Synthesis of Cationic TetraNHC Complexes of Cobalt, Chromium, Iron, and Platinum for Oxidative Group Transfer
Date of Award
Doctor of Philosophy
Ampofo Darko, Brian Long, Nitin Jain
Aziridines are the nitrogen analogue of epoxides, and like epoxides, this functional group is an intermediate in numerous syntheses. Additionally, they are found in several natural products with anticancer activity. Despite their chemical applications, aziridine synthesis faces two big challenges: functional group tolerance is limited and a large excess of one reagent is often needed. This research is focused on ameliorating these two limitations.Previous research demonstrated that square planar, strong donor metal complexes, such as porphyrin or macrocyclic tetracarbene complexes were capable of catalyzing aziridination. In our efforts to develop an ideal catalyst for aziridination, we chose to screen a series of transition metals with our tetracarbene ligand to determine which would be most effective for aziridination.A cobalt(II) tetracarbene complex was synthesized by transmetallation and tested for C2 + N1 catalytic aziridination. The complex was unable to catalyze aziridination; furthermore, it showed no reactivity towards organic azides. The oxidative chemistry of the cobalt complex was explored, and a series of cobalt(III) tetracarbene complexes were synthesized. These complexes proved ineffective at aziridination and were unsuitable for further oxidation to a cobalt(IV) complex.A chromium(III) tetracarbene dichloride complex was tested as an aziridination catalyst. The complex proved capable of performing catalytic aziridination at low alkene substrate loadings and performed aziridination with alkenes and azides containing protic functional groups, such as alcohols. The axial chloride ligand on the chromium was proposed as the key to successful catalysis. This complex represents the first ever C2 + N1 aziridination catalyst on a group 6 metal.Following our hypothesis of the penta-coordinated catalyst, an iron(II) pentacarbene complex was synthesized and characterized for aziridination catalysis. While the complex proved an effective catalyst for aziridination at low alkene loadings, the synthesis of the iron complex proved irreproducible.A series of azidoalkenes compounds were synthesized to study the organic chemistry of ring closing intramolecular aziridination. The aziridination reactions had high conversion of the starting materials. However, the reactions proved to be non-catalytic as the control and catalytic yields were identical.
Keller, Calvin L., "Synthesis of Cationic TetraNHC Complexes of Cobalt, Chromium, Iron, and Platinum for Oxidative Group Transfer. " PhD diss., University of Tennessee, 2019.
Chapter 2 of this document was previously published in the journal Chemical Communications, DOI: 10.1039/C7CC08928G.