Tandem hydroformylation-acetalization of para-menthenic terpenes under non-acidic conditions

Rhodium-catalyzed tandem hydroformylation/acetalization of a series of para-menthenic terpenes, i.e., α-terpinene (1), γ-terpinene (2), terpinolene (3), and limonene (4), has been studied in ethanol solutions in the presence of PPh3 or P(O-o-tBuPh)3 as auxiliary ligands. Limonene gives a corresponding acetal in high yields in both systems. The reaction with conjugated diene 1 occurs in the presence of PPh3 at a reasonable rate giving aldehydes and acetals only as minor products. On the other hand, non-conjugated dienes 2 and 3 has showed an extremely low reactivity in the Rh/PPh3 system. The use of P(O-o-tBuPh)3 not only remarkably accelerates the hydroformylation of all four substrates but also increases significantly the acetalization activity of the catalytic system. With the Rh/P(O-o-tBuPh)3 system, the mixtures of fragrance acetals have been obtained from substrates 1–3 in excellent combined yields, with two or three main acetals accounting for 70-85% of the mass balance for each substrate. The process is performed under mild conditions and in the absence of acid co-catalysts. The hydroformylation of all substrates in ethanol solutions was found to be much faster than in toluene.

α-Terpinene, γ-terpinene, terpinolene, and limonene can be selectively transformed in corresponding diethylacetals in ethanol solutions containing [Rh(COD)(OMe)]2 and triphenylphosphine or tris(O-tbu-phenyl)phosphite under mild non-acidic conditions through the hydroformylation and consecutive one-pot acetalization of primarily formed aldehydes. One to three main fragrance para-menthenic acetals account for 70-90% of the mass balance for each substrate.Figure optionsDownload high-quality image (29 K)Download as PowerPoint slide