Chiral Manganese Complexes Research Articles

Asymmetric Epoxidation of Unfunctionalized Olefins Catalyzed by Chiral (Pyrrolidine Salen) Mn (III) Complexes with Proline Sidearms

Four (R, R)-3, 4-diaminopyrrolidine-based chiral Salen ligands with L-Boc-N-proline, D-Boc-N-proline, L-proline, D-proline as respective sidearms, as well as their manganese complexes were synthesized and characterized. These Salen-Mn(III) complexes were applied to the asymmetric epoxidations of unfunctionalized olefins with NaClO, and m-CPBA as respective oxidants. The catalysis of the Salen-Mn (III) complexes varied with the change of sidearms. The one with D-Boc-N-proline as a sidearm showed the best asymmetric catalysis among the four chiral manganese complexes, and its catalysis was also better than that of the one with a benzyl sidearm, and comparable with that of Jacobsen's catalyst. Moderate to excellent enantioselectivities and yields were achieved in the asymmetric of various olefins over this complex. The big enhancement of the D-Boc-N-proline sidearm on the enantioselectivity of the asymmetric epoxidation might be ascribed to its large steric hindrance and appropriate configuration, allowing the Boc moiety to be away from the side-on approach pathway of olefins to the active center but near the opposite side one in some degree, thus facilitating the approach of olefins to the active center from one direction.

Photocatalytic Asymmetric Epoxidation with a Chiral Manganese Complex

Photocatalytic Asymmetric Epoxidation with a Chiral Manganese Complex

Effects of Lewis Acids on Photoredox Catalysis

AbstractThe reactivity of organic photoredox catalysts in oxidation reactions is enhanced by the addition of Lewis acids, which bind to the catalyst to increase its oxidizing ability. Redox‐inactive metal ions, such as Sc3+, also act as Lewis acids and can bind to flavins to increase their one‐electron reduction potentials and photocatalytic stability. The Sc3+ ion can bind to acridine and pyrene to enhance their photoredox catalytic activity in the oxidation of substrates by dioxygen. The binding of Lewis acids to radical anions enhances their overall photocatalytic activity by prohibiting a back electron transfer from the radical anions of substrates to the oxidized photocatalysts. The photocatalytic asymmetric oxidation of terminal olefins by using water as the oxygen source with a mononuclear nonheme chiral manganese complex has been made possible by the addition of acetic acid, which is essential to afford the products with high enantioselectivity.

Photocatalytic Asymmetric Epoxidation of Terminal Olefins Using Water as an Oxygen Source in the Presence of a Mononuclear Non-Heme Chiral Manganese Complex.

Photocatalytic enantioselective epoxidation of terminal olefins using a mononuclear non-heme chiral manganese catalyst, [(R,R-BQCN)MnII]2+, and water as an oxygen source yields epoxides with relatively high enantioselectivities (e.g., up to 60% enantiomeric excess). A synthetic mononuclear non-heme chiral Mn(IV)-oxo complex, [(R,R-BQCN)MnIV(O)]2+, affords similar enantioselectivities in the epoxidation of terminal olefins under stoichiometric reaction conditions. Mechanistic details of each individual step of the photoinduced catalysis, including formation of the Mn(IV)-oxo intermediate, are discussed on the basis of combined results of laser flash photolysis and other spectroscopic methods.

A Mononuclear Manganese Complex of a Tetradentate Nitrogen Ligand – Synthesis, Characterizations, and Application in the Asymmetric Epoxidation of Olefins

AbstractA new chiral manganese complex (C1) bearing a tetradentate nitrogen ligand containing chiral bipyrrolidine and benzimidazole moieties was prepared. The structure of C1 was confirmed by ESI‐MS and crystallography. This manganese complex is an active catalyst for the asymmetric epoxidation of various olefins with excellent conversion (up to 99 %) and high enantiomeric excess (up to 96 % ee) with hydrogen peroxide as the oxidant in the presence of 2‐ethylhexanoic acid or acetic acid. Compared with previous structurally similar manganese complexes with different diamine backbones (C2, cyclohexanediamine; C3, diamine from L‐proline), C1 showed improved asymmetric induction, especially for simple olefins such as styrene derivatives and substituted chromene. The possible reasons for the improvement of the ee values are discussed in the text on the basis of the crystal structures of the manganese complexes.

Manganese complexes with non-porphyrin N4ligands as recyclable catalyst for the asymmetric epoxidation of olefins

New chiral manganese complexes of N4 ligands derived from 2-acetylpyridine were prepared and used as catalysts in the enantioselective epoxidation of olefins, using H2O2 as an oxidant to give epoxides, with excellent conversions (up to 99%) and enantiomeric excess (up to 88%) within 1 h at 0 °C. A detailed mechanistic study was undertaken based on the information obtained by single crystal X-ray, optical rotation, UV-Vis, CD spectra and kinetic studies, and revealed that the reaction is first order with respect to the concentration of catalyst and oxidant and independent of substrate concentration. The complex (0.1 mol%) was successfully subjected to recyclability experiments over 3 cycles in the epoxidation of styrene with H2O2 as an oxidant and acetic acid as an additive at 0 °C with retention of performance.

