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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.

The rhodium‐catalyzed hydroformylation of limonene (1) in the presence of PPh3 or P(O‐o‐tBuPh)3 as auxiliary ligands and pyridinium p‐toluenesulfonate, as an acid co‐catalyst, gave two diasteroisomers of 4,8‐dimethyl‐bicyclo[3.3.1]non‐7‐en‐2‐ol (3) in nearly quantitative yield. Limonene is a cheap natural product obtained commercially from citrus fruits, in particular, as a sub‐product of the orange juice industry; whereas alcohol 3 is an expensive perfume ingredient. Alcohol 3 is formed through the hydroformylation of 1 giving a corresponding aldehyde (2) followed by the intramolecular carbonyl ene reaction resulting in cyclization of the aldehyde. The cyclization step is highly stereoselective with only one diasteroisomer of 3 being formed from each of two diasteroisomers of 2. The use of the P(O‐o‐tBuPh)3 ligand not only remarkably accelerates the hydroformylation step compared to the system with PPh3, but also increases significantly the cyclization activity of the catalytic system.

Rhodium-catalyzed hydroformylation of acyclic monoterpenic compounds, i.e., linalool and ␤citronellene, was studied in toluene and ethanol solutions in the presence of PPh 3 or P(O-o-t BuPh) 3 ligands. Although both substrates have a monosubstituted terminal double bond, they show different behavior under the hydroformylation conditions. In toluene, linalool gave almost quantitatively a cyclic hemiacetal; whereas the hydroformylation of ␤-citronellene resulted in two isomeric aldehydes also in a nearly quantitative combined yield. The reactions occurred approximately two times faster in ethanol than in toluene giving the corresponding acetals even in the absence of additional acid co-catalysts. In the absence of phosphorous ligands, linalool (differently from ␤-citronellene) was very resistant to hydroformylation probably due to the binding with rhodium through both the double bond and the hydroxyl group to form stable chelates. The P(O-o-t BuPh) 3 ligand exerted a remarkable effect on the reactivity of both substrates accelerating the reactions by 5-20 times as compared to the system with PPh 3. Several fragrance compounds were obtained in high yields through a simple one-pot procedure starting from the substrates easily available from natural bio-renewable resources.

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