Chiral diphosphites derived from D-glucose: New ligands for the asymmetriccatalytic hydroformylation of vinyl arenes

A series of novel diphosphite ligands derived from readily available D-(+)- glucose has been synthesized. These ligands have been applied to the Rh-cat alyzed hydroformylation of vinyl arenes. Both excellent enantioselectivitie s (up to 91%) and regioselectivities (up to 98.8%) were achieved under mild conditions. The advantage of these ligands is that their modular natures a llow facile, systematic variation in the configurations at the stereocenter s [C(3), C(5)] at the ligand bridge and in the biphenyl substituents, enabl ing their effects on the stereoselectivity to be studled. Results show that the absolute configuration of the product is governed by the configuration at the stereogenic center C(3), while the level of the enantioselectivity is influenced by a cooperative effect between stereocenters C(3) and C(5). Replacement of the tert-butyl substituent by methoxy substituents at the pa ra positions of the biphenyl moieties improved the enantioselectivities. We have characterized the rhodium complexes formed under CO/H-2 by NMR techni ques and in situ IR spectroscopy and have observed that there is a relation ship between the structure of the [HRh(CO)(2)(PP)] species and their enanti odiscriminating performance in hydroformylation. Enantioselectivities were highest with ligands with a strong bis-equatorial coordination preference, while an equilibrium of species with bis-equatorial and equatorial-axiaI co ordination modes considerably reduced the ee's.