Influence of Tethered, Axially Coordinated Ligands on Rh(II,II)-Catalyzed Carbene Transfer Reactions

Dirhodium (II,II) paddlewheel complexes have become ubiquitous in diazo-mediated carbene transfer reactions. The Rh(II,II)-carbene intermediate is capable of a large variety of transformations such as cyclopropanation, C-H and X-H (O, N, S, Si, B) insertion reactions, cyclopropenations, and ylide transformations. Cyclopropanation reactions resulting in the formation of functionalized cyclopropane structures has always been a major focus in Rh(II,II)-carbene chemistry. Improvements on catalytic performance in cyclopropanations has largely focused on the modification of the bridging ligands and has resulted in Rh(II,II) catalysts that exhibit high reactivity and selectivity in cyclopropanation reactions. However, high enantio- and diastereoselectivity is not easily achieved with all carbene types and other control elements on the catalyst, such as axial coordination, can possibly remedy this issue. Although there are some examples of axially coordinated Lewis Base additives affecting stereoselectivity, elucidation of the role the axial ligand plays in the reaction is still ambiguous. To gain more control over the axial stie and more accurately study the role axial coordination, the synthesis of axial ligands that are anchored to a bridging ligand site have been developed.

My work in the Darko group has focused on developing Rh(II,II) complexes that contain tethered, axially coordinated ligands (TACLs) and studying the effect the axial coordination has on diazo-mediated carbene transfer reactions, specifically cyclopropanations. Characterization in both the solid and solution state confirmed axial coordination. The complexes demonstrated higher cyclopropanation yields than control catalysts when using acceptor-type carbenes. It has also been determined that modifications to the axial ligands can have a small influence on the reaction’s sensitivity to changes in the substrate.