Synthesis of Fragrance Compounds from Biorenewables: Tandem Hydroformylation–Acetalization of Bicyclic Monoterpenes

The Rhodium-catalyzed tandem hydroformylation–acetalization of the terpenes 3-carene, 2-carene, α-pinene, and β-pinene was studied in ethanol solutions in the presence of PPh3 or tris(O-tert-butylphenyl)phosphate, P(O-o-tBuPh)3, ligands. All these terpenes are constituents of turpentine oils obtained commercially from coniferous trees. β-Pinene gave the corresponding aldehyde and acetal in excellent combined yields in both systems. 3-Carene, 2-carene, and α-pinene, which contain sterically encumbered endocyclic double bonds, showed an extremely low reactivity with PPh3. The use of P(O-o-tBuPh)3 not only accelerated the hydroformylation of all four substrates remarkably but also increased the acetalization activity of the catalyst. In the Rh/P(O-o-tBuPh)3 system, various fragrance acetals and aldehydes were obtained from these renewable substrates in nearly quantitative combined yields. The process was performed under mild conditions, in environmentally friendly ethanol as a solvent, and in the absence of acid cocatalysts.