%0 Journal Article
%T Chitosan as a Renewable Heterogeneous Catalyst for the Knoevenagel Reaction in Ionic Liquid as Green Solvent
%A Nam T. S. Phan
%A Ky K. A. Le
%A Thien V. Nguyen
%A Nhan T. H. Le
%J ISRN Organic Chemistry
%D 2012
%R 10.5402/2012/928484
%X The combination of chitosan as a renewable heterogeneous catalyst and ionic liquid as a ※green§ solvent was employed for the Knoevenagel reaction. The chitosan catalyst was characterized by various techniques, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), and elemental analysis. Excellent conversions were achieved under mild conditions without the need for an inert atmosphere. There was no contribution from leached active species, and conversion was only being possible in the presence of the solid catalyst. The chitosan catalyst as well as the ionic liquid solvent could be recovered in essentially pure form after being used in the reaction, and each of them could be reused several times without a significant degradation in efficiency. 1. Introduction Room temperature ionic liquids have been considered as potential green alternatives to conventional volatile organic solvents during the last decade [1每5]. They exhibit several advantages such as negligible vapor pressure, excellent ability to dissolve organic compounds, ease of separation from products, and potential for recycling [6每8]. A variety of ionic liquids have been investigated, generally consisting of salts of organic cations, for example, tetraalkylammonium, alkylpyridinium, 1,3-dialkylimidazolium, tetraalkylphosphonium [2, 9]. During the past few years, several organic transformations have been carried out using ionic liquids as environmentally benign solvents, such as hydrogenation [10], oxidation [11每14], Heck cross-coupling reaction [15, 16], Suzuki reaction [17], Sonogashira reaction [18], Diels-Alder reaction [19], aldol condensation [20], alkylation [21每23], Micheal addition [24], oxa-Michael addition [25], Schmidt reaction [26], ring-closing metathesis [27], esterification reaction [28, 29], and enzyme-catalyzed organic reactions [30每33]. However, since the application of the first ionic liquid sample as solvent for organic transformations, research works have been mostly focused on homogeneous catalysis in ionic liquids. Indeed, reports on organic reactions using heterogeneous catalysts in ionic liquids as solvents have been very limited in the literature [34每38]. The Knoevenagel condensation between aldehydes or ketones with activated methylene compounds is one of important carbon-carbon forming reactions in organic synthesis [39, 40]. Conventionally, this reaction is catalyzed by alkali metal hydroxides or by organic bases under
%U http://www.hindawi.com/journals/isrn.organic.chemistry/2012/928484/