2 : 4-Hexadienal

The diacetoxylation of E,E)- and ( ,Z)-2.4-hexadiene (351 and 353) is stereospecific, and 2,5-dimethylfurans (352 and 354) of different stereochemistry have been prepared from the isomers. Two different carboxylates are introduced with high cis selectivity by the reaction of 1,3-cyclohexadiene and... 

Dienes can also be used ia Friedel-Crafts cyclo alkylations. For example, treatment of phenol with 2,5-dimethyl-2,4 hexadiene gives 5,5,8,8-tetramethyl,6,7-dihydro-2-naphthol. 

Union Carbide abandoned the ketene—crotonaldehyde route in 1953 in favor of the oxidation of 2,4-hexadienal made by acetaldehyde condensation. A silver compound used as the catalyst prevented peroxidation of the ethylenic bonds (39,40). Thein plant operated until 1970. 

Sodium amalgam converts perfluoro(3,4-dimethyI-3-hexene) to perfluoro-(3,4-dimetliyl-2,4-hexadiene at room temperature in 70% yield [62] (equation 31). 

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). 

A Dimethyl butynedioate undergoes a Diels-Alder reaction with (2 ,4 )-hexadiene. Show the structure and stereochemistry of the product. 

Figure 30.4 Electrocyclic interconversions of 2,4-hexadiene isomers and 3,4-dimethylcyclo-butene isomers.

A similar result is obtained for the thermal electrocyclic ring-opening of 3,4-dimethylcyclobutene. The trans isomer yields only (2 ,4 )-2,4-hexadiene when heated, and the cis isomer yields only (2 ,4Z)-2,4-hexadiene. On UV irradiation, however, the results are opposite. Cyclization of the 2E,4 isomer under photochemical conditions yields cis product (Figure 30.4). 

Problem 30.3 fra/is-3,4-Dimethylcyclobutene can open by two conrotatory paths to give either (2 ,4 )-2,4-hexadiene or (2Z,4Z)-2,4-hexadiene. Explain why both products are symmetry-allowed, and then account for the fact that only the 2 ,4 isomer is. obtained in practice. 

Thus the reaction of the three geometrical isomers of 2,4-hexadiene with thiirane oxide afforded the three related 3-thiolene 5-oxides 154 depicted in equation 62104... 

Occasionally a colorless solution was obtained at approximately — 50°. In most eases the reaction mixture contained a small quantity of fine suspended black powder (presumably copper metal). Examination of the reaction mixture at this stage by hydrolysis of a sample at —35° and gas chromatographic analysis demonstrated the presence of approximately 10% of 2,4-hexadienes [predominantly the (E),(E)-isomer],... 

Hexadiene, 55, 109 Hexanoic acid, 55, 27 Hexanoie acid, 2-bromo-, 55,30... 

Similar divergences are found for lithium poly-2,4-hexadiene solution (1 10-3 M in living polymers) for which a sixfold decrease of viscosity upon protonation corresponding to a degree of association of 1.7 was reported 113), whereas only a threefold decrease, i.e. a degree of association of 1.4 was indicated earlier 1,8). The difference between the 1.7 and 1.4 values was tentatively attributed to a slow decomposition of the active ends over a period of two weeks U8) notwithstanding their reported good... 

The first studies on cation-radical Diels-Alder reactions were undertaken by Bauld in 1981 who showed [33a] the powerful catalytic effect of aminium cation radical salts on certain Diels-Alder cycloadditions. For example, the reaction of 1,3-cyclohexadiene with trans, iraw5-2,4-hexadiene in the presence of Ar3N is complete in 1 h and gives only the endo adduct (Equation 1.14) [33]. 

These reactions, called electrocyclic rearrangements, take place by pericyclic mechanisms. The evidence comes from stereochemical studies, which show a remarkable stereospecificity whose direction depends on whether the reaction is induced by heat or light. For example, it was found for the thermal reaction that cis-3,4-dimethylcyclobutene gave only cw,tran5-2,4-hexadiene, while the trans isomer gave only the trans-trans diene... 

A strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

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