Chiral aluminum complexes have become established as promising Lewis acid catalysts for a wide variety of asymmetric reactions. [1,2] In contrast, asymmetric oxidation catalysis by aluminum complexes has scarcely been developed. As a rare example, Bolm and co-workers reported an Alcatalyzed asymmetric Baeyer-Villiger oxidation with an alkyl hydroperoxide as the oxidant. High enantioselectivity was observed in the reaction, which required anhydrous conditions.[3] To the best of our knowledge, no chiral aluminum catalyst has been reported for a highly enantioselective oxidation with aqueous hydrogen peroxide as the oxidant.[4] Herein, we describe the development of an asymmetric oxidation of sulfides with aqueous hydrogen peroxide in the presence of a chiral Al(salalen) complex. [5][6][7][8][9][10][11][12] Recently, we reported the first synthesis of the chiral Al(salalen) complex 1 and its asymmetric catalysis of hydrophosphonylation.[13] Reactions with both aldehydes and imines furnished the corresponding products with high to excellent enantioselectivities. We decided to modify and tune the complex to further explore new catalysis of Al(salalen) complexes. During the course of this investigation, we found that the treatment of an Al(salalen) complex with water provided a new water-compatible Al(salalen) complex. Although the complex could not be isolated, it occurred to us that Al(salalen) complexes should serve as catalysts under aqueous conditions. Thus, we investigated the use of the complexes as catalysts for asymmetric oxidation with aqueous hydrogen peroxide as the oxidant.First, we examined the asymmetric oxidation of thioanisole with 30 % hydrogen peroxide (1.1 equiv) in the presence of Al(salalen) complexes (2 mol %) and found that the structure and configuration of the complexes affected significantly their catalytic and asymmetry-inducing abilities (Table 1). The reaction with complex 1 was only modestly selective and poorly reproducible. In contrast, complexes 2-5, which contain a binol-based salalen ligand (binol = 1,1'-binaphthalene-2,2'-diol), showed higher catalytic activity with good reproducibility in the presence of a phosphate buffer. The reproducibility of the reaction with 1 was not improved by the addition of the phosphate buffer. Asymmetric induction by the (aR,R,R) complexes 2 and 3 was modest; however, much higher levels of asymmetric induction [a] Yield [%] [b] ee [%]

The aluminium(salalen) complex was found to be an efficient catalyst for the asymmetric oxidation of sulfides under solvent-free or highly concentrated conditions, in which an only 0.002-0.01 mol% catalyst loading was sufficient to obtain optically active sulfoxides in high yield with high enantioselectivity.

Chiral aluminum complexes have become established as promising Lewis acid catalysts for a wide variety of asymmetric reactions. [1,2] In contrast, asymmetric oxidation catalysis by aluminum complexes has scarcely been developed. As a rare example, Bolm and co-workers reported an Alcatalyzed asymmetric Baeyer-Villiger oxidation with an alkyl hydroperoxide as the oxidant. High enantioselectivity was observed in the reaction, which required anhydrous conditions.[3] To the best of our knowledge, no chiral aluminum catalyst has been reported for a highly enantioselective oxidation with aqueous hydrogen peroxide as the oxidant.[4] Herein, we describe the development of an asymmetric oxidation of sulfides with aqueous hydrogen peroxide in the presence of a chiral Al(salalen) complex. [5][6][7][8][9][10][11][12] Recently, we reported the first synthesis of the chiral Al(salalen) complex 1 and its asymmetric catalysis of hydrophosphonylation.[13] Reactions with both aldehydes and imines furnished the corresponding products with high to excellent enantioselectivities. We decided to modify and tune the complex to further explore new catalysis of Al(salalen) complexes. During the course of this investigation, we found that the treatment of an Al(salalen) complex with water provided a new water-compatible Al(salalen) complex. Although the complex could not be isolated, it occurred to us that Al(salalen) complexes should serve as catalysts under aqueous conditions. Thus, we investigated the use of the complexes as catalysts for asymmetric oxidation with aqueous hydrogen peroxide as the oxidant.First, we examined the asymmetric oxidation of thioanisole with 30 % hydrogen peroxide (1.1 equiv) in the presence of Al(salalen) complexes (2 mol %) and found that the structure and configuration of the complexes affected significantly their catalytic and asymmetry-inducing abilities (Table 1). The reaction with complex 1 was only modestly selective and poorly reproducible. In contrast, complexes 2-5, which contain a binol-based salalen ligand (binol = 1,1'-binaphthalene-2,2'-diol), showed higher catalytic activity with good reproducibility in the presence of a phosphate buffer. The reproducibility of the reaction with 1 was not improved by the addition of the phosphate buffer. Asymmetric induction by the (aR,R,R) complexes 2 and 3 was modest; however, much higher levels of asymmetric induction [a] Yield [%] [b] ee [%]

An alternative more efficient photo-cross-linker having a d-threoninol skeleton instead of the 2′-deoxyribose backbone in pyranocarbazole was investigated to improve the photoreactivity of photo-cross-linkers.

Asymmetric oxidation catalysis with an aluminum-based complex was achieved by a combination of newly synthesized chiral aluminum(salalen) complexes (salalen = half-reduced salen = salan/salen-hybridized [ONNO]-type tetradentate ligand; salan = reduced salen, salen = N,N'-ethylenebis(salicylideneiminato)) derived from binol (1,1'-bi-2,2'-naphthol) with aqueous hydrogen peroxide as the oxidant. The combination was found to be efficient for asymmetric sulfur oxidation. Various sulfides were smoothly converted into the corresponding sulfoxides with high to excellent enantioselectivity. Thioacetals are also good substrates for the oxidation.