Coupling of monoterpenic alkenes and alcohols with benzaldehyde catalyzed by silica-supported tungstophosphoric heteropoly acid

Abstract

The reactions of biomass-based substrates, i.e., limonene, α-pinene, β-pinene, terpinolene, α-terpineol, nerol and linalool, with benzaldehyde in the presence of tungstophosphoric heteropoly acid H3PW12O40 (HPW) supported on silica give an oxabicyclo[3.3.1]nonene compound with fragrance characteristics in good to excellent yields. The reactions apparently involve the formation of α-terpenyl carbenium ion by the protonation of alkene or dehydration of alcohol followed by the nucleophilic attack of benzaldehyde. The subsequent oxonium-ene cyclization of the resulting oxocarbenium ion gives the oxabicyclic product. The process is an environmentally benign and heterogeneous and can be performed under mild conditions with low catalyst amounts and no significant leaching of active components.