The Rhodium-catalyzed tandem hydroformylation–acetalization
of the terpenes 3-carene, 2-carene, a-pinene, and b-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. b-Pinene gave the corresponding
aldehyde and acetal in excellent combined yields in
both systems. 3-Carene, 2-carene, and a-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