Fine tuning RhII complexes with tethered, axial coordination: structural studies and application to diazo-mediated cyclopropanation reactions

The cyclopropane moiety is an attractive synthetic target due to its application in pharmaceuticals and medicinal research. One effective strategy involves the formation of metal carbenoid species from diazo reagents. The carbenoid then reacts with an olefin substrate to generate the cyclopropane ring. Of the metal complexes that can facilitate this reaction, dirhodium(II) paddlewheel complexes are arguably the most prevalent catalysts. This is because modification of the bridging ligands enables control to be exerted over the catalyst’s chemoselectivity and enantioselectivity. Exploiting the axial site as a control element is often overlooked as strongly coordinated Lewis bases inhibit catalysis. Despite this, Lewis base additives have been observed to increase enantioinduction in cyclopropanation reactions and axial coordination has been suggested as a possible explanation. However, leveraging axial coordination as a control element remains a problem due to the difficulty of controlling the coordination of exogenous ligand.

The goal of my research is the development of heteroleptic dirhodium(II) paddlewheel complexes with tethered thioether ligands that are capable of axial coordination. Tethering of the Lewis base anchors the thioether proximal to the axial site to provide control over axial coordination. Solid and solution-state characterization indicated that axial coordination was present as part of a rapid equilibrium. The electrochemical analysis demonstrated the ability of different thioethers to modulate the electrophilicity of the complex. These complexes were then tested as catalysts in the cyclopropanation of olefins and diazo reagents. It was shown that the novel complexes were better suited for more reactive diazo reagents and afforded higher yields than control catalysts.