Dienes hydrogenation mechanism

Scheme 14.15 Proposed hydrogenation mechanism for the 1,4-hydrogenation of dienes by [Cr(CO)3 (arene)] (45).

Scheme 14.18 Proposed 1,4-hydrogenation mechanism for the hydrogenation of dienes by cationic rhodium complexes [P2Rh(diene)]+A (54).

Fig. 34. Mechanisms for buta-1 2-diene hydrogenation operative over Types A and B nickel catalysts [207]. (Numbers refer to species in Fig. 33.)...

The transfer of hydrogen to the alkene also occurs by a radical process,55,50 and not by the insertion reaction which is much more common in homogeneous hydrogenation. These kinetic studies were carried out using cinnamic acid as alkene. The observation of alkenyl and allyl species in diene hydrogenation suggests that a different mechanism of hydrogen transfer may... 

Spectroscopic investigations of alkenes (68,83,104,254,337,456,538) have suggested 7i-bonded or di-o-bonded species as possible reactive intermediates. However, the complexity of butadiene allows for a large number of adsorbate configurations (539,540), and future spectroscopic investigations are needed to explore the reaction mechanism of diene hydrogenation. 

HCN also adds to alkenes in the presence of an appropriate catalyst (Arthur et al., 1954 Jackson and Level, 1982). Thus, cobalt carbonyl leads to Markownikov addition, for example, 1-propene yields isopropyl cyanide in approximately 75% yield. HCN adds to alkynes in the presence of metal complexes, and the use of a nickel complex may lead to syn addition (Jackson and Level, 1983 Jackson et al., 1988). Hydrogen cyanide reacts with conjugated dienes, the mechanism involving a TT-allyl intermediate. The course of addition is complex and may lead to more than one product (Keim et al., 1982). 

The fate of the bicyclic Diels-Alder adducts derived by the interaction of oxazoles with dienophiles containing a triple bond is very different from those considered above. In such cases ready elimination of hydrogen cyanide or a nitrile by the retro-diene cycloaddition mechanism (also called retro-Diels-Alder reaction) is generally observed. Typically, the reaction of 5-ethoxyoxazoles (199) with dimethyl acetylenedicarboxylate... 

A proposed mechanism for this diene hydrogenation is shown in Scheme 15.22. After generation of the Cr(CO)3(S)3 fragment, this hydrogenation is proposed to occur by initial binding of the alkyne or diene or by initial oxidative addition of H. In the case of the diene... 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

The hydrogenation of unsaturated polymers and copolymers in the presence of a catalyst offers a potentially useful method for improving and optimizing the mechanical and chemical resistance properties of diene type polymers and copolymers. Several studies have been published describing results of physical and chemical testing of saturated diene polymers such as polybutadiene and nitrile-butadiene rubber (1-5). These reports indicate that one of the ways to overcome the weaknesses of diene polymers, especially nitrile-butadiene rubber vulcanizate, is by the hydrogenation of carbon-carbon double bonds without the transformation of other functional unsaturation such as nitrile or styrene. 

Another subject of dispute was the mechanism of the photochemical, chromium carbonyl catalyzed hydrogenation of dienes /42/. The question here was whether the catalytic reaction is started by the dissociation of CO (Equation 42) or by the dissociation of the coordinated diene (Equation 43) /42, 43/. 

The chromatograms of the liquid phase show the presence of smaller and larger hydrocarbons than the parent one. Nevertheless, the main products are n-alkanes and 1-alkenes with a carbon number between 3 to 9 and an equimolar distribution is obtained. The product distribution can be explained by the F-S-S mechanism. Between the peaks of these hydrocarbons, it is possible to observe numerous smaller peaks. They have been identified by mass spectrometry as X-alkenes, dienes and also cyclic compounds (saturated, partially saturated and aromatic). These secondary products start to appear at 400 °C. Of course, their quantities increase at 425 °C. As these hydrocarbons are not seen for the lower temperature, it is possible to imagine that they are secondary reaction products. The analysis of the gaseous phase shows the presence of hydrogen, light alkanes and 1-alkenes. 

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