Small amounts of achiral polyether binders are employed to enhance the enantioselectivity of the hydroformylation of an array of diversely substituted substrates (increase of up to 62% ee for vinyl acetate) mediated by chiral rhodium complexes derived from the α,ω-bis(phosphite)-polyether ligands 1. To the best of our knowledge, this study represents an unprecedented successful example of the positive regulation of enantioselectivity in hydroformylations.

Abstract:Herein we report the use of polyether binders as regulation agents (RAs) to enhance the enantioselectivity of rhodium-catalyzed transformations. For reactions of diverse substrates mediated by rhodium complexes of the ,-bisphosphite-polyether ligands 15,ad, the enantiomeric excess (ee) of hydroformylations was increased by up to 82% (substrate: vinyl benzoate, 96% ee), and the ee value of hydrogenations was increased by up to 5% (substrate: N-(1-(naphthalene-1-yl)vinyl)acetamide, 78% ee). The ligand design enabled the regulation of enantioselectivity by generation of an array of catalysts that simultaneously preserve the advantages of a privileged structure in asymmetric catalysis and offer geometrically close catalytic sites. The highest enantioselectivities in the hydroformylation of vinyl acetate with ligand 4b were achieved by using the Rb[B(3,5-(CF3)2C6H3)4 (RbBArF) as the RA. The enantioselective hydrogenation of the substrates 10 required the rhodium catalysts derived from bisphosphites 3a or 4a, either alone or in combination with different RAs (sodium, cesium or (R,R)-bis(1-phenylethyl)ammonium salts). This design approach was supported by results from computational studies.

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The synthesis of new P═O-disubstituted receptors with appended crown ethers and their properties as receptors for dicarboxylic acids have been studied. High affinities have been observed (oxalic and malonic acids with 4-, 5-, 6-, or 8-crown ethers). Binding of a cationic effector within the crown ether unit resulted in a positive "allosteric" effect, which has been determined to be K(rel) = 7 in the best case (binding of malonic acid with Li(+) @ rac-3b).