Hydroformylation of monoterpenic polyenes: Effect of the conjugation of double bonds on reactivity

Rhodium-catalyzed hydroformylation of a series of monoterpenic polyenes, i.e., myrcene (1), alpha-terpinene (2), gamma-terpinene (3), terpinolene (4), and limonene (5), was studied in the presence of various monophosphines. Effects of reaction variables, such as the ligand to rhodium ratio, ligand basicity, and partial pressures of reacting gases were evaluated for each reactive substrate. The hydroformylation of conjugated olefins 1 and 2 results in two main aldehydes in each case with excellent combined selectivities and can be performed under mild conditions (80 degrees C, 80 atm) using a large excess of PPh(3) (P/Rh = 20-40). The hydroformylation of I follows the trends opposite to those usually observed with simple olefins: the increase in the concentration of the phosphorus ligand, ligand basicity, and pressure of both hydrogen and CO strongly accelerates the reaction showing that the most critical step is a conversion of eta(3)-allylrhodium intermediates into much more reactive eta(1)-complexes. The hydroformylation of 2 does not seem to occur through the formation of eta(3)-complexes and at low P/Rh ratios and in systems with more bulky ligands than PPh(3), such as PBZ(3) and PCy(3), is strongly complicated by the hydrogenation of the substrate. Nonconjugated olefins 3 and 4 show an extremely low reactivity toward hydroformylation under similar conditions, whereas in substrate 5, only a terminal exocyclic double bond reacts with rhodium to give a corresponding aldehyde in near quantitative yield. Thus, the endocyclic double bond in p-menthane dienes 2-5 can be hydroformylated at a reasonable rate under relatively mild conditions only if it is conjugated with another double bond. All obtained aldehydes have a pleasant scent and can be useful as components of synthetic fragrances.