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

Abstract

A series of chiral Schiff bases (L 1 -L 5 ) with different substituents in the salicylidenyl unit were prepared from condensation of 3-aryl-5-tert-butylsalicylaldehyde derivatives and optically active amino alcohols. Bromination of 3-phenyl-5-tertbutylsalicylaldehyde gave an unexpected product 3-(4-bromophenyl)-5-bromosalicylaldehyde, from which the corresponding Schiff base ligands L 6 and L 7 , derived from (S)-valinol and (S)-tert-leucinol, respectively, were prepared. Ligands L 1 -L 7 were applied to the vanadium-catalyzed asymmetric oxidation of aryl methyl sulfides. Under the optimal conditions, the oxidation of the thioanisole with H 2 O 2 as oxidant in CH 2 Cl 2 catalyzed by VO(acac) 2 -L 1 -L 7 gives good yields (74-83%) with moderate enantioselectivity (58-77% ee). Ligand L 7 , containing a 4-bromophenyl group on the 3-position and a Br atom on the 5-position of the salicylidenyl moiety, displays an 80-90% ee for vanadium-catalyzed oxidation of methyl 4-bromophenyl sulfide and methyl 2-naphthyl sulfide.

Synthesis of enantioenriched sulfoxides

“…, Liu et al 80 and Li et al 82 also found dichloromethane, and chloroform to be superior to other solvents for this oxidation, confirming Ellman's results. 89 This gave superior enantioselectivity (98% ee) compared to dichloromethane (90% ee), however, the yield obtained (61%) was lower than when using dichloromethane (75%).…”

Catalytic Asymmetric Oxygenations with the Environmentally Benign Oxidants H2O2and O2

“…Sun and co-workers examined a series of 3-aryl-substituted chiral ligands of the type 105 in vanadium-catalyzed oxidations of aryl methyl sulfides, and reported moderate to good enantioselectivities (53–92% ee). , tert -Leucinol-derived ligands were shown to afford better asymmetric induction than valine- or phenylalanine-derived structures. The catalyst system VO­(acac) 2 / 106 exhibited enantioselectivities up to 77% ee in oxidation of aryl methyl sulfides in acetone …”

Camphor‐Based Schiff Base Of 3‐Endo‐Aminoborneol (SBAB): Novel Ligand for Vanadium‐Catalyzed Asymmetric Sulfoxidation and Subsequent Kinetic Resolution

“…In the vanadium catalyzed sulfoxidation reaction, halo‐substituted salicylaldehyde Schiff base exhibited excellent chiral induction, therefore, we synthesized and applied dibromo‐, diiodo‐, and ortho ‐bromo substituted ligands 31 (SBAIB‐8), 32 (SBAIB‐9) and 33 (SBAIB‐10) for this reaction; all ligands produced moderate enantioselectivity (entries 8—10, Table ,). In addition, extremely sterically hindered phenyl substituted ligands 34 (SBAIB‐11) and 35 (SBAIB‐12) were screened; they exhibited only moderate chiral induction (58 and 68 % ee ) for this reaction.…”