Chiral octahedral complexes of Co(iii) as catalysts for asymmetric epoxidation of chalcones under phase transfer conditions

Abstract

Stereochemically inert and positively charged chiral complexes of Co(III) were shown to catalyze the asymmetric epoxidation of chalcones with H 2 O 2 under phase transfer conditions. The reaction products had enantiomeric purities of up to 55%. It was also shown that complex 1a Icatalyzed the coupling reaction of a resulting epoxide with CO 2 (conversion 72%).Enantiomerically enriched α,β-epoxy ketones are versatile chiral building blocks for access to natural compounds and drugs in medicinal chemistry. 1,2 They can be converted into many types of useful chiral compounds, such as αhydroxy, β-hydroxy, α,β-dihydroxy carbonyl compounds, as well as epoxy alcohols. 3 The basic method of producing the enantiomerically enriched epoxy ketones is the asymmetric oxidation of activated olefins. 4 By far the most attractive method for the preparation of epoxy ketones is asymmetric epoxidation of chalcones. 5 A green and most cost effective approach is to use hydrogen peroxide as the oxidizing agent, 4e because the only by-products of the reaction is are water and molecular oxygen. The catalytic protocols usually employ either chiral metal complexes of iron 6 and manganese 7 or chiral organocatalysts, in particular, those operating under phase transfer conditions. 8 Recently we successfully elaborated chiral, positively charged, stereochemically inert complexes of Co(III) as chiral phase transfer catalysts for efficient asymmetric alkylation of a glycine Schiff base ester (O'Donnell substrate) with alkyl halides. 9a In addition, the family of the complexes could be successfully applied for the asymmetric 1,4-addition of O'Donnell's substrate to activated olefins. 9b The convincing evidence was put forward proving the complexes functioned in the reactions as "organic catalysts in disguise". 10 We believed further attempts at employing the catalysts in classical asymmetric reactions of C-C formation could be of interest.Herein we describe the use of octahedral stereochemically inert and positively charged "chiral-at-metal" Co(III) complexes 9 (depicted on Fig. 1) of both Λ− and Δ-configurations. The complexes were used as catalysts for the asymmetric epoxidation of chalcones under phase-transfer conditions and some preliminary results on the CO 2 coupling with the forming epoxides, promoted by the same complexes.

Chiral Cobalt(III) Complexes as Bifunctional Brønsted Acid–Lewis Base Catalysts for the Preparation of Cyclic Organic Carbonates

“…No kinetic resolution of terminal epoxides, assessed by analyzing the enantiomeric purity of the remaining initial epoxides at the later stages of the conversion, was detected. However, in the case of the internal epoxide chalcone oxide 8 g , kinetic resolution did occur and the enantiomeric purity of the remaining epoxide 8 g was 55 % (determined by chiral HPLC) at 60 % conversion . It may be that the positioning of the epoxide by hydrogen bonding to the chiral catalyst was more rigid than for simple epoxides owing to the presence of an additional carbonyl group, also capable of forming hydrogen bonds with the catalyst.…”

“…[e] Reaction conditions: neat epoxide, 1 mol % of complex 3 , 50 °C, 5 MPa CO 2 . [f] In 0.1 mL toluene using 10 mol % of the catalyst …”

“…[ f] In 0.1 mL toluene using 10 mol %o ft he catalyst. [36] ChemSusChem 2016, 9,216 -222 www.chemsuschem.org…”

“…4‐Benzoyl‐5‐phenyl‐1,3‐dioxolan‐2‐one 9 g (trans‐isomer): 1 H NMR (400 MHz, CDCl 3 ): δ =7.96 (d, J =7.5 Hz, 2 H), 7.66 (t, J =7.5 Hz, 1 H), 7.55–7.43 (m, 7 H), 5.98 (d, J =6.2 Hz, 1 H), 5.60 ppm (d, J =6.2 Hz, 1 H).…”

Synthesis and Investigations of Chiral NNO Type Copper(II) Coordination Polymers

“…In our earlier work we used the chiral NNO and ONO type ligands based on Schiff bases of chiral diamines (or amino acids) and salicylaldehydes to construct new type of chiral octahedral inert cobalt(III) complexes . These complexes were efficient catalysts for different asymmetric reactions …”