Metathesis transformations of natural products: cross-metathesis of natural rubber and mandarin oil by Ru-alkylidene catalysts

This study reports on the degradation of natural rubber (NR) via crossmetathesis with mandarin oil and d-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?)?(Cl)?Ru=CHPh (I) and (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) (PCy?)Cl?Ru=CHPh (II), respectively. Catalyst II bears an N-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 I. In both cases, isolated monoterpene-terminated isoprene oligomers were obtained as products of the cross-metathesis degradation of NR. In the presence of catalyst II molecular weight values around M(n) × 102 and yields of 80% were obtained; whereas with catalyst I, the molecular weights of products were about M(n) × 10? 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 A(m) = 2, 3 y 4.