Hydroformylation of endocyclic double bonds in para-menthenic terpenes under mild conditions

Abstract

Rhodium-catalyzed hydroformylation of a series of para-menthenic terpenes, i.e., terpinolene ( 1), γ-terpinene ( 2), and α-terpinene ( 3), has been studied. The hydroformylation of conjugated diene 3 can be performed at a reasonable rate using triphenylphosphine as an auxiliary ligand (P/Rh = 20), whereas non-conjugated dienes 1 and 2 have showed an extremely low reactivity in the Rh/PPh 3 system. The use of a bulky phosphite ligand, i.e., P(O- o- tBuPh) 3, significantly increased the rate of the hydroformylation of sterically crowded endocyclic double bonds in all three substrates. Dienes 1– 3 have been efficiently hydroformylated to a mixture of fragrance para-menthenic aldehydes with excellent combined yields (ca. 90%) under relatively mild conditions (80 °C, 80 atm). Two or three main aldehydes account for 80–90% of the mass balance for each substrate. Differently from the Rh/PPh 3 system, the hydroformylation of conjugated diene 3 in the presence of P(O- o- tBuPh) 3 is not complicated by partial catalyst deactivation, probably, because the ligand prevents the formation of unreactive η 3-allyl rhodium complexes. The aldehydes obtained have a pleasant scent and can be useful as components of synthetic fragrances.