Rhodium catalyzed hydroformylation of monoterpenes containing a sterically encumbered trisubstituted endocyclic double bond under mild conditions
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The rhodium catalyzed hydroformylation of endocyclic monoterpenes, that is, 2-carene (1), 3-carene (2), and a-pinene (3), in the presence of PPh3 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 PPh3, 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-tBuPh)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 8C, 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.
|Descripción||Applied Catalysis A: General 326 (2007) 219–226|