Eco-Friendly, Catalyst and Solvent-Free, Synthesis of Acetanilides and N-Benzothiazole-2-yl-acetamides

Abstract

An expeditious and green synthesis of acetamides in a solvent-free simple way is described, without catalyst or additives, and in good yield by an instantaneous reaction of anilines or 2-aminothiazoles and acetic anhydride without external heating, and with simple purification. Sixteen substituted acetanilides and four N-benzothiazole-2-yl-acetamides were formed, but aliphatic amines of low molecular weight were not as effective as aromatic ones, and only cyclohexylamine and the enaminone ethyl 3-amino-2-butenoate afforded the corresponding acetamides in good yield.Keywords: acetylation, acetamides, amides, solvente-free reaction, green chemistry IntroductionIn the modern synthetic chemistry the development of greener approaches to the functional group transformations is of continuous interest. 1 There are several ongoing efforts to develop methods which do not need solvent, additives, and without tedious purification. Even reactions carried out without heating and magnetic stirring are relevant contributions, due to the energy economy aspects. 2 The acetamide/acetanilide are functional groups whose importance is beyond of a simple protecting group. For instance, in medicinal chemistry they play a pivotal role affording chemically stable compounds as prodrugs with improved pharmacological profile, and many N-acylated derivatives are in clinical use. 3 On the other hand, acetanilides have natural aptitude to act as ortho directing group in C−H transformations to C−C bond formation, wherein functionalized benzophenone, 4 quinone, 5 bisphenyl, 6 or styrene 7,8 derivatives can be obtained by Pd or Rh catalysis, Figure 1. Besides, the reactivity of some ortho functional group of acetanilides is modulated by the presence of the N-acetyl moiety, which is thus selectively converted to more complex compound, constituting this kind of acetanilide into important synthetic intermediates. 5,[9][10][11][12][13] Due to the abovementioned applications, new developments in the acetylation procedure are still desirables, and representative contributions are described, Figure 2.14-24 Furthermore, in undergraduate courses, the parent acetanilide is extensively synthesized as classical preparation of aromatic amide.25 Therefore, we recently developed a practical solvent-free green synthesis of such compound to experimental courses. 26 However, a search in the literature revealed that when other substituted acetanilides are needed as starting material in the context of a research project, the preparation used is very similar to that of parent acentanilide, which is almost the same of one century ago, which uses large amounts of solvents and additives such as acetic acid and sodium acetate, for instance. [5][6][7][8][9][10][11][12][13][14][15][19][20][21][22][23][24] It should be pointed out that, although there are a few syntheses of acetanilides in the absence of solvent, it is always necessary the use of grinding or microwave heating in these previously described syntheses or, more recurrent, the use of catalyst such as morpholiniu...