Polyethylene Glycols as Efficient Catalysts for the Oxidation of Xanthine Alkaloids by Ceric Ammonium Nitrate in Acetonitrile: A Kinetic and Mechanistic Approach
Kinetics of oxidation of xanthine alkaloids, such as Xanthine (XAN), hypoxanthine (HXAN), caffeine (CAF), theophylline (TPL), and theobromine (TBR), have been studied with ceric ammonium nitrate (CAN) using poly ethylene glycols (PEG) as catalysts. Reaction obeyed first order kinetics in both [CAN] and [Xanthine alkaloid]. Highly sluggish CAN-xanthine alkaloid reactions (in acetonitrile media even at elevated temperatures) are enhanced in presence PEGs (PEG-200, -300, -400, -600). An increase in [PEG] increased the rate of oxidation linearly. This observation coupled with a change in absorption of CAN in presence of PEG, [H–(OCH2–CH2)n–O–NH4Ce(NO3)4(CH3CN)] (PEG bound CAN species), is considered to be more reactive than CAN. The mechanism of oxidation in PEG media has been explained by Menger-Portnoy’s enzymatic model. 1. Introduction There has been an increasing interest in the kinetics of electron transfer reactions since more than half a century because of their ever green importance in understanding the mechanisms of industrially, pharmaceutically, and biologically important redox reactions [1–11]. A special focus has been paid to single electron transfer (SET) oxidations [1–18]. In this context, ceric ammonium nitrate (CAN) has emerged as one of the most valuable and notable SET oxidants for a variety of reactions [19–30], due to its relative abundance, ease of preparation, low cost, and low toxicity. During the oxidation of organic substrates, the initial formation of a radical or radical cation is usually followed by rearrangement or follow-up reactions that led to other free radical intermediates. Typically, the free radical reacts with another substrate (olefin, etc.) to form a new C–C bond and a product radical. Oxidation of the free radical intermediate to a cation leads to capture of solvent or nitrate expelled from CAN upon its reduction to Ce(III) and these alternative mechanistic pathways result in many of the side products prevalent in oxidations. Therefore, preparative Ce(IV) initiated oxidations cannot be achieved in many instances. Chemical intuition suggests that these pathways can be depressed by understanding the interrelationship between the mechanism of oxidation by Ce(IV), the effect of solvent on the stability of the initially formed radical cation intermediate, and the rates (mechanisms) of various available pathways. Polyethylene glycol (PEG) is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its
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