Molecular building block approaches to chiral porous zirconium phosphonates for asymmetric catalysis

Abstract

Porous zirconium phosphonates containing chiral dihydroxy functionalities have been synthesized via a building block approach. Enantiopure atropisomeric bisphosphonic acids of various lengths, L1–L3, were first synthesized starting from 1,1′-bi-2-naphthol (BINOL) in multi-step sequences. Amorphous chiral porous zirconium phosphonates were then obtained by refluxing BINOL-derived bisphosphonic acids with Zr(OnBu)4 in n-BuOH, and have been characterized by powder X-ray diffraction, solid-state CP-MAS 31P NMR, IR, TGA, adsorption measurements, circular dichroism spectroscopy, and microanalyses. These zirconium phosphonates have empirical formulae of (Zr-L1–3)·xH2O (x=4 or 5), and exhibit BET surface areas ranging from 431 to 586 m2/g. In combination with Ti(OiPr)4, these zirconium phosphonates have been used to heterogeneously catalyze the additions of diethylzinc to a wide range of aromatic aldehydes with high conversions and e.e. of up to 72%. This work represents a novel approach towards heterogeneous asymmetric catalysis. The tunability of such a molecular building block approach promises to lead to practically useful heterogeneous asymmetric catalytic processes.