Stereochemically inert and positively charged chiral complexes of Co III prepared from Schiff bases derived from chiral diamines and salicylaldehydes were shown to be efficient catalysts of the asymmetric phase transfer benchmark reaction of alkylation of O'Donnell's substrate with alkyl halides. The enantiomeric purities of the reaction products were up to 92%.

Stereochemically inert and positively charged chiral complexes of cobalt(III) prepared from Schiff bases derived from chiral diamines and salicylaldehydes were shown to be efficient catalysts of the benchmark asymmetric phase‐transfer Michael addition of nine activated olefins to O’Donnell’s substrate. The reaction products had enantiomeric purities of up to 96%. DFT calculations were invoked to rationalize the stereochemistry of the addition.magnified image

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.

A general protocol for the asymmetric synthesis of unnatural α-amino acids with γ-tertiary and quaternary carbon centers via generated radicals is reported.

Chiral copper(II) and cobalt(III) complexes (1−5 and 6, respectively) derived from Schiff bases of (S)-2-(aminomethyl)pyrrolidine and salicylaldehyde derivatives were employed in a mechanistic study of the Henry reaction-type condensation of nitromethane and o-nitrobenzaldehyde in CH 2 Cl 2 (CD 2 Cl 2 ), containing different amounts of water. The reaction kinetics was monitored by 1 H and 13 C NMR. The addition of water had a different influence on the activity of the two types of complexes, ranging from a crucial positive effect in the case of the copper(II) complex 2 to insignificant in the case of the stereochemically inert cobalt(III) complex 6. No experimental support was found by 1 H NMR studies for the classical Lewis acid complexation of the carbonyl group of the aldehyde by the central copper(II) ion, and, moreover, density functional theory (DFT) calculations support the absence of such coordination. On the other hand, a very significant complexation was found for water, and it was supported by DFT calculations. In fact, we suggest that it is the Brønsted acidity of the water molecule coordinated to the metal ion that triggers the aldehyde activation. The rate-limiting step of the reaction was the removal of an α-proton from the nitromethane molecule, as supported by the observed kinetic isotope effect equaling 6.3 in the case of the copper complex 2. It was found by high-resolution mass spectrometry with electrospray ionization that the copper(II) complex 2 existed in CH 2 Cl 2 in a dimeric form. The reaction had a second-order dependence on the catalyst concentration, which implicated two dimeric forms of the copper(II) complex 2 in the rate-limiting step. Furthermore, DFT calculations help to generate a plausible structure of the stereodetermining transition step of the condensation.