Investigations of the Mechanism of a Friedel-Crafts-Like Gold and Silver-Catalyzed Microwave Reaction PDF Download

Author: Tamara Nader El-Hayek Ewing
Publisher:
ISBN:
Category :
Languages : en
Pages :

Get Book Here

Book Description
Within the last 20-40 years, gold catalysis has becoming increasingly more interesting to researchers. A particular turning point was when one group was able to perform several alcohol additions to unactivated alkynes using a gold(I) catalyst. A significant result of this success was the ability to replace toxic mercury (II) catalysts that had been previously used to perform similar transformations. Since this report, others have successfully shown that gold(I) catalysts of the form [L-Au-Cl] as well as gold(III) catalysts in various forms, are able to activate a variety of starting materials for inter and intramolecular transformations. Despite this work, the mechanism of action for gold catalysis remains unknown. There are several complicating factors including relativistic, ligand, solvent and temperature effects, among others. Of particular interest to us are silver effect complications as well as the phenomena of gold decay. Silver salts are typically used in gold catalysis as an activating co-catalyst. The silver salt is presumed to be a halide scavenger in gold(I) catalysis, removing the chloride and exposing the Au+ center. However, several groups have reported on silver and its counter ion playing a less benign role--even demonstrating that silver can catalyze transformations on its own. This at the least warrants further investigation of the nature of silver in the reaction. Gold decay also introduces complications to gold catalysis. One possibility for the explanation for gold decay is through disproportionation of the gold(I) catalyst to gold(0) and gold(III). This is a particularly exciting avenue for exploration in the work done here. In the reaction of interest, 1-phenylethol and 4-methylanisole combine to produce a Friedel-Crafts-like product. In typical Friedel-Crafts reactions, a hard acid like AlCl3 is used to generate a carbocation which then undergoes a nucleophilic addition and C-C bond formation. In our reaction, if the gold(I) catalyst that was utilized initially were to disproportionate, gold(III)--a harder Lewis Acid--would help explain the Friedel-Crafts reactivity and the gold(0) generated would explain the gold-plating on the vials seen after the reaction. Through organic synthesis methods as well as some inorganic analysis, we were able to make some interesting discoveries. First, we see gold(I) decay in the absence of substrate, visually producing black/purple particles. At extended reaction times, the analysis of these particles shows about 33% of the gold we put in by mass was recovered. This still left open the possibility of gold(I) in solution as the active catalyst, however. In fact, when the particles from the substrate-less reaction were filtered out, the filtrate remaining was still able to catalyze the reaction. This did not lead us closer to understanding if gold(I) or gold(III) was the active catalyst but did let us know that the active catalyst remained in solution despite decay. We were also able to confirm that a gold(III) salt can catalyze the reaction of interest, though the yields were less than half of what was accomplished in the optimized gold(I) reaction. Additionally, with low catalyst loadings and significantly reduced reaction times (1 minute), a secondary product was observed in the gold(III)-catalyzed reactions. The nature of this secondary product--whether it is a major/minor product or if it is some intermediate species is still under investigation. Overall, evidence showing that gold(0) is being formed and the ability for a gold(III) salt to form the Friedel-Crafts-like product is clear, but that alone does not fully support disproportionation. Indirect evidence leads us to believe that disproportionation is the mechanism for gold decay, however, without direct evidence of a gold(III) species, we cannot say for sure. Also, there is some support for gold(III) being the active catalyst but there are some necessary questions that still need to be explored. In particular, an investigation of the possible intermediate or secondary product in both gold(I) and gold(III)-catalyzed reactions would be helpful in identifying the mechanistic path for the C-C bond formation in this Friedel-Crafts-like addition.