1,5-Hexadiene hydrogenation

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The process involving aHyl alcohol has not been iadustriaHy adopted because of the high production cost of this alcohol However, if the aHyl alcohol production cost can be markedly reduced, and also if the evaluated cost of hydrogen chloride, which is obtained as a by-product from the substitutive chlorination reaction, is cheap, then this process would have commercial potential. The high temperature propylene—chlorination process was started by SheH Chemical Corporation ia 1945 as an iadustrial process (1). The reaction conditions are a temperature of 500°C, residence time 2—3 s, pressure 1.5 MPa (218 psi), and an excess of propylene to chlorine. The yield of aHyl chloride is 75—80% and the main by-product is dichloropropane, which is obtained as a result of addition of chlorine. Other by-products iaclude monochioropropenes, dichloropropenes, 1,5-hexadiene. At low temperatures, the amount of... 

One of the butadiene dimeri2ation products, COD, is commercially manufactured and used as an intermediate in a process called FEAST to produce linear a,C0-dienes (153). COD or cyclooctene [931-87-3], obtained from partial hydrogenation, is metathesi2ed with ethylene to produce 1,5-hexadiene [592-42-7] or 1,9-decadiene [1647-16-1], respectively. Many variations to make other diolefins have been demonstrated. Huls AG also metathesi2ed cyclooctene with itself to produce an elastomer useful in mbber blending (154). The cycHc cis,trans,trans-tn.en.e described above can be hydrogenated and oxidi2ed to manufacture dodecanedioic acid [693-23-2]. The product was used in the past for the production of the specialty nylon-6,12, Qiana (155,156). 

Figure 15.2 A comparison of the heats of hydrogenation for cyclohexene, 1,3-cyclo-hexadiene, and benzene. Benzene is 150 kJ/mol (36 keal/mol) more stable than might be expected for "cyclohexatriene."...

Hydrogenation. Hydrogenation of poly(5-methyl-l,4-hexadiene) was carried out with p-toluenesulfonyl hydrazide (20) in refluxing xylene (2il molar ratio of the hydrazide to the polymer repeat unit). 

Compression molded (150°C for 3 minutes press chilled with cold water immediately thereafter) samples of poly(trans-l,4-hexadiene) (14) and poly(5-methyl-l,4-hexadiene) were examined with a General Electric (XRD-3) X-ray unit. Transmission Laue X-ray photographs were taken using nickel filtered copper X-radiation. Samples were stretched to four times of their original lengths to obtain oriented fibers. The fiber patterns were obtained in a flat plate film holder with the specimen to film distance standardized at 5 centimeters. X-ray diffraction patterns were similarly obtained for the hydrogenated sample of poly(5-methyl-l,4-hexadiene). 

Further confirmation of the structure and tacticity of poly/5-methyl-l,4-hexadiene)was obtained from X-ray diffraction and u-NMR data of its hydrogenated polymer (Scheme 2). The hydrogenated polymer sample showed a highly crystalline pattern (Figure 7), with diffraction spots that were well defined. This pattern was identical to that of isotactic poly(5-methyl-l-hexene) as reported in the literature (26) (measured identity period, 6.2 A lit., 6.33 A). 

Figure 7. X-ray fiber diagram of hydrogenated poly(5-methyl-l, 4-hexadiene). Solvent cast film strip cold drawn to four times its original length. Reproduced, with permission, from Ref. 19. Copyright 1976, John Wiley Sons, Inc.

Figure 8. 13C-NMR spectra of (A) hydrogenated poly(5-methyl-l,4-hexadiene) and (B) poly(5-methyl-l-hexene).

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Hydride-promoted reactions are also well known, such as the acrylic and vinylacrylic syntheses (examples 7-10, Table VII). Some less-known compounds, which form in the presence of halide ions added to tetracar-bonylnickel, have been described by Foa and Cassar (example 11, Table VII). Reaction of allene to form methacrylates, and of propargyl chloride to give itaconic acid (via butadienoic acid), have been reported (examples 13 and 14, Table VII). 1,5-Hexadiene has been shown to be a very good substrate to obtain cyclic ketones in the presence of hydrogen chloride and tetracarbonylnickel (example 15, Table VII). The latter has also been used to form esters from olefins (example 16, Table VII). In the presence of an organic acid branched esters form regioselectivity (193). 

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. 

Hexadiene which is formed by 1,4-addition of hydrogen and a vinyl group to butadiene, is the predominant product in the codimerization reaction. However, there is always a small amount (1-3%) of 3-methyl-... 

To do so, one can take the enthalpy of formation of n -hexane from Pedley, and with the phase independence assumptions in Reference 7, employ the enthalpies of hydrogenation of 1-hexene and 1,5-hexadiene from References 11 and 12 respectively. Alternatively13, one can forget about the first quantity altogether and simply take the difference of the enthalpies of hydrogenation of the diene and twice that of the monoene. This reaction is endothermic by 1.1 1.8 kJ mol-1, a value statistically indistinguishable from the absence of any interolefin interaction in the diene. Relatedly, for the isomeric 1,4-hexadienes 14 and 15, equation 8 may be used. 

Knowles reported the hydrogenation of a-phenylacrylic acid and itaconic acid with 15% and 3% optical purity, respectively, by using [RhCl3(P )3] [P = (R)-(-)-methyl-n-propylphenylphosphine] as homogeneous catalyst [38]. Horner found that a-ethylstyrene and a-methoxystyrene can be hydrogenated to (S)-(+)-2-phe-nylbutane (7-8% optical purity) and (R)-(+)-l-methoxy-l-phenylethane (3-4% optical purity), respectively, by using the complex formed in situ from [Rh(l,5-hexadiene)Cl]2 and (S)-(-)-methyl- -propylphenylphosphine as catalyst [39]. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

Before examining the reaction of deactivated alkenes, the phosphonium salt synthesis was applied to 1,3-dienes.21 When ( )-6-phenyl-l,3-hexadiene was treated with equimolar amounts of PPh3 and CF3S03H in the presence of RhH(PPh3)4 (2.5 mol%) in THF at 0 °C for 3 h, ( )-(6-phenyl-3-hexenyl)triphenylphosphonium salt was obtained in 89% yield after anion exchange with LiPF6 and recrystallization (Scheme 16). The addition of phosphine and hydrogen occurred at the 1- and 2-carbon atoms of the 1,3-diene, respectively. The reaction of (7)-1,3-dienes was then performed for comparison. 

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