Electronic Effect on Rhodium Diphosphine Catalyzed Hydroformylation: The Bite Angle Effect Reconsidered

Abstract

The electronic effect in the rhodium diphosphine catalyzed hydroformylation was investigated. A series of electronically modified thixantphos ligands was synthesized, and their effects on coordination chemistry and catalytic performance were studied. Phosphine basicity was varied by using p-(CH3)2N, p-CH3O, p-H, p-F, p-Cl, or p-CF3 substituents on the diphenylphosphine moieties. X-ray crystal structure determinations of the complexes (thixantphos)Rh(CO)H(PPh3) and (p-CH3O-thixantphos)Rh(CO)H(PPh3) were obtained. The solutions structures of the (diphosphine)Rh(CO)H(PPh3) and (diphosphine)Rh(CO)2H complexes were studied by IR and NMR spectroscopy. IR and 1H NMR spectroscopy showed that the (diphosphine)Rh(CO)2H complexes consist of dynamic equilibria of diequatorial (ee) and equatorial−apical (ea) isomers. The equilibrium compositions proved to be dependent on phosphine basicity; the ee:ea isomer ratio shifts gradually from almost one for the p-(CH3)2N-substituted ligand to more than nine for the p-CF3-substituted ligand. Assignments of bands to ee and ea isomers and the shifts in wavenumbers in the IR spectra were supported by calculations on (PH3)2Rh(CO)2H, (PH3)2Rh(CO)2D, and (PF3)2Rh(CO)2H complexes using density functional theory. In the hydroformylation of 1-octene and styrene an increase in l:b ratio and activity was observed with decreasing phosphine basicity. Most remarkably for 1-octene the selectivity for linear aldehyde formation was between 92 and 93% for all ligands. These results indicate that the chelation mode in the (diphosphine)Rh(CO)2H complexes per se is not the key parameter controlling the regioselectivity. Mechanistic explanations of the effect of the natural bite angle on regioselectivity are reconsidered.

Application of P-Stereogenic Aminophosphine Phosphinite Ligands in Asymmetric Hydroformylation

“…This underlines the results of the NMR experiments, that there is no bis-equatorial complex present because these would lead to frequency shifts of the carbonyl bands upon the exchange. [17] In contrast to the behavior described for the ligands 6 a, 6 b, 6 e, and 6 f that gives rise to high enantiomeric excess, the ligands bearing electron-withdrawing groups in the phosphinite part show a mixture of several different species in solution. The signals obtained in the 31 P NMR under syngas pressure are very broad, indicative of a fluctional behavior of the complexes.…”

The bite angle makes the catalyst

“…Subsequent bubbling CO through a solution of (diphosphine)Rh(H)(CO)(PPh 3 ) led to displacement of the remaining PPh 3 . Recent in situ high-pressure IR experiments [37] have shown that ligands 1±9 form mixtures of bis-equatorial and equatorial-apical isomers, which rapidly equilibrate. Especially for ligands having small bite angles and electron donating substituents the proportion of thè unwanted' e-a isomer can be substantial, but the preference for linear aldehyde remains relatively high for such systems.…”

Dormant States of Rhodium Hydroformylation Catalysts: Carboalkoxyrhodium Complex Formed from Enones in the Alkene Feed

“…[6] The HP-IR spectrum of 1 has four carbonyl bands (atñ n = 2043, 1997, 1986, and 1953 cm À1 ) that have been assigned to the two isomers. [7] HP-IR studies of the hydroformylation of 1-octene showed that, while 1-octene was present in the reaction mixture, the main species observed was 1. Minor decomposition of 1 was due to the formation of an inactive dimer.…”