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

Abstract

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 PPh 3 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 PPh 3 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/PPh 3 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.