Dienes, unconjugated

Since this reaction is a unimolecular example of the selective hydrogenation of an alkene mixture, the successful saturation of the less-substituted double bond should take place most readily over those catalysts that are most effective for the preferential saturation of one olefin in a mixture. Ruthenium has not been used extensively for such hydrogenations, but P-2 Ni(B) has been effective in promoting the selective hydrogenation of one of the double bonds in 46, methylene norbornene (49) (Eqn. 15.30), dicyclopentadiene (50) (Eqn. 15.31), and 2-methyl-1,5-hexadiene (51) (Eqn. 15.32).6 9,80,81 

The hydrogenation of 4-vinylcyclohexene (46) to 4-ethylcyclohexene (47) was also reported to take place over a supported nickel arsenide, Ni-As(B), which was prepared by the sodium borohydride reduction of nickel arsenate supported on either silica 2,83 or alumina. These catalysts, however, fimction best in the presence of additives. When the reaction was run in pentane at 125 C and 25 atmospheres of hydrogen in the presence of a small amount of acetone, the product mixture at 96% conversion was 96% 4-ethylcyclohexene (47) and 4% ethylcyclohexane (48). No isomeric olefins were detected.  

The selective hydrogenation of the disubstituted double bond of limonene (52) took place over a platinum catalyst at 60°C and 3 atmospheres pressure (Eqn. 15.33). 5 1,5-Undecadiene was hydrogenated to 5-undecene with 78% selectivity at 97% conversion over a Pt/A 2eolite catalyst that was treated with diphenyldiethoxysilane. 9 

As wilt be described in the next section, the selective hydrogenation of conjugated dienes is accomplished more readily than the partial hydrogenation of 

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Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

For hydrogenation in water with an inexpensive catalyst, solutions containing cobalt salts and excess cyanide are useful10,11. The catalysts are selective for conjugated C=C bonds and are relatively unreactive with unconjugated dienes such as 1,5-cyclooctadiene. 

The isomerization catalysts are hydride complexes, and they can convert the unconjugated dienes or polyenes to conjugated systems through double-bond migration. This process occurs by an M—H addition-elimination process. 

Acyclic diene metathesis polymerization (ADMET) is a related polymerization in which an unconjugated diene polymerizes with loss of ethene [Lehman and Wagener, 2002, 2003 Schwendeman et al., 2002], ADMET is carried out using the Schrock and Gmbbs initiators at about 40-80°C. The process is a step polymerization, not a ROP chain reaction. The reaction is reversible, and high polymer MW is achieved by removal of ethene (usually by reduced... 

In the carbonylation of unconjugated dienes the nature of the products is influenced by reaction conditions. With Pd halides in ethanol at 100°C and 97 atm CO, 1,5-cyclooctadiene is successively carbonylated to the unsaturated monoester and then to the saturated diester (II). With (Ph3P)2PdCl2 in ethanol-HCl and 300-700 atm CO, the monoester is produced selectively at 60°C and the diester at 100°C (8). Finally, with (Bu3P)2PdI2 in THF at 150°C and 1000 atm CO, 1,5-cyclooctadiene undergoes transannular addition of CO to give a cyclic ketone in 40-45% yield (14, 15). The mechanism proposed involves a a-7r-cyclooctenyl... 

Pathways involving alkyl-acyl rearrangements are proposed to explain the carbonylation of a-bonded alkoxy complexes (17). The stereochemistry of the products indicates that the ester group replaces Pd with retention of configuration at the carbon to which Pd is o-bonded. In all these studies with unconjugated dienes the nature of carbonylation products to be expected is clearly influenced by the geometry of the intermediate Pd complexes. 

An interesting means of improving the selectivity of Pd for the conversion of unconjugated dienes, such as 1,4-cyclooctadiene to the monoene is to add phenylacetaldehyde to the mixture undergoing reaction (ref. 36). The mechanism of action is not established but it may involve aldehyde decarbonylation to form adsorbed CO but the addition of small amounts of CO to the reactants does not reproduce the effect of the aldehyde (ref. 37). Means to modify the metal suface in other ways can prove effective, the studies of Ni catalysts by Okamoto et al. afford an interesting example of an attempt to reach a more fundamental understanding of catalyst selectivity. 

Several recent reviews have included specific types of electrophilic cyclofunctionalization reactions.1 Important areas covered in these reviews are halolactonization u cyclofunctionalization of unsaturated hydroxy compounds to form tetrahydrofurans and tetrahydropyrans lb cyclofunctionalization of unsaturated amino compounds lc cyclofunctionalization of unsaturated sulfur and phosphorus compounds ld lf electrophilic heterocyclization of unconjugated dienes 1 synthesis of y-butyrolactones 1 h synthesis of functionalized dihydro- and tetrahydro-furans lj cyclofunctionalization using selenium reagents lk lm stereocontrol in synthesis of acyclic systems 1" stereoselectivity in cyclofunctionalizations lP and cyclofunctionalizations in the synthesis of a-methylenelactones.lq Previous reference works have also addressed this topic.2... 

Depending on the reaction conditions and workup procedures, the primary adduct, the triene, the unconjugated diene, or mixtures of these are isolated and identified. Characteristically, after some time of irradiation, a maximum concentration of ortho adduct is reached which decreases upon continued irradiation. 

Unconjugated dienes form the 1,3-diene complexes after isomerization to conjugated dienes. Formation of the stable conjugated diene complexe is the driving force of the isomerization. For example, the 1,4-diene in the synthetic intermediate 29 of prostaglandin A can be protected as the diene complex 30 after isomerization to the conjugated diene when it is treated with Fe2(CO)9. This method was applied to the synthesis of prostaglandin C (13). The diene complex 30 is stable for the oxidation of the lactol and introduction of the a-chain [7]. 

Unconjugated dienes can produce an even more complicated range of macro-molecular structures. Homopolymers of such monomers are not of current commercial importance but small proportions of monomers like 1,5-cyclooctadicne are copolymerized with ethylene and propylene to produce so-called EPDM rubbers. Only one of the diene double bonds is enchained when this terpolymeriza-tion is carried out with Ziegler-Natta catalysts (Section 9.5). The resulting small amount of unsaturation permits the use of sulfur vulcanization, as described in Section 1.3.3. 

Finally, unconjugated dienes and even polyenes can be produced (R = alkyl, aryl R = H) by the transition metal promoted coupling (Cu, Ni, Pd) with an allylic or other unsaturated halide (equations 65 and 66 Scheme... 

The unconjugated diene, 6-16, reacts to form a (yclic molecule. 

A series of unconjugated dienes having two double bonds with different reactivity were therefore synthesized. The EPDM rubbers prepared with them on a commercial scale found many important applications especially where ox> n and ozone resistance were needed. However, one drawback limited their growth. Blends of conventional highly unsaturated diene based elastomers and of low unsaturated EPDM rubbers were not readily covulcanizable owing to the large difference in double bond concentration. 

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