Doctoral Dissertations
Date of Award
12-2022
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Chemistry
Major Professor
Dr. Ampofo K. Darko
Committee Members
Dr. Michael D. Best, Dr. Konstantinos Vogiatzis, Dr. Steven C. Chmely
Abstract
Rhodium(II) paddlewheels are versatile carbene transfer catalyst that are broadly applied in insertion reactions, cycloadditions, and ylide transformations. The effects of axial coordination in rhodium(II)-catalyzed carbene transfer reactions are still little understood due to compounding factors that are difficult to isolate. Traditionally, researchers study axial coordination by addition of Lewis base additives. To ensure interaction between the Lewis base and catalyst, high molar equivalents are used. This can also have the undesired effect of hampering the activity of the catalyst and suppressing the yield of the reaction. We have developed ligands with a tethered Lewis base to overcome these issues. We found that the ligand can be selectively exchanged onto Rh2(OAc)4 to provide mono- and bis-complexes with a thioether as the Lewis base. These compounds were then characterized by NMR, UV-vis, x-ray crystallography, and cyclic voltammetry. The complexes were then studied in cyclopropanation reactions and silyl-hydrogen insertion reactions and compared to control catalyst to isolate the effects of the axial coordinating thioether. In cyclopropanations with ethyl diazoacetate it was demonstrated that reactivity was affected by tether length and the substituent on the thioether. It was also found that complexes with the tether yielded higher than the control catalyst. In silyl-hydrogen insertion reactions with methyl phenyl diazoacetate it was found that the catalyst was selective toward diazos with donating substituents and provided comparable yields otherwise. This result led us to study the catalyst in silyl-hydrogen insertions with hydrazone protected donor diazos. It was found that the control catalyst outperformed the catalysts with tethers. Density functional theory calculations, geometry optimizations and transition state searches, were performed to supplement the theoretical understanding of the role the tether has in reactivity. The calculations indicate donation of the axial ligand destabilizes HOMO of the catalyst and resulting rhodium(II)-carbene intermediate.
Recommended Citation
Abshire, Anthony Dean, "Tethered Axial Coordination as a Control Modality in Rhodium(II)-Catalyzed Carbene Transfer Reactions. " PhD diss., University of Tennessee, 2022.
https://trace.tennessee.edu/utk_graddiss/7714