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

Abstract

Titanium–salan complexes with 3,3’‐diphenyl substituents in the salicylidene rings of the salan ligand are efficient sulfoxidation catalysts, capable of catalyzing the asymmetric oxidation of bulky aryl benzyl sulfides with H2O2 with good to high enantioselectivities. In this paper, substituent effects on titanium‐salan‐catalyzed enantioselective oxidation of sulfides to sulfoxides have been systematically investigated. Titanium–salan catalysts with halogen substituents at the 5,5’‐positions (3,3’‐H2dihydrogen substituted) have been found to catalyze the oxidation of both bulky aryl benzyl sulfides and small alkyl phenyl sulfides with good to high enantioselectivities. Kinetic data witness a direct attack of the sulfide to the electrophilic active oxygen species; a consistent reaction mechanism is proposed. Copyright © 2013 John Wiley & Sons, Ltd.

Titanium Salan Catalysts for the Asymmetric Epoxidation of Alkenes: Steric and Electronic Factors Governing the Activity and Enantioselectivity

“…For instance, a peroxotitanium(IV) reactive intermediate was predicted by DFT calculations in a titanium–salan based catalyst system for (nonstereoselective) sulfoxidation, and its formation was corroborated by atmospheric‐pressure chemical ionization mass spectrometry studies 31. Talsi and Bryliakov studied the kinetics of parallel oxidation of various sulfides on titanium–salan catalysts and concluded that, under those reaction conditions (−10 °C, C Ti =10 −4 M ), the reaction is rate‐limited by the reaction of H 2 O 2 with the chiral Ti complex to form the active oxidant 21. The latter rapidly reacts further with the sulfide.…”

“…The latter rapidly reacts further with the sulfide. The oxidation of sulfide by the electrophilic active species (with Hammett ρ values of −1.35 to −1.40) was concluded to proceed through an oxygen‐transfer mechanism 21…”

“…By contrast, when a more readily oxidizing sulfide substrate, t BuSMe, was taken instead of 1‐octene, only partial disappearance of the 303 nm feature (to convert into the λ max 360 nm active species) was documented; furthermore, the concentration of the λ max 303 nm active species approached a minimum value within approximately 18 min and subsequently began to recover slowly (Figure 3 B). This behavior is characteristic of a quasi‐steady‐state situation when the active intermediate species (formed through a relatively slow reaction between Ti‐ 1 and H 2 O 2 21) rapidly oxidizes the sulfide substrate, to leave a relatively small concentration of the active species.…”

Metal-catalyzed asymmetric sulfoxidation, epoxidation and hydroxylation by hydrogen peroxide

“…Bryliakov and Talsi reported a simple and efficient catalytic procedure for the oxidation of bulky thioethers with hydrogen peroxide2 [99,100]. This has been achieved by reducing the steric bulk of the salicylidene rings [100].…”

A new supramolecular zinc(II) complex containing 4‐biphenylcarbaldehyde isonicotinoylhydrazone ligand: Nanostructure synthesis, catalytic activities and Hirshfeld surface analysis

“…On the other hand, various oxidants such as selenium oxide (SeO 2 ), nitric acid, permanganates, hydrogen peroxide, and oxygen are used for the oxidation of sulfides . Among them, aqueous hydrogen peroxide (H 2 O 2 ) is the most attractive because of safety with the low expense, the high content of effective oxygen, being environment‐friendly, easily available reagent, and also it generates water as the only by‐product . We have also examined the close intermolecular contacts between the molecules in the crystal packing of 1 using Hirshfeld surface analysis.…”