Synthesis of Unsaturated Esters by Cross-Metathesis of Terpenes and Natural Rubber Using Ru-Alkylidene Catalysts

This study reports the cross-metathesis of bicyclic beta-pinene, acyclic cis-3-methylpent-2-ene terpenes and the natural rubber with functionalized olefins, a route for the functionalization of the carbon-carbon double bond of natural products to obtain aliphatic unsaturated esters. The production of unsaturated esters from beta-pinene and cis-3-methylpent-2-ene via cross-metathesis reaction with dimethyl maleate and diethyl maleate in the presence of the ruthenium-alkylidene (Ru(Cl)(2)(=CHPh)(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(PCy3)] (I), [Ru(Cl)(2)(=CH(o-isopropoxyphenyl-methylene)(1,3-bis(2,4,6-trimethylphenyl) -2-imidazolidinylidene)] (II) and ruthenium- vinylidene [RuCl2(=C=CH(p-C6H4CF3))(PCY3)(2)] (III) was carried out. Results showed that the reaction of beta-pinene with diethyl maleate using II catalyst produced unsaturated esters with 43 % selectivity. I and III catalysts showed low activity toward the cross-metathesis of beta-pinene and dimethyl maleate. A survey about the cross-metathesis of acyclic cis-3-methylpent-2-ene with diethyl maleate by II catalyst was also studied. The formation of ethyl but-2-enoate and ethyl-3-methylpent-2-enoate products was highly selective by 63 %. The unsaturated esters formation from the cross-metathesis degradation of natural rubber (99.9 cispolyisoprene) with dimethyl maleate and diethyl maleate using I-III catalysts was accomplished as well. I and II catalysts showed high activity in the degradation of natural rubber with diethyl maleate to produce the low molecular weight of oligomers unsaturated ester products (M-n = 1 x 10(3) g mol(-1)) with isoprene units of m = 10 - 27 and yields ranging from 68 to 94 %.