Rhodium-catalyzed hydroformylation of a series of para-menthenic terpenes, i.e., terpinolene (1), gamma-terpinene (2), and alpha-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/PPh3 system. The use of a bulky phosphite ligand, i.e., P(O-o-(BuPh)-Bu-t)(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 degrees C. 80 atm). Two or three main aldehydes account for 80-90% of the mass balance for each substrate. Differently from the Rh/PPh3 system, the hydroformylation of conjugated diene 3 in the presence of P(O-o-(BuPh)-Bu-t)(3) is not complicated by partial catalyst deactivation, probably, because the ligand prevents the formation of unreactive eta(3)-allyl rhodium complexes. The aldehydes obtained have a pleasant scent and can be useful as components of synthetic fragrances. (C) 2009 Elsevier B.V. All rights reserved.