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 (PCy₃)₂(Cl)₂Ru=CHPh (<strong>I</strong>) and (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) (PCy₃)Cl₂Ru=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_n</em> × 10² and yields of 80% were obtained; whereas with catalyst <strong>I</strong>, the molecular weights of products were about <em>M_n</em> × 10⁴ 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>_m</em></strong> = 2, 3 y 4.

Keywords

• metathesis degradation
• natural rubber
• mandarin oil
• alkylidene-ruthenium catalysts