Rhodium-catalyzed hydroformylation of isoprene has been studied in the presence of various mono-
and diphosphines. Using a large excess of the ligand, the reaction can be performed under mild conditions
(80−100 °C, 40−80 atm) and results in three unsaturated aldehydes formed in a 95% combined selectivity.
The study of the effects of the reaction variables revealed remarkable trends, opposite those usually
observed with simple alkenes. The increase in the concentration of the phosphorus ligand and/or ligand
basicity strongly accelerates the reaction. Moreover, the reaction shows unusual kinetics, being first order
in both hydrogen and CO pressure under “common” hydroformylation conditions. The obtained data
confirm that an η3−η1 rearrangement, which converts an η3-allylrhodium intermediate resistant to CO
insertion into the much more reactive η1 complex, is the most critical step of this reaction.
Rhodium-catalyzed hydroformylation of a series of monoterpenic polyenes, i.e., myrcene (1), R-terpinene (2), γ-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 °C, 80 atm) using a large excess of PPh 3 (P/Rh ) 20-40). The hydroformylation of 1 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 η 3 -allylrhodium intermediates into much more reactive η 1 -complexes. The hydroformylation of 2 does not seem to occur through the formation of η 3complexes 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.
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