ELECTRONIC EFFECT ON RHODIUM DIPHOSPHINE CATALYZED HYDROFORMYLATION -THE BITE ANGLE EFFECT RECONSIDERED

The electronic effect in the rhodium diphosphine catalyzed hydroformyl ation was investigated. A series of electronically modified thixantpho s ligands was synthesized, and their effects on coordination chemistry and catalytic performance were studied. Phosphine basicity was varied by using p-(CH3)(2)N, p-CH3O, p-H, p-F, p-Cl, orp-CF3 substituents on the diphenylphosphine moieties. X-ray crystal structure determination s of the complexes (thixantphos)Rh(CO)H(PPh3) and (p-CH3O-thixantphos) Rh(CO)H(PPh3) were obtained. The solutions structures of the (diphosph ine)Rh(CO)H(PPh3) and (diphosphine)Rh(CO)(2)H complexes were studied b y IR and NMR spectroscopy. IR and H-1 NMR spectroscopy showed that the (diphosphine)Rh(CO)(2)H complexes consist of dynamic equilibria of di equatorial (ee) and equatorial-apical (ea) isomers. The equilibrium co mpositions proved to be dependent on phosphine basicity; the ee:ea iso mer ratio shifts gradually from almost one for the p-(CH3)(2)N-substit uted ligand to more than nine for the p-CF3-substituted ligand. Assign ments of bands to ee and ea isomers and the shifts in wavenumbers in t he IR spectra were supported by calculations on (PH3)(2)Rh(CO)(2)H, (P H3)(2)Rh(CO)(2)D, and (PF3)(2)Rh(CO)(2)H complexes using density funct ional theory. In the hydroformylation of 1-octene and styrene an incre ase in 1:b ratio and activity was observed with decreasing phosphine b asicity. Most remarkably for 1-octene the selectivity for linear aldeh yde formation was between 92 and 93% for all ligands. These results in dicate that the chelation mode in the (diphosphine)Rh(CO)(2)H complexe s per se is not the key parameter controlling the regioselectivity. Me chanistic explanations of the effect of the natural bite angle on regi oselectivity are reconsidered.