Relations of conjugated dienes catalysed by transition-metal complexes

Abstract

With the aim of dimerising isoprene in a head-to-tail manner to yield monoterpenoid compounds, its homogeneous reactions with palladium, nickel and rhodium complexes were studied.

Using various palladium-phosphine complexes as catalysts, isoprene was telomerised in high conversions with phenol, alcohols (methanol, ethanol and n-propanol) and (less efficiently) water, yielding linear C₁₀H ₁₆ dimers and ethers and alcohols of the type ROC₁₀H ₁₇ (R=Ph, Me, Et, n-Pr, H). Telomerisation with phenol generally yielded ethers of the head-to-head type as the major product, hut by variation of triphenylphosphine ligand concentration the tail-to-head ether was instead obtained. In contrast, telomerisation with methanol yielded either the tail-to-tail or the tail-to-head ether as major product, depending on the nature of the phosphine ligand and the reaction temperature. In no case, however, did the amount of the desired head-to-tail ethers exceed 7% of the product.

Nickel-complex catalysts gave considerably lower conversions of isoprene, predominantly to triene dimers. As in the case of palladium the use of different phosphine and phosphite ligands produced some variation in the isomer distribution of the product. These reactions were less selective than the palladium-catalysed reactions; the amount of head-to-tail dimers obtained was higher, reaching 20% in the case of the catalyst system Ni(CH₂ = CH CN)₂ - P(OEt)₃.

The reaction of isoprene with rhodium chloride in methanol gave a linear head-to-head dimer LI selectively, but in small conversions.Substituted phenols were selectively produced when isoprene or butadiene was reacted with phenol in the presence of a RhCl(PPh₃)₃-NaOPh catalyst;this is in contradiction to a previous claim that phenyl-diene-dimer ethers were obtained.