Dimerization of Isoprene by Palladium-Diphosphine Complex Catalyst

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Abstract
Oligomerization of isoprene catalyzed by the PdBr2(Ph2PCH2CH2PPh2)2-PhONa catalyst was studied. The catalyst activity was greatly enhanced by the addition of phenol. Five linear dimers were identified. Distribution of the isomeric dimers was remarkably affected by the molar ratio of phenol to isoprene. The reaction in the presence of a small amount of phenol (phenol/isoprene=1/30) gave 2,7-dimethyl-1,trans-3,7-octatriene derived from a tail-to-tail dimerization almost selectively, while the addition of a large amount of phenol (phenol/isoprene=1/1.5) afforded head-to-head dimers (2-vinyl-5-methyl-1,6-heptadiene and 3,6-dimethyl-1,cis-3,7-octatriene) as main dimers (more than 40% of all the dimers). In a 1:3 ratio of phenol to isoprene the dimers consisted of mainly 2,6-dimethyl-1,trans-3,7-octatriene derived from a head-to-tail dimerization.