Rhodium catalyzed hydroformylation of monoterpenes containing a sterically encumbered trisubstituted endocyclic double bond under mild conditions

Abstract The rhodium catalyzed hydroformylation of endocyclic monoterpenes, that is, 2-carene ( 1 ), 3-carene ( 2 ), and α-pinene ( 3 ), in the presence of PPh 3 or various diphosphines and phosphites has been studied. The unmodified Rh catalyst promotes an intense isomerization of both carenes whose hydroformylation occurs rather slowly, and results in a complex mixture of aldehydes and alcohols. The addition of PPh 3 , diphosphines or P(OPh) 3 in a P/Rh ratio as high as 20, efficiently prevents the isomerization, but the activity for hydroformylation is drastically reduced. On the other hand, the use of a bulky P(O- o - t BuPh) 3 ligand both reduces the isomerization, and significantly increases the hydroformylation rate. All three sterically crowded olefins 1–3 have been efficiently hydroformylated under relatively mild reaction conditions (80–100 °C, 40–80 atm) to a main aldehyde (2-formylcarane, 4-formylcarane, and 3-formylpinene, respectively) with good chemo- and regioselectivity, and almost 100% stereoselectivity for the trans isomers.