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Ecologically Friendly, Efficient and Catalyst-free N-acetylation of Amines in Choline Chloride Based Deep Eutectic Solvents

Acetylation is a reaction of introduction of acetyl functional group into a chemical compound [1, 2]. Amine acetylation is one of the most widely used reactions for amide synthesis and protection of –NH group [3]. Conventional method for acetylation implies refluxing of compounds with an excess of acetic anhydride or acetyl chloride in presence of catalyst [3, 4], and is often characterised by use of dangerous, expensive and non-eco-friendly solvents or catalysts, long reaction time, harsh reaction conditions and average yield. Deep Eutectic Solvents (DESs) are highly desirable „green solvents”, often used without any catalysts. Choline chloride (ChCl) is often used in formation of DESs due to its low toxicity, biodegradability and low cost, in combination with urea, thiourea, glycerol, ethylene glycol, carboxylic acid, etc. [5]. N-acetylation of various amines was performed in several choline chloride based DESs. A series of amines were acetylated under conditions optimized in our preliminary research, giving excellent results considering reaction times (10-60 min) and isolated yields up to 91%. For instance, acetylation of 4-(trifluoromethyl)aniline was over within an hour with yield of isolated pure product of 88%. Acetylation of aniline, 4-chloroaniline and naphthalene-1-amine was completed within 30 minutes with achieved yield of isolated pure product of 91, 98 and 99%. Choline chloride based DESs were proven to be an excellent media for fast, selective and catalyst- free acetylation of selected amines. Presented protocol offers several advantages such as operational simplicity, easy workup, shorter reaction time, excellent yield, economical and environmentally acceptable reaction conditions with the use of cost-effective, recyclable, non-toxic and bio-degradable solvents. [1] A. Chikkulapalli et al., Tetrahedron Lett. 56 (2015) 3799-3803. [2] S. Ouarna et al., Orient. J. Chemi. 31 (2015) 913-919. [3] K. Phukan et al., Synt. Comm. 39 (2009) 2694- 2701. [4] S. Naik et al., ARKIVOC i (2004) 55-63. [5] Q. Zhang et al., Chem. Soc. Rev. 41 (2012) 7108-7146.

amines; choline chloride; deep eutectic solvents; N-acetylation

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