Kelly A. da Silva Rocha

Phosphotungstic Acid as a Versatile Catalyst for the Synthesis of Fragrance Compounds by α‐Pinene Oxide Isomerization: Solvent‐Induced Chemoselectivity

The remarkable effect of the solvent on the catalytic performance of H3PW12O40, the strongest heteropoly acid in the Keggin series, allows direction of the transformations of alpha-pinene oxide (1) to either campholenic aldehyde (2), trans-carveol (3), trans-sobrerol (4 a), or pinol (5). Each of these expensive fragrance compounds was obtained in good to excellent yields by using an appropriate solvent. Solvent polarity and basicity strongly affect the reaction pathways: nonpolar nonbasic solvents favor the formation of aldehyde 2; polar basic solvents favor the formation of alcohol 3; whereas in polar weakly basic solvents, the major products are compounds 4 a and 5. On the other hand, in 1,4-dioxane, which is a nonpolar basic solvent, both aldehyde 2 and alcohol 3 are formed in comparable amounts. The use of very low catalyst loading (0.005-1 mol %) and the possibility of catalyst recovery and recycling without neutralization are significant advantages of this simple, environmentally benign, and low-cost method. This method represents the first example of the synthesis of isomers from alpha-pinene oxide, other than campholenic aldehyde, with a selectivity that is sufficient for practical usage.

Heteropoly acid catalysts for the synthesis of fragrance compounds from biorenewables: isomerization of limonene oxide

The liquid-phase isomerization of limonene oxide was studied in the presence of heteropoly acid catalysts in aprotic solvents in homogeneous and heterogeneous systems. Among the catalysts were bulk and silica-supported tungstophosphoric acid H3PW12O40 and its acidic Cs salt Cs0.5H0.5PW12O40 (CsPW). The reaction gave dihydrocarvone, a valuable fragrance intermediate, as the main product with turnover numbers of up to 8000. The nature of the solvent had a strong effect on reaction rate and selectivity. CsPW (0.1 mol%) was found to be a highly efficient and truly heterogeneous catalyst for this reaction, providing 82% yield of dihydrocarvone in 1,4-dioxane as a solvent under ambient conditions. This simple catalytic method represents economically attractive route to industrially important compounds starting from bio-renewable substrates easily available from essential oils.

Heteropoly Acid Catalysts for the Synthesis of Fragrance Compounds from Biorenewables: Cycloaddition of Crotonaldehyde to Limonene, α‐Pinene, and β‐Pinene

The interaction of widespread monoterpenes limonene, α‐pinene, and β‐pinene with crotonaldehyde using silica‐supported tungstophosphoric heteropoly acid H3PW12O40 and its acidic cesium salt Cs2.5H0.5PW12O40 as solid acid catalysts in dichloroethane solutions results in cycloaddition, which gives the same fragrant oxabicyclo[3.3.1]nonene product in a high yield. The product is likely to be formed through an α‐terpenyl carbenium ion intermediate, which is generated from monoterpene protonation and undergoes nucleophilic attack by crotonaldehyde. Both H3PW12O40 and Cs0.5H0.5PW12O40 are efficient and truly heterogeneous cycloaddition catalysts.

Heteropoly Acid Catalysts for the Synthesis of Fragrance Compounds from Biorenewables: The Alkoxylation of Monoterpenes

The cesium salt of tungstophosphoric heteropoly acid, Cs2.5H0.5PW12O40, is an active and environmentally friendly solid‐acid catalyst for the liquid‐phase alkoxylation of widespread monoterpenes, such as camphene, limonene, α‐pinene, and β‐pinene. These reactions provide isobornyl or α‐terpenyl ethers, useful as fragrances, in good to excellent yields. The reactions are equilibrium‐controlled and occur with high catalyst turnover numbers (TONs) up to 1500–4200 without catalyst leaching. Heteropoly acid H3PW12O40 also efficiently catalyzes the alkoxylation of these monoterpenes under homogeneous conditions with TONs up to 1500–11 300.

Heteropoly acid catalysts for the synthesis of fragrance compounds from bio-renewables: acetylation of nopol and terpenic alcohols

The cesium salt of tungstophosphoric heteropoly acid, Cs2.5H0.5PW12O40, is an active and environmentally friendly heterogeneous catalyst for the liquid-phase acetylation of nopol and several biomass-derived terpenic alcohols (i.e., α-terpineol, nerol, geraniol, linalool, menthol, isoborneol, perillyl alcohol, carveol, isopulegol, carvacrol and nerolidol) with acetic anhydride. The resulting flavor and fragrance acetic esters, which are widely used in perfumery, household and food products, are obtained in good to excellent yields. The reactions occur at room temperature with low catalyst loadings without substantial catalyst leaching and can be performed with stoichiometric amounts of an acetylating agent in solvent free systems.