Catalytic hydrogenation of alkenes

Tbe Parr hydrogenation apparatus shakes the reaction vessel (containing the alkene and the solid catalyst), while a pressurized cylinder supplies hydrogen. 

Although we have mentioned catalytic hydrogenation before (Sections 7-7 and 8-1), we now consider the mechanism and stereochemistry in more detail. Hydrogenation of an alkene is formally a reduction, with H2 adding across the double bond to give an alkane. The process usually requires a catalyst containing Pt, Pd, or Ni. 

For most alkenes, hydrogenation takes place at room temperature, using hydrogen gas at atmospheric pressure. The alkene is usually dissolved in an alcohol, an alkane, or acetic add A small amount of platinum, palladium, or nickel catalyst is added, and the container is shaken or stirred while the reaction proceeds. Hydrogenation actually takes place at the surface of the metal, where the liquid solution of the alkene comes into contact with hydrogen and the catalyst. 

Hydrogen gas is adsorbed onto the surface of these metal catalysts, and the catalyst weakens the H—H bond. In fact, if H2 and D2 are mixed in the presence of a platinum catalyst, the two isotopes quickly scramble to produce a random mixture of HD, H2, and D2. (No scrambling occurs in the absence of the catalyst) Hydrogenation is an example of heterogeneous catalysis, because the (solid) catalyst is in a diffo-ent phase from the reactant solution. In contrast, homogeneous catalysis involves reactants and catalyst in the same phase, as in the acid-catalyzed dehydration of an alcohol. 

One face of the alkene pi bond binds to the catalyst, which has hydrogen adsorbed on its surface. Hydrogen inserts into the pi bond, and the product is freed from the catalyst. Both hydrogen atoms add to the face of the double bond that is complexed with the catalyst 

A catalyst must somehow overcome this symmetry restriction. As already discussed, Hj can add to organometallic species to make a metal hydride. If one can make an olefin complex of the metal hydride, then the electrons can flow from the M—H a bond to the 7t antibonding orbital of the olefin to initiate the process of breaking the C=C bond and making a C— H bond. 

The catalytic properties of Rh(PPh3)jCl were first reported by Wilkinson and co-workers. The nature of the species present in hydrocarbon solvents was the subject of controversy until the work of Arai and Halpem, which indicated some PPhj dissociation, shown by 

However, Tolman and co-workers found that phosphine liberation was accompanied by dimer formation, which has K = 2.4x1 O M in benzene at 25°C. The reaction and suggested structure for the dimeric product are given by 

With [Hj] and [PPh3] [Rh] at 25°C in benzene, the kinetics indicate that the reaction proceeds by parallel paths involving oxidative addition to 

Values of Jfc, = 4.8 M s , Jfej = 0.71 s and kjfkj = 1.1 (25 C in benzene) were determined from the dependence of the rate on [PPhj] and [Hj]. At low [PPhj], the pseudo-first-order rate constant  

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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... 

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. 

The catalytic hydrogenation of alkenes by mixed NHC/phosphine complexes of rhodinm was also stndied. Initial resnlts of the hydrogenation of cyclohexene by /ranx-[RhCl(ICy)(L)2] (ICy = M/V -(dicyclohexyl)imidazol-2-ylidene, L = PPhj. 

The mechanistic description of catalytic hydrogenation of alkene is somewhat imprecise, partly because the reactive sites on the metal surface are not as well... 

One of the most famous catalysts, which operates through a mechanism involving formation of a mono-hydride (M—H), is [RuCl2(PPh3)3].38-40 In the catalytic hydrogenation of alkenes (Equations (1) and (2)) it shows very high selectivity for hydrogenation of terminal rather than internal C=C bonds. 

Duckett381 reported on the use of parahydrogen-induced polarization (PHIP) to delineate the pathways involved in the catalytic hydrogenation of alkenes and alkynes by [Ru3(CO)12 x(PPh3)x] (x = 1 or 2) and showed that the mechanism is highly dependent on the solvent. Bassett and... 

