A μ‐Oxo‐μ‐η²:η²‐Peroxo Titanium Complex as a Reservoir of Active Species in Asymmetric Epoxidation Using Hydrogen Peroxide

Abstract

The development of efficient methods for preparing chiral epoxides with high optical purity is a topic of great importance in organic synthesis. [1][2][3] As part of our studies on titanium-catalyzed asymmetric oxidation reactions using hydrogen peroxide as an oxidant, [4][5][6] we recently reported that the readily available salan compound 1 serves as an effective ligand for the titanium-catalyzed asymmetric epoxidation of unfunctionalized olefins (Scheme 1 a). [7] On the other hand, the corresponding titanium(salen) complexes (in which double bonds replace the single bonds between the benzylic carbon atoms and the nitrogen atoms of the ligand) have been found to react with hydrogen peroxide to form a peroxo complex that is responsible for sulfide oxidation but is inert to olefin epoxidation. On the basis of these findings, we proposed that the titanium(salan) complexes provide more reactive peroxo species, which arise from a hydrogen bond between one of the peroxo oxygen atoms and the amino proton (Scheme 1 b). Although the participation of the hydrogen-bonded peroxo species would explain the sharp contrast in reactivities, the detailed reaction mechanism was unclear. Herein, we report the synthesis of an isolable m-oxom-h 2 :h 2 -peroxo titanium complex that plays an important role as a reservoir of catalytically active species for asymmetric epoxidation.We have reported that the premade di-m-oxo titanium-(salan) catalyst with ligand ent-1 a (the enantiomer of 1 a) and the catalyst prepared in situ from Ti(OiPr) 4 and salan ent-1 a gave comparable yields and enantioselectivities in the asymmetric epoxidation of olefins, and the X-ray structure analysis of the di-m-oxo complex with ent-1 a revealed its homochiral structure, in which each cis-b titanium(salan) unit (in which one oxygen atom of the four coordinating atoms occupies an axial position) has the same chirality with respect to the titanium center (anti-L,R,R,S N ,S N -L,R,R,S N ,S N ). [7a, 8] We have also found that the catalyst prepared in situ with salan 1 b showed a higher catalytic performance. Thus, we decided to evaluate the asymmetric catalysis of its di-m-oxo titanium complex. Since ligand ent-1 a resulted only in the di-m-oxo complex with the (anti-L,R,R,S N ,S N -L,R,R,S N ,S N ) configuration, we expected the production of a single isomer. [9] Contrary to our expectations, however, salan 1 b resulted in three di-m-oxo complexes (2, 3, and 4; Scheme 2), although complex 2 was the major product. Fortunately, each structure was unambiguously characterized by X-ray crystallographic analysis.[10] While complex 2 has a homochiral (anti-D,S,S,R N ,R N -D,S,S,R N ,R N ) configuration identical to the di-moxo complex with 1 a, complexes 3 and 4 are pseudoheterochiral, as the titanium centers in the two units have opposite chirality (anti-L,S,S,S N ,R N -D,S,S,R N ,R N for 3; anti-L,S,S,S N ,S N -D,S,S,R N ,R N for 4; see Figure 1 and Figures S1 and S2 in the Supporting Information). Complexes 3 and 4 differ in the stereochemistry of the chiral nitr...

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

“…Taking into account the previous success of salalen and salan ligands6–26 bearing 3,3′‐aryl substituents, the framework of salan ligand 1 was chosen as the basis for further modifications. Several titanium(IV) salan and salalen complexes with ligands 1 – 14 (Figure 1) were prepared and tested as catalysts in the epoxidation of the same substrate (styrene) with H 2 O 2 (Table 1).…”

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

“…At the first (rate‐determining) step, an adduct is formed between the titanium complex and hydrogen peroxide (cf. other studies). Coordination of H 2 O 2 to the titanium center in the chiral ligand framework is (1) a precondition to accomplish the oxygen transfer in a stereoselective fashion and (2) believed to increase the peroxide electrophilicity and activate it for the sulfide oxidation: in the absence of Ti‐salan catalysts, no oxidation takes place.…”

“…What is the nature of the active species responsible for the enantioselective oxygen transfer to the sulfide? In 2005–2009, Katsuki and co‐workers contributed a series of papers dedicated to enantioselective epoxidations with H 2 O 2 in the presence of chiral titanium(IV) salan and salalen complexes . The authors managed to isolate and X‐ray characterize a binuclear μ‐oxo‐μ‐peroxo titanium–salan complex, formed in the reaction of the starting (also binuclear) catalyst with H 2 O 2 .…”

“…The authors managed to isolate and X‐ray characterize a binuclear μ‐oxo‐μ‐peroxo titanium–salan complex, formed in the reaction of the starting (also binuclear) catalyst with H 2 O 2 . The authors believed the μ‐oxo‐μ‐peroxo complex to be a precursor of the elusive active epoxidizing species . Using high‐resolution electrospray mass spectrometric technique, Berkessel with co‐workers identified a titanium–salalen complex (formed in situ from the salalen ligand and Ti(O i Pr) 4 ) as a mononuclear species .…”

Mesoporous Titanium–Silica Composite Encapsulated γ-Fe2O3: Novel Magnetically Recyclable Heterogeneous Catalysts for Olefin Epoxidation Reactions

“…Transition metal titanium has been widely taken as active species for epoxidation reactions [3]. Although many catalytic epoxidation reactions using homogeneous catalysts have been successfully demonstrated [4][5][6][7][8][9][10], the difficulties in separating of the catalysts greatly restrict their applications in practice. Titanium-containing mesoporous molecular sieves have been used as efficient heterogeneous catalysts in the epoxidation of alkenes [11,12].…”