UVM Theses and Dissertations
Format:
Print
Author:
Paradee, Lacey Marina
Dept./Program:
Chemistry
Year:
2009
Degree:
MS
Abstract:
Photolytically-driven carbonyl substitution reactions of Cr(arene)(CO)₃ compounds are limited by low yields and by lack of generality to the type of arene involved. Although electrochemical oxidations of such compounds provide for rapid generation of the monocation [Cr(arene)(CO)₃], traditional supporting electrolyte anions such as [PF₆]⁻ decompose the monocation by nucleophilic attack, even at reduced temperatures. Hence, cation radicals which are stable on the cyclic voltammetric time scale are unstable on the synthetic time scale.
A new approach was therefore needed to the electrochemistry of Cr(arene)CO₃ compounds which would allow more efficient use of this method of eiectrosynthesis. In order to test a new approacfi, a chromium estradiol complex was chosen, owing to the instability of its cation radical, as shown in the earlier chemical literature. In the present work, studies were performed using a steroidal arene complex, Cr(estradiol)(CO)₃,1. It was found that by changing the supporting electrolyte anions to weakly-coordinating anions such as [B(C₆F₅)₄]⁻, TFAB or [B(C₆H₃(CF₃)₂)₄]⁻, BArF₂₄, electrolyte nucleophile attacks were minimized and the monocation was stable on a synthetic time scale. The 1/1 couple was determined to be chemically and electrochemically reversible. Exhaustive oxidation yielded solutions of 1 that were characterized by FT-IR (via spectro-electrochemistry) and ESR spectroscopy.
A redox sequence was possible in which 1 was anodically oxidized in the presence L giving the cations [Cr(estradiol)(CO)₂L] (L = trimethylphosphosphite, 4). Both of these cations were characterized by ESR spectroscopy, and 2 was also characterized by FT-IR. This was followed by cathodic reduction to give the neutral products in good yields in solution. The overall mechanism is an example of an electrochemical switching process, and promises to be valuable for the electrochemical preparation of Cr(arene)(CO)₂L complexes.
A new approach was therefore needed to the electrochemistry of Cr(arene)CO₃ compounds which would allow more efficient use of this method of eiectrosynthesis. In order to test a new approacfi, a chromium estradiol complex was chosen, owing to the instability of its cation radical, as shown in the earlier chemical literature. In the present work, studies were performed using a steroidal arene complex, Cr(estradiol)(CO)₃,1. It was found that by changing the supporting electrolyte anions to weakly-coordinating anions such as [B(C₆F₅)₄]⁻, TFAB or [B(C₆H₃(CF₃)₂)₄]⁻, BArF₂₄, electrolyte nucleophile attacks were minimized and the monocation was stable on a synthetic time scale. The 1/1 couple was determined to be chemically and electrochemically reversible. Exhaustive oxidation yielded solutions of 1 that were characterized by FT-IR (via spectro-electrochemistry) and ESR spectroscopy.
A redox sequence was possible in which 1 was anodically oxidized in the presence L giving the cations [Cr(estradiol)(CO)₂L] (L = trimethylphosphosphite, 4). Both of these cations were characterized by ESR spectroscopy, and 2 was also characterized by FT-IR. This was followed by cathodic reduction to give the neutral products in good yields in solution. The overall mechanism is an example of an electrochemical switching process, and promises to be valuable for the electrochemical preparation of Cr(arene)(CO)₂L complexes.