Table 6.8 Rate constants for the catalytic hydrogenation of alkenes catalyzed by Group III and lanthanide complexes.3 ...

Biphasic systems containing an ionic liquid and supercritical CO2 have been used effectively for catalytic hydrogenation of alkenes. The ionic liquid phase containing the catalyst could be reused (2/6). 

Alkanes are prepared simply by catalytic hydrogenation of alkenes or alkynes (see Section 5.3.1). 

The reduction of double bond-containing functionalities, especially alkenes and carbonyl compounds, is an important methodology used in synthetic elaboration. In this section the stereocontrolled reduction of aldehydes, ketones and C=N-containing compounds and the catalytic hydrogenation of alkenes are covered, among other reductions. The emphasis here is placed on stereocontrolled reactions. 

Mechanism 8-8 Formation of Halohydrins 352 8-10 Catalytic Hydrogenation of Alkenes 355 8-11 Addition of Carbenes to Alkenes 358 8-12 Epoxidation of Alkenes 360... 

Any method of making such bicyclic compounds will automatically form this stereochemistry. An important method of stereochemical control that we have not used so far in this chapter is catalytic hydrogenation of alkenes, which adds a molecule of hydrogen stereospecifically cis. If the reaction also makes a fused ring system, it may show stereoselectivity too. Here is an example with 5/5 fused rings. 

The catalytic hydrogenation of alkenes, ketones, and imines is arguably one of the most important transformations in chemistry. Powerful asymmetric versions have been realized that require metal catalysts or the... 

The dihydrido complex [RhH2Cl(PPh3)2] is a very important intermediate in the homogeneous catalytic hydrogenation of alkenes.20 The monohydrido complexes (Table 63) can be made by the oxidative addition of HY species to rhodium(I) complexes (equation 187). Similar complexes can be obtained when bulky tertiary phosphines are allowed to react with alcoholic solutions of hydrated rhodium trichloride.268 269... 

Reaction of Ceo with Pd2(dba)3 yields the organometallic polymer CeoPd . Material of varying composition may be prepared, but those with n > 3 are active for the catalytic hydrogenation of alkenes and alkynes. This material is proposed to consist of catalytically active palladium atoms distributed on an inert 3D polymer CeoPds. 

Good reversible binding of O2 and CO was observed for Cu(I)-dielate (Table 18) The Cu(I)- and Cu(n)-chelates (122) show a high activity in the Michael-type addition of alcohols to acrolein Up to 66 mol /3-alkoxypropionaldehyde per mol metal center were obtained the yield decreased with lower Cu content in (122) Acrylonitrile is polymerized in the presence of (122)-Cu(II) under H2 pressure. The Co containing complex is able to polymerize styrene and to catalyze Michael additions. For a Pd-complex CO binding and afterwards catalytic hydrogenation of alkenes are reported ... 

The partial reduction of substrates containing triple bonds is of considerable importance not only in research, but also commercially for stereoselectively introducing (Z)-double bonds into molecular frameworks of perfumes, carotenoids, and many natural products. As with catalytic hydrogenation of alkenes, the two hydrogen atoms add syn from the catalyst to the triple bond. The high selectivity for alkene formation is due to the strong absorption of the alkyne on the surface of the catalyst, which displaces the alkene and blocks its re-adsorption. The two principal metals used as catalysts to accomplish semireduction of alkynes are palladium and nickel. 

Some very widely used organic reactions are catalyzed or mediated by transition metals. For example, catalytic hydrogenation of alkenes, dihydroxylation of alkenes, and the Pauson-Khand reaction require Pd, Os, and Co complexes, respectively. The d orbitals of the transition metals allow the metals to undergo all sorts of reactions that have no equivalents among main-group elements. This doesn t mean that the mechanisms of transition-metal-mediated reactions are difficult to understand. In fact, in some ways they are easier to understand than standard organic reactions. A transition-metal-catalyzed or -mediated reaction is identified by the presence of a transition metal in the reaction mixture. 

In contrast to a trace catalyst, which is only required to be present in minute amounts, e.g. the catalytic hydrogenation of alkenes by Pt. 

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