1,5-Hexadiene dicarboxylation

When the substrate contains two double bonds, either di- or tetra-carboxylatkm can be achieved.96 The reaction of 1,5-hexadiene at 5 bar CO gives the diester, while at 1-3 bar the tetraester is obtained (equation 72). These somewhat surprising results are consistent with a pressure-dependent competition of the alkene and carbon monoxide for coordination sites on palladium. Since 1,5-hexadiene is not easily displaced by CO, even at higher pressures, the carboxylation of one double bond is ensured. Competition of the resulting monoalkenic diester with carbon monoxide is dependent on the CO pressure, lower pressures allowing monoalkene coordination and thus dicarboxylation. 

Cydopentanone. 4. C, 1,3-cyclohexanedicarboxylic acid F, 1,4-cyclohexaoe-dicarboxylic acid H, succinic acid J, 1,2-cyclobutanedicarboxylic acid. 5. K, 1,5-hexadiene O, 2,5-dimethyicyclopentanecarboxylic acid. 7. (b) Intramol. aldol cond. (d) gives 3-methyl-2-cyclohexen-l-onc. 11. (a) Retro (reverse) Claisen condensation. 

Ozonolysis as used below is the oxidation process involving addition of ozone to an alkene to form an ozonide intermediate which eventually leads to the final product. Beyond the initial reaction of ozone to form ozonides and other subsequent intermediates, it is important to recall that the reaction can be carried out under reductive and oxidative conditions. In a general sense, early use of ozonolysis in the oxidation of dienes and polyenes was as an aid for structural determination wherein partial oxidation was avoided. In further work both oxidative and reductive conditions have been applied . The use of such methods will be reviewed elsewhere in this book. Based on this analytical use it was often assumed that partial ozonolysis could only be carried out in conjugated dienes such as 1,3-cyclohexadiene, where the formation of the first ozonide inhibited reaction at the second double bond. Indeed, much of the more recent work in the ozonolysis of dienes has been on conjugated dienes such as 2,3-di-r-butyl-l,3-butadiene, 2,3-diphenyl-l,3-butadiene, cyclopentadiene and others. Polyethylene could be used as a support to allow ozonolysis for substrates that ordinarily failed, such as 2,3,4,5-tetramethyl-2,4-hexadiene, and allowed in addition isolation of the ozonide. Oxidation of nonconjugated substrates, such as 1,4-cyclohexadiene and 1,5,9-cyclododecatriene, gave only low yields of unsaturated dicarboxylic acids. In a recent specific example... 

Hydrocarboxyladon of olefins [I. 721-722, after citation of ref. 8]. Nickel carbonyl reacts with 1,5-dienes in the presence of dilute acid to form cyclic ketones rather than the expected mono- and dicarboxylic acids. Thus hexadiene-1,5 is converted in 70% yield into a mixture of 2,5-dimethylcyclopentanone and 2-methyl-cyclohexanone.83... 

The reaction thus always leads to a six-membered ring, by 1,4-addition. Of the three double bonds concerned in the reaction, only one is retained in the product, and that at a different position. Rearrangements rarely occur in these reactions. Moreover, they are stereospecific on addition of maleic acid to a diene the adduct produced is a cw-dicarboxylic acid, and on addition of fumaric acid it is a frms-dicarboxylic acid. Substituents on the diene or philodiene are not wholly without effect on the course of the addition for example, 1,2,3,4-tetramethylbutadiene (3,4-dimethyl-2,4-hexadiene) reacts smoothly,33 but the tetraphenyl analog shows no tendency to add a philodiene. Bulky substituents at the 2,3-positions can completely suppress the addition, presumably because they prevent free rotation and thus disfavor formation of the con-... 

Vogel was the first to recognize that these reactions were stereospecific. He found that the dimethyl ester of c/5 -cyclobutene-3,4-dicarboxylate gives only the dimethyl ester of cis,trans-m xcomc acid at 120°C (Scheme 5.9). Similar stereochemical observations were made by Criegee who found that cw-1,2,3,4-tetramethylcyclobutene gives on c/5, ra/25 -3,4-dimethyl-2,4-hexadiene while the trans compound gives the trans,trans-6itnt at 200°C (Scheme 5.9). ... 

A - -Dihydroterephthalic Acid (1 Z-Cyclo. hexadiene-2 5-dicarboxylic acid)... 

A mixture of diethyl l,5-hexadiene-l,6-dicarboxylate. HgS, methanol, and 5 drops of triethanolamine allowed to stand 2 days at room temp, in an autoclave, then heated 12 hrs. at 100° diethyl thiophane-2,5-diacetate. Y 77%. F.e. s. A. Liittringhaus and H. Merz, Arch. Pharm. 293, 881 (1960). 

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