Investigating the Effects of Tethered, Axial Lewis Base Coordination on Rhodium(II) Paddlewheel Complexes

Dirhodium(II) paddlewheel complexes are highly renowned for their use in diazo decomposition to form a metallic-carbenoid species. This species has been used for a diverse range of chemical transformations including cyclopropanation, cycloproprenation, C-H functionalization, and X-H (Si, S, O, N) insertion reactions. Modulation of these catalysts traditionally involve the exchange of bridging ligands which have profound effects on the catalyst's reactivity, chemo, and enantioselectivity. Recent interest has turned towards to modifying the axial sites present in the complex as an additional means of modulating catalytic activity. These sites normally serve as the active site of the catalyst, but coordination of Lewis Bases one of the sites are known to be beneficial to chemo and enantioselectivity. However, a main problem encountered in this field is the lack of control of coordination to axial site. This work aims to examine the development of novel ligands containing a pendant chain containing a Lewis base, and their incorporation into the rhodium(II) paddlewheel scaffold. Inclusion of these tether- containing ligands allows for more direct control of the location of the Lewis base and effectively increases the local concentration of the Lewis base near the active site. Ligands derived from amino acids or amino alcohols that contained either a phosphite or a thioether moiety were synthesized and attempts to exchange onto the rhodium complex were investigated. Exchange of the phosphite containing ligands was unsuccessful due to complications with oxidation. The thioether ligands proved to be more robust in comparison with successful exchange onto the catalyst scaffold with a variety of thioether derivatives. Evaluation of these complexes in Si-H insertion and cyclopropanation reactions revealed that the presence of the tethered thioether does indeed provide a positive benefit, with increased yields as compared to a control with no tethered thioether additive.