On the structure of the Kagan–Modena catalysts for asymmetric oxidation of sulfides

A Combined Theoretical and Experimental Investigation on the Enantioselective Oxidation of Aryl Benzyl Sulfides in the Presence of a Chiral Titanium Catalyst

“…A mechanistic scheme was suggested for vanadium catalysts, [21] but nothing similar has been reported for active titanium species, even though titanium was the first metal that was proposed for these reactions [9] and a huge amount of literature relating to its use has been published in more than 25 years. [6,7] The proposal of a plausible reaction mechanism encountered serious problems, [35][36][37] the most significant being the experimental evidence for a fast exchange of titanium ligands in solution [35] and the formation of supramolecular assemblies. It has been demonstrated that the complexes formed between titanium and tartrates [36] and between titanium and BINOL [14,37] yield intricate patterns when observed by spectroscopic techniques.…”

Asymmetric oxidation of 2-(arylsulfenyl)pyrroles

“…Although the Kagan and Modena procedures have been widely used for the oxidation of many prochiral sulfides, the structure of the reagent responsible for the stereochemical course of the reaction is still unknown. 39,40 In some cases, the addition of additives and the preparation of the reagent are crucial in obtaining specific chiral sulfoxides with high enantiopurities and/or yields. 37,41,42 More seriously, prochiral sulfides with two sterically different substitutents are generally good substrates for asymmetric oxidation, whereas substrates with similar groups often give products with low enantiopurities.…”

A Tetradentate Ligand for the Enantioselective Ti(IV)-Promoted Oxidation of Sulfides to Sulfoxides: Origin of Enantioselectivity

“…14 A more precise clarification of the influence of water on enantioselectivity is challenging because of the presence of multiple species in the Kagan-Modena system. 4 The rates of Kagan-Modena oxidations increase with increasing hydroperoxide and sulfide concentrations with DET as ligand. 5 In the case of the sulfide reactant, the rate dependence is almost, but not strictly first order, possibly because the sulfide can complex to Ti and compete with hydroperoxide for Ti coordination to form an η 2 -peroxy complex.…”