Dimerization of Isoprene with Titanium Catalyst

Isoprene dimers are obtained in good yields when the third component, such as sulfolane or tetrahydrofurfuryl alcohol, is added to titanium tetrachloride-triethylaluminum catalyst which is used to polymerize isoprene to obtain high polymer. Main products are the cyclic dimer, 2, 4-dimethyl-4-vinyl-1-cyclohexene (bp 60.5° C/25 mmHg) and the linear dimer, 2, 6-dimethyl-1, 3, 6-octatriene (bp 70.5° C/25 mmHg). A small amount of 1-methyl-4-isopropeny1-1-cyclohexene is also produced. It was found that the third component is available merely as ligands to titanium. Hydrogen transfer in dimerization was studied using deuterated isoprene (CD2C(CH3) CH= CD2). Dimers of isoprene-d4 are 2, 4-dimethyl-4- (vinyl-0, -d2)-1-cyclohexene-2, 2, 5, 5, 6, 6-d6 and 2, 6-dimethyl-1, 3, 6-octatriene-1, 1, 4, 5, 5, 8, 8, 8-d8. Possible mechanism was discussed ih relation to the oligomerization of butadiene.