Picket-fence ruthenium(II) phthalocyaninates bearing (1R,2S,5R)-menthoxy groups as prototype of chiral catalysts

Abstract

We report design and synthesis of novel picket-fence phthalocyanines to access the ruthenium(II) complexes carrying di-(1R,2S,5R)-menthoxy substituted aryloxy-groups. Owing to bulkiness of such groups located either at peripheral (β) or non-peripheral (α) positions, they are nearly orthogonal to the plane of the phthalocyanine thus creating a chiral environment around the metal center. This orthogonality was supported by X-ray analysis of corresponding phthalonitrile precursors (86.0° and 81.4° angles between the planes of aromatic moieties for α- and β-substituted phthalonitriles, respectively). As a proof of concept, the synthesized complexes were investigated as catalysts in a benchmark reaction of the cyclopropanation of styrene by ethyl diazoacetate. While the β-substituted complex showed very low enantioselectivity, the α-substituted analogue afforded a moderate asymmetric induction towards ethyl (1S,2R)-2-phenylcyclopropane-1-carboxylate. This result emphasizes the importance of appropriate arrangement of the chiral groups relative to the phthalocyanine catalytic center and provides guidelines for further elaboration of phthalocyanine catalysts for asymmetric transfer of carbenes.