Hexadiene, - and

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). 

And other dienes, eg, hexadiene and butadienylacetylene also methylacetylene. 

In special cases aHyl compounds, such as diaHyl [592-42-7] (C H q), (1,5-hexadiene) and diaHyl ether [557-40-4] (C H qO), can form high polymers... 

Less reactive dienes such as cyclohexadiene can be employed efficiently, giving the adduct in 90% yield in 93% ee. Acyclic dienes such as piperylene, 2,4-hexadiene, and 1-phenylbutadiene also react with the acryloyloxazolidinone derivative to afford Diels-Alder cycloadducts in high optical yields (Scheme 1.38, Table 1.17). 

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."...

What stereochemistry would you expect for the product of the Diels-Alder reaction between (2 ,4 )-2,4-hexadiene and ethylene What stereochemistry would you expect if (2 ,4Z)-2,4-hexadiene were used instead ... 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

Materials. 5-Methyl-1,4-hexadiene was obtained by the codimerization of isoprene and ethylene with a catalyst (18) consisting of iron octanoate, triethylaluminum and 2,2 -bi-pyridyl. The product mixture which contained principally 5-methyl-1,4-hexadiene and 4-methy1-1,4-hexadiene was fractionated through a Podbielniack column to yield high purity (>99%) 5-methylxhexadiene, b.p. 92.80C,njj 1.4250 (Lit. (19) b.p. 88-89°C, np 1.4249). 1-Hexene (99.9% purity), 1-decene (99.6% purity), 4-methyl-1-hexene (99.5% purity) and 5-methyl-l-hexene (99.7% purity) were obtained from Chemical Samples Co. 6-TiCl3 AA (Stauffer Chemical Co.j contains 0.33 mole AICI3 per mole of TiClj). Diethylaluminum Chloride was obtained from Texas Alkyls (1.5 M in hexane). 

To clarify the tacticity problem, trans-l,4-hexadiene and 5-methyl-l,4-hexadiene polymers were examined by X-ray diffraction. Fiber diagrams were obtained from samples stretched to four times their original lengths. Eight reflections from the poly(trans-1,4-hexadiene) fiber pattern may be interpreted on the0basis of a pseudo-orthorhombic unit cell with a = 20.81 + 0.05 A b =... 

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

Figure 10. 300 MHz IH-NMR spectrum of a deuterobenzene solution of an equimolar copolymer of 5-methyl-l,4-hexadiene and 1-hexene prepared with a EttAlCl/ S-TiCl, catalyst at 0°C in pentane solvent.

In contrast to the behavior of 1,4-hexadiene and limonene, the regioselectivity of the norbomene derivatives 29 and 32 strongly depends on the catalyst. A... 

These cooligomerization reactions can be explained by the following mechanism. First, insertion of butadiene to palladium hydride gives the methyl-substituted 7r-allylpalladium complex 125. Subsequently, insertion of the olefin to the unsubstituted side of the 7r-allyl system and /3-elimination give the 1,4-hexadiene and palladium hydride ... 

The simplest diene that satisfies this requirement is 1,4-hexadiene, and indeed it has been adopted as the cure site monomer in commercial ethylene-propylene-diene rubber. Because 1,4-hexadiene exists in both trans and cis configurations, significant amounts of work have been devoted to find ways to control the selectivity of the catalysts for one of the isomers over the other. 

Effect of Change of Ethylene Pressure on the Ratio of trans- to cis-Hexadiene and Reaction Rates ... 

Although the reactions in the above ligand effect studies are designed to minimize the formation of isomers of 1,4-hexadiene, approximately 10% isomers are still formed. These are 3-methyl-1,4-pentadiene and 2,4-hexadiene and a very small amount of higher oligomers. Whatever effect the donor might have on the rate of formation of these isomers (i.e., with respect to the 1,4-hexadiene formation) or on their distributions appeared to be insignificant (24). 

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]. 

The hydrogenation of simple alkenes, such as hexene, cyclohexene, cyclo-hexadiene and benzene, has been extensively studied using biphasic, alternative solvent protocols. These hydrocarbon substrates are more difficult to hydrogenate compared to substrates with electron withdrawing groups. Benzene and alkyl substituted aromatic compounds are considerably more difficult to hydrogenate... 

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, AHr, as determined from the heats of formation of the species involved, as well as the heats of formation of the isomeric radical cations themselves clearly reveal the favourable stability of the cyclic isomers and/or fragment ions. Thus, instead of the linear pentadienyl cation (3), the cyclopenten-3-yl cation (2) is eventually formed during the loss of a methyl radical from ionized 1,3-hexadiene (1). Since 1,2-H+ shifts usually have low energy requirements (5-12 kcalmol-1), interconversion of the linear isomers, e.g., 4, and subsequent formation of the cyclic isomers, in particular of the ionized methylcyclopentenes 5 and 6, can take place easily on the level of the... 

Later on, product distribution studies15 of the ionic addition of chlorine to conjugated dienes, and in particular to cyclopentadiene, 1,3-cyclohexadiene, cis,cis-, trans,trans-and c ,fraws-2,4-hexadienes, and cis- and trans-1,3-pentadienes have supplied the first stereochemical data, showing that the stereochemistry of 1,4-addition is predominantly syn, although to an extent smaller than that of bromine addition. Moreover, the 1,2-addition is generally non stereoselective, except for the addition to the 3,4-bond of cis-and trans-1,3-pentadienes where the attack is 89-95% anti. Finally, appreciable amounts of cis- 1,2-dichlorides were obtained from the two cyclic dienes, whereas 2,4-hexadienes showed a preference for anti 1,2-addition, at least in the less polar solvents (carbon tetrachloride and pentane). On the basis of all these results the mechanism shown in equation 29 was proposed. 

Terminal dienes such as butadiene, isoprene and 2,3-dimethylbutadienereactregiospeci-fically with I(Py)2BF4, in the presence of a nucleophile, to give 1,2-iodofunctionalization (equation 58)87. In contrast, internal dienes such as (Z, )-2,4-hexadiene and 1,3-cyclooctadiene yield the 1,4-addition products under similar conditions (equation 59). 

Halogenation of conjugated dienes proceeds chiefly by 1,4-addition with molecular halogens (equation 3). 1,2-Addition is favored in the presence of pyridine-halogen complexes and amine tribromide salts (equation 4)9. The stereochemistry of 1,4-bromine addition with 2,4-hexadienes and cyclopentadiene is primarily anti in the presence of amine, but syn with molecular halogen in the absence of amine. 

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