Immobilized aza-bis(oxazoline) copper catalysts on alkanethiol self-assembled monolayers on gold: Selectivity dependence on surface electronic environments

Aza-bis(oxazoline) copper complexes have been immobilized onto alkanethiol self-assembled monolayers on gold utilizing five background tail groups with different electronic characteristics. The catalyst was tested in the standard cyclopropanation reaction of ethyl diazoacetate and styrene. The five different tail groups were hydroxyl, bromine, carboxylic acid, ester, and nitrile. Enantioselectivity improved to 95% when the surrounding tail groups were hydroxyl- and bromine-terminated surfaces. The carboxylic acid and ester tail groups reduced the enantioselectivity compared to the homogeneous phase. Additionally, the homogeneous cyclopropanation reaction was performed in methanol, acetonitrile, ethyl acetate, and acetic acid to determine whether similar trends in selectivity could be obtained by varying the homogenous electronic environment. However, the cyclopropanation reaction in these solvents gave greatly reduced selectivity and yield of the cyclopropane products demonstrating the positive aspects of immobilization of self-assembled monolayer supports.