Designing Ruthenium Phthalocyanine with Chiral Pockets Formed by (1R,2S,5R)-Menthoxy Groups for Enantioselective Catalysis

Unprecedented chiral ruthenium(II) complexes with phthalocyanines having chiral motifs near the catalytic metal site have been prepared by cross condensation of the chiral 3,6-bis-aryloxy-phthalonitrile (α-Ar*O)₂Pn bearing two (1R,2S,5R)-menthoxy groups orthogonal to the aromatic plane and (15-crown-5)phthalonitrile. Four complexes containing chiral menthyl groups (M) and 15-crown-5 units (C), notably RuPc[MC₃](CO), RuPc[opp-M₂C₂](CO), RuPc[adj-M₂C₂](CO), and RuPc[M₃C](CO), were isolated in pure form and fully characterized by UV–vis, circular dichroism, HRMS, and various ¹H NMR and ¹³C NMR techniques. Their evaluation in the benchmark asymmetric cyclopropanation reaction of styrene derivatives by ethyl diazoacetate indicated that the RuPc[opp-M₂C₂](CO) complex was the most efficient in terms of diastereo- and enantioselectivity. Further study revealed the strong dependence of the stereoselectivity on the solvent nature and salt additives, which caused conformational rearrangement of the flexible chiral surrounding, as evidenced by multinuclear NMR and CD spectra. For instance, upon moving from commonly used CH₂Cl₂ to EtOH with the addition of NaPF₆, a significant enhancement of enantioselectivity (from 35 to 84% with p-methylstyrene) was obtained. Of particular importance is a very high diastereoselectivity of cyclopropanation of many substrates promoted by the incorporation of sodium cations into crown ether cavities of phthalocyanine to attain a trans/cis ratio up to 499:1. Such a regulating effect in chiral catalysis involving tetrapyrrolic complexes has not been previously observed, rendering this complex a prominent example of the phthalocyanine tunable catalyst. The developed synthetic strategy paves the way to phthalocyanine complexes with a chiral environment around the metal site and crown ether receptors to tune the catalytic properties.