RHODIUM-CATALYZED ASYMMETRIC HYDROFORMYLATION WITH CHIRAL DIPHOSPHITE LIGANDS

Chiral diphosphites have been synthesised starting from 1,2:5,6-diisopropylidene-D-mannitol, L-alpha,alpha,alpha,alpha,-tetramethyl-1,3-dioxolan-4,5-dimethanol and L-diethyltartrate. The diols react in moderate to good yields with 2,2'-bisphenoxyphosphorus chloride and 4,4',6,6',-tetra-t-butyl-2,2'-bisphenoxyphosphorus chloride (32-92%) to the corresponding chiral diphosphites. These compounds all exhibit C2 symmetry and have been used as ligands in the rhodium catalysed asymmetric hydroformylation of styrene. The catalytic activity of the diphosphites strongly depends on the bulkyness of the ligand. With a bulky ligand enantiomeric excesses up till 20 % have been obtained under mild reaction conditions (25-40-degrees-C, 40 bar syngas). It was found that both enantiomeric excess and regioselectivity to the branched aldehyde strongly depend on the hydroformylation reaction conditions.