Isoprene telomerization with alcohol

Telomerization of Isoprene.—Reviews have appeared on isoprene and chloro-prene, and on the complex reactions of isoprene to form terpenoids (in Japanese). Isoprene reacts with magnesium, especially in the presence of Lewis acids, and the resulting complex gives adducts with aldehydes. As usual in this type of reaction, a very complex mixture is obtained. The palladium-chloride-catalysed reaction of isoprene with acetic acid gives different products in different solvents. Monomers predominate in benzene [2-methylbut-2-enyl acetate (5) and 3-methylbut-2-enyl acetate (6)] while dimers [(7), (8), neryl (9), and geranyl (10) acetates] tend to be formed in tetrahydrofuran. Further details of the synthesis of Cio alcohols from isoprene and naphthyl-lithium are available, as well as of the in situ oxidation,but there is little of novelty (see Vol. 1, p. 17). 

The Pd-catalyzed telomerization of isoprene with alcohols is an appealing route for the preparation of terpenoid compoimds. Dos Santos and coworkers demonstrated that the catalytic system Pd(OAc)2/SlPr was capable of promoting the trimerization of isoprene producing sesquiterpenes (Figure 4.28) [84]. Switching to IMes promoted the telomerization of isoprene with methanol, resulting in methoxy-dimers with a head-to-head coupling of the isoprene units at unprecedented activity and selectivity. 

The results in Table VI were obtained by the reactions at 80°C with Pd(acac)2 using different ligands in a mixture of methanol and isopropyl alcohol. From these results, it seems likely that reaction temperature has large influence on the regiospecificity of the telomerization. As another example, a mixture of isomeric telomers was obtained by the reaction of methanol and isoprene at 100° (95). 

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