Rhodium-catalyzed hydroformylation of isoprene: Unusual accelerating effects of phosphorus ligands and gas pressure

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 degrees 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 eta(3)-eta(1) rearrangement, which converts an eta(3)-allylrhodium intermediate resistant to CO insertion into the much more reactive eta(1) complex, is the most critical step of this reaction.