Eliska Leitmannova and Libor Cerveny, Institute of Chemical Technology, Prague, Czech Republic

Abstract: cis-Hex-3-en-1-ol and trans-hex-3-en-1ol are called leaf alcohols, though their aromas resemble freshly cut grass. Perfumers define their aroma more precisely: cis-hex-3-en-1-ol has powerful and intensely green grassy odor.1 Traces of cis-hex-3-en-1-ol are used in refreshing top notes in delicate floral fragrance types such as muguet and lilac; in addition, the alcohol is often used alongside geranium oil, galbanum, oakmoss, lavender and mint oils.

trans-Hex-2-en-1-ol is powerful, fruity-green, slightly caramellic-fruity in its undertone, with an overall odor often compared to that of chrysanthemum foliage, or wine leaves. In flavors, traces are used in fruit complexes such as artificial strawberry.

The alcohols’ specific fragrant properties are determined by the presence of a double bond and geometric isomery on the molecule. cis-Hex-3-en-1-ol has a very pleasant fragrance, though the trans isomer has an oily touch and is not used in fragrance compositions. trans-Hex-2-en-1-ol lacks typical “green” odor, but is more fruity.

Leaf Alcohol Preparation

Leaf alcohol syntheses are difficult due to desired stereospecificity, the many steps involved in processing and low yields. The most common technological processes consist of a hexyn-1-ol preparation followed by selective catalytic hydrogenation or partial chemical reduction. Hydrogenations using poisoned palladium catalysts primarily produce cis isomers, while reductions using sodium in liquid ammonia yield trans isomers (F-1). Remains of hexynol can be banished using mercury salts.2 Partial reduction of the triple bonds in acetylenic compounds can be stereoselectively realized using Zn powder activated by 1,2-dibromethan in absolute ethanol.3,4 The short reaction times and simplicity of method are its primary advantages, contrary to catalytic hydrogenation. Another selective reduction of alkynes to cis-alkenes is based on a reduction system of hydrosilane functions immobilized on silicagel with acetic acid and a palladium catalyst, or on using an insoluble Pd(II) salicylidenethylendiamine (salen) complex.5,6.

Other topics discussed: cis-Hex-3-en-1-ol Preparations; trans-Hex-2-en-1-ol Preparation; Leaf Alcohol Preparations from Sorbic Acid and its Derivatives; Hydrogenations Catalyzed by Heterogeneous Catalysts; Hydrogenations Catalyzed by Homogeneous Catalysts; Conclusion