Alkene hydrogenation reaction

The trinuclear cluster [(/i-H)2Ru3(/i3-0)(C0)5(DPPM)2] is also an efficient catalyst for alkene hydrogenation reaction, for which Bergounhou proposed the catalytic Scheme 73.38... 

A few experiments have been performed to evaluate the potential of the P-menthyl-substituted class of phosphe-tanes 76 in rhodium-mediated alkene-hydrogenation reactions <1998S1539>. Within this series, monodentate ligands show low catalytic activity and poor enantioselectivity when employed in the hydrogenation of model dehydroamino acid derivatives. This observation is clearly consistent with the observed lability of their rhodium complexes, due to steric hindrance. 

To complete the alkene hydrogenation reaction sequence, the first hydrogen transfer must be followed by a second, which results in the reductive elimination of the alkane product. This proceeds through a three-centered transition state. The catalytic cycle is shown in Fig. 22-1 but the process is actually more complicated since the equilibria are dependent on phosphine, alkene, rhodium concentrations, temperature, and pressure. 

An jj (S) equilibrium (equation 6) is observed in the benzo[fc]thiophene (BT) complexes Cp (CO)2Re(BT), where Cp is Cp or Cp. As for selenophene, the Cp ligand favors j -BT, while Cp favors the rj (S) form it should be noted that 2-MeBT and 3-MeBT form only the jj (S) complexes. The formation of the r] form from the is important for understanding the first step in the HDS of BT, which gives dihydrobenzothiophene (equation 7). This alkene hydrogenation reaction is catalyzed by homogeneous catalysts and may occur similarly on heterogeneous HDS catalysts. While there is no evidence that there is an r] (S) equilibrium in thiophene complexes, both thiophene species may exist on a catalyst surface. Low metal oxidation states would favor r] coordination, which could lead to the initially observed alkene hydrogenation products. 

In your study of chemistry thus far, you have probably encountered a number of catalyzed reactions. Those who are familiar with organic chemistry will recognize equation 9.2 as an alkene hydrogenation reaction that occurs at a reasonable rate only in the presence of a catalytic amount of Pd or Pt deposited on an inert solid. The synthesis of esters from alcohols and carboxylic acids (equation 9.8) is catalyzed by mineral acids such as H2S04 or HC1. 

The simplest reaction of alkenes is the addition of hydrogen to form alkanes. Hydrogenation is a nonpolar process, unlike many of the addition reactions, to be described later, that involve electrophiles attaching to the nucleophilic n electrons of the alkene. Hydrogenation reactions require catalysts, such as platinum oxide (Adams s catalyst), palladium on carbon (Pd-C), or Raney nickel (Ra-Ni). —, ... 

Instead, biochemical redox reactions involving the oxidation of alkane to alkene require the participation of a coenzyme such as flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). The reduced forms FADH2 and FMNHj act as hydride donors in the alkene hydrogenation reactions (Figure 1.41). 

In the following section we 11 see another method for converting alkynes to alkenes The reaction conditions are very different from those of Lindlar hydrogenation So IS the stereochemistry... 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydrogenation of the caibon-oxygen double bond. Like the hydrogenation of alkenes, the reaction is exothennic but exceedingly slow in 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 ... 

The reverse reaction (formation of metal alkyls by addition of alkenes to M-H) is the basis of several important catalytic reactions such as alkene hydrogenation, hydroformylation, hydroboration, and isomerization. A good example of decomposition by y3-elimination is the first-order intramolecular reaction ... 

Alkene hydrogenation occurs on the surface of metal particles which act as a catalyst for the reaction. This usually means that both hydrogens are added to the same face of the alkene syn addition). 

Draw the other stereoisomer that might have been obtained from syn addition of hydrogen to each alkene. Is the observed product for each addition also the thermodynamic product Compare energies for alkene A+ H2 observed and not observed and alkene B+H2 observed and not observed. What structural factors seem to be responsible for the relative stabilities of the two products of each hydrogenation reaction ... 

Figure 7.7 MECHANISM Mechanism of alkene hydrogenation. The reaction takes place with syn stereochemistry on the surface of insoluble catalyst particles.

In this type of reaction, called hydrogenation, the regiochemistry will always be irrelevant, regardless of what alkene we use (we are adding two of the same group). However, we do need to explore the stereochemistry of hydrogenation reactions. In order to do this, let s take a close look at how the reaction takes place. 

When the addition is initiated by attack of the jr-electrons in an unsaturated bond on an electrophile to form a carbocation the reaction is an electrophilic addition, a very common class of reactions for alkenes. The reaction is governed by Markovnikov s rule, which states that in addition of HX to a substituted alkene, the H will form a bond to the carbon of the alkene carrying the greater number of hydrogen atoms. 

Until recently, iron-catalyzed hydrogenation reactions of alkenes and alkynes required high pressure of hydrogen (250-300 atm) and high temperature (around 200°C) [21-23], which were unacceptable for industrial processes [24, 25]. In addition, these reactions showed low or no chemoselectivity presumably due to the harsh reaction conditions. Therefore, modifications of the iron catalysts were desired. 

Conjugated Alkene Hydrogenation Investigated in Micro Reactors Cas/liquid reaction 19 [CL 19) Hydrogenation of Z-(a)-acetamidocinnamic methyl ester... 

The hydrogenation activity of the isolated hydrides 3 and 6 towards cyclooctene or 1-octene was much lower than the Wilkinson s complex, [RhCKPPhj) ], under the same conditions [2] furthermore, isomerisation of the terminal to internal alkenes competed with the hydrogenation reaction. The reduced activity may be related to the high stability of the Rh(III) hydrides, while displacement of a coordinated NHC by alkene may lead to decomposition and Rh metal formation. 

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