X‐Ray Structural Studies of Highly Enantioselective Mn(salen) Epoxidation Catalysts

Abstract

Abstract:The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn (sa1en) complexes (1 -10) was examined. The Xray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6 = -fBu; 4,4 = -tBu]+CIO;), and 10 (6,6 = -fBu; 4.4 = -Br ) were determined. Catalysts 1 -9 were derived from (R,R)-1 ,tdiaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethyIenediamine. Catalysts 1-9 differ in the stereoelectronic substitution of the orfho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five-and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1 -10 was effected. Systematically increasing the steric bulk on the ortho and vtynordp asymmetric eponidations Cstalysis * manganese complexes * structure elucidation

Oxidation of olefins catalyzed by new binaphthyl-ruthenium(III) complexes

“…In the case of classical Mn-salen complexes, it has been proven that the geometry around the metal center was square planar in the case of cationic complexes with two residual ligands in apical positions, trans to each other or slightly square pyramidal in the case of neutral complexes with the residual ligand perpendicular to the plane. 24 Che has shown 15d that, in the case of binaphthyl-salen manganese complexes, the geometry, established by X-ray crystallography around the manganese atom, was octahedral with the Schiff base ligand bound to the metal center in a cis-β configuration, as shown on Scheme 5. The binaphthyl moiety enforces the complexes to adopt a non-square-planar geometry.…”

Strengths, Weaknesses, Opportunities and Threats: Computational Studies of Mn- and Fe-Catalyzed Epoxidations

“…It has thus been suggested that the most stable electronic configuration of Mn(III) would involve having all four d electrons unpaired, yielding a quintuplet state. However, the fact that the salen ligand assumes a slightly bent conformation [28] and the effect of axial coordination by a donor ligand may alter this outcome. A triplet (two unpaired d electrons) or a singlet (no unpaired electrons) ground state may therefore be a viable hypotheses [29][30][31].…”

Diphenylmethane Oxidation Catalyzed by Mononuclear Mn(III) Complexes

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] Recent studied have demonstrated that manganese complexes in combination with various oxidizing reagents catalyze a range of oxidation reactions, oxygenation of saturated [18][19][20][21][22][23] and aromatic hydrocarbons. [24][25][26][27] Although many studies performed on the oxidation of hydrocarbons by using Mn(III) complexes [28][29][30][31][32][33][34][35] and various peroxide substances as oxidants, there are few which used H 2 O 2 as a source of oxygen in the C-H oxidation of alkanes. 36 In recent decades, hydrogen peroxide has provoked significant interest in synthetic community and demonstrated to be a promising oxidant in organic synthesis since the oxidizing agent is economic, environmentally benign and powerful.…”