Influence of electrostatic repulsive force and electron-withdrawing effect in ionic diphosphine on regioselectivity of rhodium-catalyzed hydroformylation of 1-octene

• Neutral diphosphine with two-PPh2 units located in cis-position.
• Ionic diphosphine with two-PPh2 units located in trans-position.
• High regioselectivity to linear nonanal in Rh-catalyzed hydroformylation of 1-octene.
• In situ high-pressure FT-IR characterization of active Rh–H species.

The neutral diphosphine of 1 and the corresponding ionic diphosphine of 2 after quaternization 1 by MeOTf were prepared and investigated as the wide bite angle and bulky steric ligands to control the regioselectivity in Rh(acac)(CO)2-catalyzed hydroformylation of 1-octene. As for 2, due to the electrostatic repulsive force of the two positive charges, the involved two-PPh2 were located in trans-position with the relatively longer P1…P2 distance [8.0231(11) Å] to prefer to aa-site coordination in trigonal bipyramid H(alkene)Rh(CO)(P–P) intermediate, which renders less bulky steric hindrance for the regioselectivity to linear aldehyde as a result of low L/B ratio of 1.9. Due to the strong electron-withdrawing effect from the positive-charged imidazolium rings on the neighbored P-atoms, 2 with strong π-acceptor ability can develop π-backdonation interaction between Rh and P linkage to favor the dissociation of CO ligand and facilitate the selectivity to nonanals. In contrast, 1 with no electrostatic repulsive effect and less steric hindrance, the two-PPh2 were able to be conveniently located in cis-position with the relatively shorter P1…P2 distance [P1…P2, 6.2096(10) Å], which corresponded to the preferential linear regioselectivity with L/B = 7.1. The in situ high pressure spectroscopy analysis indicated that the involvement of 1 in Rh(acac)(CO)2-catalyzed hydroformylation of 1-octene could greatly facilitate the formation and stability of the active Rh–H species (ν2047 cm−1).

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