Mechanism conjugated diene hydrogenation

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 conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

Conjugated dienes also undergo addition reactions by radical-chain mechanisms. Here, the addition product almost always is the 1,4 adduct. Thus radical addition of hydrogen bromide to 1,3-butadiene gives l-bromo-2-butene, presumably by the following mechanism ... 

The hydrogenation of trans-l-phenyl-1,3-butadiene has been discussed49 in terms of the mechanism originally proposed,45 and 773-allyl species were invoked. NMR studies on the hydrogenation of butadiene have shown that Tj -but-2-enyl and r 3-l-methylallyl complexes are present in the reaction mixture. With isoprene, an q1 -methylbut-2-enyl complex was observed.50 By using a two-phase system and a phase-transfer catalyst, the turnover numbers attainable with conjugated dienes can be increased to about 30.51... 

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. 

The picolyl hydrogens of 2- and 4-alkylpyridines are more acidic than the corresponding benzylic hydrogens. For that reason, the reactions of alkylpyridines with olefins occur under mild ccHiditions, and with conjugated dienes, styrenes, and vinylpyridines at below room temperatures. The mechanism of side-chain addition of alkylpyridines to olefins is very similar to that described for alkylbenzenes. 

The mechanism for homogeneous hydrogenation of methyl linoleate by Fe(CO)s based on kinetic evidences and radioactive tracers involves monoene- and diene-Fe(CO)4 and diene-Fe(CO)3 complexes as important intermediates. Contrary to our previous postulate (7) the free conjugated diene is only a minor intermediate. Confirmatory evidence is needed for the occurrence of oleate- and linoleate-Fe(CO)4 complexes during hydrogenation and isomerization with Fe(CO)r>. Also, the species of iron carbonyl hydrides formed during hydrogenation should be elucidated. 

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). 

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