Highly regioselective homogeneous isomerization-hydroformylation of 2-butene with water- and air-stable phosphoramidite bidentate ligand

Highly selective isomerization-hydroformylation of 2-butene was achieved with the presence of Rh(acac)(CO)2 and a phosphoramidite bidentate ligand which bearing 2,2 '-dihydroxy-1,1 '-binaphthyl backbone and N-indolyl substitute. The molar ratio of n- to isovaleraldehyde (217) is distinctly higher than the reported systems. NMR and IR revealed that the five-coordinate HRh(ligand)(CO)2 was an equatorial-equatorial configuration which contributed to the n-selectivity of valeraldehyde. The strong pi-acceptor ability of ligand was suggested to play a key role in fast isomerization of 2-butene. Hydrolysis and oxidation experiments demonstrated that the ligand was water- and air-stable. Cyclic voltammetry measurement confirmed that this phosphoramidite ligand is more difficult to be oxidized, compared with the phosphine, phosphinite and phosphite ligands. Inspiringly, recycling experiments showed the catalytic system could work for at least 7 runs with unchanged selectivity.

Automated summary

The highly selective isomerization-hydroformylation of 2-butene was achieved using a specific ligand and catalyst, resulting in a higher molar ratio of n- to isovaleraldehyde compared to reported systems. The structure and strong pi-acceptor ability of the ligand played a key role in enhancing reaction rate and selectivity, while the catalytic system proved to be stable during multiple recycling runs.