%0 Journal Article
%T Polyethylene Glycols as Efficient Media for Decarboxylative Nitration of ¦Á,¦Â-Unsaturated Aromatic Carboxylic Acids by Ceric Ammonium Nitrate in Acetonitrile Medium: A Kinetic and Mechanistic Study
%A K. Ramesh
%A S. Shylaja
%A K. C. Rajanna
%A P. Giridhar Reddy
%A P. K. Saiprakash
%J Advances in Physical Chemistry
%D 2013
%I Hindawi Publishing Corporation
%R 10.1155/2013/146585
%X Polyethylene glycols (PEGs) were found to be efficient media for decarboxylative nitration of ¦Á,¦Â-unsaturated aromatic carboxylic acids by ceric ammonium nitrate (CAN) in acetonitrile to give ¦Â-nitrostyrene derivatives. Kinetics of the reaction exhibited second order kinetics with a first order dependence on [CAN] and [substrate]. Reactions were too sluggish to be studied in the absence of PEG; therefore detailed kinetics were not taken up. Reaction times were reduced from 24 hrs to few hours. The catalytic activity was found to be in the increasing order PEG-300 > PEG-400 > PEG-600 > PEG-200. Mechanism of PEG-mediated reactions was explained by Menger-Portnoy's scheme as applied in micellar kinetics. 1. Introduction Cerium (IV) ammonium nitrate (CAN) is one of the most important reagents used for organic synthesis [1¨C4]. The formula of cerium (IV) ammonium nitrate, [(NH4)2[Ce(NO3)6], reflects that the cerium (IV) ion is surrounded by six nitrate groups and the ammonium ions are counterions to compensate for the negative charge of the hexanitratocerate (IV) coordinating unit. CAN is a one-electron oxidation reagent. Cerium (IV) ammonium nitrate can also be used as a nitrating agent [5, 6], initiator for radical polymerization reactions [7], and also as a reagent to remove protecting groups [8]. Even though cerium (IV) reagents are milder oxidation reagents than the other metal-based oxidation reagents such as Mn(VII) and Cr(VI) salts, they are relatively much less toxic. Because of their high molecular mass, large quantities of cerium (IV) salts are required for stoichiometric reactions. Therefore, indirect and catalytic reactions using Ce(IV) have been developed [9, 10]. The main advantage of CAN over other cerium (IV) reagents is its higher solubility in organic solvents. The most popular solvents are (in decreasing order of importance) water, acetonitrile, dichloromethane, THF, and methanol [11]. Often mixtures of these solvents are also used as reaction medium. Other solvents have found only marginal use for this type of reactions. Being good Michael acceptors, ¦Á,¦Â-unsaturated nitroalkenes are widely applied in organic synthesis [12]. Among various methods reported for their preparation, a method involving the use of CAN provides a practical way to the synthesis of ¦Á,¦Â-unsaturated ¡°nitroalkenes¡± in good-to-excellent yields [13¨C23]. Recently polyethylene glycols (PEGs) have been used as catalysts and catalyst supports and also have been found to be an inexpensive, nontoxic, and environmentally friendly reaction medium, which avoid the use of acid or
%U http://www.hindawi.com/journals/apc/2013/146585/