Crucial role of additives in iridium-catalyzed hydroformylation

Novel Ir-Complexes for Hydroformylation of Olefins with H2O as the Hydrogen Source

“…These features indicated that the enhanced π-accepting ability of L2～L4 in comparison to that in PPh 3 facilitated a more intensive π-back donation interaction between the Ir(I)-center and the phosphine, leading to a more consolidated Ir-P linkages. In comparison to Ir-L3 and Ir-L4 wherein L3 ( 1 J 31 P-77 Se＝735 Hz) and L4 ( 1 J 31 P-77 Se＝781 Hz) possessed a similar steric effect but differ in the electronic effect, the two Ir -P distances in Ir-L4 (0.23147(31), 0.23152(30) nm] were much shorter than those in Ir-L3 (0.23177(10), 0.23301(10) nm) due to the imidazolium-tailed phosphines L4 with a stronger π-accepting ability. In comparison to Ir-L2 and Ir-L4 wherein L2 ( 1 J 31 P-77 Se＝786 Hz) and L4 ( 1 J 31 P-77 Se＝781 Hz) possessed a similar electronic effect but only differ in the steric hindrance, the Ir-P distances in Ir-L4 were also shorter than those in Ir-L2 (Ir-P 0.23145(19), 0.23173(19) nm) due to the chelating effect of the diphosphine in L4.…”

“…As the market's (in particular, the automotive industry) demands for rhodium continues to increase, the price of the precious metals in general and of rhodium in particular has also risen dramatically. [9][10] Therefore, it is highly desirable to develop catalysts using metals other than rhodium for hydroformylation.…”

“…In addition, Beller et al have shown that iridium, as an alternative metal to rhodium, can be used to prepare efficiently catalysts for the homogeneous hydroformylation of olefins. [10][11][12][13] It is well known that iridium-complex catalysts are often active for the hydrogenation of olefins. [14][15][16][17] Therefore an important issue to improve the performance of iridium-complexes for catalytic hydroformylation is to suppress the hydrogenation process.…”

“…A frequently used method is to add lithium salts to the reaction systems to inhibit the hydrogenation of olefins during hydroformylation. [10,13] Another method is to control the amount of H 2 in the reaction system. [18][19] The water-gas shift reaction (WGSR, H 2 O＋CO → H 2 ＋CO 2 ) over the Ir-catalyst reported by Peter in 1984, [20][21] is a good method to inhibit hydrogenation of olefins in hydroformylation [22] by in situ generation of the desired amount of H 2 .…”

“…The P-Ir-P chelation in these two complexes was presented in the trans-position due to the existence of the bis(imidazolyl)-butyl bridge in L3 and L4. In comparison to Ir(CO)(PPh 3 ) 2 Cl [Ir-P and Ir-CO distances are 0.23309(10) and 0.17911(13) nm],[18] Ir-L2, Ir-L3 and Ir-L4 were all featured with a much shorter Ir-P linkage (0.23145～0.23301 nm), and a longer Ir-CO bond distance (0.18309～0.18629 nm) (Table1). These features indicated that the enhanced π-accepting ability of L2～L4 in comparison to that in PPh 3 facilitated a more intensive π-back donation interaction between the Ir(I)-center and the phosphine, leading to a more consolidated Ir-P linkages.…”