Alkenes, hydrogenation mechanism

Scheme 4.17 Simplified alkene hydrogenation mechanism using Wilkinson s catalyst...

Fig. 15.1. Horiuti-Polanyi alkene hydrogenation mechanism showing double bond isomerization steps.

Fig. 15.2. Alkene hydrogenation mechanism proceeding through a n-allyl intermediate.

Fig. 20.4 A cyclopropane hydrogenolysis mechanism based on the Horiuti-Polanyi alkene hydrogenation mechanism.

In the mechanism for alkene hydrogenation shown m Figure 6 1 hydrogen atoms are transferred from the catalyst s surface to the alkene Although the two hydrogens are not transferred simultaneously they both add to the same face of the double bond... 

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

The stereochemistry of addition depends on the details of the mechanism. The addition can proceed through an ion pair intermediate formed by an initial protonation step. Most alkenes, however, react via a complex that involves the alkene, hydrogen halide, and a third species that delivers the nucleophilic halide. This termolecular mechanism is generally pictured as a nucleophilic attack on an alkene-hydrogen halide complex. This mechanism bypasses a discrete carbocation and exhibits a preference for anti addition. 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Scheme 1 Fundamental steps in Rh-catalyzed alkene hydrogenation involving a dihydride-based mechanism...

Scheme 2 General strategies for hydrogenative C-C coupling predicated on the mechanism of conventional Rh-catalyzed alkene hydrogenation...

The most accepted mechanism for alkene hydrogenation is mainly due to Hal-pern [23], and is supported by careful kinetic and spectroscopic studies of cyclo-... 

The old and lasting problem of heterogeneous catalysis, the mechanism of alkene hydrogenation, has also been approached from the viewpoint of structure effects on rate. In 1925, Lebedev and co-workers (80) had already noted that the velocity of the hydrogenation of the C=C bond decreases with the number of substituents on both carbon atoms. The same conclusion can be drawn from the narrower series of alkenes studied by Schuster (8J) (series 52 in Table IV). Recently authors have tried to analyze this influence of substituents in a more detailed way, in order to find out whether the change in rate is caused by polar or steric effects and whether the substituents affect mostly the adsorptivity of the unsaturated compounds or the reaetivity of the adsorbed species. Linear relationships have been used for quantitative treatment. 

A multistep pathway analogous to the mechanism of alkene hydrogenation has been shown to be operative in the rhodium-catalyzed hydroboration of alkenes.363 Deuterium labeling studies furnished evidence that the reversibility of the elementary steps is strongly substrate-dependent. The key step is hydride rather than boron migration to the rhodium-bound alkene. 

The hydrogenation mechanisms discussed above indicate the reversible formation of intermediates. The degree of reversibility depends on the nature of the catalyst and the alkene, and on reaction conditions. Some nonreducible internal alkenes undergo slow hydrogen exchange and isomerization, indicating that reversible steps can occur in these cases. The reversible formation of alkylmetal intermediates provides a ready explanation for the isomerization of alkenes in the presence of certain metal complexes (see Section 4.3.2). 

Hydrogenations with [Co(CN)5]3 as catalyst have been long thought to involve radical reactions. Hydrogen is activated prior to reaction with the alkene, the mechanism involving homolytic fission by two [Co(CN)5]3 ions (Scheme 3).54... 

Scheme 9 Proposed mechanism and substrate scope for tandem alcohol oxidation/Wittig reaction/alkene hydrogenation sequence...

The mechanism of the Lindlar hydrogenation is analogous to the mechanism of the heterogeneously catalyzed alkene hydrogenation (Figure 14.65). It is for this similarity... 

Various kinetic studies on these processes have been carried out.388 391,393,395"196 For example alkene hydrogenation in [CoH(alkene)(PPh3),] follows the rate law kobs = fc[H2][alkene][PPh3] 1 and this has been interpreted in terms of a dual pathway originating from the coordinatively unsaturated [CoH(alkene)(PPh3)2] intermediate (Scheme 30),396 but other mechanisms are possible. Similarly... 

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