Self‐Supported Heterogeneous Titanium Catalysts for Enantioselective Carbonyl–Ene and Sulfoxidation Reactions

Abstract

A new strategy for the heterogenization of chiral titanium complexes was developed by the in situ assembly of bridged multitopic BINOL ligands with [Ti(OiPr)4] without using a support. The assembled heterogeneous catalysts (self-supported) showed excellent enantioselectivity in both the carbonyl-ene reaction of alpha-methylstyrene with ethyl glyoxylate (up to 98 % ee) and the oxidation of sulfides (up to >99 % ee). The catalytic performance of these heterogeneous catalytic systems was comparable or even superior to that attained with their homogeneous counterparts. The spacers between the two BINOL units of the ligands in the assembled catalysts had significant impact on the enantioselectivity of the carbonyl-ene reaction. This demonstrates the importance of the supramolecular structures of the assemblies on their catalytic behavior. In the catalysis of sulfoxidation, the self-supported heterogeneous titanium catalysts were highly stable and could be readily recycled and reused for over one month (at least eight cycles) without significant loss of activity and enantioselectivity (up to >99.9 % ee). The features of these self-supported catalysts, such as facile preparation, robust chiral structure of solid-state catalysts, high density of the catalytically active units in the solids, as well as easy recovery and simple recycling, are particularly important in developing methods for the synthesis of optically active compounds in industrial processes.

Modified Ta/MCM-41 catalysts for enantioselective oxidation of thioanisole

“…Ti-binol has been supported on polystyrene [20], on an ionic liquid film deposited on a modified SBA-15 silica [21], or self-supported by using a polytopic binol ligand [22]. Very high enantioselectivities were obtained in the sulfoxidation with TBHP or CHP, combining the direct enantioselective sulfoxidation with a partial kinetic resolution by over-oxidation.…”

Influence of substituents in the salicylaldehyde‐derived Schiff bases on vanadium‐catalyzed asymmetric oxidation of sulfides

“…[4,5] Therefore, asymmetric oxidation of prochiral sulfides is an attractive subject in organic synthesis. In the past two decades, asymmetric oxidation of sulfides has been extensively investigated, either promoted by Ti(O-i-Pr) 4 -chiral tartrates [6,7] and chiral organic compounds such as chiral oxaziridines, [8,9] or catalyzed by transition metal complexes of various chiral ligands, such as ironporphyrins, [10,11] manganese-salens, [12,13] titanium-BINOL, [14,15] titanium-C2-symmetric diols, [16,17] zirconium-trialkanol amines, [18] and vanadium- [19 -21] and iron- [22] tridentate Schiff base catalyst systems. Among these catalyst systems for asymmetric sulfoxidation, Bolm's catalysts, that is, VO(acac) 2 -and Fe(acac) 3 -Schiff base systems, have received considerable attention in recent years because of three attractive advantages: (1) the simplicity, convenient preparation, and easy modification of chiral Schiff base ligands; (2) the utilization of cheap and environmentally benign terminal oxidant (H 2 O 2 ); and (3) the facile reaction conditions and easy workup.…”

Ordered Mesoporous Silica as Supports in the Heterogeneous Asymmetric Catalysis

“…Bruce et al have reported the Polystyrene-bound cyclo-BINOLs for the Ti-catalyzed asymmetric oxidation of sulfides to sulfoxides with 78% ee and reusability up to three cycles [140]. Wang et al has utilized the self-supported strategy for the heterogenization of Ti-Binol complex which was highly stable and could be readily recycled and reused for over 1 month (at least eight cycles) without significant loss of activity and enantioselectivity (up to [99.9% ee) [141]. Fraile et al have immobilized Ti-diethyltartarate complex onto silica (Scheme 9) and got very good yield of sulfoxide with small (13%) asymmetric induction [142].…”