Isoprene → 1,4,6-Octatriene, 2,7-dimethyl-, (E)-

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50 ml of distilled isoprene, 1 kg of dioxane, 5 g (16.5 mmol) of palladium acetylacetonate, 23.1 g (82.5 mmol) of tricyclohexylphosphine and 1.9 g (16.5 mmol) of 85% phosphoric acid in a four-necked flask are held under reflux under an argon atmosphere until the internal temperature exceeds 90°C. 1 kg of isoprene is then added dropwise at an oil-bath temperature of 120°C in such a manner that the internal temperature does not fall below 88°C (ca 30 ml/hour) but remains in the range of 90°-95°C. After 50 hours, the isoprene addition is terminated, the product now consisting essentially of 2,7-dimethyl-1,3,7-octatriene. The mixture is left to react for a further 40 hours, the internal temperature rising to 105°-108°C. The dioxane is now removed on a rotary evaporator at 40°C/20 mm Hg. The black-coloured residual oil is steam-distilled, 780 g of distillate being obtained. The residue (trimers and polymers) amounts to 215 g. By fractional distillation of the oil on a Raschig-ring column at 63°C/10 mm Hg, there are obtained 550 g of 2,7-dimethyl-1,4,6-octatriene; nD20 =1.4885. Odour: fatty, green, soapy, terpene-like.

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