Mechanisms of alkenes

The mechanism of alkene epoxidation is believed to be a concerted process mvolv mg a single bimolecular elementary step as shown m Figure 6 14... 

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

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... 

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. 

In general, the mechanism of alkene hydroformylation with an [RhH(CO)P3] catalyst in water or in aqueous/organic biphasic systems (P = TPPTS) is considered to be analogous [61] to that of the same reaction in homogeneous organic solutions (P = PPh3) [84], a basic version of which is shown on Scheme 4.8. 

Early work on the mechanisms of alkene cleavage by RuO has been briefly reviewed [50]. In the oxidation of 1,5-dienes to cA-tetrahydrofurandiols by RuO / aq. Na(10 )/EtOAc-acetone it is likely that there is cyclo-addition of RuO to one double bond of two 1,5-diene molecules to give a Ru(lV) diester this is oxidised by Na(lO ) to a Ru(Vl) diester, which is then hydrolysed to the organic product (Fig. 3.12) [345], and indeed Ru(Vl) diesters RuOlO R) have been isolated (Fig. 1.31) [323, 346]. ... 

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. 

A. A. Frimer. Chemical Reviews, volume 79, American Chemical Society (1979), p. 3S9. The reactions and their mechanisms of alkenes with singlet oxygen are the subjects of this review. 

The reaction of (148) with norbomene produces a stable, well-characterized bright yellow netallacyclic compound (152) which, upon standing at 25 °C in toluene, slowly decomposes to pve exo-epoxynorbornene in almost quantitative yield (equation 199).210 The particular decomposition, which is related in some aspects to the mechanism of alkene epoxidation by d° metal-peroxo... 

Mechanism of alkene epoxidation by oxometalloporphyrins containing hypervalent Fe, Cr, and Mn atoms 92ACR314. 

These mechanisms should remind you of the mechanism of alkene bromination (p. 504)—except that here the attack on the bromine is assisted by an electron pair on oxygen. The product, instead of being a bromonium ion (which would undergo further reactions), loses a proton (or the Lewis acid) to give a ketone. 

As mentioned earlier, the first reaction in the second stage is the isomerization of 3PN to 4PN, reaction 7.18. The mechanism of this reaction is very similar to the mechanism of alkene isomerization discussed in Section 7.2.1, and shown by Fig. 7.14. The following points need attention. 

The work of Davis was, however, unable to distinguish which oxygen was attacked on the titanium peroxo complex when the alkene co-ordinates. Therefore, Neurock and Manzer conducted a theoretical study of the mechanism of alkene epoxidation using TS-1 with aqueous hydrogen peroxide.32 The workers concluded that their calculations to predict both the structure and relative bands in the IR spectra for TS-1 were in good agreement with experimental data. The calculations indicated that the oxygen closest to the titanium centre was the active site for alkene attack. The result was the direct formation of... 

Scheme 11. The mechanism of alkene epoxidation catalyzed by chromium-salen complexes...

Scheme 4.56 Mechanism of alkenation of aldehydes and ketones with tosylhydrazones in the presence of a catalyst (ClFeTPP)...

That alkenes are formed from those alkyl groups containing a jS-hydrogen atom strongly implies that the mechanism of alkene formation involves a /3-hydride abstraction step. There is a very pronounced kinetic isotope effect when C6D5CD2CH2COCI is decarbonylated, which indicates that not only does a jS-deuteride abstraction take place but that it is also rate determining. Further evidence for the participation of a /3-hydride abstraction comes from the decarbonylation of erythro- or t/ reo-2,3-diphenylbutanoyl chloride, where the former yields the (ii)-alkene and the latter the (Z)-isomer. 

Alkyl iridium compounds are also accessible via insertion (see Migratory Insertion) of alkenes into Ir H bonds. Analogously, alkenyl iridium compounds may be formed via insertion of alkynes into Ir-H bonds. These types of reactions have been studied to shed tight on the mechanism of alkene and alkyne hydrogenation processes. For example, HIr(CO)(PPh3)2 (65) will react with ethylene and higher olefins to produce the alkyl iridium compounds (equation 17). 

The mechanism described in Scheme 1 (the Chauvin mechanism) is the accepted mechanism of alkene metathesis, and its validity has been demonstrated in two ways. First, classical kinetic studies, including isotopic labeling and crossover experiments performed using poorly defined catalysts, conclusively demonstrated that the carbene mechanism was consistent with the experiments, while the pairwise mechanism was not. More recently, the synthesis of isolable carbene complexes that catalyze the reaction has allowed a more direct observation of the reaction. Each individual step in the Chauvin mechanism has now been observed spectroscopically for several of the well-defined catalyst systems. 

Scheme 1 The Horiuti-Polanyi mechanism of alkene hydrogenaiion...

Mechanism of alkene hydroboration. The reaction occurs in a single step, in which both C-H and C-B bonds form at the same time and on the same face of the double bond. Electrostatic potential maps show that boron becomes negative in the transition state, as electrons shift from the alkene to boron, but is positive in the product. 

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