A Versatile Approach to Planar Chiral Monophosphane Ligands

Abstract

The enantioselective reaction of a chiral bisamide and an electrophile with benzyl ether complexes of arene tricarbonyl chromium(0) was used to introduce the asymmetry into a synthesis of novel enantiopure planar chiral ligands. Elaboration of the resulting ether complexes [η⁶-4-t-BuC₆H₄CHE¹(OR)]Cr(CO)₃ via a diastereo­selective ortholithiation/chlorophosphane quench protocol led to the synthesis of planar chiral monophosphane complexes with high enantiomeric purity (typically 96-98% ee) for assessment in the palladium-catalyzed asymmetric hydrosilylation of styrene. The identity of benzylic substituent E¹ (installed using the chiral base reaction) was found to have a significant effect on the enantioselectivity of resulting catalysts. Removal of the tricarbonylchromium(0) unit from the structurally optimized phosphane ligand was accomplished using a high-yielding borane protection-aerial oxidation-deprotection strategy.

Key words

arene complexes - asymmetric synthesis - chromium - hydrosilylation - phosphane

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14

All HPLC analyses were carried out with racemic samples created using t-BuLi instead of the diamide of (+)-3.