Asymmetric Epoxidation of Alkenes Catalyzed by a Porphyrin-Inspired Manganese Complex

A novel strategy for catalytic asymmetric epoxidation of a wide variety of olefins by a porphyrin-inspired chiral manganese complex using H2O2 as a terminal oxidant in excellent yield with up to greater than 99% ee has been successfully developed.

Chiral manganese complexes with pinene appended tetradentate ligands as stereoselective epoxidation catalysts

A novel family of chiral manganese complexes Lambda-1(CF(3)SO(3)) and Delta-2(CF(3)SO(3)), have been stereoselectively prepared, characterized and studied as epoxidation catalysts. The complexes are structurally related to [Mn(II)(CF(3)SO(3))(2)(alpha-MCP)] (MCP=N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)cyclohexane-trans-1,2-diamine), recently reported as a very efficient epoxidation catalyst in combination with peracetic acid. Pinene rings have been fused to the 4 and 5 positions of the two pyridine groups of the ligand, giving rise to complexes where the two labile binding sites of the manganese ion are confined in a better-defined chiral pocket than in the parent [Mn(II)(CF(3)SO(3))(2)(alpha-MCP)]. Chirality in these complexes arises from the stereochemistry of the trans-diaminocyclohexane ring, from the pinene ring and also from the topological chirality adopted by the ligand upon binding to the manganese ion. While previous studies have demonstrated that small modifications in the structure of the MCP ligand result in a dramatic loss of efficiency, Lambda-1(CF(3)SO(3)) and Delta-2(CF(3)SO(3)) exhibit comparable catalytic activity to [Mn(II)(CF(3)SO(3))(2)(alpha-MCP)]. In addition, the complexes exhibit a remarkable stereoselectivity (up to 46% ee) in the epoxidation of selected substrates. The results reported in this work point towards modification of the 4 and 5 positions of the pyridine groups as a new strategy towards the design of stereoselective versions of this family of highly active and environmentally benign epoxidation catalysts.

Catalytic asymmetric sulfoxidation by chiral manganese complexes: acetylacetonate anions as chirality switches.

Three manganese(II) complexes, namely [Mn(1)(ClO(4))(2)] (3), [Mn(1)(acac)(2)] (4), and [Mn(2)(1)(acac)(4)] (5), were isolated from solutions of Mn(ClO(4))(2) or Mn(acac)(2), and an easily accessible diimine ligand (1S,2S)-N,N'-bis-pyridin-2-ylmethylene-cyclohexane-1,2-diamine (1). Their structure was determined by X-ray crystallography, and these complexes proved to be catalysts for asymmetric sulfide oxidation by H(2)O(2). Enantiomeric excesses ranging from 5% to 62% were obtained with a variety of aryl alkyl sulfides. We also observed an interesting "chirality switch" effect by the achiral acac anion reversing the enantioselectivity of the complex [Mn(1)(ClO(4))(2)] from the S to the R sulfoxide enantiomer.

Synthesis and Characterization of New Chiral Schiff Base Complexes with Diiminobinaphthyl or Diiminocyclohexyl Moieties as Potential Enantioselective Epoxidation Catalysts

New chiral Schiff base complexes have been obtained by condensation of 2,2'-diamino-1,1'-binaphthalene or 1,2-diaminocyclohexane and various salicylaldehydes and by subsequent metalation with manganese, iron, cobalt, nickel, copper, or zinc. The complete (1)H and (13)C NMR characterization of the ligands is reported, as are the X-ray crystal structures of (1R,2R)-(-)-N,N'-bis[3-(N,N-dimethylamino)salicylidene]-trans-1,2-cyclohexanediimine and [(1R,2R)-(-)-N,N'-bis(salicylidene)-trans-1,2-cyclohexanediiminato]copper(II). The new chiral manganese complexes have been evaluated in the oxygenation of prochiral olefins and sulfides using sodium hypochlorite, hydrogen peroxide, or N-methylmorpholine N-oxide/m-chloroperbenzoic acid as oxidant.

ChemInform Abstract: Diastereoselective Nucleophilic Addition to Coordinated Cyclohexadienyl Rings in Chiral Manganese Complexes.

AbstractThe addition of malonate (II) to (I), in which the metal is the chiral center, proceeds with significant asym.

Diastereoselective nucleophilic addition to coordinated cyclohexadienyl rings in chiral manganese complexes

Diastereoselective nucleophilic addition to coordinated cyclohexadienyl rings in chiral manganese complexes