Molecularly enlarged S,S-BnTsDPEN ligands for iron-catalyzed asymmetric olefin epoxidation reactions using hydrogen peroxide

摘要

A convenient approach for the anchoring of S,S-BnTsDPEN ligands (S,S-N-tosyl-1,2-diphenylethylenediamine) to branched carbosilane scaffolds was investigated. It is based on a high-yielding reductive amination reaction between commercially available S,S-TsDPEN and readily accessible carbosilanes furnished with benzaldehyde terminal fragments. These molecularly enlarged ligands, bearing four S,S-BnTsDPEN units, and their simplified monomeric and “dimeric” analogues were evaluated in iron(III)-catalyzed asymmetric trans-stilbene epoxidation reactions using hydrogen peroxide as an environmentally benign oxidant. The catalytic investigations showed a large degree of variation in the activity and stereoselectivity of the series of DPEN catalysts. In combination with ESI-MS investigations, these data revealed an important role of the ligand orientation in determining the overall activity of the catalyst system. Accordingly, a suitable design of the molecularly enlarged ligands resulted in fully retained activity and selectivity in catalysis. Finally, a number of strategies for the recovery and reuse of the best performing carbosilane-tethered DPEN ligands were explored.