Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation with Hydrogen Peroxide

Abstract

We report the synthesis and the solution and solid-phase characterization of several titanium(IV)–salan complexes. The structures of the ligands H2sal(R,R-chan), H2ovan(S,S-chan), and H6pyr(R,R-chan)4+·4Cl–·H2O were determined by single-crystal X-ray diffraction and their main features are compared and discussed. Single crystals suitable for X-ray diffraction studies were also obtained for [Ti4{sal(R,R-chan)}4(μ-O)4] and showed a tetranuclear core bearing μ-oxo bridges between the titanium atoms and the ligand wrapped around the Ti in a fac-mer geometry. The Ti–salan complexes were tested as catalysts in the benchmark oxidation of thioanisole with H2O2 as oxidant in organic solvents. High H2O2 conversion and moderate enantioselectivities (up to 51 % ee) were obtained. In an effort to perform greener catalysis the complexes were tested in selected ionic liquids with several oxidants. [Ti{5-MeO-sal(R,R-chan)}] was also covalently bound to a polystyrene matrix. tert-Butyl hydroperoxide was found to be the most suitable oxidant for use in ionic liquids (100 % conversion vs. 80 % with H2O2) and in general the Ti–salan complexes showed better activity in ILs at 25 °C although with reduced induced asymmetry (up to 18 % ee)

Thirty years through vanadium chemistry

“…Taking into account their much higher intrinsic stability and much lower susceptibility toward hydrolysis, several publications were released either with characterization of metal complexes [153,[160][161][162][163][164] or using these very versatile ligands to prepare suitable complexes to prepare metal-based catalytic systems [163,[165][166][167][168][169][170][171][172].…”

Mn(III)‐salan/graphene oxide/magnetite nanocomposite as a highly selective catalyst for aerobic epoxidation of olefins

“…Reduction of the CN bond to a CN bond yields a tetrahydro derivative of salen (salan; Fig. ) . The presence of an amine backbone (instead of an imine) results in a more flexible structure, which is also more resistant than the salen CN bond to hydrolysis .…”

Titanium–salan‐catalyzed asymmetric sulfoxidations with H2O2: design of more versatile catalysts

“…Previously, we reported that the enantioselective oxidation of various sulfides with 'green' 30% aqueous hydrogen peroxide could be performed in the presence of catalytic amounts of titanium–salan complexes (0.2–1.0 mol%); the asymmetric induction was generated in a tandem stereoconvergent asymmetric oxidation/kinetic resolution process . Pessoa and Correia with co‐workers synthesized a series of related titanium catalysts with 1,2‐cyclohexanediamine and 1,2‐diphenylethylenediamine as chiral moieties and reported the catalytic oxidation of thioanisole, with moderate enantiomeric excess (ee) values . Further studies revealed that titanium–salan catalyst 1 had an optimal structure only for the highly enantioselective oxidation of bulky aryl benzyl sulfides, while the oxidation of smaller sulfides (such as thioanisole derivatives and alkyl phenyl sulfides) proceeded with lower chemo‐ and enantioselectivity …”

“…[1][2][3][4][5][6][7][8][9] On the other hand, several titanium-based catalyst systems appeared in the last few years, some of them surpassing the Kagan-Modena systems in terms of chemo-and stereoselectivity, as well as in ease of handling and sustainability, especially those relying on H 2 O 2 as the oxidant. [10][11][12][13][14][15][16][17][18] Previously, we reported that the enantioselective oxidation of various sulfides with 'green' 30% aqueous hydrogen peroxide could be performed in the presence of catalytic amounts of titaniumsalan complexes (0.2-1.0 mol%); the asymmetric induction was generated in a tandem stereoconvergent asymmetric oxidation/ kinetic resolution process. [16] Pessoa and Correia with co-workers synthesized a series of related titanium catalysts with 1,2-cyclohexanediamine and 1,2-diphenylethylenediamine as chiral moieties and reported the catalytic oxidation of thioanisole, with moderate enantiomeric excess (ee) values.…”