Molybdenum(VI)-promoted sulfoxidation in the presence of tridentate sugar-based Schiff-base ligands

• Chiral sugar-based Schiff-base ligands.
• Molybdenum-promoted sulfoxidation reaction.
• Electrophilic character is observed for the active species.
• Chirality of enantiomeric excess triggered by the substitution pattern of the salicylidene moiety.

A series of Schiff-base ligands derived from 6-aminoglucose and various salicylaldehyde derivatives introducing different electronic and steric effects were synthesized and fully characterized. These ligands were used in molybdenum-promoted sulfoxidation reactions and tested for their influence on the activity of the employed catalytic system. It was found that in situ generated catalysts perform superior to pre-prepared complexes. The presence of a benzylidene protecting group in position C4 and C6 of the sugar backbone leads to a better enantioselectivity of the catalytic system. The in situ catalytic systems with benzylidene protecting group at the sugar backbone have been tested for the influence of electronic and steric effects by substituents at the salicylidene moiety. Electron withdrawing groups were found to increase the catalytic activity, indicating the electrophilic nature of the active species during catalysis. Steric effects were found to play only a minor role as the overall reactivity is concerned. However, steric effects seem to determine the observed chirality of the enantiomeric excess obtained in the sulfoxidation reaction. Overoxidation leading to the sulfone is generally observed, but decreases with the steric demand of the supporting Schiff-base ligand.

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