Hydrogenation towards Synthetic Sandalwood Odorants in Fragrance Industry

Abstract

The use of metal hydrides as historical reagents for selective carbonyl group reduction is often still a really highly convenient solution for laboratory‐scale transformations due to the lack of requirement for specific equipment. Nevertheless, based on safety, environmental, but also economical industrial constraints, metal hydride reductions were replaced by hydrosilylation, hydrogen transfer, and mostly hydrogenation reactions. Among many other advantages, the use of hydrogen as a reducing agent indeed represents the most sustainable solution for implementation of such chemical transformation on industrial scale. Whereas heterogeneous catalysts have been initially sparsely employed, the most recent developments towards the synthesis of highly active and efficient homogeneous catalysts have become an indisputable solution for selective hydrogenation of both ketones and aldehydes in sandalwood odorant production according to green chemistry principles.

Direct Heterogenization of the Ru-Macho Catalyst for the Chemoselective Hydrogenation of α,β-Unsaturated Carbonyl Compounds

“…The chemoselective hydrogenation of α,β-unsaturated carbonyl compounds to their corresponding unsaturated alcohols is an important class of organic reactions because the resulting allylic alcohols are important intermediates in the synthesis of numerous fine chemicals and pharmaceutical agents, agrochemicals, and perfumes. − The chemoselective hydrogenation of a carbonyl group of the α,β-unsaturated carbonyl compounds is challenging because reduction of the olefin group is kinetically faster than that of the carbonyl group. Additionally, hydrogenation of the CC group relative the CO group is thermodynamically preferred by a factor of 35 kJ mol –1 .…”

From Pine to Perfume

“…[10,11] Campholenic aldehyde (3) is the precursor of a wide range of renewable synthetic sandalwood ingredients obtained by aldol-condensation, alkylation and reduction reactions (Scheme 5). [12]…”

“…Under these conditions, the obtained terpineols contain mainly the α-isomer. [14] Terpineol (11) is readily acetylated to its corresponding acetate (12) in the presence of acetic anhydride. Hydrogenation of terpineols (11) over Raney nickel leads to menthanol which in turn can be acetylated to menthanyl acetate.…”

Biocatalytic approaches for a more sustainable synthesis of sandalwood fragrances

“…4 Alternatively, a new category of synthetic sandalwood-scented fragrances ( 1 ) like Brahmanol®, Firsantol®, Sandalore®, Ebanol®, and many others was developed. 5 These compounds are efficiently synthesized starting from α-pinene, which is an abundant and renewable natural source (Scheme 1B). As a result, their production can be considered more environmentally sustainable than the sandalwood oil extraction from endangered Santalum trees.…”

“…Currently, these reductive steps are accomplished either by hydrosilylation, hydrogen transfer or hydrogenation reactions, mostly catalysed by precious transition metals. 5,6 Although these methodologies were preferred to the metal hydride-based procedures, mainly for practicality reasons, they are not yet fully chemoselective, leading to possible over-reduced products and to CC double bond positional isomerization.…”