Kinetics and mechanisms of homogeneous catalytic reactions. Part 14. Hydroformylation of 1-hexene with formaldehyde catalyzed by a cationic bis(diphosphine)rhodium complex

Abstract

Complex [Rh(κ2-P,P-dppe)2]acac ([1a]acac, dppe: 1,2-bis(diphenylphosphino)ethane) showed to be a highly efficient precatalyst for the hydroformylation of 1-hexene with formaldehyde under relatively mild reaction conditions (353–403K) in 1,4-dioxane as the solvent; the corresponding C7 aldehydes were obtained in linear to branched ratios (l/b) close to 2. A kinetic study for this reaction lead to the experimentally determined rate law: ri={K1k2/(1+K1[CH2O])}[Rh][C6H12][CH2O] [K1=(1.47±0.04)M−1 and k2=(0.47±0.01)M−1s−1 at 373K). The experimental activation parameters for the overall reaction were also calculated [Ea=(20.1±0.9)kcalmol−1, ΔH≠=(19.5±0.9)kcalmol−1, ΔS≠=(−9.3±0.5)calK−1mol−1 and ΔG≠=(22±2)kcalmol−1]. The kinetic study together with results on coordination chemistry performed with 1a and with the BF4 salt of its iridium analogue, [Ir(κ2-P,P-dppe)2]BF4 ([1b]BF4), as well as theoretical DFT-calculations allowed us to propose a mechanism for this reaction. The catalytic cycle involves the reversible oxidative addition of formaldehyde to 1a to yield [Rh(H)(CHO)(κ2-P,P-dppe)2]+, followed by the insertion of the olefin into the Rh-H bond to generate [Rh(alkyl)(CHO)(κ2-P,P-dppe)2]+, which is considered as the rds of the process [Ea2=(23.9±0.8) kcalmol−1]; the cycle is completed by the reductive elimination of the aldehydes to regenerate 1a and restart the cycle.