Molecules (May 2012)
Metathesis Transformations of Natural Products: Cross-Metathesis of Natural Rubber and Mandarin Oil by Ru-Alkylidene Catalysts
Abstract
This study reports on the degradation of natural rubber (NR) via cross-metathesis with mandarin oil and <em>d</em>-limonene, an abundant compound in essential oils; that were used as chain transfer agents (CTAs) and green solvents. Reactions were performed in the presence of the ruthenium-alkylidene catalysts (PCy<sub>3</sub>)<sub>2</sub>(Cl)<sub>2</sub>Ru=CHPh (<strong>I</strong>) and (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) (PCy<sub>3</sub>)Cl<sub>2</sub>Ru=CHPh (<strong>II</strong>), respectively. Catalyst <strong>II</strong> bears an <em>N</em>-heterocyclic carbene ligand (NHC) bounded to the ruthenium atom, which has a strong basic character; therefore it is more active toward trisubstituted olefins in comparison with catalyst <strong>I</strong>. In both cases, isolated monoterpene-terminated isoprene oligomers were obtained as products of the cross-metathesis degradation of NR. In the presence of catalyst <strong>II</strong> molecular weight values around <em>M<sub>n</sub></em> × 10<sup>2</sup> and yields of 80% were obtained; whereas with catalyst <strong>I</strong>, the molecular weights of products were about <em>M<sub>n</sub></em> × 10<sup>4</sup> with yields ranging 70 to 74%. The composition and yield of NR degradation products were determined by GC/MS (EI) analysis and it was found that the oligomers obtained have primarily one vinyl group and one terpene-monocyclic group at the chain end, with isoprene units <strong>A<em><sub>m</sub></em></strong> = 2, 3 y 4.
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