New hydroformylation rhodium catalysts with dithiolate chiral ligands

Asymmetric hydroformylation of styrene is performed using the bridge dithiolate chiral rhodium complexes [Rh₂(μ-(−)-DIOS)(cod)₂]_n n=2 (1); and n=1 (2); ((−)-DIOS: 2,3-O-isopropylidene-1,4-dithiO-L-threitol and cod: 1,5-cyclooctadiene) as catalyst precursors. The catalytic system 1 provides high conversion in the corresponding aldehydes (100% at 30 bar, 65°C) with a selectivity in 2-phenylpropanal of 64%. Addition of PPh₃ to the catalytic system 1 increases the selectivity in 2-phenylpropanal to 91 % in the same conditions. Although the ee's obtained using 1 were low (3–5%), the addition of a chiral phosphine, (+)-DIOP ((2S,3S)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis (diphenylphosphino)butane), improves the ee in (S)-2-phenylpropanal to 17%. However the addition of (−)-DIOP gives a lower ee (3% (S)). Comparative results were obtained using 2 as catalyst precursor indicating that similar species were formed during catalytic cycle.