Dienes alkene hydrogenation

This section contains dehydrogenations to form alkenes and unsaturated ketones, esters and amides. It also includes the conversion of aromatic rings to alkenes. Reduction of aryls to dienes is found in Section 377 (Alkene-Alkene). Hydrogenation of aryls to alkanes and dehydrogenations to form aryls are included in Section 74 (Alkyls from Alkenes). 

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

Metallocene complexes of early transition metals [Cp2MR R2] (R1, R2 = H, alkyl, M = Zr, Ti, Hf) are active and selective catalysts in semi hydrogenation of dienes.431 Two ruthenium-carbene complexes display high activity in alkene hydrogenation, which may be further enhanced by the addition of HBF4-OEt2.432 A turnover number of 12,000 h 1 was obtained. 

The pentacyanocobaltate(II) ion has long been known to catalyze alkene hydrogenation, mainly of conjugated dienes. A review of the early work is available.45 The catalyst system shows negligible activity for the hydrogenation of non-activated monoenes. A major disadvantage is that the system is inhibited by excess substrate, and the turnover numbers obtained are generally less than 2. 

The mechanism of 1, 4-addition starts off in the same way as a normal electrophilic addition. We shall consider the reaction of a conjugated diene with hydrogen bromide as an example (Following fig.). One of the alkene units of the diene uses its n electrons to form a bond to the electrophilic hydrogen of HBr. The H-Br bond breaks at the same time to produce a bromide ion. The intermediate carbocation produced has a double bond next to the carbocation centre and is called an allylic carbocation. 

Several complexes of Pd° and Pd containing PR3, DMSO, etc. catalyze hydrogenation of monoal-kenes. " Salen complexes of Pd and Ni are employed as catalysts for alkene hydrogenation. The cluster complex of palladium [Pd5(PPh)2], is a catalyst for the hydrogenation of alkenes, dienes, alkynes and many other kinds of unsaturated compound. ... 

A different stereochemical behavior has, however, been observed in methanol . In this solvent XeF, reacts with the solvent to form an unstable reactive species (CHsOXeF), which gives quantitatively formaldehyde by disproportionation in the absence of unsaturated hydrocarbons or with unreactive alkenes. Hydrogen fluoride generated in situ complexes the electrophilic CHsOXeF species to form a protonated derivative a which reacts with activated dienes such as 2,3-dimethylbutadiene, as an apparent fluorine electrophile to give 1,4- and 1,2-fluoromethoxy products, together with 1,2- and 1,4-difluoro derivatives (equation 25). 

Dienes are hydrogenated to give alkenes, but over-reduction is a problem, as it was with alkynes. Homogenous catalysts can be of value here. The diene unit in 382 was hydrogenation to give the non-conjugated alkene unit in 383 (80% yield) in the Serebryakov et al. synthesis of faranal. Both 1,2- and 1,4-reduction are possible, and the major product depends on the catalyst used, as well as the substituents on the C=C units of the diene. 

Even [Rh(diene)L3]A salts were shown to be active, probably because alkene hydrogenation requires no more than three sites at the metal, two for hydrogen and one for the substrate. Induction periods are sometimes observed in these cases, no doubt because L(or cod) does not have to dissociate before H2 can add. Compared to the Wilkinson system, the side-reaction leading to isomerization of the alkene (e.g., 1-butene 2-butene) is a more serious competitor with hydrogenation itself. In applications requiring specific deuteration or where the isomerization product is nonhydrogenable, this can be a problem... 

Cationic [Rh(diene)L2]+ species are also catalyst precursors for the hydrogenation of ketones and aldehydes (49). The mechanism presents analogies with that proposed for alkene hydrogenation, with the formation of alkoxy species by insertion of the ketone or aldehyde into a metal-hydrogen bond (Fig. 14). 

Besides the addition of halides and hydrogen-halide adds to alkenes or alkynes, other industrially relevant electrophilic addition reactions involve hydratization reactions (addition of water to alkenes and alkynes, forming alcohols), cationic polymerization (addition of carbocation to an alkene), hydrogenation (addition of hydrogen to alkenes to form alkanes), and Diels-Alder reactions (addition of an alkene to a conjugated diene to form complex, unsaturated hydrocarbon structures). 

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