Hydrogenation, catalytic, alkene mechanism

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. 

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... 

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... 

By contrast with the radical systems, this mechanism is the most common and most studied. It is also historically important for giving the first practical demonstration of catalytic homogeneous hydrogenation of alkenes. 

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. 

Proposed mechanism for the catalytic hydrogenation of alkenes using Wilkinson s catalyst. 

Surface Reaction Mechanism. The mechanism of catalytic alkene ammoxidation is invariably linked to allylic oxidation chemistry. Allylic oxidation is the selective oxidation of an alkene at the allylic carbon position. Selective allylic oxidation and ammoxidation proceed by abstraction of the hydrogen from the carbon positioned a to the carbon-carbon double bond. This produces an allylic intermediate in the rate-determining step. In the case where propylene is the hydrocarbon, the reaction is as follows ... 

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