nine categories of Chapters 10-18. An exception to this rule was made for reactions that involve elimination of hydrogen (19-1-19-6), which were not treated in Chapter 17 because the mechanisms generally differ from those in that chapter. 

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